# My 1st Bill Since New AC Installed - WAS:100% Positive Flow AC in the Shop



## HorizontalMike (Jun 3, 2010)

Like the title says, I am interested in installing a 100% positive flow AC in the shop. What I mean is NO RETURN AIR TO THE AC UNIT.

For all the detractors, I understand that THAT is not efficient or cost effective, so please bite the bullet on that portion. What I am trying to find is a unit that only provides NEW air (AC'd new air) into the shop. In other words, I DO NOT want to be cleaning air filters all of the time.

My shop:

24' x 30' x12' = or roughly 1100 sq ft for an equivalent 8ft tall shop.
R-10 & R 7.5 Insulation via Solar Guard in a metal shop.
South Texas environment, very hot summers…
Marginal Winters…

WHAT I WANT/DESIRE:

100% New air into the shop, in order to eliminate dusty filters.
At least marginal cooling/dehumidifying while working in shop.

*DO THEY MAKE SUCH A CRITTER?*

I have read A1Jim's post ( http://lumberjocks.com/topics/165146#reply-textbox ) and found much information from Jim and Randy, but really looking for an alternative…

SO THE BOTTOM LINE IS:
Is there an AC solution that will ONLY pump fresh air into an environment/room, without sucking in all the bad dusty air?


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## GR8HUNTER (Jun 13, 2016)

what about a split system Mike ….would that work for you ???? :<))

1250 square feet = 30,000 BTU

https://www.ecomfort.com/cooling/fujitsu-30000-btu-mini-splits.html


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## RichT (Oct 14, 2016)

Well, like you said, it's not efficient. You can take in air for the AC from anywhere with custom ductwork. The advantage of a closed system is that you are recirculating cooler air that's already been dehumidified. With an open system, you'll be cooling/dehumidifying outside air, so the cost to operate is going to be significantly higher.

I can appreciate your not wanting to be changing filters all the time, but is it possible to construct a multi-stage filter with a large, cheap fiberglass first stage filter that could be taken off and knocked clean? You could then taper down to a pleated filter to keep the AC innards clean.


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## Desert_Woodworker (Jan 28, 2015)

> what about a split system Mike ….would that work for you ???? :<))
> 
> - GR8HUNTER


This was my thought as well.


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## clin (Sep 3, 2015)

I'm not sure if what you want exists. But I ran some basic calculations to see if it makes any possible sense.

Lots of assumptions are needed. Your shop has a volume of 8,800 cu-ft. Let say the AC unit moves 1,000 cu-ft/min (CFM). So you would exchange the air about every 9 mins. This exchange rate directly affects how much cooling you need, and of course affects the filtering affect.

A 1,100 sq0ft shop is pretty big. I used a Jet room filter that moves 1,000 CFM. While making dust it keep the air pretty clean, though it will peak. My shop is 1/3 your size. So I don't think 1,000 CFM is excessive and may not even be enough. In any case, let use that.

1,000 CFM = 28 cu-m/min Air is about 1.2 kg per cu-meter. So that's about 34 kg of air you need to cool every minute.

Air has a specific heat of very nearly 1 kJ/(kg-K) = 1 kW-sec/(kg-K)

Let's assume you need to cool the outside air by 18 F = 10 C = 10 K.

So we have:

34 kg/min * 1 kW-sec/(kg-K) * 10 K = 340 kW-sec/min.

Canceling the minutes with seconds.

340 kW-sec/min * 1 min/60 sec = 5.67 kW of cooling power.

Converting to BTU/hour (common AC capacity units)

5.67 kW x 3412 BTU/1 kW-h = 19,300 BTU/hour

Now, AC units have a quite a range of efficiency. And I'm sure there would be quite a difference between one recycling room air (air that's just a little warm) versus drawing in hot outside air.

While SEER ratings (BTU/W-h) are based on average seasonal operation. They can be as low as 3-4 for a typical AC unit and as high as 20+ for super efficient. But lets use 10 SEER.

Power (W-hours) 19,300 BTU/hour / 10 SEER = 1,930 W = ~2 kW.

So this is just a rough estimate at how much electric power you might need to cool 1000 CFM by 18 F (10 C).

This would be continuous average power at this temperature difference. You would of course need more capacity if it is hotter outside, less when it is cooler.

But let's say you averaged 4 hours of peak (2 kW) usage a day, that's 8 kW-hr and at say $0.15 per kW-hr $1.2 per day.

That certainly is not excessive.

But keep in mind, this is just the energy needed to cool the incoming air to room temperature. But your shop space has all sorts of heat that needs to be removed. I.E., what a normal AC unit needs to do. So this is an estimate of the extra cost of not recycling the air.

So you might pay in the range of $40 more each month doing it this way. Lot's of assumptions here, so I think this cold easily go 2X in either direction.

I get what your are trying to do. But here's another idea. You're not going to need to be replacing your air to keep it clean (effectively filter), all the time.

Why not do a conventional recycling AC system? And put in a separate exhaust fan and air intake you run only when you need to replace (clean) the air.

It's not uncommon for people to exhaust there dust collection outside. This is effectively the same thing, you are drawing in outside air. Obviously not the most efficient thing, but those that do this say they don't notice that much of a problem with added heating or cooling.

Of course, it all depends on how often and long you run ventilation. Moving 1,000 CFM of outdoor air is certainly a major air exchange. So if you did this a significant percentage of the time, you're going to notice it and need more AC.

Also, there are air exchangers that do help retain some of the cool in the air. These are typically meant to ventilate a whole house at a relatively low rate. But perhaps a whole house exchanger would be a good fit for your shop. Something to at least consider.

Also, you aren't likely going to be able to dehumidify the air very well. First off, you are drawing in outside air, and in your location I assume that is very humid air. I think a typical recycling AC system relies on the air passing over the cooling coils over and over to draw out the moisture. I'm not sure it would be possible to do that much with a single pass.

This is another reason a traditional system with separate air venting may work better. At least while not venting the shop the AC can be continually dehumidifying.

What you are proposing is unusual for residential use, and if it is done anywhere, it would be some sort of commercial application. I'm sure there must be commercial operations that need to clean the air of dust or fumes. So you might be better off checking with a commercial AC contractor.

Anyway, I hope my back of the envelope calculation provides some perspective. Your idea doesn't sound crazy, but there may be some better ways to approach it that a pro AC guy could tell you about.


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## clin (Sep 3, 2015)

> Well, like you said, it s not efficient. You can take in air for the AC from anywhere with custom ductwork. The advantage of a closed system is that you are recirculating cooler air that s already been dehumidified. With an open system, you ll be cooling/dehumidifying outside air, so the cost to operate is going to be significantly higher.
> 
> I can appreciate your not wanting to be changing filters all the time, but is it possible to construct a multi-stage filter with a large, cheap fiberglass first stage filter that could be taken off and knocked clean? You could then taper down to a pleated filter to keep the AC innards clean.
> 
> - RichTaylor





> what about a split system Mike ….would that work for you ???? :<))
> 
> - GR8HUNTER
> 
> ...


Mini-splits are great, I use one in my shop. But, he's wanting to ventilate and not recirculate to avoid filtering.

However, by running a room filter (like a Jet), you won't get much dust in a mini-split. But then you do have to clean the room filter. I think that's what he is wanting to avoid.


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## Desert_Woodworker (Jan 28, 2015)

Very informative reply Clin- thanks


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## Redoak49 (Dec 15, 2012)

Very nice calculation…I think that the actual cost will be more as you need to add in the power to dehumidify the air.


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## JoeLyddon (Apr 22, 2007)

It seems to me that you need a source of Fresh Air…

BUT…
If you just do not want to replace filters, why not just Leave The Filters OUT and let it circulate?

Just get the unit that will COOL the place and use it… w/o filters.

Too simple? LOL


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## HorizontalMike (Jun 3, 2010)

I have an HF DC-System with a Wynn filter. I move the 20' intake hose from tool-to-tool as needed. Things like lathe and 1/4-sheet sanders don't get the DC (sweep and suck). Here in South Texas Summer, the heat is high and dry for the most part. My biggest problem is dripping sweat on all my cast iron, literally. If it is only 80-90F then I can often get away with opening the 16' x 8' garage door and running 5 x fans in a circular in/out pattern.

The fans actually create a bigger dust mess in the long run. I am trying to find a more "pleasant" way to spend time in the shop during these really hot months. In the Winter, I can usually get by with a 1500W base-board heater. BTW, insulation is R10 roof, R7.5 walls, using Solar Guard on my metal building. IMO, spending more on upgrading insulation would be too tedious since the shop is already "full" of stuff.

So bottom line is trying to make things comfortable for the few hours, at a time, that I spend in the shop. The more comfortable it is, the more I can spend time there… ;-) A dry 80F would be nice…


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## HorizontalMike (Jun 3, 2010)

> ...[snip]...
> I can appreciate your not wanting to be changing filters all the time, but is it possible to construct a multi-stage filter with a large, cheap fiberglass first stage filter that could be taken off and knocked clean? You could then taper down to a pleated filter to keep the AC innards clean.
> - RichTaylor


Am wondering about doing that, though real estate is very tight and not sure where/how to do that. What ever gets done, I will need to have access to it at ground level if changing filters etc. FWIW, me, ladders and deer on MCs don't get along very well any more (+50 fractures), so closer the to ground level the better. BTW, HorizontalMike wouldn't mind "earning" a different name eventually…;-)


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## jonah (May 15, 2009)

Why not get a system with a washable/reusable filter? You'd save tons of money in operation, and you could buy two filters. You use one until it's dirty, then put the other in and clean the first one. Some of them even go in the dishwasher.

Either that or a mini split system and a ceiling mounted air cleaner.


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## clin (Sep 3, 2015)

> Why not get a system with a washable/reusable filter? You d save tons of money in operation, and you could buy two filters. You use one until it s dirty, then put the other in and clean the first one. Some of them even go in the dishwasher.
> 
> Either that or a mini split system and a ceiling mounted air cleaner.
> 
> - jonah


This is exactly what I have and it works well. As is commonly done, my room filter hangs from the ceiling, and therefore doesn't consume any floor space. Changing filters does require using a ladder. However, I find it works pretty well to just use the shop vac to vacuum the dust off the filter. That can be done standing on the floor.

Not saying you'd never have to climb a ladder or step stool to replace the filter, but it wouldn't have to be all that often.



> BTW, insulation is R10 roof, R7.5 walls, using Solar Guard on my metal building. IMO, spending more on upgrading insulation would be too tedious since the shop is already "full" of stuff.
> 
> - HorizontalMike


Insulation really should always be the first step. You have about 1/3 what you should have. Is your ceiling really that inaccessible that you can't upgrade the insulation?

Is your humidity low enough for an evaporative cooler? If so, that would fit your original requirement. These are specifically designed to continuously draw in outside air and exhaust the structure.

But, I'm guessing you'd already be doing this if it were an option in your area. And you already mentioned dehumidifying, so again, I'm guess you really aren't that dry.

Bottom line, if people use evaporative (swamp) coolers in your area, that's a perfect option for what you want. Very cheap to install, can move a huge amount of air, much cheaper than refrigerated AC to run. They just don't work well or at all if the humidity isn't low.


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## clin (Sep 3, 2015)

I found this table for evaporative cooler use. As you can see, if you have outside temps of 100 F or so, you need humidity in the range of 20% or less for it to work very well.


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## hotbyte (Apr 3, 2010)

Don't mini splits have filters?


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## mer500 (Jun 22, 2017)

Great Thread


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## HorizontalMike (Jun 3, 2010)

Sorry, but an evaporative cooler would only exacerbate rusting of the cast iron in the shop, that and (from above chart) the best that they can do is about 86F to 88F on my "way-too-often" 90-100F days. IMO, that results in hot AND humid in the shop, so I choose not to go that route.

I may think about doing the two-step, using a "room filter system" with the AC/heater. Maybe that and a window AC unit, but would need to be in corner of shop. So from corner of shop, how much BTU would be needed for that placement?

At my age/shape I will most likely pay for the installation.

Front Exterior of Shop:









Exterior Space for possible AC Unit:









Inside view of AC Spot only 28in. wide between I-Beam and door frame.









The rest of the interior of shop. All that space in the center needed to move my bike in & out. Also needed to roll my BS, lathe, and planer table to open area for use, as needed.









UPDATE:
Also wondering about how much of a filter box to put in front of a window unit to catch ambient airborne dust? Hepa filters, how many to use on a window unit in order to allow free flow of fan blower air?


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## GR8HUNTER (Jun 13, 2016)

BEAUTIFUL SHOP inside and outside …..BEAUTIFUL BIKE …... I am still thinking a split system is the only way go …...GOOD LUKE Mike :<))


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## jonah (May 15, 2009)

I'd do a mini split system. I'd also spend some time and money insulating and air sealing the building better. That will pay for itself very, very quickly.


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## nkawtg (Dec 22, 2014)

> I m not sure if what you want exists. But I ran some basic calculations to see if it makes any possible sense.
> 
> Lots of assumptions are needed. Your shop has a volume of 8,800 cu-ft. Let say the AC unit moves 1,000 cu-ft/min (CFM). So you would exchange the air about every 9 mins. This exchange rate directly affects how much cooling you need, and of course affects the filtering affect.
> 
> ...


The reason why the do return ducting is because of the 18 - 20 degree cooling delta.
Cooling 80 degree return air nets you 60 out the duct.
Cooling 100 muggy degrees might net you 80 degrees out the duct, with almost no de-humidification.


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## Hermit (Oct 9, 2014)

Based on your info, size shop, insulation etc….one through wall a/c won't be enough to cool your shop. I don't even think 2 units would cool your shop. If it was me I'd also lean towards a split system if you spend a lot of time in your shop. I'd have an a/c guy come out and give you an estimate/ideas. I think you're looking at a 3-4 ton unit and that could run about 8k-10k or more since you have no ductwork.

I have a 20×22 shop, 9' ceiling with r-13 insulation. A through wall a/c heater. It works fine for my shop but you still have to clean the filter occasionally.


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## HorizontalMike (Jun 3, 2010)

QUESTION:
Since this shop AC system will be an On-Demand system that will be used intermittently, how important is the SEER rating? Am I wrong in thinking that the higher the SEER, the longer it will take to cool the shop, when combined with the tonnage of the system?


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## Hermit (Oct 9, 2014)

Mike this link may help you out. https://servicechampions.com/difference-between-eer-and-seer/


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## oldnovice (Mar 7, 2009)

This sounds an awful lot like a clean room!
In my 45+ years in industry I worked in a number of different classes of clean rooms.
The last clean room, before I retired, was for laser optcal assembly where a "bunny" suit was required.

*This is definitely an interesting topic!*


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## HorizontalMike (Jun 3, 2010)

All I know is as I type this, it is 100.5F outside my office window. On days like this the shop has gotten as high as 107F. That said, I am willing to pay the electrical penalty for more humane hours in the shop…


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## jonah (May 15, 2009)

The higher the SEER rating, the less electricity it uses. SEER rating doesn't have any direct relationship to the system's cooling capacity or speed of cooling. To ensure it doesn't take forever to cool the shop down, you'd want to size the system appropriately. You could likely get away with a smaller system if you insulate more.

No matter the system, though, it will take a decent amount of time to actually dehumidify the air. You'll almost certainly want something with the ability to control remotely. That way, if you know you're going out to the shop in two hours, you start the system and it's ~80 with manageable humidity and not ~100 when you walk in there.


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## curliejones (Mar 12, 2012)

To hotbyte, yes mini-splits have filters and, though pleasantly quiet, they are 2-3Xs the cost of window units. Keep the filters clean if you want performance. 
Mike, it's not as hot here in SE Louisiana, but perhaps even more humid. If you choose to not recirc the air, you still must have an exhaust to allow incoming air in. I have a 900 sq. Ft. shop and average 10 ft. Ceilings on a slab and I cool it just fine with one (1) 8000 btu window unit. There is no insulation in the walls or ceiling but I did use reflective foil on the outside of 1/2" sheathing (walls and roof) in attempt to abate radiant heat gain. It does not get very cold, but it dehumidifies very well and that's what keeps ME from sweating. I am amazed that my tools do not sweat in the winter when the weather warms up dramatically. I use a d.c. on major tools only and I have a homemade air cleaner using a 20" box fan w. A 20" square a/c filter. My ac unit only needs the foam filter washed occasionally. I would cut holes and buy 2 units at 8000 btu each (about $350) or start w. 1 and see if you need the other. They have washable foam filters and you can replace them a few times before you reach mini-split money. Another thought-investigate shade cloth for your building?... Cheap and effective, like is used on greenhouses.?.


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## clin (Sep 3, 2015)

When it comes to heating and cooling is there is no rule of thumb that is worth a damn. You have to run the proper calculations to figure out what you need. And as always, insulate first.

Unless an AC contractor runs proper calculations, you'll likely end up with more capacity than you need. However, if you just want to cool the space down quickly, then you are sizing for this not for the actual heat load. Then more is better.

Sort of like the difference between a dragster and a car designed for cruising down the highway.

Given the fact that you have about 1/3 the typical insulation, you'll need about 3X more cooling capacity. This will cost you more to buy and more to operate. It's definitely cheaper to insulate. But if this is too difficult, then of course you adjust accordingly.

It's always a bit complicated to figure out what is the best way. A properly insulated space would allow you to run a mini-split 24/7 for a modest operating cost. Doesn't mean you have to keep it at 75 F all the time, but you could keep it from heating all the way up. Then you don't have to anticipate when you're going to be using the shop.

You don't really save much energy by turning an AC unit off when you aren't there. The space still absorbs heat that you have to remove later. To be clear, I'm talking about daily. In other words, you might as well keep it cool rather than turn the AC off when you leave the shop, just to turn it back on the next day and have it running constantly removing all the heat that has built up.

Of course if you are not in the shop for days at a time, then it makes sense to set the thermostat back.

Another thing to consider. A properly sized (read not oversized) AC unit will work best at removing humidity. An oversized unit will short cycle. It will only be on a short time, before the thermostat turns it back off. This is very inefficient, and it doesn't move enough air for a long enough period of time for the moisture to condense out. Then you get the classic, cold but damp situation.

This is one reason mini-splits work so well. They typically have variable speed compressors. So rather than just turn off, they slow down. This increases efficiency and de-humidification.


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## jonah (May 15, 2009)

I can't even imagine working in a shop in 100 degree heat. This thread is making me sweat just thinking about that.


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## firefighterontheside (Apr 26, 2013)

Without return air, I don't think you'll see much cooling and even less dehumidifying. I have a thru the wall/window unit with no filter. I just blow compressed air thru the coils a few times each season.


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## Gilley23 (May 9, 2017)

In all honesty and seriousness, this whole no return air and no cleaning a filter business is ridiculous. Slap a little prefilter on there and hit it with the shop vac for 3 seconds when you're through for the day and you're done….not to mention you'll be cooler and more comfortable while working.

Control the humidity and you control your comfort. You can't control the humidity with any kind of effectiveness without returning the air through the unit.


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## TheWoodenOyster (Feb 6, 2013)

I work as a general contractor and have actually been pricing these for one of my jobs. The ones we are pricing are for locker rooms, which presumably are pretty close in size to your shop, maybe a little bigger. The ones we have specified by the engineer are by AAON. They run about $40k installed. So do they make them?... yes. I don't know if they make them smaller or for residential applications, but they do exist. But they are expensive.


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## sawdustdad (Dec 23, 2015)

Your idea is not practical. I know you said not to say that, but that's the answer. You NEED to recirculate the air in your shop if you want any kind of useful system. The ONLY way to what you want to do with readily available equipment, is to run two systems in series, to get adequate cooling without recirculating.

I put this 1.5 ton heat pump in my 28×46 x 10 foot shop. R38 ceiling, R19 walls. Virginia. Keeps the shop at 72 deg effortlessly in 95 degree heat. Located entirely within the shop envelope to maximize efficiency.

The filter is at floor level under the air handler. ( behind wood pile in pic) So easy to change. Cheap pleated filters. buy them in bulk.


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## jonah (May 15, 2009)

Very nice looking shop!


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## HorizontalMike (Jun 3, 2010)

> Your idea is not practical. I know you said not to say that, but that s the answer. You NEED to recirculate the air in your shop if you want any kind of useful system. The ONLY way to what you want to do with readily available equipment, is to run two systems in series, to get adequate cooling without recirculating.
> 
> I put this 1.5 ton heat pump in my 28×46 x 10 foot shop. R38 ceiling, R19 walls. Virginia. Keeps the shop at 72 deg effortlessly in 95 degree heat. Located entirely within the shop envelope to maximize efficiency.
> 
> ...


Since you mention it, I am NOT totally against recirculating, but was wondering if it were possible. OK, nix that and go with recirculating. Will probably buy air filter something like what Grizzly offers(these are their high end units):

Floor model-http://www.grizzly.com/products/Large-Floor-Air-Filter/G0573 
Hanging model-http://www.grizzly.com/products/Heavy-Duty-Double-Air-Filter-with-Remote-Control/G9956

I like the looks of your heat pump setup. I have two such 1.5 ton units in my 2-story house, one for each 720sq.ft. story/level. Granted that the house is much better insulated than the garage/shop, maybe 2x to 3x more guessing. Looking at the walls of my shop, ~3/4 of the walls are covered with pegboard mounted on 2×4 frames screwed to the perlons. While no additional insulation was used behind the pegboard, I am assuming that the dead-air space behind the pegboard offers a bit of insulating effect but how much who knows.










Also, the entire wall on the right, as you look at the image, has an 8ft concrete retaining wall just 3ft from the building. That means that that wall only sees the sun maybe 1 or 2hr/day, the rest being perpetually in shade.









Mounting the ducting for the cooling system might be challenging since I do use the center cross I-Beam for my chain hoist, so a split-mini plus a stand-alone air filter system (above) might be a better/best? solution… THAT setup would keep the cleaning of the split filter to a minimum, since the majority of filter cleaning would be with a floor model air filterer.

I know that this will not be cheap by any sense, but at 65 I am tired of being alienated from my shop, just waiting on "better" weather. At least it will not be like I will be running the AC 24/7, and this will allow me to "plan" shop time instead of waiting and rushing when weather allows… Our winters are mild, so heating issues are pretty much nil.


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## jonah (May 15, 2009)

I'll say it again: not spending the time and money to insulate better now is something you will regret later. "Dead air space" behind pegboard is not insulation. If it's 2×4 construction, R-13 is going to be the best you can do with fiberglass. Foam would be better, but it'd be more expensive.

At the very least, I would do R-13 in the walls and R-30 in the ceiling. It's not clear from the pictures how thick the ceiling is. Either way, insulate as much as you possibly can.


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## sawdustdad (Dec 23, 2015)

Certainly dust control is a challenge in any woodworking shop. I have a good dust collection system, and an overhead HEPA filter unit that runs most of the time I'm working wood. With these, the filter on the heat pump does not clog up frequently. I think if I were you, I'd just buy the hanging unit, you don't need both. You can always buy the second one later if needed, but I doubt you'll ever need it.

A conventional heat pump does not NEED all the duct work I installed. In a previous shop I literally hung the air handler from the ceiling with NO duct work attached at all, and it worked great. It pulled in hot air at one end and discharged cold air at the other. The air flow was such that it circulated throughout the shop completely. It also did a great job heating the shop. With this setup, there was a filter in the air handler, so I had to get a ladder to change it, but I was younger then. 8^)

You could simply provide a return duct that came down from the ceiling hung air handler to a filter that you can reach easily. The ceiling mounted air handler would save floor space and the return duct (suction for the air handler) with a filter can be custom fabricated of sheet metal or built from plywood. No floor space used.

Second the suggestion to try to do some insulation.


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## cebfish (Jan 15, 2011)

A/C absorbs heat into the unit. An A/C takes heat from where it not wanted and moves it to a place that its does not matter. That is the basic rule of A/C. That is the first thing they taught us in basic refrigeration.


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## HorizontalMike (Jun 3, 2010)

Got a local AC & Electrical contractor coming out to discuss my needs and a possible quote, later on this afternoon. I'll post what I find out later. Told him I was considering a Mini-Split System, but maybe he has a better idea…

Anyway, back to the insulation thing… 
Where I live we get roughly 2-3weeks of below freezing nights in a typical Winter. On such days I can heat the shop with just a small 1500W baseboard heater and get the temps to ~60F, which is plenty for a shop. My guess is that I can get a +30-35 delta in our mild Winter heating season with that small heater.
SUMMER, we get ~3-4months north of +90F and some summers +100F. Not sure how the Winter +delta rise might infer on an achievable Summer -delta drop, but assume it would be somewhat less, depending on size of the AC unit used. At worst, I might have to restrict the AC and use to the first half of the day before the AC might start lagging, but gaining a full 4-5hr morning use of the shop would still be worth it IMO. Just have to wait and see how much this will cost.


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## DS (Oct 10, 2011)

First, my apologies for not reading closely all the comments in this thread before posting.

I really don't know how 100% positive flow AC would work.
It seems like you want it to work similar to how Evaporative coolers work.

Out here in AZ, "Swamp coolers", as we call them, work pretty well until about late July and August when the dew point gets a bit too high for them to be very effective. (Much like you describe at your location, well it sounds like, pretty much all the time.)

One side effect of shops with only evaporative coolers and inadequate dust collection is that there forms what I will call a "dust halo", for lack of better words, around each opening in the building. The dust is forced to be expelled from the doorways, windows and other leak points and gathers on the walls, ground, vehicles, trees and what-not outside the building.

It seems like your motivation for wanting 100% positive flow is to avoid dealing with the dust management side of the equation. (dust in the air-return filters) 
If you were to deal effectively with dust management in your environs, (admittedly I don't know how you are set up for that now), the 100% positive air flow would be far less of a concern.

just my 2 cents.


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## nkawtg (Dec 22, 2014)

To save space on the inside, can your roof handle a package unit?


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## HorizontalMike (Jun 3, 2010)

> To save space on the inside, can your roof handle a package unit?
> - nkawtg


nkawtg,
Looked at package units online, and don't think that would be an option in my case, but thanks for the suggestion. I learned something by the search…

DS,
Yeah, swamp coolers don't work that well when you really, really, really need the cooling capacity on hot humid days. And besides, swamp coolers + sawdust = a tacky mud-like coating that does not blow free with compressed air, leading to even more rusting of iron… 8-(

UPDATE:
Have not gotten the written quotes yet, but the AC contractor suggested two options:

The mini-split, mentioned earlier. At least a 2-ton with variable fan/BTU output. Just have to run the coolant line to blower.
 A full traditional central unit with wall mounted blower. Again 2-ton, but maybe more since less blower unit is less variable.

Can't remember which would require 2-240v outlets and which only one 240v outlet. I already have 3-240v breakers in my sub-panel, one for each machine needing one. With experience, I now know better and can double up these machines on same breakers SINCE I only use one machine at a time…;-) All I need to do is run some and armored 10-3 (grounded to metal building/box). All of my existing 240v is run this way.


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## HorizontalMike (Jun 3, 2010)

Well I got some quotes today. I always figured that the quotes would be somewhat comparable to installing a complete central AC unit (did that ~5yr ago for $5400).

Right now, I am considering options #2 and #3. I see a really big drop-off on the SEER rating from 20.50 to just 14.50 when jumping up from the 24K unit to the 30K unit. Looks like that extra ~$600 purchase cost increase would also cost more on the monthly utilities.

All I know is that when I had two 2-ton units (24K BTU) in my 2-story house, that they would short cycle and I ended up rusting out a furnace/blower because of that. Granted, that my house is probably 3x better insulated, as suggested above,... SOooo… maybe option#2 -24K BTU 20.50 SEER might be the best choice in the long run. Maybe it will actually be enough for most of the day, even though I would be happy getting my mornings up to ~1pm.

Thoughts…?


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## kelvancra (May 4, 2010)

Of course they make such critters. Well, half of them, anyway. You find them in single-wides all over the country. We call them fire places. Now that I've solved half your problem, we can move on to the rest. (insert knee slap here)

My small shop has three collectors that get used religiously, including for vacuuming. Generally, the dust isn't too bad, but I want to install one or two squirrel cages for the occasional blow out with the electric leaf blower.

Currently, I heat with a small ceiling mount unit and cool with a window mount. I have to blow the pretend filter on the air conditions and coils out about every two weeks.

I'll be going to a full blow HVAC system in the not too distant future and have the same concern as you. I'm thinking of building my own filter box so I can install [or leave out] whatever I want. Of course, everything would be a standard size. 20"x20" for example, but able to accept 1" to 5" width filters. As such, I might have to build spacers for the 1" and such.

Something as simple as the cloth off one of my old bazillion micron collector bags might make a good pre-filter. For that matter, I used to work for REI and have a quarter ton of Polar Plus that would trap large particles and still blow or wash out.

My expectation is, with a decent filter arrangement, I should be able to get by with a monthly filter swap or clean, even though my system tends to run eight or more hours a day.

If you do go the positive route, have an automatic flapper to allow an air exit. From first hand experience, I know a squire cage will draw so much pressure it can take nearly a hundred pounds force to pull the door open of the house you left your ex with, and "might"l crack a concrete foundation.

They used to test our HUGE concrete fruit storage buildings for air leaks with a leaf blower. They blew out 4,000 square foot walls that way, so use hair dryers, carefully, now.

As to insulation, I have the same problem. However, I figure I'll just focus on an area at a time. For example, the southern walls take a hit here, but the west really take a beating from the heat. Those areas and the ceiling, a bit at a time, will get my attention.

I've got six inch insulation in now, but with no rock. Already, there has been a twenty degree difference between in and out. Currently, it's about 105 out and 81 in the shop.
__

A little background on my shop and my dust battles:

- My planer, jointer and router table kick up the most debris, but leave the least behind with the 3hp Jet on them.

- Most my lathe work is on dry wood. I run my collector on them religiously. Since I do mostly spindle work, that works well. The hood I made from 8" pipe somewhat wraps around the work and slides across the table to focus at where I'm sanding or cutting.

If I take up bowls more, I might cage both lathes and do a positive air flow there. I took that route refinishing wood floors a few years back and it worked far better than when I drew air out of the room.

- Hand sanding isn't too much of a problem, since I built a sanding table with sides, a back and a top. Too, quality sanders with good dust collection allow me to work mask free much of the time. The only time the allow dust to escape seems to be when I'm working rounded edges.

- The edge sander and the drum still need work. Maybe a collector bigger than the 1-1/2" hp Jet. Probably bigger ports too.

- The miter is getting better. A nylon cage would tend it nicely. As it is, I swapped a 1hp Delta for a 2hp Harbor Freight unit and I'm pleased with the latter's performance.

- The carver kicks up very little with a 4" near the router.


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## jonah (May 15, 2009)

It seems clear that option 2 is the better one. Did he give you any idea how well he thought 24k BTU would cool the building during the day?

Also: will you be insulating more? I'd do #2 and insulate more.


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## HorizontalMike (Jun 3, 2010)

> It seems clear that option 2 is the better one. Did he give you any idea how well he thought 24k BTU would cool the building during the day?
> Also: will you be insulating more? I d do #2 and insulate more.
> - jonah


Not planning on any additional insulation at this point. I guess if things don't work as planned then maybe, but not until then. Not looking to live out there, just spend some/more productive time out in the shop during the 1st half of the day or so. All I know is that in my insulated house,either-floor, a 2-ton unit cooled things down regardless (short-cycling), so the guess-ti-mate is that that kind of setup might work well in my less than adequately insulated shop. Not going to bitch if I get run out of the shop after 1pm or so. This is my retired hobby, not a job… ;-) Jumping up to 30K just doesn't seem to be cost effective in the long run,... just hope I'm not wrong. If I had the funds ~15yr ago when I built the shop, I would have done things right, but as it is now it is all after the fact and I need to adjust for those really high temps during those 4-5 months where I normally stay away from the shop.


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## HorizontalMike (Jun 3, 2010)

UPDATE:
Well, I finally got the 2-ton Mitsubishi Mini-Split AC installed and operational.


Currently provides a 10-11F Delta in cooling-- +6-7 here . Shop temperatures stay below 80F until noon, or 1pm. Highest outdoor temp this week was 101F and resulted in a shop indoor temp of 90F at about 3pm in the afternoon. This gives me a full 4+ hours at temps in the 70Fs during the hottest part of the Summer here in South Texas. IMO, this is an immediate big plus for me.


Works great at lowering the humidity in the shop--+10 very BIG **. This aspect really adds to the ability to work in the shop, even as temps rise.

All said and done, I will wait until late Fall to early Winter, before adding to the ceiling insulation (R30 Fiberglass roll with 4×8 hardboard holding it between the metal roof perlons. That will eventually make my ceiling R40, and will more than offset that direct radiation from our Summer Sun. Don't really expect to do anything with the walls, except for maybe the top 3ft above the wall perlon (which is still accessible-lower walls are all pegboard).

For now, I am more than happy to pay for the extra electricity… **


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## Sparks500 (Jun 30, 2017)

BIG exhaust fan, open all the windows and doors.


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## duckmilk (Oct 10, 2014)

Very interesting read Mike. I'll bet it will work for you especially with the added roof insulation. Congrats 



> BTW, HorizontalMike wouldn t mind "earning" a different name eventually…;-)
> 
> - HorizontalMike


How about "NoLaddersMike or Two-StepMike"?


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## HorizontalMike (Jun 3, 2010)

> Very interesting read Mike. I ll bet it will work for you especially with the added roof insulation. Congrats
> BTW, HorizontalMike wouldn t mind "earning" a different name eventually…;-)
> - HorizontalMike
> 
> ...


Well,... it has been suggested that I adopt… "...Deer Killer…" since I have now hit 3 deer on the bike. HOWEVER, I don't want to jinx myself. *;-)*

BTW, if you look closely at the last image on the upper right, you can make out two dead windshields/trophies from numbers #1 and #3 deer. Got lucky on #2 deer and stayed upright. Oh yeah, don't worry about all the "used" crash-bars on the left, they are just "consumables" anyway… *;-)*

And YUP, more insulation comes this Winter.


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## HorizontalMike (Jun 3, 2010)

> BIG exhaust fan, open all the windows and doors.
> - Sparks500


Yeah, that is how I have been operating. I have 5 fans creating a counter-clockwise rotation while the 8'x16' roll-up door is open. Now, with the added AC and closed overhead door, I may have to consider additional lighting for these old eyes… But I do like the cooler temps!


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## oldnovice (Mar 7, 2009)

Mike, just a question, how about foam instead of fiberglass?


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## HorizontalMike (Jun 3, 2010)

> Mike, just a question, how about foam instead of fiberglass?
> - oldnovice


Yeah, I did check out the various foam sheets, but they pretty much top out at R 10 and less. I thought they would be better than that, but found out otherwise. The "unfaced" fiberglass is R 30 (can you say itchy…), so I am going to wait for winter and long shirtsleeve weather. I am thinking of using a spray contact cement/glue to stick the fiberglass to hard board, and then install, one 4×8 hard board sheet at a time between ceiling perlons. I'll have to cut it in 8ft strips, but figure that is better than trying to deal with 30ft long strips, especially when I have to feed it over and around existing conduit, lights, and center I-Beam.

BTW, I can get the hard board with a white siding for ~$10/sheet. That should keep my existing lighting bright since my fixtures do not use reflectors.


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## HorizontalMike (Jun 3, 2010)

Just spent 3hr from Noon-3pm in shop with thunder clouds in the sky and 96F outside under intermittent sun. Temperature in shop held at 86F, and since AC has been running +4days the humidity is quite low in shop and I did not even have to run any additional fans. Have the AC set at "Econo Cool" at 85F and that was it!

Color me a happy camper, for sure! And yeah, I'm sure these first couple month's electric bills will surge, but at least it will give me data going forward until I can add insulation this Winter. I guess I'll find out what that 21 SEER rating on the new Mini-Split is worth.

All I can say is that my Sargent hand planes have never been happier, since I am no longer dripping sweat all over them while I'm restoring them to their old glory days...

Geez, 86F in shop with no other fans blowing and NOT sweating! Life is goooood!... **


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## oldnovice (Mar 7, 2009)

Mike, speaking of your lighting, are you/have you considered LED?
I really love mine, no heat generated, no hum, instant on, and the bugs can't see them.


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## HorizontalMike (Jun 3, 2010)

> Mike, speaking of your lighting, are you/have you considered LED?
> I really love mine, no heat generated, no hum, instant on, and the bugs can t see them.
> - oldnovice


No, not recently. Still have the six double 8ft bulb T-12s as my lighting. Thought about adding to, though not about replacing. Replaced bulbs once, even though not really needed, in past 15 years. Have not spent enough time in shop to be concerned about energy saving$ per se.

I have noticed that my lighting is not enough for my old eyes at this point. Considering a couple of 4×4FT T-8 or T-5 fixtures directly over my work areas, but only shopping at this point. More interested in getting the right prescription for my "shop" glasses. I am finding out that in low light, I need more magnification than in bright light. Probably related to my eye's shifting focal length in varying light conditions.


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## HorizontalMike (Jun 3, 2010)

Well I just got my first monthly electric bill since I installed a 2-ton Mini-Split AC in my shop. Remembering that I have rather low insulation values (R10-ceiling, R7.5-walls), the bill was surprising. This was for July-August, the hottest time of year, and the bill was ONLY +$50 higher!

That means it cost me all of $1.66/day to AC the shop in the dead of summer! COOL! to say the very least… ** The way I figure it, that will be roughly $200-$250 more per year. I find that quite doable. I am exploring adding insulation to the ceiling and contacting different contractors.

I explored the cost of just the materials (insulation, plywood etc) and came up with ~$1,200 alone. Adding labor, who knows. Most folks down here in Texas only want to sell you "blown-in" insulation. Seems few if any will lay batts/rolls in existing buildings.

Anyway, happy camper here. Time to head back out to the shop and fire up the lathe… ;-)


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## AlaskaGuy (Jan 29, 2012)

I found it to be much easier to move to a cooler climate.


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## cathode (May 18, 2014)

Congrats on the new comfort, Mike.

This thread has given me more hope for keeping my new 350sqft shop space cooled down. I have a 10k BTU window unit that I can use for the shop. Before, I wasn't sure if it would even make a dent in the temps but I guess it should be fine.

On a side note, I was really lamenting the heat yesterday; I tried to drill a couple holes through 14ga steel with my cordless 18v drill, and the drill and batteries kept overheating and shutting off because the ambient temps were too high. I'm in Portland, OR and we're having a pretty miserable heat wave for the last few weeks.


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## wapakfred (Jul 29, 2011)

I do find that surprising, thinking it (the electric) would be much higher….sounds like it really worked out well. I'm considering a mini split to AC my shop, this thread has certainly been useful to me.


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## HorizontalMike (Jun 3, 2010)

I have to say, I was wondering just how efficient that 24,000 Btu 20.5 Seer rating would be. I am very impressed.

I love that air handler/fan. It oscillates from narrow to wide and a full 90-degrees up and down, both at the same time. And it does this 24/7 unless there is a cold snap that shuts the entire thing down. When I leave the shop, I adjust it to 82-82F and that keeps the humidity down overnight. I get up around 4am and when I check the shop it has been 78-80F inside while outside may be 74-76F. I'll kick it on full cooling and when I come out after breakfast, ~7:30-8am, the temp inside the shop will be down to 69-70F. That cools down all my cast iron (saw, bike, etc.) and that makes it much easier for the unit to hold back the rising temps outside.

It got up to 83F inside the shop today by 2pm. Came back in because I was starting to make mistakes. Minor mistakes, but I could tell it was time to call it a day, for me at least. A 6-hour day, minus lunch, is more than enough for this old man. After all, this IS retirement… *;-)*


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## RogerM (Oct 31, 2011)

Sure, Just put the return to the air handling unit in another room separate from your shop or to the outside.


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## putty (Jan 1, 2014)

That indoor unit is sucking shop air to blow over the coils just like in your house, be sure to keep the filters clean so the coils don't get caked with dust.

I have a mini split like that in our sun room, the filter gets plenty dirty and that's without shop dust.


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## wapakfred (Jul 29, 2011)

> It got up to 83F inside the shop today by 2pm. Came back in because I was starting to make mistakes. Minor mistakes, but I could tell it was time to call it a day, for me at least. A 6-hour day, minus lunch, is more than enough for this old man. After all, this IS retirement… *;-)*
> 
> - HorizontalMike


I wish I only made mistakes when it got to 83˚, for me it doesn't seem to be temperature related.


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## JoeLyddon (Apr 22, 2007)

SUPER GOOD!

Glad you got working good for you!

The cost wasn't bad at all! GREAT results!


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## HorizontalMike (Jun 3, 2010)

> That indoor unit is sucking shop air to blow over the coils just like in your house, be sure to keep the filters clean so the coils don t get caked with dust.
> I have a mini split like that in our sun room, the filter gets plenty dirty and that s without shop dust.
> - putty


Yeah, I have been checking those a lot, at least until I determine just how frequently i need to do that. So far so good. There are two filter systems, one for the bigger dust and one for the dual replaceable Hepa filters (purported to be an annual replacement), By and large, it has me sweeping/sucking up all of the dust must more frequently, out of guilt… Over all, not a bad thing… *;-)*


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## wapakfred (Jul 29, 2011)

The annual replacement may be predicated on use in a house or room that are living quarters…much more run time than you will probably have on your unit. They may well last longer than a year. That would probably be a good thing since I'd bet they aren't cheap.


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## HorizontalMike (Jun 3, 2010)

> The annual replacement may be predicated on use in a house or room that are living quarters…much more run time than you will probably have on your unit. They may well last longer than a year. That would probably be a good thing since I d bet they aren t cheap.
> - Fred Hargis


Yeah, I just checked and they run $65.00 for a 2-pak (it takes two so this is a single replacement):
http://hepafiltersales.com/mm5/merchant.mvc?Screen=PROD&Store_Code=HFS&Product_Code=MAC-408FT-E&Category_Code=Mitsubishi

As far as run time, the indoor fan unit runs pretty much 24/7, but is a variable speed with a wide range. All I know is that the AC system won't be used for nearly half the year. When it's cooler I prefer to work with the 8×16 roll-up door open, for the air and sunlight.


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## Rayne (Mar 9, 2014)

Nice to hear the not-so-large increase in the electric bill. I'm still waiting for my first Full month bill to hit so I know what it really looks like in the dead of summer in Central FL. My garage has been amazing though. 86 is the "sleep" mode for mine and it feels a million times better than it did before. I usually set it to 81 when I'm in there working and it's cooler in there than my home at 78. My first major project with the A/C is starting today, so I'm looking forward to that. Anyways, good luck with your A/C and your insulation project. It can only help your bill.


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## HorizontalMike (Jun 3, 2010)

> Nice to hear the not-so-large increase in the electric bill. I m still waiting for my first Full month bill to hit so I know what it really looks like in the dead of summer in Central FL. My garage has been amazing though. 86 is the "sleep" mode for mine and it feels a million times better than it did before. I usually set it to 81 when I m in there working and it s cooler in there than my home at 78. My first major project with the A/C is starting today, so I m looking forward to that. Anyways, good luck with your A/C and your insulation project. It can only help your bill.
> - Rayne


Great to hear that I am not the only one going to the "cool side!"... *;-)*
The very best thing I have discovered thus far is just how much lowering the humidity increases the comfortably in the shop! I am sure that in your Central Florida shop you are definitely going to notice that. For me, I can stay in the shop until the shop temps get into the 90s, and that usually means +100F outside. I made it to ~2pm the other day and that was fine for me. Having 6hr shop time in the summer is more than enough in retirement.

Keep us posted on where your electric usage ends up. Enjoy!


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