Air Filtration exhaust

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Forum topic by pauldye posted 01-03-2013 12:45 PM 1744 views 0 times favorited 21 replies Add to Favorites Watch
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68 posts in 2288 days

01-03-2013 12:45 PM


I have this air filtration system hanging in my shop garage. It works fine, except for the exhaust of very fine dust. I change the filters often, but even with a 2-stage filter system, the extra fine dust gets through.

I would like to pipe the exhaust out of the garage. They sell plenty of metal ductwork at Lowes and HD, but I need to reduce the 8.5”x9.5” vent down to a 4” round connector.

Does anyone have advice on where to buy or howto build this?

My plan so far is to reduce the square down to a 4” round, then vent at a right angle up into the attic. The dust can accumulate up there, since I don’t use the attic for much.


21 replies so far

View kizerpea's profile


775 posts in 2572 days

#1 posted 01-03-2013 12:49 PM

sounds like a bad idea to me..venting into the attic


View JoShmoe's profile


15 posts in 2303 days

#2 posted 01-03-2013 01:51 PM

I agree. I don’t know how much dust you plan on making over the years but if you vent in the attic – be prepared to have to go up there and potentially empty it out over time.

Do you have a way to pipe the exhaust outside through a wall? I would send it outside and put a trashcan under it to collect the exahust. You might want to consider putting a cover over the pipe outlet to keep rain from getting into the trashcan.

In answer to your question about reducing the vent – you should be able to find a duct reducer at any of the big box stores (e.g. duct reducer) or create your own using some sheet metal and duct tape.

-- Joe

View tenontim's profile


2131 posts in 3948 days

#3 posted 01-03-2013 02:41 PM

Not to mention sucking the heat out of your shop. You might try gluing a rubber weather strip seal around both filters and see if that stops your dust problem. If your bag filter is full of dust, you need to replace it.

View pauldye's profile


68 posts in 2288 days

#4 posted 01-03-2013 03:36 PM

Here is a picture of the air filtration hanging from the ceiling.

View DIYaholic's profile


19709 posts in 2879 days

#5 posted 01-03-2013 04:02 PM

Can you upgrade to a better filter, that will remove the fines?

-- Randy-- I may not be good...but I am slow! If good things come to those who wait.... Why is procrastination a bad thing?

View Bill White's profile

Bill White

5130 posts in 4165 days

#6 posted 01-03-2013 04:14 PM

What filters are ya using? You said 2 stage, but the type of filter is important.


View taoist's profile


124 posts in 2695 days

#7 posted 01-03-2013 04:20 PM

Not a good idea to vent it to the attic as if there are any electrical wires there, you may risk an explosion.
I personally would try to “tighten” up the filtration system with tape and new filters. I have a Jet air filtration system hanging from my shop ceiling also. I buy the best filters I can at Walmart and HD to prefilter the intake air and I replace them often.
If you do plan to vent the exhaust, as others have said, take it outside. Usually, a local HVAC business can build you a plenum and then you can duct it with 4” ducts.

View derosa's profile


1577 posts in 3040 days

#8 posted 01-03-2013 06:39 PM

When I bought my house the building inspector told me of a gentleman who he had dealt with that had taken his wood shavings over the years and deposited them in the attic to add insulation. Although it ended up doing an amazing job of that over the years of accumulation it was considered a massive fire hazard and had to be completely removed in order to sell. Although the dust may be fine it will add up fast and may be seen as just such a hazard.

-- --Rev. Russ in NY-- A posse ad esse

View teejk's profile


1215 posts in 2889 days

#9 posted 01-03-2013 06:54 PM

I don’t think one can even vent a bath fan into the attic!

I was involved with a large grain-mill construction project a long time ago…dust was a big issue and as I understand it, is self-combustible. worse yet is they explained the first “boom” is the least of the problems…it raises any dust and the second “boom” is worse.

View crank49's profile


4032 posts in 3175 days

#10 posted 01-03-2013 07:28 PM

Let me get this straight.
You have an air filter hanging from the ceiling in your shop.
It works fine except it bypasses some fine dust.

If the filters are not catching the fine dust, it is useless to filter out the big stuff. It’s the fine that causes all the health issues.

Putting it in the attic won’t do any good either because it will come out into the house through light fixtures and such, so you will just be moving the problem from the shop into the house. Not to mention the horrendous fire hazard it would create.

Add a third stage with a HEPA cartridge and catch the fine stuff. Or at least fix the problem with the existing filters to stop the bypassing of dust.

Now this machine must have a switch, why not just turn it off if you don’t like what comes out of it?

Pushing it into the attic is the worst thing you could possibly do.

View MarkwithaK's profile


370 posts in 3382 days

#11 posted 01-03-2013 10:26 PM

You’re best bet is to work on the filtration system. If you were to vent this up into your attic (other than being a massive fire hazard) you are putting the space into a negative pressure. All of that dust is going to find it’s way right into your shop which negates any venting work you did. By venting it outside you are also putting the shop into a negative pressure. This is going to draw in outside air and dust/dirt from any and all cracks and crevices….as well as suck any heat to the outdoors.

-- If at first you don't succeed then maybe skydiving isn't for you.

View pauldye's profile


68 posts in 2288 days

#12 posted 01-04-2013 01:19 AM

everybody thanks for the advice about using the attic exhaust. That wasn’t my main question, but it is great that you all picked up on my error. I can either go out the wall, or through the attic to a roof vent.

JoShmoe: I am new to duct bending. The idea of going from square to round would be work-in-progress for me.

tenontim: I live in the sub-tropics. Sucking the heat out of my garage would be a good thing.

I have used whatever filters I can find at HD/Lowes. Lately, the quality has not been too good. Even when I was getting the more expensive filters, there still was fine dust. I will try taping around the intake filter to see if that is where the leakage is happening.

And thanks all for the mountain of info. I will report back when I get to work again.


View Dallas's profile


3599 posts in 2691 days

#13 posted 01-04-2013 01:32 AM

Not sure if this helps, but put the filters on the intake side of the fan.
I use MERV13 filters on mine and they help a lot.
I would use MERV15’s but our local places don’t carry them.

Putting the filter on the intake forces the fines to stop at the filter, not be processed through the fan and spit out any place they can go.

-- Improvise.... Adapt...... Overcome!

View Grandpa's profile


3261 posts in 2880 days

#14 posted 01-04-2013 04:19 AM

The transitions from round to rectangle are available at any good heat/air supply house. They are made in sizes that should go over the wall into the soffit. It is not to code to empty in the soffit either. You have to go to the outdoors. No systems empty int he attics or soffits for over 10 years now. get rid of it. get a better filter. I see them for sale in many catalogs.

View pauldye's profile


68 posts in 2288 days

#15 posted 01-07-2013 04:00 PM

I put in a merv 13(mpr 1900) filter, and taped the 30-day old filter (merv 11) on the outside of that. Now it has 3 filters in total, with the intake taped for the small gaps.

I will run with this system for awhile and see if the fine dust gets exhausted. If needed, I will revisit the duct to outside.

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