LumberJocks

HVAC questions to the Pro's

  • Advertise with us

« back to Coffee Lounge forum

Forum topic by ScottKaye posted 11-14-2018 05:57 PM 304 views 0 times favorited 4 replies Add to Favorites Watch
View ScottKaye's profile

ScottKaye

711 posts in 2152 days


11-14-2018 05:57 PM

The time has come to replace our 23 year old Propane furnace. its a 90k btu upright unit with a single speed motor and just a primary stage gas valve. It ran flawlessly for 20 years but the last three has been a band-aid nightmare.

I live in the Mid Atlantic (40-45 btu’s per square foot ) and we need to heat approximately 2200 square feet of space which includes and unfinished basement that we have plans to finish one of these days. We are currently in the process of getting estimates to replace just the furnace and thermostat and not the evaporator coil above as its only 3 years old.

I’ve had two estimates so far and waiting on the third. The first two estimates are for exactly the same equipment. its a HEIL Quiet Comfort 96 G9MXT. Its a 2 stage variable speed unit. I specifically requested the variable speed for de-humidification purposes as we have that feature on our upstairs heat pump and absolutely enjoy the comfort it provides. My regular HVAC guy came in at $4100 which i think is over priced. The other guy I found on Angie’s list has great reviews and came in at $2700 for the same 100k btu unitl and $2300 for the same unit but at 80k btu’s he felt the 80k unit would do the job and save me some fuel. here is my first question. whether or not i install and 80k vs 100k until wont both units burn the same amount of fuel to get me to my desired temp? It will just take the 80k a little longer to get there vs the 100k. I’m leaning towards the 100k as my house doesn’t have the best insulation and according to the HVAC sizing estimate 2200sq x 40 btu sqft = 88k or 2200sq x 45btu sqft = 99k. Definitely going with 100k. Do i even need de-humidification if my upstairs unit already has it?

And now for the other question. The lower estimate contractor says he wouldn’t normally sell me a variable speed fan or a two stage burner. He felt single stage is perfectly fine for gas. Am I being the gadget freak that I know I am and wanting all the bells and whistles or am I right in my thinking that two stage gas valve is better along with the variable speed blower for summer de-humidification? The one Caveat to the de-humidification thinking is that the compressor/condenser unit outside is only a single stage until and not two stage like the unit upstairs is so its going to run at max speed IE dollar bills flying out of my wallet because of all those extra pixies running thru the compressor! Should i nix the variable speed and stick with the 2 stage?

thanks

Scott

-- "Nothing happens until you build it"


4 replies so far

View OSU55's profile

OSU55

1964 posts in 2189 days


#1 posted 11-14-2018 09:24 PM

I’ve worked for the largest global scroll compressor mfr (commercial, industrial, residential AC & heat pump systems) for about 30 yrs, in product design, reliability, mfg, and quality. Copeland is the brand name.

Bit hard to tell from your description, but it sounds like you have an upstairs heat pump with some type of variable speed for AC dehumidification, and a downstairs AC unit with a propane gas furnace. This is a bit of a strange combination. It is unclear if you are only replacing the gas furnace or the entire indoor unit or what, so I will speak with some generalities.

It would help to know your location closer than “mid-atlantic”, but most likely you are in ASHRAE climate zone 4A, which is great for heat pumps (could depend whether you are several 1000 ft in elevation). From an operating $ perspective, a heat pump typically is far better than propane, but you would need to do the calculation based on your electric vs propane $. Natural gas is usually a different story – depends on $. Then it depends on upfront $, and I can’t decipher what the choices are.

There isn’t much if any benefit to a two speed/capacity gas furnace.

Many of our system mfr’s use our 2-step or 2-stage scroll in their systems, and there are var speed systems out there (many different flavors). My company would scream blasphemy, but I don’t recommend var speed to anyone – the payback period is measured in decades. The only time I recommend the 2-stage system is in a humid climate, and for comfort only, not really operating $ savings. For the added upfront $, the 2-step systems aren’t cost effective, but they do help comfort a bit more vs lowering the setpoint a degree or 2. FYI, I don’t recommend programmable thermostats either. Jacking your temp setting up and down daily doesn’t save any $ – I’ve done the calculations.

Without knowing the house layout and ducting, and how well the two systems are separated, I have no idea how much the upstairs system would help whole house dehumidification. The more the 2 systems mix air the more it will help. Typically a 2-stage system would be used on the lower floor and not needed for the upper floor unless the 2 systems were very isolated, and possibly not then. Heat rises, so the upstairs system will usually run more and dehumidify better. It is also possible your systems are oversized – a common problem. System installers usually don’t get called for an oversized system, but they do for an undersized one, so they typically go a little extra, which the customer pays for. For a design day, i.e. outside design temperature, the unit should not cycle – it should run all day. This maximizes dehumidification and minimizes starts (electrically expensive) and a smaller unit uses less electricity when running. Of course that does not solve humidity issues on cooler cloudy or rainy days.

View GR8HUNTER's profile

GR8HUNTER

5108 posts in 912 days


#2 posted 11-14-2018 10:45 PM

not a pro just like to say 1 thing compare apple to apple and not apple to lemon :<))

-- Tony---- Reinholds,Pa.------ REMEMBER TO ALWAYS HAVE FUN

View DrDirt's profile

DrDirt

4510 posts in 3942 days


#3 posted 11-14-2018 10:57 PM

You are definitely on track with the variable speed unit… For us it wasn’t about humidity, but circulation, where our unit will run the blower, w/o heating or cooling, to help keep the temperatures level in the house… especially downstairs would get cold w/o kicking on the heat, as the thermostat is upstairs in the bedroom hall.

Unit size – - look at how cold or Hot do you like it. those are as a difference between outside. If you and SWMBO, like to have the heat at 75 in the winter… stay with the larger unit. If you are frugal and keep to 68 or so, the smaller unit will work.

The Variable speed will prevent the need for cycling on and off with an oversize unit. But the old single speed, that was a big issue because the heat would kick on… Spike the temp, shut off, then equalize and get cold. Your High Efficiency new unit wont have that issue. We dealt with this on the AC side… and would err on having more capacity, then a small unit needing to run and run and run to keep up.

-- “The two most important days in your life are the day you are born and the day you find out why.” Mark Twain

View OSU55's profile

OSU55

1964 posts in 2189 days


#4 posted 11-15-2018 01:13 PM

There ar t-stats available that turn on just the fan for an amount of time, say 10 min, every hour to prevent hot and cold spots. Also important how the heat stages are wired for a heat pump. Only 5kw is needed for makeup, then an additional 10-15kw available for 2nd stage and emergency. Mine is wired this way so that minimal resistance heat is used and the heat pump does the manority of the work plus prevents the big hot blast, and saves $. The 5kw is all that ever comes on even in single digit weather. Improper t-stat locations create lots of issues. An oversized variable speed unit = burning $ bills.

Have your say...

You must be signed in to reply.

DISCLAIMER: Any posts on LJ are posted by individuals acting in their own right and do not necessarily reflect the views of LJ. LJ will not be held liable for the actions of any user.

Latest Projects | Latest Blog Entries | Latest Forum Topics

HomeRefurbers.com