Did your contractor do a manual j or equivalent. There are other options for tstat wiring you could relocate or go wireless etc. First find a contractor to give you a good system evaluation. Square feet for sizing is not correct.
I'm replacing an old 1966 200,000 Btu input furnace with a 95% efficient furnace to get the tax credit. So far I have one estimate and it has generated a few questions. The dealer is recommending a 90,000 Btu output Amana AMH95 two stage burner with multispeed blower.
Questions:
1) I did a manual J calculation and also back calculated heat loss based on 2008 gas consumption, 60% efficiency and 2008 Heating Degree Days for my area (SE Michigan). Answer was 32,000 Btu / hour heat loss. Assuming this calculation is correct, how big should I go on the furnace? Dealer says he only uses house square footage to size the furnace, independent of insulation, air infiltration etc. He also said that since it is a two stage, normal sizing rules don't apply.
2) Since the basement ceiling is drywalled, it would be a bear to rewire the thermostat, so I think I'm stuck with a single stage thermostat. The dealer quote says I can have any brand of furnace I want for the same price with the same basic specifications (95%, two stage burner, multispeed). Does being stuck with a one stage thermostat limit my choices of other equivalent furnaces?
Thanks in advance for your advice and feedback.
Did your contractor do a manual j or equivalent. There are other options for tstat wiring you could relocate or go wireless etc. First find a contractor to give you a good system evaluation. Square feet for sizing is not correct.
If he says 2 stage furnaces get sized differently, get another contractor.
At the top of the page is a $50 load calc for homeowners called HVAC Calc. A proper load calc should have yeilded the size needed. Finished basements can be a bear, but we have run new stat wire through a nearby return in similar cases.
Theres more to consider then just the size of the heater!
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i was gonna take a look at your link but for some reason it wont open.
Thanks for the replies so far. I did use a Manual J calculator and that is how I came up with the 32,000 btu /hour loss for the house. As I indicated, back calculating from last year's gas usage verified this number. But aren't furnaces generally sized with a safety factor to take into account especially cold periods? For instance I believe the Manual J calculator assumes 5 degrees F as a baseline, but the loss would be greater on a -10 degree day, which we get occasionally here.
I was disappointed that this contractor did not even offer to do their own Manual J, as they are highly recommended by our home handyman radio show guy, our local building inspector and have been in business for 40+ years with a 70 truck fleet.
I appreciate the suggestions for thermostat options. I didn't realize that a wireless option existed, but it might be the easier solution.
Thanks again.
I'm not a pro, but when watching the demonstration of HVAC-Calc heat gain/loss calculator, it suggested a 15-25% safety factor to add to the heat loss. Still, that's putting you at 40k btu.
Sizing by square footage is never the right way to do it.
Make who ever do a heat loss calc or move on to the next. As far as the thermostat, ask about the Honeywell wireless RedLink series. A little pricier but it will practically vacuum your house it's apps are that versatile.
A good HVAC tech knows how, an educated HVAC tech knows why!
DEM
Find a competent contractor that will perform the Man J calculation. They should have no problem showing it to you and leaving a copy when you sign the sales agreement. Very important this is done correctly.
As stated there are many options for the T'stat. In addition to wireless H/W VP IAQ uses only 3 wires between the wall unit and interface module.
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Thats probably a PAID endorsement.
You don't size for the occasional coldest temps.
A a 40,000 BTU input 90% would give you 36,000BTUs output, A 50,000 BTU input 90% would give you 45,000BTUs output.
That would cover you on those few colder days you get every couple years.
Travelover, A good estimator will know how to size a system for your home and your area instinctively, but he/she will always check.
Most 2 stage equipment will work off of a single stage t'stat. Carrier has 1 system that only needs 4 wires.
I suggest that you call a couple of manufactures, ask who they consider is the best 3 dealers in your area and go from there.
In my opinion 2 stage variable speed( D.C. drive ) is the way to go. Do yourself a favor and add 10 yr. labor warranty.
Thanks all. Contractor #2 is coming tomorrow - I'm sure I'll have more questions.
Contractor #2 came and went and I must say that he spent a lot more time looking at the job and asking questions. Unfortunately, he also bases furnace size on square footage, though he did agree to take a sheet of data (dimensions, wall and ceiling construction) about my house back to the office and do a calculation.
Who ever put in the AC created a bumped out area on the furnace plenum to get the A coil in and he said the A coil needs to be reoriented to be correct. So that adds a little to the price, though he is also quoting a Lennox vs an Amana.
I think I'm convinced that I want to go with a two stage burner with a variable speed motor.
I appreciate the feedback from all of you.
See what size his load calc says.
2 stage does improve comfort.
Like Ryan, I'm also not a PRO, but did my own ManJ using HVAC-calc after several PROs gave 'varying' quotes.
The program doesn't assume any particular OD temperatures like in your case, but relies on you telling it where you live. The result is a more accurate 'starting point' at which to add any safety factor if you should so choose.
The OD (design temperatures) are rarely exceeded, so on very cold days, you will enjoy long heating cycles.
Amp
The load calc's can be very accurate and uses the OD design temp in your area. Here is a chart for for the US.
http://www.comfort-calc.net/outdoor_design_temps.html
All due respects sir, just trying to point this customer in the right direction and be profesional. Yes , Carrier 2 stage equipment will work very well with a single stage t'stat, and the Infinity controls are even better but I shouldn't have to remind you of that. Oh by the way, having worked for 2 of the top 3 Carrier dealers in the St. Louis area, beleive me, the area distributors know who they can rely on! Why such the negative attitude?