wildly different HVAC proposals
Hi all. I’ve got a question regarding HVAC for a new construction house that I’m building myself, and I’m not too sure that there is an easy answer. So far, I’ve gotten two proposals; one from a large, well known company and another from a small, family owned company. Both are for two zones; one being a heat pump w/ gas backup on the first floor and finished, walk-out basement (approx. 2700 sq. Ft. total) and the other being a separate heat pump for the second floor loft, bath and bedrooms (about 700 sq. Ft.). But, the capacities vary wildly. One proposes 3.5 and 4 ton units and 130K BTU while the other only 2-ton units and 60K BTU. The house is 2×6 construction, but they both know that and both did a walk through. I don’t want to get a system that is undersized, but then again, I don’t want something that is too powerful either. Is there a way that I can check to see whose numbers are more in line? FYI, I’m in Maryland, if that makes a difference.
Thanks in advance for your input.
Replies
I'm not an expert on this by any means, and I know this isn't the preferred method of sizing a unit, but I was always told for a rough estimate, 1 ton/600 square feet. I know that doesn't take into account heat loss, tall ceilings, window types and amounts or anything, but generally speaking it's a good rough estimate. If that were the case, I think the 3.5 ton unit for hte first floor would be about correct, and a 2 ton unit for the second floor would be about correct as well. Even in a case were the home were insulated perfectly, 2700 square feet seems like an awfull lot of space for a 60K btu 2 ton unit. I think it would run way too long. And on the flip side, 600 square feet seems way too small for a 3.5 tom 130k btu unit. the thing would click on and off way too fast to circulate the air.
Sq ft guestimates are worthless.
Way too many variables.
Ask them to show you the manual J calculations.
Or you can use this program to do your own calculations.
http://www.hvaccomputer.com/
Having an AC oversized is much more of a problem than having it undersized.
To clarify, both contractors are proposing two separate systems. Each system is a heat pump with a gas furnace. One contractor is proposing a 4 ton cooling system for the 2700 sq ft of first floor and walk-out basement and another system that is 3.5 tons of cooling for the 700 sq ft second floor/loft. The other contractor is proposing (2) identical 2 ton (cooling) systems for the same areas as the first contractor proposed 4 & 3.5 ton systems, correct?
As the others have correctly pointed out, "square footing" HVAC systems is not an acceptable manner to size these systems. On the other hand, there are some basic checks to look at in terms of reasonable-ness of systems. The climate where you live is very important, as is the construction of your home in determining the proper size of the equipment.
For instance, 3400 sf of house and 130,000 btuh of gas heat, works out to 38 btuh/sf (a little on the high side for your climate and good contrstuction). With 60,000, that is less than 18 btuh/sf (probably not enough to keep you warm on the coldest days of winter without supplemental heat). For cooling, 7.5 tons in the space works out to 453 sf/ton (this is suspect!)and 4 tons of cooling in the space works out 850 sf/ton (not bad for dehumidifcation, but maybe a little light). Design conditions for Andrews AFB is 10 deg for heating and 95 deg for cooling. In that environment, with 2x6 walls (r-19) and similar better than average insulation in the ceiling and good (not Pella) double pane, insulated windows, I have to beleieve that neither has done a good job of proerly sizing the systems for your home. Look for more bids or ask to see the sizing calculations.
My best guess (and I do this for a living) is that no more than 5 tons total cooling capacity (both systems combined) and around 100,000 btuh of gas heat. This is ONLY A GUESS.
"To clarify, both contractors are proposing two separate systems. Each system is a heat pump with a gas furnace."
Actually, only the unit for the first floor and basement has the gas furnace backup. The other unit, for the second floor only, is strickly a heat pump.
Otherwise, as stated, 2x6 walls and triple pane windows.
get a third proposal
As it ends out, only the company with the smaller sized equipment had done the manual J calculations. The other company said that the calculations were too time consuming and would do it if we accepted their contract. Needless to say, that company is no longer being considered.Thanks to everyone for their input.