Sizing thoughts - DIY Chatroom

moneymgmt · 10-23-2007, 11:16 AM

I've had a few HVAC people out to look at my current system and prepare estimates. One quality that all these sales people have lacked is conviction in their recommendations and that really bugs me. The entire house isn't more than 1200sf and I need a 90%+ forced air unit . I've had recommendations of everything from 60,000btu to 115,000btu. How can one guy say 60 and the next guy twice that? The 60k guy said "I have double this footage and I have a 60k unit that works fine", i think he's insane. The old furnace was an 80% 70,000btu and I thought it ran a little too much. Since we're increasing efficiency common sense tells me 70k should be about right as a replacement. Any thoughts?

HVACDave · 10-24-2007, 06:32 PM

Sizing depends on a lot of factors. What is the insulation factor in your house, how many air changes per hour is your house experiencing (do you have a heat recovery ventilator) how tight are your windows and doors. What is your exposure like. Does your house sit in the middle of an open field, or is it surrounded by a good wind break.
Many guys will just replace with the size of unit you already have. Others will use rule of thumb calculations. Even others may just pick the closest size that thier particular manufacturer makes in the equipment models they are trying to sell you. Most high efficient models come in 40, 60, 80, 100,000 btu's. If you currently have a 70,000 mid effficient, which would they recommend?
As far as run times go, you would expect your furnace to run fairly constant on the coldest days. Generally you would expect that on the coldest day you would ever expect to see in your area, you should expect to see your furnace run 85-95% of the time if it is sized close to correctly.

Dave

coolmen · 10-28-2007, 03:30 PM

a load calculation is the only true way. You can do it your self on some websites for a small fee

hvactech · 10-28-2007, 08:57 PM

like others here said, a heatloss calculation is needed to be 100% on the mark.
my house is bigger than yours and has a 60K 92% rheem unit, it runs like a charm.