I’m going to be replacing the furnace in the house we just bought, and I’ve had three estimates done for installing a new high-efficiency gas furnace. The problem I have is that the size of the furnaces in the quotes is quite different.
An Aire Flo Rep quoted me a 75,000 BTU unit (I’ve pretty much eliminated this company, since the sales rep who came out did almost no measurements and didn’t take much time to explain anything. I’ve also found some information that Aire Flo furnaces are not very good quality).
A Carrier Rep quoted me a 60,000 BTU furnace.
A Lennox Rep quoted me a 80,000 BTU Trane or a 90,000 BTU Lennox. He claimed a 70,000 BTU furnace (the next size down in a Lennox) was only good for up to 1200 ft[sup]2[/sup]
I’m trying to figure out what the correct size should be, so I thought I’d turn to the expertise of the Teeming Millions. Here is some relevant information:
The house is about 24 yrs old, and is 1,450 ft[sup]2[/sup] (including the basement). It’s a raised bungalow, and faces south (roughly). I’m in the Kitchener/Waterloo area of Ontario.
I have also had an EnerGuide for Houses assesment done. This is a new standard developed by the Government of Canada. According to it, the house has a “Design Heat Loss” of 35,021 btu/h, and uses approximately 3006 m[sup]3[/sup] of Natural Gas. I suspect the Design Heat Loss figure may help me size my new furnace, but I’m not sure how - I assume it doesn’t mean I need only a 35,000 BTU furnace. I’ve tried contacting the company that did my Energy Rating, but so far I’ve only been able to talk to their answering machine.
Any advice the Dopers can give would be most appreciated! Thanks.
Proper sizing takes into account your region, building facing, and construction details. In a 24 year old dwelling, there are many variables such as: insulation in walls/ceiling/attic-total R value for each area. You’ve mentioned basement-is it above/below/or partially below grade, and is it considered part of the conditioned space? Do you have discharges and return vents within the space?
Unless there are many houses nearly identical to yours in an area, a good contractor has to do a heat loss calc from scratch. There are some software sources out there which request the data, allowing you to plug in your own figures. One manufacturer is sending me same, but they’ve not arrived yet. A visit to Google would turned up the following: search=heat+loss+calculation
I looked at the links you provided, in particular the 1st one. From what it said, it looks as though if I take the DHL factor for my house (which is 35,021 BTU/h) and double it, that will give me the maximum size for my furnace (70,000 BTU/h). Can I safely assume that when a furnace is advertised at 70,000 BTU, that it’s the same as saying 70,000 BTU/h? Or is there a difference between BTU and BTU/h?
Well, you can never be sure about advertising claims, but the specification only makes sense if it is BTU per unit of time. And for the figure 70000 BTU, BTU/hr fits.
The number of BTU’s is an amount of heat. This amount can be distributed over some time, 1 sec., 1 min., 1 hr., 1 day., etc. For furnaces the standard is BTU/hr and many brochures leave off the hr.