Furnace sizing...What makes sense????

[blackwolfrun]

I live in the Chicago suburbs and am looking to have a 95+ furnace installed. I currently have a 125K BTU Janitrol that's around 19 years old (standing pilot).

I've had 4 reputable contractors give me quotes on the Goodman 2-stage 95. Two guys say I can get by with the 90K and the other two say I need 115K. I've asked all 4 to do a load calc to no avail. And three guys eventually changed their mind when I questioned the size.

Anyone have thoughts which makes more sense? I live in a typical 2-story (2400 sq ft) w basement. This shouldn't be so complicated, right?

[amd]

125k x 75% - 80% steady state efficiency = 94k - 100k output

When was the house built?


If your house is properly insulated and has double pane windows (average surface area), a 70k BTU furnace @ 95% efficiency will probably do fine. (Design temp 0F or higher assumed)

Your best bet is to find someone who's willing to do a load calculation.

It is important to note that the 90k GMV95 needs to move a minimum of around 1400 CFM (considerably more than equivalent natural draft units) so duct modifications will probably be required.

The 70k version needs to move at least 1100 CFM on high.

Take a look at the existing unit's data plate:

If the blower motor size is 1/3 HP, the duct system was most likely designed for 1200 CFM or less.

CFM = cubic feet per minute.

[blackwolfrun]

House built in 1991. You're saying 70K? So is 115K too big?

[amd]

It depends on what minimum code was in your area.

If it was anything like Ontario's at the time (R32 attic, R11 walls, double pane windows, R8 half height basement in late 80s/early 90s), 115k is absolutely insane for 2400 sq ft.

Even 85k output (90k @ 95% efficiency) is very high for a well built house of that size. Usually you'll only find 80k+ output furnaces in Mcmansions (3200+ sq ft) and old poorly insulated homes.

[Au-en-boy]

Have someone do a Man J. You may have to pay a little for it, but it will pay off huge dividends later when you are comfortable in your home. Make one of the contractors PROVE to you that they are right, and not just TELL you. Doing the calc is the only way to be sure.

I replaced a furnace for a lady last winter. 4 contractors total for bidding - I was the only one that did the Man J and was accused (by the competition) of being "undersized". Well, I got the job and within a week, the lady CAME DOWN TO THE SHOP to deliver brownies because she was finally comfortable in her house. There is no substitute for doing the job properly the first time. You are sure to have customers that are happier, and the brownies are really good.....

[beenthere]

You can do your own Load Calc. Its a 49 dollar fee for the program.

Well worth it.

[DanW13]

Yep do your own and still ask all those who submitted bids to prove you need X BTU's, if you take good measurements and give good inputs for the values of yoru insulation in your home you will come very close to what your home needs and you will then be in control of who gets the job and who will install the right equipment.

If when you run your calc and duct work needs to be modified by all means rework the ducting thru out the home your going to be much happier and more comfortable by making the necessary changes. Not to mention the amount of money your likely to saving in operation cost over the long haul.