Furnace size

Old 05-05-06, 05:52 AM
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Join Date: Apr 2006
Location: Arlington
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Furnace size

I downloaded the HVAC Calc program that I've seen mentioned here to size my AC unit & furnace. Can anyone tell me if this sounds correct?

I live in Indianapolis, winter design temp of 3 & we keep it about 68 inside, the house is 106 years old but in the process of being updated, 2 story, 2000 sqft., R-19 exterior walls with Tyvek & vinyl siding, floor is over a crawl space with R-19 insulation, mostly carpet flooring, ceiling will be R-49 under ventilated attic. All new doors & windows. I hope this is enough info to get an idea if it's right or not.

The program spits out 46,067 heat loss, which if I add 20% gives me about a 55,000 btu furnace that I need.

Old 05-05-06, 02:29 PM
Join Date: Mar 2006
Location: Wet side of Washington state.
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It is utterly impossible to state with any authority if that figure is correct from the scant information you have supplied.

If you have not overestimated the installed value of your insulation, doors and windows (installed value is rarely the advertised value) and you have not underestimated your inward/outward air exchange rate (have you had a blower door test since making your changes?) then the heat-loss calculations should be quite close.

Why have you added another 20% to the calculated heat loss? Is the 3 degree outside temperature used in the program typical for your immediate area? Heat loss programs often use a long-term historical average for outside design temperatures and just as often these particular "design" temperatures do not correlate to average wintertime temperatures. For example, in the Seattle area (where I live) the "design" temperature is considered to be zero but in reality it is rare for the temperature to drop below 20 degrees and even then it is rare for it to stay below 20 for more than a few days.

Allowing for additional heating capacity can sometimes make sense depending on what type of system (forced air or hydronic) and the ability of the furnace or boiler to operate below maximum output via a multiple stage or modulating burner.

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