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Old 09-17-2008, 09:30 AM   #1
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Furnace Sizing


Situation:

Heat load caclulation: 59,100 BTU

Proposed Furnace #1 (two contractors):
Efficiency: up to 96.7% @ 60,000 BTU = 58020 BTU (max.)
Two stage / variable blower

Proposed furnace #2 (one contractor)
Efficiency: 95% @ 80,000 BTU = 76000 BTU
Two stage / variable blower

How close does the actual output need to be to the calculated Heat Loss? I don't want to be stuck with a furnace running full time on the second stage and yet not being able to keep up with heating the house on cold days.
On the other hand, if the larger furnace is 17,000 BTU higher than the heat load, will it short cycle on those "colder" days?

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Old 09-17-2008, 09:57 PM   #2
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Furnace Sizing


I would go with the second furnace despite the fact you think it is oversized. Remember that for the most part, the furnace will be able to heat your house and satisfy your thermostat while running in first stage anyhow.

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Old 09-18-2008, 11:07 AM   #3
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Furnace Sizing


What is the fuel: natural gas or propane?

How was the heat load calc done? If a meticulous job was done with a Manual J then #1 may be okay. If you're not sure then go over the numbers again with whomever did the calc.

Being slightly undersized won't hurt too much, but it hurts when you pay more for a furnace initially and then pay more again due to short cycling. Short cycling may also affect comfort.

Don't forget Manual J is setup to be conservative, so your actual heat loss may lower than that.

Last edited by dac122; 09-18-2008 at 11:12 AM.
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