Furnace size VS. Man. J

[AccurateHT]

After calculating with man. J (on paper) a house shows 70400 (71* HTD). I am looking at goodman 90+ equipment. The 70 would undersize by 4000, the 90 would over by 14. Man. J says stay tight to results. Opinions?

[beenthere]

Go with the 90.

Nothing worse then finding out that the gas company is suppling gas with a 100 BTU lower content then you thought when its at design temp outside. That could put you at 10,000 BTUs light.

[mayguy]

I'd go less. I did that on mine, and it keeps up just fine when it's -25˚ out.

[beenthere]

I'd go less. I did that on mine, and it keeps up just fine when it's -25˚ out.

Shows that your calc was inaccurate.

Not knowing how inaccurate his calc is, and in which direction, undersizing could mean he has a 10° cooler house at design.

[21degrees]

If the furnace is 70,000 x 90% = 63000 output and a 90,000 x 90% = 81,000.
I would go with 90,000 just incase R-valve was high on something. Have a small safety factor.

[deejoe]

If the furnace is 70,000 x 90% = 63000 output and a 90,000 x 90% = 81,000.
I would go with 90,000 just incase R-valve was high on something. Have a small safety factor.

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My thoughts exactly. Go with the larger one and get the benefit of shorter, but efficient, comfortable cycles, like my furnace with only about 2 six minute burner cycles an hour even at design temperature.