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How many btu's do I need?

Posted by smtrwl (My Page) on
Tue, Sep 15, 09 at 21:58

I obtained several estimates for replacing my current furnace. I am leaning toward the American Standard 95% two stage system. However, the btu's needed for my home seem to vary with each service company.

I live in Metro Detroit...it gets rather cold in the winter. Our home is approximately 1600 sq feet. It is an aluminum sided tri level with average, or less then average, insulation. The current system is a Rheem 75,000 BTU's. My air conditioner was replaced a couple of years ago it is 2.5 ton. The estimates range anywhere from 60,000 to 80,000 btu's on a replacement furnace.

I have found some information based on the region I live in that I should mulitply 50 by the sq ft. (50 x 1600 = 80,000 btu's). Is this based on a higher efficiency furnace?? Because I found other information saying that my BTU requirement should go down with a higher efficiency furnace. Soooo much information and it is frustrating deciphering it. Any opinions or suggestions where to obtain the most up-to-date and accurate information?


Follow-Up Postings:

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RE: How many btu's do I need?

There is only ONE way to know. You need a professional Manual J calculation done on your home. It might cost a few hundred or maybe even free, but you get what you pay for in many cases.


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RE: How many btu's do I need?

this is how you find out how many btu's you need. The 75,000 that is on the existing furn is for the imput and the output would be 90000. The way you should find btu's is take your sq. ft. 1600 and multiply it by 40. 1600x40=64,000btu's. The formula is reversed for finding the taonage of your A/c. Take 1600/40=4 ton.


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RE: How many btu's do I need?

That is funny!

Not only does crob manage to make a 75,000 input furnace and produce 90,000, it that a 125% efficient furnace?

His sizing suggestion is exactly how people get caught oversizing. 4 tons of AC in a 1,600 sq ft house in Detroit?

Too start with the exiting is 2.5 and was probably sufficient, please don't take any short cuts and get a competent appraisal with a detailed heat loss.


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RE: How many btu's do I need?

crob's formula for sizing a new furnace assumes 2 things, 1) the original furnace was sized correctly and 2) the heat loss has not changed at all. New doors, windows, insulation, weatherstripping, etc will lower the heat loss, which will reduce the size needed for the replacement furnace. His figures for in/out are reversed.


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