BTUs and blower motor

mangogirl3August 7, 2012

I'm replacing both the a/c and gas furnace due to a dead blower motor (system is 17.5 years old). Current furnace is 90,000 BTUs.

Two contractors are proposing 90,000 BTUs, one is proposing 100,000. The fourth is saying it should be 70,000 and that 90,000 is oversized for my home. House is 2280 sqft (not including basement). One contractor said that using the square footage, considering windows and insulation, about 108K was about right, but if I had been comfortable at 90K he would recommend leaving it there. (he acknowledged that his calculation was old-school and not perfect). It seems to me that 70K would be low. I'm in Maryland use the heat from sometime in October through April or early May.

One of the contractors thinks a variable speed furnace isn't good for my situation since I have bad air flow on the second story. His theory is that it would cycle on and off too much.

Any thought or suggestions are appreciated -- my head is swimming!

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The correct way to determine the size of the furnace is to do a load calcualtion. Has any of the four contractors done this? Guessing or using rules of thumb are not advisable.

I assume your current furnace was a 80% efficiency model. So if the input was 90,000BTU, then the output was aabout 72,000 BTU. This is a reasonable size for house your size. If you get a 95% efficiency model, then you only need a input of 75,000 BTU to maintain the same output you currently have.

I don't understand the comment that a variable speed furnace will cycle on and off. Variable speed furnaces are designed not to do this.

You need to address the air flow issue on the second story. You can buy the best furnace on the market but it will never work well if there are duct issues.

    Bookmark   August 7, 2012 at 4:38PM
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Mike, no, none of the contractors has done a real load calculation.

I'm not positive, but I think the current furnace is 90% efficiency.

I agree that the air flow needs to be addressed, but am not sure how. Suggestions have been using dampers or installing a mini-split system (which i'm not crazy about).


    Bookmark   August 7, 2012 at 4:57PM
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Why would anybody replace a furnace and A/C because of a failed blower motor?

Would you scrap a car because it needed brakes?

    Bookmark   August 9, 2012 at 3:05PM
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because it's 17.5 years old and the repairs are over $1000 -- also needs a capacitor and a few other parts for the a/c. much higher repair cost v. replacement cost than brakes (unless maybe the car was 20 years old).

    Bookmark   August 9, 2012 at 3:49PM
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"The correct way to determine the size of the furnace is to do a load calcualtion. "

Or go back through utility bills and determine how much energy was actually used compared to the actual degree heating and cooling days.

You already have run the measurement for heat loss and gain, far better than you can calculate, especially with an older house that probably has infiltration as the largest single factor.

    Bookmark   August 9, 2012 at 4:41PM
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