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What does Bonnet Cap mean for a furnace.

Posted by woody_007 (My Page) on
Wed, Jul 11, 07 at 14:03

Does anyone know what the term "Bonnet Cap" means for a furnace?

I looked inside my 70's furnace and it says: Input 110 MBTU, Bonnet Cap 88 MBTU.

For a finding a replacement furnace do I go off the Input BTU or the Bonnet Cap BTU.


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RE: What does Bonnet Cap mean for a furnace.

The bonnet is the actual btu's you'll get after the loss from the flue and ambient loss etc. I find it puzzling that furnace has a 80% difference.
When calculating a heat loss load in a home, it's best to use this bonnet to decide the size heater needed.
*Much like using the sensible capacity in an a/c instead of the total btu capacity.


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RE: What does Bonnet Cap mean for a furnace.

Thank you for the response.

If I'm not mistaken doesn't the heat loss work out to be 20% as opposed to 80% as was stated in the previous post.
110 BTUH in, 22 BTUH loss, and 88 BTUH left over to heat home. The loss is 22/110 * 100 = 20%.

If I understood correctly, I should use the Bonnet Cap BTUH to guide me on the size of the replacement furnace.
88,000 BTUH * 60% AFUE(70's furnace) = 49,800 BTUH. My new furnace should have an ouput of no less than 49,800 BTUH. Is my thinking correct here?


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