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Bonnet capacity

Hello to all

What does bonnet capacity mean.

I have a friend that wants to replace his forced hot air system that says input 100000 btu bonnet capacity 80000 btu

I know it differs from afue but is this an 80% furnace?

Thanks Michael


  • Robert O'Brien
    Robert O'Brien Member Posts: 3,451

    Bonnet is output
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  • charliechicago
    charliechicago Member Posts: 124
    Chimney liner

    So if I replace it with an 80% I don't need a liner?
  • Ironman
    Ironman Member Posts: 6,868
    Not 80%

    That's not an 80% furnace, it's probably 65% at best. Any thing that would have "bonnet capacity" stamped on it is probably at leas 35 years old, maybe much more.

    A new 80% furnace must have a liner installed with it to prevent flue gas condensation. By the time you pay for that, you could have gotten a 95% one and been better off in most cases.

    Also, a heat loss calculation needs to be done as the old furnace is probably way over-sized for the house and a higher efficiency furnace would have more output.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • charliechicago
    charliechicago Member Posts: 124
    Iron man

    Thank u for the response. You are right that it's an old furnace as you can see from the picture. I had been told exactly what u said but I wanted confirmation. Also that's a great idea about a condensing unit. But I'm still wondering what exactly does bonnet capacity actually mean. Thanks mike