I hear all the time that furnace has 90% - 95% efficiency.

How is the efficiency of furnace counted? When it says 90 % efficiency, what's meant by that? 90 % measured against what? What's 100 %?

Thanks,

Jignesh

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Any of the various energy modeling programs would give you a starting place. Model the ducts in various places and with different insulation levels.   I know you will not forget to seal the ducts.

It is called the Distribution Efficiency Table. Here is a link to a PDF of it:

https://energy.mo.gov/sites/energy/files/bpi-distribution-efficienc...

Hope that helps

10-12" liners are not made. Biggest made I find is 6" Flue temp on gravity is above 800' most 80% gas or lp are in the 350-380'. If a low end replacement is put in goes from quite to very very noisy

For example: if a furnace has an AFUE rating of up to 98%, which means that up to 98 cents of every $1.00 of heating energy expense warms your home.

Before purchasing this appliance, read important information about its estimated annual energy consumption, yearly operating cost, or energy efficiency rating that is available from your retailer.

The efficiency of a furnace is the ratio of useful output to heat input. Even efficiency of the furnace can be determined by direct and indirect methods. The efficiency of the furnace can be computed by measuring the amount of fuel consumed per unit weight of material produced from the furnace.

Efficiency of furnace is calculated or measured by AFUE(Annual Fuel Utilization Efficiency).

AFUE is the ratio of annual heat output of the furnace or boiler compared to the total annual fossil fuel energy consumed by a furnace or boiler.

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