Construction
Gas Furnace Input Calculator
Manufacturer rates output BTU; you need to know input BTU for gas line sizing and venting.
Input BTU/hr
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- CFH (natural gas)
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- CFH (propane)
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- Annual therms
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FAQ
Why care about input? +
Gas line sizing (NFPA 54) and vent sizing (Cat I/IV) are based on input BTU. Pump sizing, gas meter capacity — all input-based.
Modulating furnaces? +
Use rated max input. Modulating units fire down but the gas line must support full fire.