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Koven Air Tool 3
Direct vs Indirect Operating-Cost Estimator
Compare annual gas cost of a ~92%-efficient direct-fired unit against a ~80%-efficient indirect unit, using your own numbers.
Illustrative estimate from your inputs and published efficiency figures (~92% direct / ~80% indirect). Actual results vary with load profile, controls, and fuel. Direct-fired introduces combustion products into supply air; indirect keeps them sealed — efficiency is one of several selection factors.
Gas input rating of the make-up air unit.
Approximate hours the burner runs over a heating season.
1 therm = 100,000 BTU. Use your utility rate.
Direct-fired (~92% efficiency)
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Indirect-fired (~80% efficiency)
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