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What is turndown ratio and is it different for direct vs. indirect gas heaters?

April 26, 2017different Gas Heaters indirect ratio turndown vs

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What is turndown ratio and is it different for direct vs. indirect gas heaters?

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Turndown ratio compares the maximum to minimum heat output. Turndown ratio for direct gas burners is 25:1, which means that the burner can modulate from 4% to 100% of full fire. Indirect gas turndown ratio changes based on the controls. Example based on a single furnace unit (< 400 MBH). 4:1 electric modulation limits the heat output from 25% to 100% of full fire. 8:1 stage limits the heat output from 12.5% to 100% of full fire. See the Indirect Gas-Fired Technical Guide for more information.