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Boiler Input / Output Calculator
Calculate boiler output from input and efficiency, or determine the required input for a desired output. Understand the relationship between gross input, net input, and heat output.
Boiler Input vs Output
Boiler input (gross) is the total heat energy from burning the gas. Boiler output is the heat transferred to the water. The difference is lost through the flue.
Modern condensing boilers achieve 90–94% efficiency, meaning 90–94% of the gas energy goes into the water. Gross input is always higher than output; net input is gross ÷ 1.11 for natural gas.
Frequently Asked Questions
What is the difference between gross and net input?
Gross input includes all the heat from combustion. Net input subtracts the latent heat of water vapour in the flue gases. For natural gas, net = gross ÷ 1.11.
What efficiency should I expect from a condensing boiler?
Modern condensing boilers achieve 90–94% efficiency when operating in condensing mode (return water below 55°C). At higher return temperatures, efficiency drops to 85–88%.
Boiler Input / Output Calculator
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