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Under firing a boiler

moose5 Member Posts: 7
I have an H B Smith 5 section cast iron boiler with a Carlin 100CRD burner.The boiler manual for that FD-12 model says it should be fired at 1.35 GPH.The boiler is oversized for the house it is in.I would like to fire it at a lower rate{1.0 GPH or 1.1 GPH}.If I make the necessary air adjustments does anyone have any thoughts why this wouldn't work.I think it would make the boiler run longer during each cycle and therefore be more efficient.


  • Under firing a boiler

    You can under a boiler. Just how much is a question that you need to ask the boiler munufacturer.

    Many boilers can be under fired more than 25%.

    As I understand the limit to under firing a boiler is if you under fire to much condensation can occur at the wet walls of the boiler. Condensation will eventually rot out the boiler. Additionally, you need to generate enough heat to allow the breeching and the chimney to get hot enough to create the proper draft to allow the products of combustion to exit the building.

    You can lower the nozzle size to 1 GPH and be O.K. and you were correct, the boiler will run a bit longer to make steam and provide heat, but that will not cause you to burn more fuel.

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