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Can chimney liner be sized based on combustion level only?
Some background - the boiler is a Weil GO-6 fired at the level of a GO-5 using a Carlin EZ-gas propane. Even the current output is too high so I am now dropping the input from 203K to 150K. Partly this is to test "how low can you go" since I am replacing this with a Burnham ES2 (5 or 6) after this heating season. I'd like to get the chimney work done soon so that's not a problem if the boiler suddenly needs to be replaced this winter.
Stack is currently 7" but that was for an input of 210K (oil). The new boiler will have a 6" stack. NFPA says that the liner can therefore be 5" and according to the liner manufacturer, that will handle at least 200K. Chimney is over 40 feet.
Chimney guy prefers to use the 5" since he'll have to navigate some offsets and mortar lumps. I have no question that the 5" will be fine for the new boiler with input between 140K and 175K.
In the interim this new chimney needs to work with the current boiler for a few months. Am I right to think that the liner needs to handle the actual combustion level and since I have dropped this boiler to the size of the smaller future unit the smaller liner will suffice?
The EZ-gas is fan driven (like an oil burner) if that matters.