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Venting gas and oil?

ZerbeAM Member Posts: 6
Hey all,

I have a customer looking to convert to gas. They currently have a monster oil boiler and a bock 32e direct fired water heater. Due to some financial constraints (and the fact that the HWH is only a couple years old) they would like to go ahead with the boiler now, add HW later.

Here is the Question... can i tee the gas boiler to the hwh flue pipe or do i have to open up a new hole in the chimney? I know that some people say the safety controls on the boiler and water heater are acceptable as safety shut offs, but i think i should add some type of spill switch because all they do is flame prove... am i right?

Thanks for any help on this subject.

"Friends applaud, the comedy is finished."


  • Bob Harper
    Bob Harper Member Posts: 977
    common venting

    The IRC does allow common venting into a single manifold before the chimney even though elsewhere it contradicts itself by calling for the lower BTU input rated appliance to enter the chimney above the larger one(s).

    You need to perform a Level II inspection per NFPA 211 to ascertain if the chimney is suitable for this service. If not, reline & repair as needed or seek alternatives such as power venting. Note that all positive vent pressure combustion appliances must have their own dedicated separate vents.

    The code requires "primary safety controls" when common venting. It is upon the AHJ to determine what satisfies this requirement. Generally, it is intended to mean spill switches. Flame proving devices such as safety pilots and flame rectification systems do not protect against backdrafting.

    As for sizing the chimney connectors, refer to the sizing charts in the gas code. You can calculate oil input BTU ratings and apply them to gas for sizing purposes. The sizing tables in the back of NFPA 31 are Not part of that Std. but merely for informational purposes only unless specificallly voted into local ordinance.

    I would advise planning on a listed stainless steel liner and switching to an indirect DHW tank when possible.

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