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Community and Q&A

single air intake for water heater and furnace

prometheanfire | Posted in Building Code Questions on

I have both a gas furnace and tankless hot water heater (I plan on switching to electric when I can).  They currently both source their combustion air from within the conditioned envelope.  If both require a 3 inch intake, would it be possible to oversize (say to 4 inches) an only have one exterior penetration instead of two?

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  1. Expert Member
    PETER Engle | | #1

    Like most things discussed here, it depends. If the appliances are installed in a closet that is vented to the exterior, the code and manufacturer's standards tell you what size vents are required to provide combustion air to the room. Effectively, this makes the room "exterior" space and the room should be sealed and insulated as such. In this case, the vents are sized to provide adequate air to provide both appliances and to provide some passive cooling of the appliances. However, if the appliances are designed to be direct-vent, sealed combustion, then they each need their own intakes. Trying to share an intake may not work, as the vent fans for the appliances will fight with each other causing one to shut down or even enter an unsafe operating mode.

  2. prometheanfire | | #2

    Yeah, they are both sealed combustion high efficiency units that directly vent individually. So I think they'll need their own combustion air intakes.

    I wonder if it would be possible to convert the existing exhausts to a combined exhaust with intake if I use a larger hole. That way it's still just two holes.

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