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Question DetailsAsked on 7/2/2012

Given 2000sq ft residence, will installing atmospheric gas water heater require combustion air ducting from outside? Energy effect?

Erroneously thought gas water heater used direct vent (dual wall) piping for atmospheric vertical venting. Apparently, the term "atmospheric" implies similar to an open hearth fireplace; combustion gases exhaust via piping while the air supply air is drawn from the surrounding space.

All dual wall venting appears sidewall venting or use "power venting" vertically with power fan(s). We require vertical venting since sidewall venting runs directly into a covered patio.

Will we be required to install piping to get external air for combustion? If so, will this not defeat any energy savings...since that room is connected to the outside?

If only vertical atmospheric venting could function with direct vent type (dual wall) piping. I presume the colder (outer pipe) air intake stops the natural convection emission gases from exhausting (via inner piping); thus, short sidewall venting or power based fans for vertical runs.

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2 Answers


Duh pretty complex question, If you use conditioned air ie from inside the home where it exhasts is your call, But that installation gives you NO energy savings. Using outside non conditioned air is an energy savings tactic. Beware that exhaust gas will be acidic and tends to kill bushes. As to venting under a patio cover that is a code question for our HVAC gurus. Using an existing chimney that previously was used for exhasting your furnace (in a cold climate) the verticle solution, can cause a backdraft scenario since flue may not get hot enough to draw exhaust air out a (masonary) chimney.

Jim Casper

Old Energy Conservation Auditor (not HVAC contractor)

Answered 8 years ago by jccasper


Thanks for the response, your energy conservation information was one portion of my concerns. Three concerns exist, one consuming the oxygen inside a sealed house (the people tank), impacting other competing devices (gas cooktop and gas dryer), and third the energy costs incurred if required to admit fresh air into the room where the water heater exists. My gas fireplace is 45,000 Btu/hr and this water heater is 40,000 Btu/hr, both natural gas. That's a lot of heat and combustion air.

A manufacturer representative asked if we had experienced problems with other gas appliances, I reported no. She expects us to have no problems by adding the selected atmospheric heater and does not expect any need for an external air supply. I did not address costs associated with discharging conditioned air via combustion exhaust. Another representative verified we could not direct their other heater, (sidewall vent designs), into a covered porch and then through that roof.

Modifying the exhaust piping (jury rigging to make it a dual wall concentric pipe) violates the warranty by violating the design purpose. No design engineers were made available for discussion; thus, no design explanations were offered and I was only told what violated the warranty without any cited engineering principles.

I have found not source explaining why manufacturers do not sell a modification kit converting an atmospheric heater to a dual wall heater, when piping is run vertically without bends or with (required slanted) bends. It works for my fireplace, gas technicians could install it, why not design and sell those kits?


Answered 8 years ago by etcss

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