I have this I think standard gap between water heater and chimney. Due to house being colder inside than outside in summer time, I think the pressure difference is not allowing the exhaust to go up the chimney. If I open a window it helps...really helps if I open sliding door and window but we can't do that when we are gone and I hate coming home to a house full of exhaust.

What do others do about this problem? I have thought about drilling a hole from the garage into that room and put a fan on a timer to run during the day when there is a pressure difference (note: doesn't happen at night when it's much cooler outside...we live in the mountains) to blow air into the room forcing it out the chimney. Thoughts?

copy of somebody else's heater with same gap

  • 23
    Check your carbon monoxide detectors. First check, do you have them? Second check, are they outdated and no longer working? This sort of thing can kill you and your family.
    – Ecnerwal
    Commented Sep 14, 2022 at 18:27
  • Welcome! My home is sealed very well. I installed a 4" PVC pipe from outside to the floor between my two heaters. Reason it has the least temperature impact on the room. I cut 16" off each side with the openings facing each heater. I placed a large screen on the inlet and have had no problems. If you have anything else on the chimney you may want to check if it is capable for your load. Mine was large so they installed a 4" stack up for each heater. The furnace they just left be.
    – Gil
    Commented Sep 14, 2022 at 22:21
  • I'm no plumbing expert. Is it normal to plumb the output of one water heater into the input of a second one as it appears has been done here? Unless the second heater is set hotter than the first, this seems to make the 2nd heater totally useless in this situation.
    – FreeMan
    Commented Sep 15, 2022 at 12:25
  • 1
    @FreeMan "Normal" is a relative state of being. The problem solved is likely that one tank does not supply enough hot water so a second was added to double the hot water capacity instead of buying a single monstrous tank. Maybe we're looking at two 20-gallon tanks because 10 years ago the plumber had a surplus of these and offered a great deal compared to a single 40-gallon.
    – MonkeyZeus
    Commented Sep 15, 2022 at 13:21
  • Makes sense, @MonkeyZeus, thanks. I suppose, also, that in a high-demand situation, there are actually two burners heating the water, so it should replenish more quickly as well.
    – FreeMan
    Commented Sep 15, 2022 at 13:40

2 Answers 2


What you've described is a very serious carbon monoxide hazard called chimney backdraft.

You likely need to explicitly provide a fresh air intake so that chimney stack can work properly.

That water heater room needs non-blockable fresh air source year-round ASAP. Two 4-inch holes or one 6-inch hole could potentially suffice.

Warm air from the burners rises out the chimney so fresh air from the outdoors is supposed to replace it.

That vent relies on your house's natural breathability. Your house, or at least this room in particular is unable to pull in enough fresh air from the outdoors to replace the hot air trying to escape out of the chimney.

Your other option would be to upgrade to high-efficiency units which have an active blower. However, you would still need to ensure that enough fresh air makes it to the burners.

Above all else, install a carbon monoxide detector in that room.

  • Comments are not for extended discussion; this conversation has been moved to chat.
    – Niall C.
    Commented Sep 15, 2022 at 3:18

Proper venting of a gas appliance should have nothing to do with the temperature indoors or out: the exhaust gas is hotter than either of those. It should naturally rise (draft) up the chimney (or "flue") regardless of the indoor and outdoor temperature conditions. There's something more amiss here. Any kind of "helper fan" is a dangerous cover-up that avoids finding and fixing the real problem.

So long as the chimney is clear, there's an adequate source of make-up air, and the room is not under negative pressure, the draft should function as expected.

You mentioned summer time -- outdoors being warmer than indoors. That implies to me that you have air conditioning. I suggest you have a good look at that the blower and duct work in that system, especially the air return portion. If the blower is not able to draw air through the normal return path, due to the inlet grill being covered, an air filter being dirty, a cover panel being removed, etc then the blower may pull air out of its own room instead. This could create a negative pressure zone in that room -- and air may be drawn down through the flue.

  • No AC....we are in the mountains. Weather here is different. Mornings in summer are 45 ro 52. Afternoons 78 ro 86. House stays in the 66 ro 80 range. It is always hotter outside in summertime Afternoons. House insulated very well Commented Sep 15, 2022 at 22:16
  • Temp matters not because of air going out, but replacement air coming in. When cooler inside, air doesn't come in well to replace air going out, so exhaust find another route instead of out the chimney. Commented Sep 15, 2022 at 22:18
  • I should add that it's not all the exhaust. I think my smaller pipe and dome let some escape. Commented Sep 15, 2022 at 22:21

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