GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Water Heater Backdraft

user-6603947 | Posted in Mechanicals on

I have a gas-fired natural draft water heater and furnace in our 15 year-old home in Lincoln, NE (zone 5). After doing some attic air sealing a couple years ago, our leakage rate dropped to 1.5 ACH50. I did a combustion backdraft test and everything was fine.

Skip ahead to this year, and the radon levels in our basement were elevated. I installed a sub-slab mitigation system and sealed the sump pit and all visible cracks in our concrete slab. Good news is, the radon levels dropped to almost nothing. Bad news is, I did another backdraft test and found that our water heater is now starved for combustion air. It doesn’t backdraft all the time. The dryer typically has to be running, and the weather has to be just right, but under certain conditions it does backdraft.

Honestly, I would love to buy a new heat pump water heater and replace the furnace with an ASHP or a couple mini-splits so we could get rid of gas altogether, but that’s not exactly in the budget right now. Even if it was, I’m not convinced an all-electric home is the most cost effective in our heating dominated climate.

What is the most cost-effective way to introduce make-up air for our natural draft appliances? I thought about sleeving a 4″ inlet through the mechanical room wall then wiring a 24V damper off a relay to open the damper when the water heater electric ignition fires. I’m not crazy about the idea of making another penetration after all the air-sealing we’ve done, but I haven’t come up with a better solution yet. Any ideas?

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. user-6603947 | | #1

    Correction, our blower door test was 2.5 ACH50, not 1.5 ACH50.

  2. GBA Editor
    Martin Holladay | | #2

    The best solution, as you know, is to replace the astmospherically vented appliances with new sealed-combustion appliances.

    The second best solution is to make sure that your atmospherically vented appliances are all located in the same spot (a mechanical room) and to install ducted combustion air to that room. The ducted combustion air can come through a passive (always open) duct, or through a Fan-in-a-Can type of system (with control interlocks that only turn on the fan when an appliance burner is operation). You need to follow published guidelines for the size of these ducts and these systems; the published guidelines are based on the BTU ratings of the appliance burners.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |