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

Powervent water heater in a relatively tight house

thomaskansas | Posted in Mechanicals on

Hello GBA,

I’m finishing a spec house in climate zone 4A. It is a 1653 sqft slab house with a blower door test of 3.97ACH50 (15085cuft of volume, [email protected]).

My range is electric and furnace is a sealed natural gas type with intake and exhaust piped to the outside. I didn’t realize when I bought this power vent water heater that although its exhaust is sealed, it draws intake air from the house.

My question is how possible is it for a powervent water heater to backdraft combustion gasses into the house when the house is being depressurized?

My range hood’s max CFM is 350. According to RedCalc this depressurizes the house to 10.5Pa without either of the two bath exhaust fans running. If it’s relevant, Rheem appears to have no idea how many CFM their water heaters draw.

Thanks so much,

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. Expert Member
    BILL WICHERS | | #1

    There should be a "proving switch" that watches differential pressure across the blower and will lock out the unit if there is insufficient combustion airflow. The proving switch looks like a sort of metal pancake with two tubes comeing out of it and a small microswitch in the center of one of the faces of the "pancake" that is driven by the diaphram inside the "pancake".

    I don't think you'll have a problem with this unit, especially not with backdrafting, but it will depressurize your house somewhat. Sealed combustion units are better/safer, but also more expensive. you could potentially add a makeup air intake for your mechanical room to provide combustion air for the water heater from outdoors, but it won't be a sealed system -- the makeup air would just enter the room the water heater is in, not the water heater combustion chamber itself the way it works with a sealed combustion unit.


    1. thomaskansas | | #3

      Thank you Bill. Very interesting about the proving switch, I'll look for it. I sort of had a knee jerk reaction to keeping all the mechanicals inside the house based upon my reading but this seems like a situation where having the water heater in the garage makes some sense.

  2. walta100 | | #2

    I don’t think you have a problem as the blower fan on the water heater should easily overcome the pressure differential you described. I would also guess something is off in your calculation making it overstate the pressure from the range hood.

    For you own peace of mind I would create the worst-case event turn on every fan and the dryer make the water heater run at the same time. Move a burning incent stick all around the heater. If it is working correctly the smoke will be drawn into the heater and sent out the vent. If you find smoke being blown away from the heater when the heater is running, I would be surprised and concerned.


  3. thomaskansas | | #4

    Great idea Walta. Like the blower door, I suppose the only way to truly know is to test!

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |