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Green Building Curmudgeon

Why Are People Still Using Open-Combustion Water Heaters?

With backdrafting concerns and better options for tight houses, the author makes an argument for phasing out atmospherically-vented appliances

There can be a lot of negative pressure at the return plenum of the furnace or air handler which can easily cause backdrafting of an atmospherically-vented water heater installed nearby.

John Straube, one of my favorite building science comedians, describes atmospherically-vented water heaters as “faith-based ventilation,” a description I use regularly in my presentations, always with appropriate credit given. While an amusing description, it is also quite accurate, particularly when these appliances are installed in homes with well-sealed building envelopes. Since atmospherically-vented water heaters rely on convection from hot air rising in the flue to remove all the combustion gases from the building, even a small amount of negative pressure can cause them to backdraft, drawing carbon monoxide inside the home.

Backdrafting is for real

Check out the video below showing theatrical fog used to simulate what a bath fan or kitchen range hood will do to atmospherically-vented combustion gases. Watch what happens when an exhaust fan is turned on about 12 seconds into the video, reversing the flow of the combustion gases, drawing them back into the house.

Code requires combustion air to be provided to these type of appliances, and while indoor air is allowed to be used, isolating the water heater from the indoor air and using supplied outdoor air is highly recommended.

I see these appliances installed next to furnaces and air handlers, where the negative pressure from the often poorly-sealed return plenums can aid in backdrafting. Leaky return plenums and ducts can create significant negative pressure in a mechanical closet, enough to draw combustion gases back into the house through the flue of an atmospherically-vented water heater.

Here are three ways to mitigate the risks of backdrafting atmospherically-vented water heaters:

  1. Best practice: Don’t install atmospherically-vented water heaters inside conditioned spaces where backdrafting would be a concern.
  2. Next best practice: If you must install atmospherically-vented water heaters inside conditioned spaces, place them in a sealed and insulated combustion closet that is vented to the outside.
  3. Alternate options—Install a power-vented or direct-vent gas water heater or electric water heater instead of an atmospherically-vented water heaters inside conditioned spaces.

If not safety, economics

While we are seeing more homes moving to direct-vent and electric water heaters, we still do see open combustion units in many commercial projects. In one recent project we certified under the EarthCraft Sustainable Preservation program, the client installed an atmospherically-vented water heater that had to be replaced as it was not in a sealed and vented combustion closet.

Typical gas water heaters vent carbon monoxide and moisture by convection up through the metal flue. A slight negative pressure can backdraft those gases back into the house.

The primary reason we find that direct and power vented water heaters are not installed is upfront cost. Most spec builders don’t spend their money on high-performance or healthy alternatives to standard construction materials and methods. They just don’t see the same return they get from high ceilings and nice finishes. In most custom homes and renovations neither the designer nor the contractor is focused on high performance equipment, and unless the owner requests it, the water heaters decision falls to the plumber who selects whatever they are comfortable installing.

Choosing between gas and electric appliances is often controlled by the cost and availability of each fuel. In high-cost electric regions, natural gas may be the logical choice, but we are seeing a big push to more electric equipment because of the increased efficiency of heat pump space and water heating.

When homes use heat pumps for HVAC, the economy of gas water heating declines rapidly. If you have heat pumps and you pay a base charge for gas service as we do in Georgia, you will pay dearly for the privilege of using about $2 worth of gas most months.

In my last house, after I installed minisplits for heating and cooling, I found that I was using about one dollar’s worth of gas for my water heater, dryer, and range, but paid a $30 monthly base fee. This led me to getting rid of gas altogether. We try to talk people into induction ranges and skipping fireplaces to eliminate their gas meter, but that can be a hard sell.

As with most building performance questions, there is no simple answer to what type of water heater to install. The one choice I would make is to avoid an atmospherically vented model—the downside risk is too great.

-Carl Seville is a green builder, educator, and consultant on sustainability to the residential construction industry. After a 25-year career in the remodeling industry, he and a partner founded a company, SK Collaborative. Photos courtesy of the author.


  1. Jon_R | | #1

    Another mitigation: avoid creating significant negative pressure in the building. This is good practice for other reasons too.

    Also install a CO alarm.

  2. CraigRo | | #2

    I had two in my home when I bought it, along with a natural vent boiler, to the tune of 250,000 BTUs, and a giant 8" hole in my mechanical room wall for combustion air.

    Replaced the system with a smaller 105,000 mid-efficiency sealed combustion boiler and indirect tank, sealed the hole in the wall. Gas bills dropped by 60%, and I can still heat the whole house at -15F while taking two hot showers.

    Perfect example of grossly oversizing to right sizing.

  3. KauaiBound | | #3

    Is there any data that suggests that backventing is an 'actual' problem? Or is this an isolated condition where multiple failure modes have to occur?

  4. Expert Member
    Dana Dorsett | | #4

    An issue with the video:

    The water heater wasn't full of hot water (unless the plumbing was cut immediately after heating it up), which means no convective updraft from the center-flue heat exchanger, and a dead-cold flue (unless there were some other heat/exhaust source.)

    While I have absolutely no doubt that backdrafting of atmospheric drafted water heaters is real, the test in the video isn't anything like real-world conditions.

    >"Is there any data that suggests that backventing is an 'actual' problem? Or is this an isolated condition where multiple failure modes have to occur?"

    It absolutely IS an actual problem, especially in tight houses in cold climates with a perfectly code-legal chimney on an exterior wall. Backdrafting can occur even without exhaust fans or air handler depressurization of the mechanical room. What sort of "" would you need?

  5. walta100 | | #5

    The big appeal is the water heater is dead simple easy to fix with low cost generic parts and it will work when the power is out.

    Many of the power vent units are exhaust only and depressurizing the house drawing in 6 times as much cold air in and blowing 6 times warm air out.

    When I Google “water heater backdraft deaths” all I get is energy nerd stuff no obituaries.

    On average more deaths by lightning than by furnace or water heater CO. Also 93% of the time it is the furnace not the water heater.

    You may find this article interesting.


    1. Expert Member
      Dana Dorsett | | #8

      >"When I Google “water heater backdraft deaths” all I get is energy nerd stuff no obituaries."

      Why the focus on mortality numbers?

      Backdrafting atmospheric drafted appliances have health and indoor air quality consequences, even if it's only rarely (if ever) fatal. Sure, people run gas ranges without benefit of exhaust ventilation too, but in a tight house that's just stupid.

      Legionnaire's disease isn't usually fatal either, but there are no good reasons to invite it into your life.

    2. Expert Member
      BILL WICHERS | | #22

      I’ve seen backdrafting from my own water heater. I don’t have a super tight house (although I’ve been gradually improving things), but one day when I was bringing some lumber into by basement through the bilco door on the side, I noticed combustion smell in the basement. It was pretty bad too! My CO alarms (I have two, good to have redundancy on something as important as this) didn’t go off, so apparently no CO, but a VERY unpleasant atmosphere in the basement. As far as I can tell, something about how the winds were that day was causing a low pressure condition in the basement while the bilco doors were open.

      I’ll be replacing my natural draft water heater with a sealed combustion unit in the future, and I wish they weren’t so much more expensive! It’s mostly just a blower and a baffle after all.

      Don’t ever thing backdrafting isn’t a problem. It happened to me with only a certain combination of doors and wind as a cause. In a really tight house, there are many other possible causes.


  6. Expert Member
    CARL SEVILLE | | #6

    Thanks for the presentation, Walter. I respect Brett Singer and always learn from him. I agree that death from a backdrafting water heater is rare, but less severe health problems from low level, long-term CO exposure does happen. The video may not reflect real world conditions, but it is an excellent way to explain the potential problems with atmospherically vented appliances to both professionals and lay people who do not understand how they work, and the potential downside of installing them, particularly in tight buildings.

  7. Yupster | | #7

    This is like the argument regarding legionnaire's disease. Do I want to take the risk of coming down with a potentially fatal disease, even if it is low, to save some money on lower water temperatures? Do I want to take the (increased in a tight home) risk of death or long term health problems by CO poisoning (my brother almost died of CO poisoning, terrifying thing to see happen), or should I eliminate that risk entirely?

    You won't find me spending much time pondering that question.

  8. tommay | | #9

    That video is bogus. How can you compare hot rising flue gases to stagnant theatrical fog? Stop the BS and getting on this bandwagon to change everybody over to electric. Pretty soon, generations won't even know what combustion is, but this seems to be the new agenda here. For those who fight for tight houses remember all energy and all its forms come from the earth and its surroundings and is virtually free so being so energy conscious is only in one's mind, learned, taught, believed...Maybe we should put our energy into restricting those who wish to charge us for such things.

    And who piped that relief valve and gas piped that furnace???

    1. ohioandy | | #10

      Tom, your initial point was already made in comment #4 with no vitriol, and considerably more rigor. As for the rest of your comment, I am at a loss to understand why you are present here; you seem to reject the basic tenets of this site. Which by itself is not a bad thing--we welcome debate, but civility goes a long way in getting your views heard.

      1. tommay | | #11

        Well Andy, this site, years ago, was once what its title expressed, green building. Now it just promotes new tech, mostly focusing on electric do dads along with toxic insulation and water piping and everything else they promote, rather than natural, free, green energy and products.. If you are at a loss of understanding.....realize that is the problem.

        1. maine_tyler | | #16

          The tenets of GBA appear to include fighting climate change. Is that something you believe is a threat to our human-environmental relationship? Many of those who do, view electrification and energy efficiency as viable paths away from carbon intensive energies.

          I'm not sure what 'virtually free' energy coming from the earth you're advocating for. We've built our modern society on the energy of the sun stored in dense packets beneath the ground. These packets are not infinite, nor free, and they come with tremendous externalities.

          If/when you berate folks for touting technologies that are intended to shift us away from reliance on this stored solar energy (fossil fuels), bear in mind that attacking people for trying to overcome this challenge isn't an effective way to convert them to your form of environmentalism (whatever that may be).

          1. tommay | | #17

            Well Tyler, I geared part of my career towards solar energy and still work in the solar hot water and off grid field. Unfortunately, the SHW field is ignored and down played even though the world's waters are primarily heated by the sun and SHW is basically free for life, something they don't want. The PV market has been hijacked by corporate thugs...remember solyndra?
            Climate change, really, you have obviously allowed yourself to be deceived and blinded by the agenda. So where does all your clean electrical energy come from? Fossil fuels, do you believe that is where oil and gas comes from, fossils? Stored solar energy, so are you implying the sun creates all this virtually free energy? Who owns the sun and where's the meter? Those free packets are not infinite or renewable, are you sure? They're not free either? So who pays the earth and how much does it charge? You think you are shifting away from reliance, insane, you are becoming more reliant on those who are more than willing to sell you something they get for free and will shut you off if you don't pay up. Do you know how easy it is to flip a switch ? How long before any type of fuel other than what they force you to use is illegal, eg no more charcoal or gas grills, camp fires, kerosene lamps, gasoline (better sell that old mustang)......
            And I am not berating or attacking anybody are... try to understand your argument before you present it.

    2. capecodhaus | | #14

      That also bothered me. Sloppy.

  9. exeric | | #12

    "If you are at a loss of understanding.....realize that is the problem."

    Look up the definition of the Dunning Kruger effect, Tom. I think it may apply to you.

    1. tommay | | #13

      Nothing like personal attacks on a person you do not know. I attack the technology and the agendas not people, unless they are associated with the agendas. Sorry if your loss of understanding includes not knowing a persons credentials, knowledge or experience through the internet.

    2. capecodhaus | | #15

      Hey Schmuck, Eric, did you learn that from youtube? Go zip tape your mouth shut.

      1. exeric | | #18

        I've known about the theorized Dunning Kruger effect for over a decade. But the formal theory only corroborated what I already knew from my observation of some people. I agree with how so many people responded to Tom's comments. The comments didn't and don't make sense. The article explained well why open combustion water heaters don't make sense. Nothing Tom said refuted that but instead indicated to me that he did not follow the article's reasoning. Yes, the Dunning Kruger effect implies a judgement on the person who it is thought to be relevant to. I did not say it lightly. But like others have said better than myself the inner opinion that I would normally not say was given permission to me to say by Tom's (and yours) own lack of restraint and lack of respect for others. This is something that is also implicit in individuals who are unaware of their incompetence and also shows up in the DK effect.

        James, if you don't expect people to say what they really think about you then don't show the lack of respect to others that you and Tom are showing. I stand by my opinion. If you hadn't been so disrespectful and lacking in restraint I would never have said this out loud. It is what I've always thought.

        What it comes down to is this: at some point a person's destructive behavior to others must be stood up to. You're really childish if you expected that others wouldn't defend this good article. What were you two thinking?

        1. tommay | | #19

          "...Tom's comments. The comments didn't and don't make sense"..Once again, this goes back to your loss of understanding.....understand?

          "Nothing Tom said refuted that but instead indicated to me that he did not follow the article's reasoning" ...Why is that?

          Sounds like that effect is happening to you...better self shelter...and comply.

  10. tommay | | #20

    Faith based that that the new normal? Guess science vented out the window...

  11. jberger | | #21

    Just a homeowner's experience to share on the topic.
    I've tried to get a sealed unit installed in our house to replace a 20+ year old gas model.
    I really wanted one that pulled in dedicated outside air and had sealed combustion for the resulting exhaust.

    I have not found a plumber who will really even consider my request.

    They are willing to talk about a power vent, but when we get to dedicated intake, they decline the job. They want to install tankless units, rather than power vent 0r other.

    We are talking about a very simple job in this case, walk in basement, high ceilings, on slab and replacing an existing gas unit.

    The mechanical room already has dedicated intake and exhaust for the HVAC unit that is less than 7 feet away from the water heater and it's only a 20 ft straight run to the outside via a drop ceiling. Maybe 3 hours of work even with the new fresh air intake.

    The average plumber in the service business doesn't appear to want this type of business. I assume it's just a lot more profitable to just bang in straight replacements or upsell to tankless so that is what they want to do.

    1. tommay | | #23

      "We are talking about a very simple job in this case" why not just do it yourself, that is if you can find such a unit...

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