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Two backdrafting appliances and dust — Do I need an HRV?

jpritzen1 | Posted in Mechanicals on

Hello, I have a problem with my house.

1) My house is probably air-tight

2) My water heater backdrafts whenever the bathroom fan is on. It’s probably backdrafted every single morning (windows fog up & I feel the warm air at the flue when it kicks on….it only stops once turning off the bathroom fan).

3) Opening specific windows in the basement relieve the backdrafting of the water heater.

4) There was a lot of dust on a return-air duct that was right above the water heater. Another supply duct near the water heater had rust.

5) My clothes dryer has humid air coming in around the vent where it goes through the wall when clothes are drying. This tells me the exhausted air is coming back into the house.

Some quick specs:
– 40 btu high efficiency furnace, dual PVC pipes for combustion
– 2 ton a/c
– 40 gal water heater, flue connected to a same-diameter metal chimney sleeve
– ducts are rectangular metal in the floor joists, 4″x8″ duct to 2.5″x12″ opening

Since unbalanced air pressures seems to be the underlying problem in this house, is this something an HRV would fix? Or what type of person can I call who would really investigate this (not just some basic visual inspection but really figure out how to stop the air pressure problems).

I really don’t want to leave the windows open in the basement…winter temps can get below 0 (fahrenheit).

Thank you!

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  1. GBA Editor
    Martin Holladay | | #1

    You may need to hire a home performance contractor or an energy rater (someone equipped with a blower door) to help you diagnose your problem.

    Although you describe your home as "probably airtight," all homes leak. What you need to know is the rate of air leakage -- and that can be determined with a blower door.

    Your guess may be correct, however -- it's certainly possible that your home is tight enough to have backdrafting problems when you turn on your exhaust fan.

    Some of the symptoms you list may not be related to backdrafting problems. A rusty supply air duct is often a sign of high indoor humidity (which can have several possible causes). Dust on a return air grille may simply be an indication that your home is dusty, or that your furnace filter needs to be changed -- although it's also possible that your house is so humid that dust is accumulating on the return air grille because the grille is damp.

    If a blower-door test (and other types of tests) confirm your guess that your water heater is backdrafting, the best solution is to swap your atmospherically vented gas water heater with a sealed-combustion water heater or an electric water heater.

    An HRV won't solve your backdrafting problems. An HRV is a balanced ventilation system -- it brings in the same volume of fresh air as it exhausts -- so (if it is commissioned properly) it does not change the indoor pressure with respect to the outdoors. Every HRV manufacturer states that these appliances are not intended to provide makeup air.

    If you want a special fan to provide air to the house when an exhaust fan is causing a depressurization problem, the type of appliance you need is called a makeup air unit. However, I don't recommend that you try to install a makeup air unit to solve a problem with a backdrafting water heater, for several reasons. Instead, you probably need to get a new water heater.

    If you have evidence that you have high indoor humidity -- a problem that is different from a backdrafting problem -- you may want to install an HRV. If you operate an HRV during the winter, the HRV can help lower your indoor humidity levels (as well as provide fresh air for your house).

  2. jpritzen1 | | #2

    Hello and thank you for your helpful reply Mr. Holladay.

    Home performance contractor - thank you. While HVAC guys have been helpful it seems my problems are even when the HVAC system is off which makes me perplexed as to who to call.

    I just want to say that the dust was on top of a return-air duct, not the grille. In other words, while still in the basement, the metal duct had a layer of dust on top of it on a section that was _only_ right above the water heater. Other exposed return air ducts had minimal amounts of dust, but the amount right above the water heater, I could not feel any bare metal of the duct the dust was so thick.

    Also, when the water heater backdrafted, the humidity (at least in one corner of the basement) shot up 20%. So if this happens every morning, maybe that caused the rust? And likewise with the clothes dryer - humidity at the vent shot up a lot. So Iguess I need to look at fixing those before looking into the HRV it sounds.

    So let's say the house is tight & I did change the water heater to electric. That would be more of a safety thing because the (existing) environment is not safe for an atmospheric vented water heater. So are you saying you would just keep the environment the way it is & just remove the "pollution producer"? And then I shouldn't worry about negative air pressures & such after that?

    I googled the blower door & I see what it does. I can see how it can tell you how much air it can pull out of the house. But isn't that a little artificial to determine air pressure problems if it's massively forcing the air out the house? I just go back to all the detective work I did when opening various windows didn't stop the backdrafting while the fan was on, but other ones did.

    And the clothes dryer - if that's pulling humid air back in through its exhaust vent, don't I still have a problem there?

    I'll look for a home performance contractor. I'm not looking for energy saving so I will avoid the energy rater. Is there any specific credentials I should be looking for?

    Thank you.

  3. davidmeiland | | #3

    IMO you specifically need someone--probably a BPI certified Building Analyst--who can perform not just a blower door test, but a "worst case depressurization" test. This test compares the negative pressure created in your flue by exhaust gases to the strongest negative pressure created in the house by natural conditions and by your heating system and the various exhaust appliances like bath fans, range hood, clothes dryer, built-in vacuum, etc.

    If the "worst case" created by these items exceeds what the flue can overcome, you have a problem, and it sounds like you do have a problem that you've already identified. However, an experienced tech is going to be able to pinpoint the best solution, whether it's somehow adding combustion air to the water heater area, changing the location of a return air opening, rebuilding the vent, getting rid of the water heater entirely and replacing with direct vent, etc. You might have an "orphaned" water heater, which is often a problem.

    A blower door is the tool that (a) helps you determine how much ventilation is needed for the house overall, and (b) helps you identify the location of air leaks. The usual approach is to use the blower door pressure gauge setup as part of the worst-case depressurization test.

    As part of the WCD test, the tech should be measuring the CO created by the water heater as it warms up and runs, using a combustion analyzer.

    All of the details of this can be found on the Building Performance Institute website, and there are also plenty of Youtube videos showing WCD testing.

  4. Dana1 | | #4

    The ventilation requirements are somewhat orthogonal to the backdrafting issue, and ventilation is not a solution to a backdrafting, as others have noted. With a chronic backdrafting problem getting rid of the pollution source is necessary, but not always sufficient.

    A blower door test doesn't reveal the true natural ventilation rates, which can vary considerably in a tighter than average house. A simple blower door test only measures the some of the size of the air leaks, but not their location or number. The location and number DO matter. A large single hole at mid height in the house leaks almost nothing, while a pair of holes of equal total size, one at grade, the other at the attic level will have a large stack-effect drive. Chasing leak locations may be possible while the house is pressurized/depressurized, but isn't always going to tell you enough. The location of random leaks rarely places the ventilation air where it's most effective or needed. At some level of air tightness mechanical ventilation becomes a necessity, but even leakier houses can benefit from active ventilation.

  5. GBA Editor
    Martin Holladay | | #5

    In my answer, I recommended "a blower-door test (and other types of tests)" to "confirm your guess that your water heater is backdrafting." Although I didn't mention it, the "other types of tests" include a worst-case depressurization test. Thanks, David, for providing that information.

  6. GBA Editor
    Martin Holladay | | #6

    J. Pritzen,
    Q. "So let's say the house is tight & I did change the water heater to electric. That would be more of a safety thing because the (existing) environment is not safe for an atmospheric vented water heater. So are you saying you would just keep the environment the way it is & just remove the pollution producer?"

    A. The need to address the backdrafting issue is a health and safety issue -- so it is definitely priority number one. Swapping your gas water heater for an electric model would address this problem.

    After you do that, you can consider any indoor air quality issues. If you feel that you still need a mechanical ventilation system, by all means install one. Here is a link to an article about mechanical ventilation: Designing a Good Ventilation System.

  7. jpritzen1 | | #7

    Thankyou for the additional information. I have been doing a lot of reading & video-watching on these topics from this web site & others.

    So it seems that ventilation is different than the air necessary for combustion, based on what I read in the article & to DDorsett's comment. It sounds like the blower door is more for ventilation. I know I probably should be concerned about ventilation if the house really is airtight, but is it wrong to deal with combustion air & ventilation as separate issues, to isolate the solutions that are truly necessary in my case? Or is it impossible to separate the two? DDorsett I think you're alluding to the same thinking that I have - but you're obviously a lot smarter than me.

    What I don't understand about the worst case depressurization test is where it helps a technician in solving the problem. I see how it will verify if there is a problem. But then what? But I see it like mold - if I can see it, why test for it? I feel warm moist air on my hand, the flue is ice cold, and I hear the rumble & hiss of the water heater for 10 minutes straight - it's definitely backdrafting. I just want to make sure I'm paying for a test that helps with finding the solution because initial inquiries into a blower door test have been expensive.

    Is there some use of a sensitive-enough manometer to isolate actual air flows during the backdrafting under worst case to see where the imbalances are? Surely there has to be a way to find air paths without a big (costly-for-a-homeowner) blower door?

    I'll eventually get rid of this water heater, but is there anything else I can do in the short-term aside from opening a window in the basement? I'm afraid of pipes freezing.

    Thank you.

  8. davidmeiland | | #8

    A properly conducted WCD will identify the condition(s) that lead to the problem. Sometimes that's as simple as a bath fan running, and nothing else matters. Other times it's complex, requiring certain doors to be open or shut along with the air handler running, etc., or something like that. Each house is different. Based on the actual problem conditions, you identify a solution. For you I'm 90% sure it just means you should get rid of your natural draft water heater and install a sealed combustion model. You could probably save yourself the hassle and expense of further investigation just by doing that. Then, look at your ventilation scenario if there are signs of trouble with IAQ. Right now you have a combustion safety problem.

  9. jpritzen1 | | #9

    Thank you David. I'd agree with you there replacing the water heater is most-likely step 1. Just wasn't looking at replacing something so soon that by all purposes never had a problem spitting out hot water.

    I guess what I'm trying to get at is that, okay, we established the water heater might be good, but it's bad _in this house_. It just makes me wonder what other issues I might have meaning I'm not done.

    Some of the articles I read said that a bathroom fan could be used as a simple means of ventilation. But if my bathroom fan depressurizes the house to cause issues with other equipment, this is where I'm confused if ventilation is what I need.

  10. GBA Editor
    Martin Holladay | | #10

    In a tight house, you don't want to have any atmospherically vented combustion appliances. Period. It's as simple as that. So yes, you chose the wrong type of water heater for your house.

    We're taking your word for it that it's a tight house, and all of the symptoms you describe point to the fact that it's probably a tight house. So, if you want to save the expense of a worst-case depressurization test or a blower-door test, you can.

    As several commenters have explained, once you have solved your backdrafting problem, you can choose the most appropriate type of ventilation system. Even a bathroom exhaust fan might work in your case -- once you get rid of the inappropriate water heater. Or you may prefer to install an HRV.

  11. jpritzen1 | | #11

    Got it. More like I chose the wrong house...bought it last year. A new water heater + HRV = expensive, but I guess there's no way around that. This was marketed as an energy efficient house, but I guess it's another thing I failed to research before buying. Oh well.

    Thank you for the tips. I think I know where to go from here.

  12. davidmeiland | | #12

    Where in the house is the water heater located?

  13. jshima | | #13

    You should also look into your heating system. If it is not sealed combustion it can also be back drafting. This is why a BPI certified inspector is important. They will perform a combustion test on ALL your combustion appliances. It is easy to miss a piece of the puzzle without the proper training and experience.

  14. jpritzen1 | | #14


    The water heater is in the basement, right next to the furnace, and they are right against the exterior wall where the chimney is.

    Edit: Jeremy, yes the furnace is sealed (2 PVC pipes).

  15. davidmeiland | | #15

    Well, a quick 'n dirty likely solution would be to make an opening to the outside through the basement wall, right at the water heater if possible. If the water heater vent is 3", make a 3" hole. Obviously, don't make it a water leak, consider the direction of the prevailing wind, etc., and if you can make it something that's easily plugged if/when you replace the water heater, so much the better.

  16. BobHr | | #16

    Are there any vents in the ceiling in the upper level. That is there duct work in the attic. If so some of the problem may be leaky supply ducts pushing air out cause air to leak in somewhere else.

    Dust on top of the trunk lines is normal. Lots of drywall dust from construction. You should see how much drywall dust is inside the ducts.

    Does the home have a humidifier? Turn it off and if the air gets very dry you home is leaking air.

    The humid air around the dryer vent could be excessive back pressure and a poorly sealed vent. If it is pulling air back into the house from the outside then you need to find the air leak and seal it up.

    If the home is tight then you will probably have excess humidity in the winter.

  17. jpritzen1 | | #17

    David, the hesitation I have with the proposed solution is that there is a window by the chimney and opening that window all the way didn't relieve the backdrafting. Minimally cracking a window on the opposite side of the house, parallel to the water heater, relieved it. I have a hypothesis that the air travelled through the floor joists to reach the water heater.

    Also, it's a brick wall so I don't want any holes unless they're gonna be permanent.

    Robert, yes there is a supply that runs high, and it runs up a wall. If it is leaking, I'd have to tear the wall out to seal it up just like all the other ductwork I can't see. The other day I went & got some spray foam to use behind all the register grilles...and there was already some stuff sealing the gaps there. The terminations are sealed good but the ducts probably aren't since it's just metal linked together at joints. And the joints that I can see in the basement have foil tape around them already.

    I will have to take a picture of this dust. It's more like fibers than specs of dust. I don't think this is normal for there to be an unusually thick layer of soft furry dust right above a short section of return duct right above a water heater and nowhere else.

    There is no humidifier in the house. I will check the dryer vent but I do remember seeing some red foam around it already. Humidity in the house is usually in the 60% range outside of winter. Last winter I believe something in the 40% range is what I saw on the wall. No idea how accurate the thermometer is.

  18. davidmeiland | | #18

    Opening a window near the water heater did not stop the backdrafting issue? That needs an explanation. Was there wind blowing such that the window was on the lee side of the building, and was somehow subject to a vacuum? Also, I mentioned "orphaned water heater" earlier, which refers to a water heating using a vent that previously had a furnace connected to it, but no longer does. Sometimes those do not draft well or at all. Can you describe the water heater vent from bottom to top, including pipe size and types, horizontal and vertical runs, etc.? And what are the chances the vent is somehow blocked?

  19. jpritzen1 | | #19

    David, I don't know about the wind question. I don't remember it being a windy day the day I tested that.

    The vent to the water heater:
    - Top of water heater is 58.5" off the ground
    - flue is 4" diameter, smooth metal, crimped, screws
    - 5.5" total rise (including the gap) then has a 45deg elbow
    - 70" run towards the chimney in 3 sections, 8" rise
    - when it hits the chimney, it's fits into a chimney sleeve (metallic, perforated)
    - can only see the first 3 or so inches of the sleeve from inside, so I'd estimate less than a foot of horizontal -> vertical shift for the chimney sleeve within the chimney
    - from outside, there's a cap that connects to the sleeve.

    The good thing about asking me to describe this vent was I noticed the drain valve has a slow leak. I've never drained it myself so it's probably been leaking ever since. Maybe all the more reason to just get rid of it.

  20. davidmeiland | | #20

    If I'm reading that right, you have a ~70" long single-wall vent connector with minimal rise that disappears into a masonry chimney. You say "sleeve" but do you know that there is metal pipe continuous all the way to the top of the chimney? Regardless, there's a good chance that it's a setup that's never going to work reliably. The vent connector is fairly long and probably lets the exhaust gas lose too much heat--by the time it turns vertical there's not enough driving force to establish draft against any significant negative pressure.

    One thing a BPI tech would do that you can do is check whether or not it ever establishes draft. Turn on the exhaust fan that's a known problem. Take a small make-up mirror down in the basement. Turn the water heater temperature up a bit so that the burner lights. Hold the mirror right next to the gap under the draft hood. If there's spillage, the mirror will fog quickly. If it later establishes draft, the mirror will clear. Knowing the time that this takes is useful.

  21. jpritzen1 | | #21

    David, I read that the flue has to rise 1/4" per foot of horizontal run. Mine has a rise of 8/5.83 or 1.37" per foot. How is that insufficient?

    No I can't say for certain the metal sleeve the flue slips into goes all the way up because I'm not going on the roof, but it just looks like a lot of metal at the base of the chimney cap that it connects to.

    It establishes draft fine when the bathroom fan is not on, even when the furnace fan is on. It never establishes draft when the bathroom fan is on and the identified windows are closed. It instantly establishes draft when the bathroom fan is on and the identified windows are cracked. Perfomed when outside temperatures are in the 30s. Basement temp around 60. Main floor temp 68 (but differs from room to room).

    My "draft gauge" was putting a thermometer by the flue, but not too close. When backdrafting, it starts to rise. When not backdrafting, it cools off.

    What I have noticed with leaving that basement window open for make-up air is that the humidity in the house on the main floor is pretty low now to the point of an itchy throat.

    This sounds conflicting - if the house is tight, won't it keep the humidity in even with some holes here & there? Or is that window just in the right spot that it's forcing air into the main floor instead of the basement? And that the bathroom fan is getting its makeup air from that window (as opposed to the water heater getting makeup air from the window)?

  22. GBA Editor
    Martin Holladay | | #22

    Q. "What I have noticed with leaving that basement window open for make-up air is that the humidity in the house on the main floor is pretty low now."

    A. That makes sense. Opening a basement window increases the stack effect. When a source of outdoor air is provided in the basement, the rate of air leakage through existing ceiling leaks on the top floor of your house is greatly increased. More air escapes through these ceiling leaks when there is a source of incoming makeup air in the basement. So the stack effect increases; your house has a higher rate of air leakage; and the entering outdoor air (which at this time of year is dry) is lowering your indoor humidity level.

  23. jpritzen1 | | #23

    Hopefully last question on this issue -

    Is a power vent water heater recommended in a tight house? First few companies I've run into don't seem to think direct-vent water heaters exist for residential & want to sell me a power vent (intake is still from inside building but forced/induced).

    One thing I probably should've mentioned - the bathroom is right by the chimney, though the chimney opening is physically higher. Could there be some recirculation effect there?

  24. GBA Editor
  25. jpritzen1 | | #25

    Unfortunately the issue is that plumbers refuse to believe that I need a direct vent heater. This is why I'm asking if a power vent heater (something they will install) is sufficient in a tight house? Or is this a black & white situation - intake air must come from outside?

  26. Expert Member
    Dana Dorsett | | #26

    You are the one controlling the purse strings- who cares what the plumbers believe or don't believe? It's your house, not theirs!

    Tell the plumbers what you want installed, and let them know if they're not willing to bid on it you can't use them this time around.

    If that causes them to walk away in a huff, you're better off without them. Some might throw out some outlandish bids, but some might actually bother to read the manual on how to install one and quote something reasonable.

    (I'm no plumber, but) my personal preference is to never use or specify combustion appliances that are not direct vent even on not-very tight houses, since it eliminates all question about where & how the combustion air is coming into the house (it isn't coming in at all), with no possibility of dragging unhealthy contaminants in with it. It also eliminates passive stack-effect infiltration drives from the unit. Even though they are not perfectly sealed (and thus not exactly direct vent), even wood stoves that don't have direct outdoor air connections to the firebox don't make the cut, and that includes those with "proximity air" connections near but not air-tight to the stove. YMMV.

  27. jpritzen1 | | #27

    It's just been a constant battle of having to fight between what you were told on a website vs. what these guys have been doing for their whole lives.

    I confirm that they're able to install direct-vent or power-direct vent water heaters, we setup an appointment, and then they just want to sell me a power vent or tankless. Waste of time. If a power vent would suffice, then this whole thing would be done with. The water heater articles on this web site don't make it entirely clear if power vent is such a bad thing.

  28. jpritzen1 | | #28

    Thankyou Dana Dorsett - I missed your line about "with no possibility of dragging unheaelthy contaminants in with it."

    That helps clear it up that not even power vent is sufficient.

    Ever since I've been in this house I've developed a bad cough and have been suspicious of all the dust in the house. Maybe it was just from the water heater. Thanks all for the advice.

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