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Nitrogen Dioxide in home

jesse_m | Posted in General Questions on

I live in a 2 story home with a basement that was built in 2020.  We are the second owners and due to air quality issues in our home, we purchased an indoor air quality device called uhoo (https://getuhoo.com/home).  The device has been great, however, we continue to see higher than expected Nitrogen Dioxide levels in our home, especially when the gas furnace is running, but also other times that are unexpected.  

We have a unvented Gas fireplace that we never use, a vented tankless water heater in the basement room a few feet from the air handler/HVAC and a gas stove that is only contributing to high levels of N02 when we are cooking. The original owner (also the builder) told me when we purchased the home from him that the house is very tight.  Could our HVAC system be pulling N02 from the basement waterheater when it is running and if so, what are some suggestions on resolving that issue?  I am not convinced that our HVAC is sealed properly and am concerned that the water heater is not venting properly although we do not smell any exhaust fumes.

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Replies

  1. Expert Member
    NICK KEENAN | | #1

    First, I applaud you for taking actual measurements, there's no way to diagnose a problem like this without measurements.

    Whenever you burn fuel inside your house you have to vent the combustion products. In a very tight house you have to provide makeup air to replace the air that is vented. If you don't you create negative pressure in the house which can cause all sorts of problems. One of the problems that can result is that if there is negative pressure the venting of other devices fails and the combustion products go into the house.

    Does your HVAC have a gas furnace? What kind of vent does it have?

    What kind of vent does the water heater have? Does it have a blower fan? Does it have an air intake?

    Your gas stove should have a vent hood, and somewhere in the kitchen there should be an makeup air intake to provide air to replace the air that is exhausted. You can assess the effectiveness that a makeup air intake would have by opening a window a few inches while running the vent hood and seeing if your measurements change.

    Ultimately you want to see if one appliance is responsible for the N02, and then see if you can mitigate that appliance.

    1. jesse_m | | #2

      Thank you for your insight!

      Our HVAC does have a gas furnace and it is vented outside using a concentric vent system that also allows outside air to come in and burn in the furnace.

      The water heater also has a concentric vent going outside and I assume it has a blower fan to exhaust. Its a Rinnai system.

      I do not believe our range hood has makeup air connected to it, and we are considering an aprilare dehumidifier system with a fresh air intake to help produce some makeup air into our home.

      1. Expert Member
        BILL WICHERS | | #3

        Check that all the joints on your furnace and water heater vents are actually sealed, which would mean "glued" with solvent. I have seen joints get missed before, and then you can have leaks. Leaking vent lines could be part of your problem, and that's a relatively easy thing to check. I would also check for any cracks or loose clamps around the area where the vent piping connects to the furnace/water heater too, since those tend to be areas under some stress, which means more chance of cracks and leaks.

        Also check that the vents aren't obstructed since critters sometimes like to build nests in those, especially during the off season in the case of the furnace. Since you have both "in" and "out" vents here, you have what are known as "sealed combustion" appliances -- and that's a good thing! With sealed combustion, everything involved in the combustion part of the system is supposed to be done completely with outside air, so there shouldn't be anyplace for indoor air to get mixed up here. The "shouldn't" part is important, since things can still break. In most cases, significant obstruction of the lines will cause the unit to trip and not run, since there is a sensor for differential air pressure in the system, but it's still worth checking to be sure.

        Assuming that's all good, I think you need more makeup air. You need makeup air for the exhaust hood as DC mentioned, otherwise it will draw negative pressure inside your home which will tend to make all the leaks leak "inward" towards your indoor air space. That means any contaminents will be drawn into your home. Makeup air can be as simple as a damper that opens when the exhaust fan runs, just make sure you can bring in sufficient amounts of air to at least match whatever the exhaust fan can draw out.

        I would look into an HRV over a plain dehumidifier, since an HRV can be used for year round fresh air exchange.

        Bill

  2. walta100 | | #4

    It might be interesting to find out what your monitor reads in different environments. Something like inside a large cardboard box in your back yard and a different house or two.

    Walta

    1. Expert Member
      NICK KEENAN | | #5

      Good point. Basically zero the scale.

      1. Expert Member
        BILL WICHERS | | #7

        And get a rough idea as to reliability of the meter in general. Having worked with professional level "sniffers" at work on ocassion, the price point of some of the home market meters makes me question how well they actually perform. A meter that is always reading elevated levels of something doesn't necassarily mean there are acutally elevated levels of that thing everywhere -- it might just mean the meter isn't reliable.

        I agree it's a good idea to sanity check that meter before investing lots of efforts to fix a "problem" that might just be due to problems with your measuring equipment.

        Bill

  3. Expert Member
    PETER G ENGLE PE | | #6

    If you suspect that exhaust from a combustion appliance is being drawn into the house, you should see an increase in carbon monoxide (CO) at the same time as the NO2. Probably a humidity spike as well.

    You should also look at orders of magnitude and what the general "safe" levels are for NO2. For example,. NIOSH sets a limit of 1 ppm (parts per million) that shouldn't be exceeded, even though the "action" levels are around 3 ppm. If your meter is showing an increase from 4 ppb (parts per billion) to 8 ppb, then, yes, the level has doubled. But at either level, it's still close to 1/1000th of the generally accepted "safe" limit. Also, any of these over-the-counter meters that claim ppb accuracy are kidding themselves. To get anything close to accurate at that concentration level, you need lab-quality, calibrated equipment. At best, consider your meter a general qualitative guide. If the level of a given chemical doubles, there is a pretty good chance that it actually doubled. Even that might not be true. At very low levels, there can be "crosstalk" from one channel to another and other weird effects. For example, changes in humidity could cause a false reading of a spike in CO or NO2, or something else completely unrelated. Suffice it to say that at very low levels, the meter might be useless or worse.

    And finally, you should look at your outdoor air quality, ideally in parallel with the indoor air. A truck idling in the driveway next door can cause a spike in your indoor air contamination and you wouldn't be able to identify the source without both indoor and outdoor readings.

  4. jesse_m | | #8

    Thank you all for the very good ideas. I am going to start by determining the reliability of the meter, checking the outside levels to get a baseline and then taking a look at the way the water heater is sealed. I suspect that the HVAC is pulling in exhaust fumes somehow. I may also measure the IAQ in the basement room, because it will be close to my suspected source.

    We do not ever have high levels of carbon monoxide and the humidity seems decent other than in the basement.

  5. HumidHome | | #9

    I have a newly installed Rinnai tankless water heater and we are noticing respiratory symptoms. We bought the same monitor and notice a decrease in NO2 every morning when the hvac turns off and increase when it turns back on. Did you ever get to the bottom of this? I wonder if the common factor is the Rinnai, my air handler is on a separate floor.

    1. Expert Member
      BILL WICHERS | | #10

      First, check your meter. Many consumer-grade test meters are unreliable, so you don't want to start doing anything until you rule out your test gear as a possible problem in terms of erroneous measurements.

      If your meter checks out, you could have a venting issue. Note that those tankless water heaters can only be a problem WHILE RUNNING, so just "being there" isn't causing any issues. If the tankless water heater hasn't been running, but you still see a noticeable change in NO2 levels that is not attributable to a problem with your meter, then something else is going on -- possibly something outdoors that might not even be yours. If the tankless water heater is found to be running when you see the NO2 issue, then I'd suspect venting, then makeup air, in that order. There are a number of rules that need to be followed with atmospheric vented equipment. If you have a "sealed combustion" unit (seperate intake and outlet PVC vent lines to the outdoors), then you should be safe UNLESS there is a leak in one or both of those vents. Atmospheric vented equipment will only have one metal vent pipe, and those are the ones that you have to be more careful with, especially if you have a very tight home.

      Bill

      1. HumidHome | | #11

        I hate to hijack the thread but I cannot figure out how to PM on this site. We had a rogue hvac guy unknowingly chop my make up air duct and turn off the system make up air system. We are waiting to install an ERV and do it the right way. Assuming the NO2 is accurate why would NO2 only increase when the hvac is running?

        1. jesse_m | | #12

          Hi- Sorry that I am late in responding. As an update, here is what we have discovered and have done to help address the issue.

          Since we previously had mold in our home, we attacked that problem first and foremost and subsequent to remediation, installed a whole home dehumidifier from Aprilaire. Our contractor recommended that we pull in makeup air from outside as well. This helped us kill three birds with one stone as it brought in fresh air from outside, helped contribute to positive pressure and helped maintain humidity in our home without needing to install an ERV.

          One thing that really helped us be successful in our mold remediation project was purchasing a vollara unit that releases a small amount of ozone (we control the level) into our air. Ozone can be considered controversial and I realize I am writing this on a green building site, but my doctor recommended putting a unit next to one of our main returns so that it would flow through our HVAC and help maintain fresh air in the home. It worked so well that we bought multiple units and I signed up to become a distributor to save a little on the cost of future units. We also bought one for our Son's room at school (he is mold sensitive) and we put one in our basement that does not run ozone. Here is a link if you want to check it out.
          (https://www.vollara.com/productdetail/SurfaceProPlus/?u=JesseMonza).

          This being said, we realized over time that higher ozone levels correlate with increased NO2 (see attached picture) on our UHOO device. When I posted my original comment last year, I did not know that ozone and NO2 had any correlation. I haven't done a ton of research on this, but we just try to maintain the ozone at a low setting knowing that it increases the N02 reading that we are getting. When we have pumped the ozone to a high level while out of town, we noticed that VOC's go to zero, ozone goes up, and NO2 also tends to climb. We also noticed that when we run the heat in the winter, the N02 is much worse, which makes me think the issue is related to our furnace more than our water heater. We took steps to address this potential concern and had our HVAC contractor do a heavy duty seal around our air handler in the basement, sealing any gaps with tape and glue to help prevent exhaust fumes from being sucked back into our system through gaps where ducts connect to the main housing unit and where panels were not sealed perfectly. I think this helped some, but we continued to have issues with NO2 in the winter so we just kept one of our windows cracked, even when it was very cold outside, to help pull in air from outside so that the system was not pulling exhaust back in when the furnace was running. The last thing I plan to do to address the NO2 is to use wool insulation in the room where our furnace is located. I have not done this yet, but I have read that wool insulation absorbs gasses and VOCs, helps maintain moisture and lasts for a long time. Its expensive though which is why I have not yet tried this.

          In summary, I have learned that fresh, filtered and humidity controlled air coming in from the outside has been a huge help with our problem. We don't see a lot of No2 issues during the summer months, which leads me to believe that our main culprit is our furnace, not our water heater. I have been told that our unit is oversized, and although legal per the code, I think it doesn't have enough air to suck through it which is causing it to somehow pull exhaust into the home. Either that, or the internal components of the box are not properly sealed which allows cross airflow to occur.

          I hope this helps!

          1. HumidHome | | #13

            Wouldn’t an ERV contribute to a positive pressure environment and help exhaust the NO2?

  6. jesse_m | | #14

    Yes, I would assume that would help. Another option that you may want to consider is moving your air device into the space where the water heater is. I would be curious to know if the NO2 spikes are significantly higher there than somewhere else in your home.

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