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Summer, windows, VOC, stack effect?

pshyvers | Posted in General Questions on

We track PM2.5 & VOC’s in our house with a foobot. Over winter it was extremely effective, interlocked with the ERV it managed IAQ with aplomb.

However as summer arrived the VOC’s have gotten out of control, & we are trying to figure out what is going on. The ERV frequently can no longer bring the house down to desirable levels.

It’s been hard to isolate variables. But one effect in particular has been fairly repeatable & pronounced. VOC levels will be, say, in the 400-600ppb range. We open the windows during the day to bring in fresh air, and VOC levels drop to 150ppb or so. Then we close the windows and within a matter of ten to twenty minutes, VOC levels will rocket to 1000-1200ppb. They stay there & the ERV can’t recover.

I’ve been trying to figure out what is going on, and it seems like it must be a building science thing. I’m theorizing that having the windows open gets a lot of air flowing from one floor to the next in a significant stack effect. Then, when the windows are closed, the momentum is there & it wants to keep flowing. It begins powerfully drawing air in through the cracks, picking up VOC’s from who knows where. The part that doesn’t make sense here is that it seems to keep going for many hours through the night- the ERV is steadily running but cannot make a dent.

Hoping someone has heard of an effect like this. Points of reference- ZoneĀ  5 in Colorado, 2-story, leakage of ~650cfm50, with a 110CFM ERV. Have had the ERV & foobot since about last fall. Started having issues around the time outside temperatures began reaching the 70’s. ERV’s filters need replacement & it’s possible this is contributing to our struggles, but seems unrelated to this particular phenomenon. Other thought is that the roof is new & the asphalt shingles could be releasing a lot of VOC’s as it warms in the summer sun, but that doesn’t really explain how they show up inside!

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Replies

  1. Expert Member
    Akos | | #1

    My guess is opening the windows is stirring up the the air enough that some of the nooks and crevices like carpet fibers and coverings release more VOC. You don't see this while the air is flowing, but notice it once you close the windows. VOCs will always be higher in the summer as hotter plastics tend to release more VOCs.

    Out of curiousity how are you controlling the ERV with the Foobot?

  2. pshyvers | | #2

    Hmm that's a good suggestion but it doesn't explain why the ERV is never able to budge the high readings after the windows close. If it was just stirring up a lurking puff of VOC, the ERV ought to show a clear trend of removing it.

    It's not perfect, but IFTTT triggers a wemo smart outlet based on the foobot readings. Main issue is IFTTT is limited.

  3. Jon R | | #3

    An increase in humidity (common with open windows) increases VOC emission and once materials absorb humidity, it takes awhile to release it. Stack effect reverses direction when it's warmer outside than inside.

    Also note that many VOCs are harmless (ie, measured level could have nothing to do with health).

  4. pshyvers | | #4

    Thanks Jon, definitely know that many VOC are fine or even positive. We have indoor plants- they make VOC! Sadly I can't measure only formaldehyde or benzene, so this is my best proxy to drive good ventilation.

    I do know that the stack effect can reverse which is why I wonder about the new asphalt roof, could those fumes be drawn down into the house- but I can't yet satisfactorily explain how the window behavior I described would relate. Part of what I'm inquiring about here.

    The bit about humidity I didn't know, that's interesting. Although, in the winter when things were fine a whole house humidifier was holding the place around 40%, similar to what it is in summer.

  5. Reid Baldwin | | #5

    I have noticed the same effect with my foobot. The VOCs spike shortly after closing windows. It can stay high for a day or longer. I also notice that if I unplug the foobot and plug it back in, the readings go back to normal right away. I asked foobot about this. They advised me not to unplug it and plug it back in as that makes the sensor go into a 3 hour warmup and doing it often is bad for the sensor. They didn't really address the question of why the readings 6 hours later are different when I reset it and when I don't.

    Next time it happens, I will try to take note of the humidity levels in the house.

    Have people with other types of VOC sensors noticed this same pattern? I would like to understand whether it is a VOC issue or a sensor issue.

    1. Reid Baldwin | | #7

      Tuesday evening, we had our windows open from about 6 p.m. until about 9 p.m. Before opening the windows, the VOC reading was 200 +/- 20 for the preceding day. While the windows were open the VOC readings went up to about 400. After closing the windows, the readings shot up and varied between 900 and 1200, despite using the ERV at 90%.

      The relative humidity has been 45% +/- 2% the entire time, including while the windows were open. The temperature has been 74 +/- 2 during that time. Both of these are the same as before. These readings come from the foobot itself which is the only device I have that records readings. They are consistent with the readings from the thermostats whenever I have cross checked.

      This morning, I wondered if running the ERV was actually making it worse, so I set it back to 20% while nobody is home. The readings have gone up to 1350, so I conclude that the ERV was at least helping.

      I suspect that this is an anomaly with foobot. Something about having the windows open seems to change the calibration of the VOC sensor. I don't have an independent way to measure VOCs to verify that theory.

      1. Reid Baldwin | | #9

        I have had quite a few back and forth messages with foobot support about this. At one point, they told me that they had been monitoring my house for three days and all seemed fine. However, the windows were never open during those three days. I have been very frustrated that they don't seem willing to treat it as a potential problem with their sensor or algorithms. The only "advice" that they have given me is to stop unplugging it and plugging it back in - basically telling me to stop doing the one thing that is a semi-solution.

        At this point, I would NOT recommend the foobot to anyone. I don't trust the VOC measurement. The "equivalent CO2" has never correlated well enough to actual CO2 to deserve its name. The index is useless if the constituents are not reliable. It appears to measure temp and humidity ok, but many cheaper devices do those. I have no specific reason to doubt the PM2.5 readings. But when a company deceives me about some things, it impacts my level of trust in everything else.

  6. Josh Durston | | #6

    Is your ERV setup to slightly pressurize the house?
    Perhaps if your house is negative it's pulling VOCs from unwanted areas like your attic or through the walls.
    Is the ERV inlet on a wall being baked in the sun, over fresh asphalt, etc?

    Heat definitely changes how things off gas.
    I was once working in a school where the hydronic balancing contractor had overridden the infloor heating valves open and forgot to close them. The floors were about 40degC or warmer on Monday morning. The whole building smelled like some sort of flooring glue because of the hot floors and the kindergarten kids couldn't sit on the floor.
    Once the floors cooled off the smell went away. I wonder what the VOC levels were?

  7. pshyvers | | #8

    Reid, I am relieved to hear someone else has seen the same thing.

    Josh, I had similar ideas in the past. Our intake is over mulch. Put the foobot over the fresh air vent in the stream; excellent readings. I had a depressurization problem nine months ago and it was really apparent in the undulation of the readings- they would rise when vent was on, fall when off. I have balancing dampers now and fiddle continually (to no effect)

    My wife & I have noticed the ERV seems to be flowing significantly less air than when it was new so we've prioritized replacing the filters- if, e.g. it's only at 30% of original cfm, that could possibly explain why it can't clear the house anymore. I don't have high confidence it will solve things, but it was due for filter change anyway.

  8. pshyvers | | #10

    An update for folks, we've discovered:

    - the VOC's appear to be coming from the two south-facing carpeted bedrooms (rest of house is wood)
    - removed nearly everything from the bedrooms, still plenty of VOC's, so we suspect the 2yo carpet
    - we realized that closing the windows, for us, tends to mean closing the blinds & curtains too. I remembered that ozone pollution is caused by sunlight destroying VOC's. We started leaving the curtains & blinds open, and now the VOC's plummet to good levels during the day.

    Unfortunately they still rocket back up at night when the sun is gone. I'm waiting to see how things change as it cools down; historical data suggests this problem started up when ambient temperatures climbed above 65F, so I'll be looking to see if it disappears when fall brings temperatures back down.

    If it is indeed the carpet all we can do is encourage offgassing, so we have closed off the room, left the window open, and allowed the sunlight in. We also ran a carpet cleaner through with super hot water. Unfortunately there isn't much sign of improvement yet, the "highs" are still as high as ever.

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