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Community and Q&A

When Are CO2 Levels Too High?

user-5946022 | Posted in General Questions on

I have a PGH and have been measuring CO2 (among other things) for almost a year.  Prior to occupancy the CO2 was consistently in the 500-550 range with very few spikes. After occupancy it has settled in the 700-750 range and spikes to over 1000 at night with the minisplits running.  When the minis were totally off for 2 days (so no air movement) it spiked to close to 1500 at night.  All measurements (pre & post occupancy) are taken in the master bedroom.  The ERV is a spot ERV in another part of the house.  It seems to have no impact on the master. Are these readings high enough to warrant concern?   If so, is the only solution to add another ERV?

Update 2 (Update 1 is comment #14 below):
I changed the air filters and went from 2″ to 1″ after reading Allison Bailes article on that issue.  Immediately after the change the peak CO2 level in the bedroom overnight dropped to the high 800s/mid 900’s.  Nothing over 1000 since the change. 

My theory is the changed air filter allows more cfm to circulate when the ducted minisplit is running.  The higher CFM allows more air mixing which lowers the CO2. 

Still working on the other issues.

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  1. marktrumpet | | #1

    I'm interested in what others have to say here! Pardon my ignorance, but what is a PGH?

    Edit: Oh, I see. "Pretty Good House"

  2. Expert Member
  3. prometheanfire | | #3

    What is the ERV rating?
    What is the square footage and bedroom count?

    The placement doesn't seem optimal for the ERV.

    1. user-5946022 | | #6

      First floor ERV rated for 30 cfm supply (Panasonic Whisper Comfort). It is on the high setting. 1900 sf two bedrooms on first floor. Concur placement is not optimal.

      So is the best solution to add an ERV? There is no optimal place for that either...

  4. brad_rh | | #4

    Have you calibrated you CO2 meter lately? Is the bedroom door closed? Gas stove & heat? Do you have a kitchen exhaust?
    I'd be curious what the reading is close to the ERV. That seems like a huge spike just due to the minisplits being off. 500-550 non occupied seems high to me also. How many people & dogs living there?
    For comparison, my house drops drops to the low 400's when it's been unoccupied for more than a week. No ERV/HRV.

    1. user-5946022 | | #5

      1. Have you calibrated you CO2 meter lately?
      - It is an Airthings Wave + CO2 logger, so I don't think I can calibrate it. Happy to be corrected.
      2. Is the bedroom door closed?
      - I've tried both ways - keeping the door open helps somewhat. Co2 rises with the door closed.
      3. Gas stove & heat?
      - Neither. Induction stove, ducted minisplit heat/cooling, with a separate ducted transfer duct from the master to the balance of the house.
      4. Do you have a kitchen exhaust?
      - Yes and use it when kitchen is used
      5. I'd be curious what the reading is close to the ERV.
      - That is a good idea; I have also been thinking about moving it outside to get a baseline there.
      I will try both over the next two days.
      6. That seems like a huge spike just due to the minisplits being off.
      - Agreed. I was also surprised.
      7. 500-550 non occupied seems high to me also.
      - 500-550 was before anyone lived in the house. It is also approx what I got when I left the windows in the master wide open for an entire day.
      8. How many people & dogs living there?
      - 2 people, no dogs or other pets, no visitors for days at a time, especially during the week.
      9. For comparison, my house drops drops to the low 400's when it's been unoccupied for more than a week. No ERV/HRV.
      - Mine has never been that low consistently. According to the app it averages 737 since Feb 2021. Peaked at 1449 the night the minisplit was off and the house was occupied. Lowest ever was 408 in May and 428 one day in October. Looks like it peaked to 908 that same night in October. General unoccupied was 460's-530's

  5. brad_rh | | #7

    Something isn't right. I'd put it outside to get a baseline. Put it near the ERV for a while.
    How tight is your home?
    Some might say 30CFM ERV isn't enough, but I don't have any ERV and my house stays below 900 unless we have company.
    I'd buy another CO2 meter before I bought another ERV
    Mine is from, I calibrate it by putting it outside and pushing a button.

    1. user-5946022 | | #10

      House is supposedly 1.1 at 50 ACH. I think it is a bit leakier than that, but probably under 2.0 at 50 ACH.

      Outside for several hours, the CO2 readings were 491 to 515
      Inside about 8' from the ERV, for several daytime hours and overnight the CO2 readings ranged from 648 to 707.
      In the bedroom during the day with practically no occupancy the CO2 readings range from 638 to 705.
      I looked at the monitors from the site you referenced. Which one do you have? If I get one, I'd like it to have the ability to connect to Zwave or Zigbee so I can control (whatever I install) based on the results.

    2. Trevor_Lambert | | #15

      Brad, your experience is the outlier, not the normal. I'm more suspect of your CO2 meter than I am of C L's numbers. Just putting it outside isn't really a valid way of confirming its accuracy. It's a single data point, and there are local variations in outdoor CO2 levels, not to mention seasonal variations even in global averages.

      C L's numbers are about what I'd expect. He has a vastly undersized ERV delivering air to the wrong place. Maybe his CO2 monitor is off a bit, but that's not the real problem.

      1. brad_rh | | #16

        Perhaps that is true, but I don't know of another way to get another data point do you? Another thing I tend to forget about is the Radon fan I have running constantly. It's pulling from under a membrane in the crawl space, but I know the membrane is not perfect, so it is a constant exhaust
        Can you point me to some studies or analysis that would justify numbers as high as C L's?
        I can't read this anymore: since it's behind the paywall and I've dropped my subscription.

  6. Jon_R | | #8

    For two people, you typically need 30-40 CFM delivered directly to the bedroom.

    +1 on your calibration being off by around 100 ppm.

  7. Deleted | | #9


  8. CMObuilds | | #11

    I do occasional logging in my house and get very similar results, with ventilation running my house seems to settle around 650-700, if I turn off ventilation bedrooms at night would spike into the 1500’s.

    .6 ach50, 2 adults, 2 kids and a dog. Spot ventilation in bathrooms, kitchen hood and HRV running 55 cfm continuous when occupied (4pm - 8am) tied to the forced air furnace.

  9. CMObuilds | | #12

    To answer your question, this is from my state, WI.

    1. user-5946022 | | #13


  10. user-5946022 | | #14

    UPDATE: - Interested in replies regarding whether the below is a reasonable approach

    Based on above data, outside should be around 400, and my sensor reads 491 to 515.
    Thus, step 1 is to obtain another sensor to get a baseline. In the meantime will estimate my sensor is reading 50-100 high.

    This exercise caused me to turn the ERV in my first level on high. The combination of L1 ERV on high, bedroom door open at night (which is not a permanent solution due to both noise issues and fire safety), and ducted Mini is running all night (which it should be >99% of the time) results in single peak bedroom CO2 readings over the last approx 2 weeks of 1030 (I'm filtering out all nights where any of the above 3 variables were different) with regular peaks in the high 900's.

    - IF (big if) sensor is off by 50-100, peaks are still under 1000, so with the above three variables I can keep the sleeping quarters safe for the time being.

    Before I implement a long term solution, I am going to have another go at sealing the 6-8 spots in the crawl space where the vapor barrier was poorly installed and may not be sealed, and the 2 spots I recently saw where it was damaged.

    After the sealing exercise, I may install a radon fan, which may impact the CO2, so I will not permanently address the CO2 until other variables are in place, as I suspect a radon fan may impact baseline CO2 (happy to be corrected if that is wrong). Radon levels averaged 2.6 in the last year, but had a 4.1 reading in the last month. I'll post separate about the radon fan.

  11. exeric | | #17

    You are right that your CO2 sensor is inaccurate. It's tempting to just assume that it's reading 75 to 100 degrees high at all points. However, you do not know that. You only know that it's reading that much too high at the lowest levels of CO2 (atmospheric) It may also be reading even more inaccurately high at higher levels of CO2. Before you extrapolate that there is a real problem in your house you need to replace that sensor with an accurate one. Then you'll know for sure and can make the right judgement.

    1. user-5946022 | | #18

      Does anyone have recommendations on specific affordable + accurate CO2 sensors?
      + if they connect to Zwave or Zigbee networks and not the internet.

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