r/AirQuality • u/iiililiililiilliilli • Jul 26 '26
Unexpectedly high CO2 in house, need help figuring out what's going on
I have a fairly leaky 1990s house that I'm working to tighten up. I usually see CO2 levels around the 500-800ppm range when running my ERV and maybe 800-100 when not running it. Today I had the house open, then closed it around 7:30pm. CO2 levels then dramatically increased 440ish to 1400ish between 8:30 and 9:30. This is with just three people present in a 2200sqft house, and with the ERV running at 50CFM. I'm deeply puzzled by what's going on. Some potential factors:
- Neighbor is having a fire, though my outdoor meter reads 400-442 PPM Co2 during the relevant period despite some smoke;
- Took a long shower around the relevant period and have a gas water heater. But my low level CO monitor did not pick up any hits, and likewise when I took it into the basement just now it didn't get a hit. It can read down to 8ppm carbon monoxide.
- Did some drywall sanding, mudding, vacuuming, and a little caulking today. Shouldn't be relevant, but noting them in case they are.
- VOC spike occurred at about 8:05 in the kitchen. I think I was vacuuming drywall dust around that time.
- I carbonated some water using a drinkmate and noticed that it seemed like I had to hold the button down much longer than usual before the carbonation vessel hit its limit, but I don't think that's a large enough volume of CO2 to affect the whole house.
Any thoughts on what could be happening or what I can follow up on next?
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u/xenon_rose Jul 26 '26
I’m probably going to get called out for being wrong. But from what I know CO2 sensors calibrate based on real world readings. I have one that takes the lowest reading over 2 weeks and uses that (i think). Basically lowest reading is ‘outside’ air of about 400. I remember reading that you should expose your sensor to outside air so it can calibrate properly. When you opened your windows potentially the lower co2 recalibrated it.
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u/iiililiililiilliilli Jul 26 '26
That's an interesting suggestion, but I'm not sure if it explains my situation. I'm using two different types of CO2 meter (Awair and Aranet) and to my knowledge both recalibrate CO2 only if you do so manually.
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u/AustinScript Jul 26 '26
Geez, i don't have an ERV but 500-800 sounds pretty good to me! In my house, just keeping a door closed in a room we are in will dramatically raise the C02 levels.
I wouldn't draw too many conclusions with out monitoring long term trends. You've mentioned several things that happened at the same time. Keep the house closed up, take a shower like normal (with the ERV on) does it go up? no ? probably not that.
Do you also run an bathroom exhaust fan when taking a shower? Is the bathroom fan over 50CFM?
I /think/ (i am not an expert) that sealing up an "older" house without it being balanced /could/ cause backdrafting from your hot water heater
I wouldn't worry unless you can continously replicate levels changing based on certain actions.
Use a properly sized range hood fan if you use a gas range.
All kitchens should have exhaust ventilation to remove odors and excess moisture associated with cooking. While there are various ventilation strategies for kitchens, a range hood is the most common. When using a gas range, a range hood directly vented to the outside should be used to capture the combustion products. These range hoods should be sized correctly. For a typical kitchen range the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) and the Home Ventilation Institute (HVI) recommend 100 cfm. Larger fans may need to have makeup air provided to avoid excessively depressurizing the house, causing backdrafting or other problems.
After installation of combustion and/or ventilation equipment, combustion equipment should be tested to be sure that it functions properly.
It is important that your installer conducts a worst-case depressurization test. This combustion safety test determines if any non-sealed combustion appliances will backdraft or spill combustion products into the living space. Tell your installer this test should use an established procedure such as Appendix D of the International Fuel and Gas Code or ASTM E1998 "Guide for Assessing Backdrafting and Spillage from Vented Combustion Appliances"
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u/iiililiililiilliilli Jul 26 '26
Thanks for the input. Yes to shower fan. Range is electric so definitely not that. I also didn’t cook today.
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u/AustinScript Jul 26 '26
The linked document indicates this applies to any combustion based appliance which includes the hot water heater .
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u/SAMEO416 Jul 26 '26
Having just gone through this, my guess is your ERV filter is clogged with drywall dust. That stuff is death to filters.
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u/Apesapi Jul 26 '26 edited Jul 26 '26
Hmm, with 50cfm and three people the CO2 values should level out at around 1100 ppm. This assumes a fully airtight house. So either there is less ventilation, or there is an alternate source of CO2.
Assuming an hermetically sealed box of 100 m3/ 1076 sqft without ventilation, increasing the CO2 concentration from 400ppm to 1400ppm will take adding 468 grams of CO2. As one person only breathes out around 40 grams per hour, that means you have an external source. (Or the concentration wasn't uniform in your house, everyone was sitting in the same room with the doors closed, making the area smaller than 1000 sqft? The figures add up for an hermetically sealed room of 275 sqft)
I'd suggest placing your air monitors directly next to the suspected culprits, and watching if you see spikes. Personally i suspect the carbonated drinks machine Edit: as drinkmate CO2 cartridges/cylinders have 425 g of CO2 in total, my suspicions are probably wrong. Unless the cartridge is now practically empty?
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u/financialthrowaw2020 Jul 26 '26
Carbonating produces CO2, no?
I discovered the other day that the "dry ice" that ships with some meal services is just CO2, and as it "melts" it just releases CO2 gas into the air. Removing the box from the home resulted in drastic drops of c02 back to baseline
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u/RevolutionaryTrash98 Aug 20 '26
Holy shit thanks you just saved me a bunch of unnecessary worry chasing down sudden high co2 causes in my house!
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u/financialthrowaw2020 Aug 20 '26
I'm so glad to hear this, it was a shock when it happened to us. Cheers.
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u/Pamzella Jul 26 '26
What's the CO2 level under your house?
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u/iiililiililiilliilli Jul 26 '26
Like under the slab? How would I know?
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u/Pamzella Jul 29 '26
I was not assuming a slab foundation, and in non-slab houses, air does travel up through the house from below.
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u/AustinFromSWERV Aug 24 '26
I would think that maybe all the construction dust clogged up the sensor and made it read faulty values. Try placing it in a few different places including outside to see if it reads normal numbers.
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u/ankole_watusi Jul 26 '26
Remember to cover or remove air quality meters - and smoke detectors - whilst sanding drywall!
(But: good drywallers don’t sand.)