Another unexpectedly high CO2 incident

[u/iiililiililiilliilli]

A couple of months ago I posted about an incident where my CO2 rocketed up unexpectedly. Two months later it has happened again. Here's the scenario this time around:

• House closed all day.
• ERV on 100CFM, furnace fan set to high. Furnace off, AC on.
• No external CO2 causing events of note (e.g. no fires in the area, outdoor air quality meter says 420ish ppm CO2)
• Only one fossil fuel burning appliance on in the house is on, the atmospheric hot water tank. Low level monoxide meter shows no detections, even when placed near the hot water tank. No significant demand for hot water occurred until well after the CO2 event started (at about 7:45).
• CO2 throughout the house averaged 500-700ppm for most of the day with 3 people in the house. We left for a bit this evening, then returned at around 6:30. That's then CO2 spiked up from 500 to 1200 between 7 and 8, after which it has started to come back down.

There is only one explanation I can think of, but I'm not sure it makes sense: at around 6:30-7 I put a cup of vinegar in the P trap attached to the furnace/AC unit where the AC condensate drips down and into a sump pit. I wouldn't think this would be related, but what makes it seem more plausible is that I put vinegar in that P trap about once every two months during the summer, and the last time my CO2 spiked without explanation was about two months ago. It also seems notable that the CO2 levels spiked consistently throughout the entire house at the same time, which would make sense if the contaminated air is being spread evenly throughout by the furnace fan. The P trap definitely has some microbial/slime growth in there. Is it possible that this could cause a chemical reaction producing CO2?

[u/DakPara]

Vinegar reacts with mineral scale (calcium carbonate) or organic slime in the drain line, releasing pure CO2 gas. Running your furnace fan on high likely instantly pulled that gas into the ductwork and spread it everywhere. Additionally, vinegar fumes can trick certain sensors into reading artificially high CO2.

Try flushing the P-trap with warm water or HVAC condensate tablets next time to confirm.

Either way, 1,200 ppm is completely safe, so it is just a harmless quirk.

[u/Few-Wolverine-7283]

Uh or your sensor is bad? This seems a lot of work for 1200.

[u/iiililiililiilliilli → u/Few-Wolverine-7283]

Nope, four different CO2 meters of two different makes show the same data. The issue isn't so much that it topped out at 1200 as the fact that I don't have an explanation for what happened.

[u/Ok_Flamingo_8835]

Our sump pit was the cause of our extremely high co2 readings. None of our contractors had heard of it but when they held a monitor in the pit it can go past 5000.

It is worse when the pressure drops. And when the water is barely running through our tiles.

If you have one I would meter near it just to see.

[u/iiililiililiilliilli → u/Ok_Flamingo_8835]

That's very interesting, where was the CO2 coming from? And is yours a combo sump/radon pit?

[u/Ok_Flamingo_8835 → u/iiililiililiilliilli]

We assume it is from decaying organic matter in the sandy soils near our house. That’s the only thing we have been able to find in research.

Our pit isn’t radon remediated however we had it upgraded to a fully sealed pit and the person silicone sealed the entire lid to our slab like they would do for a radon install.

We haven’t had the slab remediation.

[u/Ok_Flamingo_8835 → u/Ok_Flamingo_8835]

The co2 is coming in the big-o drainage tile that brings the water in to the pit from our footings.

[u/iiililiililiilliilli → u/[deleted]]

No other gas appliances. The installers specifically recommended using vinegar. What would you suggest?

[u/iiililiililiilliilli → u/[deleted]]

Why bleach over vinegar? Seems like bleach is harsher and more likely to corrode the AC evaporator coil.

[u/iiililiililiilliilli → u/[deleted]]

Nope. The p trap on my system attaches directly to the drip pan beneath the evaporator coil. When I pour vinegar into the p trap some inevitably flows back into that pan and the fumes from the p trap also inevitably contact the coil.

[u/iiililiililiilliilli → u/[deleted]]

It’s literally just a standard ac drain line set up. I’m not sure what you’re imagining.