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wondering if anyone knows why when I turn my recently installed high-efficiency air conditioner on, CO2 levels rocket up in my house? (In just 4 hours they go from around 500ppm to 1200ppm, and that's across all three floors of two-storey house plus the basement).

This effect doesn't happen when we have the furnace running (the house is usually at around 700ppm CO2 in Winter. What could be causing CO2 to climb so high, so quickly, across a whole 3-storey home with only 3 people in it, and only with the AC? Any ideas or suggestions for what to do to fix this welcome!

I have tried cracking a basement window open a couple of inches, and turning the fan on too. With the furnace/ winter cycle this gets our CO2 down to around 500-700 ppm AOK. But today with the AC/summer cycle turned on, it seems to have only a negligible effect, our CO2 is still stubbornly at around 1100. We had the same problem last summer. I have tried measuring CO2 levels all over the house but can not find a "leak" anywhere. Not sure what the cause might be. Please help as I know this is bad for my family's health. Many thanks - Adam

Rohit Gupta
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Adam Lopez
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    To be clear - does the CO2 level remain acceptable when neither the furnace nor the AC is on? As in, is it the AC being on that is causing the problem, or the furnace being off? – psmears Apr 14 '23 at 13:03
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    Make sure your CO₂ detector isn't actually just estimating CO₂ from another cheaper-to-measure variable like VOC. For example, your new air con might have a "new equipment smell" (aka VOCs), and it might be erroneously confusing your detector's estimation – 小太郎 Apr 14 '23 at 13:19
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    What @小太郎 said. Most "CO2 detectors" are fake and detect VOC. Among common consumer-product CO2 detectors, only Aranet and Vitalight are the real thing. – R.. GitHub STOP HELPING ICE Apr 14 '23 at 15:58
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    Even if you would actually have 1000 ppm CO2 in the ambient air in your house, is there any credible source that says this is harmful to breathe? – Jim Stewart Apr 14 '23 at 17:08
  • @JimStewart Sort of circularly, CO2 itself is fine until higher levels, but CO2 is used in commercial HVAC systems as a proxy for VOCs and odors. This is called demand control ventilation. ASHRAE Standard 62-2001 says to keep CO2 at outdoor+700 ppm, or about 1,000 ppm. – user71659 Apr 14 '23 at 17:40
  • What is the refrigerant being used? If it's CO2 you might have a leak! Yes, knowing about your CO2 detector would be good - obviously cheap Cheese junk is sold on eBay/Banggood/Aliexpress - less well known is that cr*pstream also shows up all over Amazon due to Amazon Marketplace (3rd party items mixed in with Amazon listings) and FBA (3rd party items ship with Prime). [This advertisement was shoved in my face while watching Youtube, and it lays out how it's done.](https://www.youtube.com/watch?v=GCTiU2ZHVwI). – Harper - Reinstate Monica Apr 14 '23 at 19:24
  • If you're wondering "CO2 refrigerant", they solved the "hole in the ozone layer" problem when they banned a few refrigerants like R-12 and R-22, but most of them are [extremely potent greenhouse gases](https://ww2.arb.ca.gov/resources/documents/high-gwp-refrigerants) - thousands of times worse than CO2. For those, the government requires certified techs with recovery equipment. To avoid that, the market is developing alternative refrigerants like CO2 (R-744) or propane (R-290). Flammable refrigerants are only allowed in small quantity, e.g. refrigerator or window A/C. – Harper - Reinstate Monica Apr 14 '23 at 19:51
  • Are there any sources of CO2 near the air handler, the return vents, the end of the condensate line, or any of the return ducts? – jay613 Apr 14 '23 at 21:52
  • Thank you all so much for your answers, comments and questions. You are wonderful helpful people! I am fighting cancer right now, so really trying to improve my air quality so I have better health. I really appreciate your support. To answer your q's posed: CO2 remains within normal inside range when either AC is off OR when furnace runs. Just something about turning AC on that causes this problem. Our house is not overly tightly sealed, (we have old cracks etc.) but we do have a lot of "dummy returns" in our 30 year old detached two-floor house that don't seem to do much . – Adam Lopez Apr 15 '23 at 05:05
  • The AC installed last summer was high efficiency Lennox brand (we also replaced furnace at the same time, also high efficiency Lennox). I had the contractor come over and look at our house last Summer after we saw rocketing CO2 levels (scary 1,500 ppm on many Summer days!) . It didn't make sense to me when the highest CO2 we get in winter (furnace) or spring (nothing running) is around 800 ppm. He said there was nothing wrong with his AC and tried to explain it away with saying how CO2 just gets "sucked in" or held in naturally due to unequal pressure. – Adam Lopez Apr 15 '23 at 05:06
  • I had no idea what he was talking about. We do have some open drains in the basement - could it be that our AC is "sucking up" / blowing out drainage CO2 into the house? My really good detector could not find any higher CO2 near the drains though. I have am going crazy just trying to solve this problem. I am just really worried about the poor air quality. I would hate to have to buy a very expensive ERV/ HRV to bring in fresh air, but if that's the only way to get fresh air in the house to clear out the CO2 so be it. – Adam Lopez Apr 15 '23 at 05:06
  • As a cheaper stop gap to buying an ERV/HRV, as a stop gap, I have bought a window venting fan (the Vornado TRANSOM Window Fan with 4 Speeds, Remote Control, Reversible Exhaust Mode, Weather Resistant). Question: should I set it to blow out inside air or suck in outside air? What will be best for bringing down CO2 levels? Any other ideas? Much appreciated! – Adam Lopez Apr 15 '23 at 05:09
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    What is the make and model of your co2 meter? Co2 is a tricky gas to measure and different instruments have different idiosyncrasies that can play tricks on you – Joel Keene Apr 15 '23 at 07:49
  • Set the Vornado to suck in outside air and see what this does for the CO2 readings. Of course this will make the a/c run more to compensate for all the unconditioned air being pumped in, but since you already have this you might as well try it. – Jim Stewart Apr 15 '23 at 14:17
  • I've actually heard a few people opine that getting a CO2 meter was one of the worst purchases they ever made because now they constantly worry and act superstitiously about fixing something challenging that was never a problem before the meter... I would get a bunch of fast-growing indoor plants if you want to lower levels, a grass you can frequently trim is ideal. – dandavis Apr 15 '23 at 19:07
  • Thanks so much all for your comments and answers, very helpful I will explore further – Adam Lopez Apr 16 '23 at 04:14

4 Answers4

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One possibility is that you have:

  • a very well sealed home
  • a heat recovery ventilator (HRV)
  • the HRV is only on when the system is configured for heating and not when it is configured for cooling

In the old days, there was generally little concern about natural CO2 from respiration accumulating to a dangerous level in a typical home, or about providing sufficient oxygen for combustion appliances. That is because most homes had significant amounts of air entering around windows and doors and through other places.

However, that outside air entering (and conditioned inside air leaving) is a major source of energy loss. Sealing a house better, though improved materials and construction methods, results in far lower heating/cooling costs. But that comes at the cost of insufficient airflow to remove CO2 generated by people, pets and combustion appliances and provide sufficient oxygen for people, pets and combustion appliances. Actually combustion appliances are usually not so bad as far as CO2 - the same venting used for heat and other byproducts of combustion also takes care of CO2 if everything is installed and working properly. But people breathe everywhere, so if the house is really well sealed ("tight") then makeup air is needed.

A heat recovery ventilator (HRV) is a way to bring in outside air (with more O and less CO2) and send out inside air (with more CO2 and less O) while recovering most of the heat that would be lost if you simply opened a window. If the HRV is set to turn on only in heating mode then CO2 levels will be under control in heating mode but could rise significantly in cooling mode.

manassehkatz-Moving 2 Codidact
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  • Thanks for this, I have looked into HRV/ ERV but it seems it would be very expensive and difficult to install on our house. – Adam Lopez Apr 15 '23 at 04:52
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Since the AC only recirculate the air inside the home, the possible source could be outside. Neighbors barbecuing ? burning dry grass or wood, sewage tanks, increased car trafic... and many other sources.

Please measure the outside and at the possible gaps/leaks in windows ect..

Ruskes
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Most common source of CO2 at home is usually breathing. In our well isolated flat just 2 persons breathing reach those CO2 levels in 4 hours (no burning, all electric appliances, no ventilation or air-conditioning started). We need to open windows or start the kitchen air extractor to reduce CO2 levels. I am planing on installing a new air-conditioning with ventilation.

So, in your case, if there were people breathing in the house when measuring, I would think more on why the furnace provides some ventilation. Maybe, your ventilation system is designed so that hot air can escape by some chimney but not the reverse.

If that is not the case (just 1 person in a large house), check if the refrigerant gas of the machine is CO2. In that case, it can be a leak, so call a technician.

By the way, for CO2 measurements trust only on devices using NDIR (near distance infra-red). If it is not of this type, there may be measurement errors introduced by estimations.

arauzo
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Following your observations it mainly happens when the cooling is on but not with heating.

The cooling comes from a new high efficiency AC, while the heating comes from existing Furnace.

The difference would be in the air flow coming from those two.

The new AC is probably set to higher fan speed in the air handler.

The air return ducts were not modified to handle the higher air flow.

As the fan blows more air in the rooms, the return air has to come from somewhere. By suctioning the outside air from all the cracks and openings like windows, doors etc..

That would explain the raise and the difference in CO2 between Furnace and AC.

One remedy to reduce CO2 intake is to reduce the Cooling Fan speed, the other would be to close all sources of leaking.

Ruskes
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