I am 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.

  • 3
    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, 2023 at 13:03
  • 13
    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, 2023 at 13:19
  • 4
    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. Apr 14, 2023 at 15:58
  • 5
    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? Apr 14, 2023 at 17:08
  • 2
    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, 2023 at 7:49

7 Answers 7


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.

  • 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, 2023 at 4:52

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..

  • Thanks I have done this and not identified any significant outside CO2 sources.
    – Adam Lopez
    Apr 15, 2023 at 4:51

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.

  • Thanks I will look into all above!
    – Adam Lopez
    Apr 16, 2023 at 4:10
  1. Professional installers of AC units would work out the number air of replacements the unit would do. And in a modern system with HRV (which is what makes it efficient), outside air is brought in. The unit itself can not produce CO2, unless it has a leak and your detector is measuring whatever is leaking. The buildup of CO2 would be from what you breathe out.

  2. I would be surprised if your measuring device is accurate.

  3. 1000ppm is not really a problem. These figures are guides, not absolute limits

    Carbon Dioxide Levels and Your Health

    250 – 350 ppm + background (normal) outdoor air level
    350- 1,000 ppm – typical level found in occupied spaces with good air exchange.
    1,000 – 2,000 ppm – level associated with complaints of drowsiness and poor air.

    From Why Carbon Dioxide Matters

  4. If you are really feeling drowsy or have headaches, then I would worry about it.

  5. To check the CO2 meter, measure the levels outside, in the garage, in a mall and in someone else house.

I have a Mitsubishi Losnay (HRV) unit with 3 split Mitsubishi Heat Pump systems ducted around the house. There are 4 of us in the house and we have been here for 5 years. And I am not even going to measure the levels of CO2. There are enough leaks in the house from various sources, and front doors and laundry doors are being opened and closed that it would be hard for it to build up to dangerous levels. The boys are always soldering or running 3d printers and it's not a problem.

If you are really worried about it then get a professional to come and measure it.

Just for fun, I looked at some devices on Amazon. A Temtop unit at NZ$313 sounds like a decent unit. But if you look at its spec, it has an accuracy of ±5% (of reading) plus ± 50ppm. At 1000ppm reading, it's anywhere between 900 and 1100. And, if it is designed and made in China, then I would doubt that specification.

I used to design Lab instruments, as my first job in the 1980s. And it concerns me that it does not mention anything about calibration procedure or drift (with temperature, range and time). I would suggest that these are only good for comparative readings and not absolute readings.


blaming a device is not the solution, I actually have the same problem. The person asking the question specifically said during the winter the CO2 levels stayed at low 500 600 ... same for me... and with a brand new central AC installation running for 4 hours CO2 levels skyrocket to 1200 in my upper floors and basement... these are different sensors located across the house with all the same readings... and monitoring the sensors closely I see the CO2 going up for the duration of the AC running. I had the tech come in this morning as the AC unit was icing up. And he found that the coolant wasn't even in both sending and return lines. This tells me there is a leak somewhere... but im not the hvac expert as he said it's not a leak it's an adjustment... ok go with that... he left my home 20 min ago, and we've let the AC running and sure enough while it was off for the last few days and my CO2 was around 600 and 500 in the basement, it's now going back up again... so only explanation I can make out of it... is either the coolant has some level of CO2 production if it touches the air... and if it ices up then most likely the issue... i'll post back if I get mine resolved and same for you... btw, I even had it connected in with my air exchanger on the return exhaust which was sucking outside air, and still CO2 reaching high levels. And yes definitely feel drowy when it reaches the high levels... so not a false reading...

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    – FreeMan
    Jul 13, 2023 at 16:48
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    – Community Bot
    Jul 13, 2023 at 17:34

I solved mine by by putting in an over 1500 filter (like a Merv 13) in the HVAC system and I change it a month sooner than it says to do it. We tried a washable filter and had people staying in our house while we were on vacation and the washable filter was full of grease when we got back from them cooking a lot and the filter was plugged with grease and the air conditioner was running so hard I think it was producing some of the the high Voc and CO2 reading.So I asked my husband to put back in a regular paper filter and sure enough by that night the air cleared and it was easy to breath and smelled fresh and the air conditioner was not working hard and it a very o oil lid air conditioner.

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    – Community Bot
    Sep 5, 2023 at 20:51

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.

  • 1
    Thanks so much this makes a lot of sense
    – Adam Lopez
    Apr 15, 2023 at 4:50
  • 5
    It doesn't make sense, outdoor CO2 is almost always lower than indoor. Sucking in outdoor air will lower it.
    – derobert
    Apr 20, 2023 at 22:33

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