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High levels of carbon dioxide in house

My house has high levels of CO2 / carbon dioxide, every room is between 1100 ppm to 1200 ppm according to an air quality test I had. There are only 2 grown occupants and it's a 1000 sq. ft. brick house. No pets or plants, gas stove / furnace / water heater.

The basement was the only area that had between 900-1,000 ppm of CO2. I've read ASHRAE likes to see under 1000 ppm of CO2. We do have headaches & drowsiness but aren't sure if it's strictly from CO2. For reference, our CO (carbon monoxide) numbers were all under 2 ppm.

According to an energy audit, the house is 0.32 NACH, so I believe that is pretty airtight. There is no mechanical ventilation aside from a bathroom exhaust fan.

I've read that more occupants = more CO2, and poor ventilation can also lead to more CO2 levels (as compared to the outdoors). How would a building pro decide if the levels are high due to low ventilation, or if perhaps there is some source of CO2 within the house aside from the occupants?

I am questioning if it is purely occupancy & low ventilation because rooms that we aren't in as frequently had higher levels of CO2 than our bedroom. Do people commonly monitor CO2 levels within the home?

Asked by Jeff Watson
Posted Sun, 03/09/2014 - 15:21
Edited Mon, 03/10/2014 - 05:04


19 Answers

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Who did the test, and with what?

Answered by David Meiland
Posted Sun, 03/09/2014 - 16:02

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We used to live with very much higher levels of CO2 in submarines, for weeks on end, and came to no harm. I seem to recall that CO2 is harmless at the levels you have measured.

Put a few house plants on your sunny window-sills, and see if that reduces the levels.


Answered by Anthony Ratliffe
Posted Sun, 03/09/2014 - 16:04

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Your air leakage rate of 0.32 (nat) ACH is roughly equivalent to 5.4 ach50. Many homes are tighter.

Nevertheless, your home should have a mechanical ventilation system -- especially because you are concerned about indoor air quality. For more information, see Designing a Good Ventilation System.

ASHRAE sometimes recommends monitoring or measuring indoor CO2 levels, but the reason for the monitoring is not because the CO2 is particularly dangerous; it's because CO2 levels are a useful indicator of whether or not the building has a functioning ventilation system.

Here is information from an ASHRAE technical document: "CO2 at the concentrations commonly found in buildings is not a direct health risk, but CO2 concentrations can be used as an indicator of occupant odors (odorous bioeffluents) and occupant acceptance of these odors."

Here is some information from the Minnesota Department of Health: "Carbon dioxide is not generally found at hazardous levels in indoor environments. The MNDOLI has set workplace safety standards of 10,000 ppm for an 8-hour period and 30,000 ppm for a 15 minute period. This means the average concentration over an 8-hour period should not exceed 10,000 ppm and the average concentration over a 15 minute period should not exceed 30,000 ppm. It is unusual to find such continuously high levels indoors and extremely rare in non-industrial workplaces."

Answered by Martin Holladay, GBA Advisor
Posted Mon, 03/10/2014 - 05:23

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David, Anthony, and Martin - thank you for your advice. Overall, I would just like to say I am not trying to approach this from a health point of view or that I think the levels are approaching hazardous ranges. The 1200ppm readings I've read can cause drowsiness. However, my primary concern is if these CO2 measurements are a direct result of occupancy & low ventilation and how would one be able to verify that. Are there some building science or simple ecological factors going on that may be also contributing to CO2?

David - these measurements were performed by a Professional Engineer (PE) who was using a "GrayWolf Advanced Sense IAQ" device.

Anthony - I understand. I'm not at the point where I believe there is a serious health hazard as I know I am far away from dangerous levels.

Martin - The one thing that I wish I had was a control sample of the CO2 outside of the house. I don't exactly live on a quiet dead-end street. I'm on a 4 lane busy city street. If the area just has more CO2 in the air, and if I were to invest in something like an HRV, wouldn't I potentially be increasing levels of CO2 & other contaminants?

I guess what I am trying to say is I am interested in what is happening within / around the building from a technical perspective, as opposed to the usual health/money-concerned homeowner. If there's a source of CO2 aside from occupancy that can be reduced, I'd just like to find it. I'm just a curious person & like to learn how things work so that I can be able to describe it.

Answered by Jeff Watson
Posted Mon, 03/10/2014 - 08:08

Helpful? 0

It would certainly be useful to know the outdoor CO2 level--surprising he didn't check that. Any humidity readings? Knowing indoor RH compared to outside would be another indicator of whether ventilation was the issue, or perhaps something else. Personally I doubt you have an unusual indoor source of CO2. What analysis did your PE offer?

Answered by David Meiland
Posted Mon, 03/10/2014 - 11:51

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David, humidity readings were performed. With an indoor temp of 73-74 degrees Fahrenheit, RH was between 28% and 29%. For reference, outdoor temperature was 20 degrees @ 62% humidity (absolute?) according to Weather Underground historical/hourly data. The only analysis provided was more in the form of "crack a window" vs. identifying the source of CO2. Maybe he didn't want to speculate to protect himself or something.

Answered by Jeff Watson
Posted Mon, 03/10/2014 - 14:19

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Hit up and look into their units. They're not the
cheapest thing to have, but it's a nice bit of reassurance you can
sit on a prominent shelf in your living space and monitor what's
going on.

With the minimum levels set for CFM and time-interval set in my
HRV I'm at maybe 0.1 ACH or less, single occupant, no pets or plants,
I run between 700 - 900 PPM when the house is closed up. But I've
let it run up over 2400 PPM with the ventilation shut down on occasion,
if for nothing else but to see how I could drive it, with no ill
effects. Place smelled a little stuffy, but I probably had similar
conditions for much longer periods before the HVAC retrofit [that's
a guess, as I didn't have the meter back then].

Don't worry about room-to-room. Partial pressure of any gas tends to
disperse fairly evenly until it finds a hard air barrier. It's not
intuitive, but I found that opening windows in seemingly unrelated
areas of the house would start lowering CO2 levels pretty rapidly
where the meter is. I'd consider CO2 levels as a strictly secondary
priority to monitoring interior humidity, which is likely to have
a more profound effect on where you live, but it is nice to be able
to quantify how your own existence is affecting the IAQ.


Answered by Hobbit _
Posted Wed, 03/12/2014 - 00:17

Helpful? 0

Thanks _H*. I think I'll invest in a CO2 meter - taking measurements myself to study steady-state as well as cause & effect situations are probably the only thing that will settle my nerves. I'm a numbers nerd so I need to quantify things. There are a couple of areas I was interested in hearing if they would be mentioned in being contributors (e.g., sump pit that serves a basement half-bathroom, high efficiency furnace venting). Guess I'll just have to buy a meter & measure.

Answered by Jeff Watson
Posted Thu, 03/13/2014 - 09:39

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I bought a CO2 meter. Some observations & request for comments:

1) Right before going to sleep it read 1100ppm in our bedroom and when we woke up, it was 1300ppm. This is with no heat running overnight, all windows closed up.

2) With the furnace on, the CO2 levels start to steadily rise. I need to do a test to see if the levels rise with only the air handler on (no gas) to see if this is just diffusion, or if the burning of gas is dumping CO2 back into the equation.

3) The kitchen had the highest levels...about 1200ppm steady-state. Rose to 1900ppm with the oven on & one window open.

4) In my laundry room which has a sink & ejector pit (which serves a basement bathroom), the levels were high as compared to the rest of the basement.

Can I get some ideas as to CO2 contributors? Searching the internet, Google always thinks I meant to say carbon monoxide.

Any comments?

Answered by Jeff Watson
Posted Fri, 03/28/2014 - 09:30

Answered by David Meiland
Posted Fri, 03/28/2014 - 09:55

Helpful? 0

I have a CO2 meter in my house.
My experience is that the major CO2 contributors/generators are gas ranges and people. I was surprised to learn how quickly a gas oven will increase indoor CO2 concentrations and how slowly it dissipates. In our house, running the gas oven for an hour, even with the kitchen exhaust fan on, the indoor CO2 would jump from 500 ppm to 2,000 ppm. And it would not get back down below 1,000 until the following afternoon. We moved since then and don't have gas appliances anymore and do have an ERV and now our CO2 usually ranges from 400 ppm to 600 ppm. But the CO2 can still shoot up over 1,000 and even 2,000 ppm if we have a bunch of people in the house and the widows are closed.
I didn’t have the CO2 monitor in the previous house (the one with a gas range) over a Thanksgiving holiday, but since then have wondered whether high CO2 levels from running the gas oven and gas stovetop burners for hours that day with a crowd of people in the house ever contributed to the lethargic after-Thanksgiving-turkey-dinner feeling!

Answered by Nelson Labbe
Posted Fri, 03/28/2014 - 15:02

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Great info, Nelson.

Answered by David Meiland
Posted Sat, 03/29/2014 - 20:40

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Thanks for the links David & thanks for sharing your experiences, Nelson. I can only dream of the day an HRV is within reach in my old house. For now I'll just have to stick with leaving a window open before I leave for work in the morning....that's the only way my CO2 concentration reaches 500ppm. Seeing how high the levels rise when using the oven probably explains why I always feel sleepy when the oven is on, and why 75% of the time, I fall asleep after eating if it's something we cooked at home. Definitely keeping a window open if the stove is ever on.

Ventilation rates seem to only account for people and/or number of bedrooms. I guess the only way to really test if I have contributors aside from people & lack of ventilation is to vacate the house & expect that the CO2 level will eventually reach equilibrium. If my house's ventilation is rated at 0.32 NACH, after 3.125 hours, I should expect equivalent CO2 readings inside & outside the house.

David, your links mentioned CO2 in soil, sewer gas, and decomposition as other sources. I might be going crazy but all of those I wouldn't cross off my list: I have a damp basement, not sure I have a properly vented sewage ejector, and there is a dead tree in my yard. All three of those things could add up & possibly introduce CO2 within the building if it gets in through the concrete foundation.

For example, while sitting here typing this I am the only one home with all windows closed. I had been sitting here for 20 minutes and CO2 was at ~1000ppm. I cut the furnace on and the CO2 is now rising faster. After 10 minutes it's at 1300ppm. If I am the only contributor to CO2 I would have assumed the room I'm in would have the highest level and once the furnace is on, that the level would drop as the air is distributed throughout the house. But it seems like the opposite happens.

Answered by Jeff Watson
Posted Sat, 03/29/2014 - 23:01

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what type and age is your furnace and water heater? Direct or atmospheric vents?

Answered by Bob Irving
Posted Sun, 03/30/2014 - 12:36

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Bob, furnace was installed in October 2013 and is a nat gas 92% model with intake & exhaust pvc pipes. Water heater is the typical 40gal atmospheric vent.

One hypothesis is that once the air handler is on, leaky ductwork pulls in CO2 from the basement and the CO2 concentration equalizes between basement & main floor. To test this, I left all the windows open in my basement yesterday to let it naturally ventilate. When I cut the furnace on after some hours, the CO2 on the main floor did not rise as fast as I had previously observed. So I'm wondering if this is just a simple case of leaky ductwork that pulls in basement air. After all CO2 is denser than O2 so maybe it just pools in the basement.

I've never heard anyone recommend ventilating their basement (let's just assume it's unfinished/unoccupied), but I presume this is only a concern since I don't have balanced ventilation. I guess when people say exhaust-only ventilation doesn't always give you good air is true - I've noticed that running my bathroom fan increases CO2 concentration just like running the air handler does. Eventually I do reach a point where the meter stops rising as fast.

I did perform a separate test where I aired out the house til it was down to 700ppm, closed all the windows, set furnace to OFF, came back 3 hours later and the meter was sitting at 720ppm...negligible. Once we got home is when it started to rise.

So I guess this really is just about ventilation; just that I didn't think about my basement & leaky ductwork & how they might impact ventilation strategies (such as exhaust-only) on the main floor.

In another eye-opening test, I had pretty much all of my windows open, and my meter refused to decline in its reading. I thought the meter was broke, so I then placed it right next to a window to which it started to fall sharply (as expected). Put it back in the middle of the room and it shot right back up to over the 1100ppm reading it had before. Even turned on the bathroom fan & it went nowhere. Not sure how to explain that one.

To really conclude if I have an additional CO2 contributor, I need to air out the basement, close all the windows in the basement, and start the meter; leave and come back after some time to see the rise, fall, or absence of change.

In my case, I guess the only guaranteed way of reducing CO2 in my house is by cracking a couple of windows (on the main floor AND the basement)!

Answered by Jeff Watson
Posted Mon, 03/31/2014 - 09:54

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Jeff... have your tested CO2 and shutting off water heater? Also... somewhere you might have posted but do you have a gas stove/oven and do you have pilot lights anywhere burning?

Answered by aj builder, Upstate NY Zone 6a
Posted Mon, 03/31/2014 - 12:17

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Good point....water heater was not shut off during any of my tests. It does have a pilot so I guess that's one contributor that I didn't think about.

I do have a gas stove but the burners don't have standing pilots; electronic ignition.

Answered by Jeff Watson
Posted Mon, 03/31/2014 - 21:51

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After that last post, I did some closer monitoring in the utility room in the basement and I think I've found possible mold.

My CO2 meter pointed out an area in the utility room right under my bathroom where the tub would be. A couple of the wooden floor joists have some black powdery-looking hairy stuff on them. The main supply & return ducts for my furnace are less than 2 feet away from this area.

So it kind of adds up with what I was experiencing; basement was not necessarily poorly ventilated, I just think I have pollution (if it's mold that gives off CO2). With the (unsealed) ductwork in close proximity, probably explains why I would see a jump in CO2 levels on the main floor which then evened out.

As one of David's links above mentioned decaying vegetation as a source of CO2, wondering if the high CO2 is from the wood rotting, or if mold gives off CO2. I have seen articles that only seem to indicate there's a correlation between CO2 and mold, but not necessarily if mold is the causation of CO2.

For now, I am just going to leave the window to the utility room open if we are home & figure out if I really do have mold. If it's true, that $150 CO2 meter surely served its purpose!

Answered by Jeff Watson
Posted Tue, 04/01/2014 - 08:19

Helpful? 0

Your posts raise many questions. I'm not convinced that the CO2 levels you are recording are high enough to be worrisome.

That said, if you are worried about the indoor air quality in your basement utility room, it makes sense to seal your duct seams with mastic.

Answered by Martin Holladay, GBA Advisor
Posted Tue, 04/01/2014 - 08:42

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