# Thread: Using CO2 meters to determine fresh air ACH

1. ## Using CO2 meters to determine fresh air ACH

Most accept blower testing as a method of determining the theoritical building air infiltration. This is great but the test only provides the theoritical amount of fresh air infiltration during the average winter temperature/wind. As temperature rise and fall along with wind, we more or less fresh air in the home. All so the mechanicals on the home are not operating while the blower test is conducted.

With some care, measuring CO2 concentrations during steady state occupancy and internal conditions, we have real data regarding the actual amount of fresh air entering the home. The number of occupants generate a specific amount of CO2. The CO2 is mixed with the fresh air infiltration/ventilation (typically 450 ppm), generating a part per million (ppm) concentration of CO2. The resulting ppm determines the cfm of fresh air being mixed with occupants CO2. Reliable CO2 meters are becoming inexpensive, providing confirmation of the amount of fresh entering the home.
http://www.co2meter.com/?gclid=CMbLh...FRPyDAodqTeS3A

I recently found a blocked fresh air inlet by observing the slow rise of the CO2 levels in my home.
I like the idea of checking the amount of fresh air infiltration/ventilation to match the ideal building air change rate to purge indoor pollutants and occupancy density. Building need an air change in 4-5 hours for ideal purging of pollutants.
Determining Fresh infiltration/ventilation by monitoring CO2 PPM levels. Locate the CO2 meter in the middle of the home. Operate air circulation with occupants away from the meter. Monitor for several hours until the CO2 levels stablize. With an interior volume of 32,000 cubic ft. and CO2 meter reading of 800 ppm indicates 27 cfm per occupant of fresh air infiltration.
With 2 occupants, 2 X 27 cfm/ = 54 cfm total effective fresh air blending with the CO2 from the occupants. 32,000 FT^3 / 54 cfm = 592 minutes or 10 hours to change the air in the home with perfect air mixing. I measured 60 cfm of fresh make-up air plus infiltration. My guess is 50% of the fresh air gets out of the home without mixing with the CO2 because of slow mixing. I am under-stating the amount of fresh air by 505. I could use a little more fresh air, but its ok for me. As the outside temps drop and wind rises (winter), the CO2 levels decline, indicating an increase in fresh air infiltration. I can discontinue fresh make-up air during the coldest wind months of winter while maintaining low CO2 levels.
Attached is a table of various CO2 levels and the amount of fresh air infiltration/ventilation per occupant and the square footage of the home with a 9 ft. ceiling. These numbers are based on perfect mixing of the fresh air and CO2 prior to being metered. Because of poor mixing of the fresh air and the CO2 from the occupants, expect an underestimate of the amount of fresh by 25-50%. Operating the air handle and allowing the CO2 levels to stablize a couple hour while the occupants are located away from the meter will provide a more accurate reading.
This an interesting concept to provide real time data of air change depending on the different conditions home are exposed to.
This the best tool we have to monitor levels of fresh air in the home.
I am attaching a chart illistrating the concept with typical fresh air/CO2 levels.
Regards TB

2. need it to hold withing 100 PPM for a few hours to be sure

need to measure it outside as well, it goes up and down all the time

I believe in getting enough fresh air and CO2 numbers verify it, I find it amusing that the 'pollutant purging' band wagon tapers off when the dehumidifying season is over

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TB:

Finally getting my backordered Extech CO250 meter tomorrow.

4. Here is look at the difference of the CO2 levels with one occupant verses two. The house has 60 cfm of fresh make-up air 24/7. The CO2 levels are half as much increase with one person as two. There was also a little wind. The dehu was off for a couple days but humidity got to high. The last 5 days the dehu was on and removed 170 lbs. of water while suppling the 60 cfm of fresh air. The main point is that the CO2 levels are indicating about 60 cfm of fresh air when the occupants are in the home but not next to the CO2 meter. When next to the meter, the CO2 read higher which indicates less fresh air to the home. This also indicates that the CO2 is not getting mixed with the fresh air when next to the meter. While in the bedroom, the fresh air is getting mixed with the CO2. The graph is interesting.
Using the CO2 as fresh air infiltration/ventilation level indicator looks ok when CO2 and fresh air are mixed well in the home.
Regards TB

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TB:

I got my Extech CO250 but I guess I didn't research it too good! Darn thing only datalogs while connected to a PC. The software manual wasn't available online and I "assumed" wrong! They do have a higher priced model which I think logs on its own but this one should be good enough for my needs.

It has a DB9>3.5mm RS232 cord. Although I have a notebook PC, I don't like the idea of moving it around all the time. Looking into a real long 3.5mm mono to female extension cord but I'm not sure if I'd run into problems with it unless I go into a premium cord used for digital cameras.

Leaving the meter in a centeral location away from my location. Using your chart, I should see about 800ppm, I think. The lowest I've seen was 850 to a high of 1200/1300. I have no clue as to why it varies so much. No fancy ventilation system here just a 28 year old forced air system in a home built in 1956. The heats been ON for the past several because of cool weather enough to have a freeze/frost warning the other night!

6. I data log every 5 mins. Take a reading 1st thing in the morning. Activate your CO2 meter where your computer is locatted as long as it is in an area where the mixed air reaches. Operate your furnace fan for 15-30 mins. while being away from you computer. Read the CO2 ppm every 15 mins. You will see the effect of being next to the meter verse being away. You are idealy looking for data when you are remote from the meter and allowing CO2 and fresh air mix during typical weather/temps conditions. As the temperature declines/wind increases expect the fresh air infiltration increase signifacantly. No wind and neutral temps means very little fresh air.
Regards TB

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TB,
Could one of those CO2 bds be integrated into a ERV/HRV in place of say a percentage run timer and let the CO2 bd turn the ERV/HRV on or off when necessary ? Would the ERV/HRV know the difference it there was a percentage run-timer or the CO2 bd in place?

8. They were selling that in the early 90s with HRVs Dan.

I am adding a telaire ventostat to run an ERV in my own home whenever CO2 gets a little high.

People exhale CO2 and water vapour. In tight homes in the winter, having the HRV kick on to keep humidity from building up and condensing on your windows will automatically keep your CO2 levels down low.

You would not really need the CO2 unless you wanted to make sure you were getting enough fresh air in the cooling seasons.

I am mainly adding the telaire to see how low I can keep the CO2 in my own home and not having to worry about excessive humidity.

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Carnak,

I'm getting a EV-130 hopefully this coming week, it will be running on a percentage run timer but since seeing that CO2 bd. I was thinking it might be nice to have it run off the CO2 bd instead or would that not be a good idea or not worth the time ?

10. Dan

Heating season is coming soon and I think you have an older home. Maybe you run a humidifier even in the winter.

I think the tight homes can need the timers or even the CO2 controls.

A tight home in the winter will be plagued with window condensation. You need more air in winter than you do in summer as you have to keep humidity down to keep windows clear.

A looser home can get a lot of air in winter just because of infiltration, to the point that it gets so dry you have to run a humidifier.

11. Besides being away from the meter when sleeping teddy, the activity level has dropped unless of course someone were to get the best bang for the buck with those four hour blue pills.

Just kidding but when you are sleeping, your metabolic rate drops down, you do not exhale so much CO2. The teleaire meter you use assumes 400 PPM outside and also a standard met rate like could be expected in an office environment.

So when you are in the same room as the meter, your met rate would be similar to what could be expected in an office, higher while you are active such as cooking and cleaning, and even lower if you are just sitting there watching TV.

But at night it will get a fair bit lower. I think if you are running constant fan on your furnace, the higher levels building up in the bedroom get dispersed through out the house somewhat.

In my own home, I have a high air change rate in my master bedroom, because of its western and southern exposures close to 10 air changes an hour.

In your own home you could be half of that when the furnace or AC fires up, and then perhaps half of that again with low speed constant fan, but it still tends to mix it through the home.

at 400 PPM outside and 600 PPM inside you are about 50 CFM per person

at 450 PPM outsode and 600 PPM inside you are about 67 CFM per person.

Just pointing out how little fluctuations in ambient CO2 make a 34 percent difference on how much fresh air you THINK you are getting.

I am a semi rural location and ambient CO2 floats around all the time drops when it is really windy, changes with wind direction, and goes up when it rains.

At my office I am more prone to traffic, even rush hour traffic and it swings by almost 100 PPM outside.

I had a graph before how CO2 rose in my office with the AC and ventialtion shut off for the weekend, coinciding with early morning traffic.

I also have no shortage of data on educational, commercial and institutional buildings with high ventialtion rates, even 100% outside air systems, where you can see how the CO2 levels go up and down when the space is unoccupied, from ambient changes.

Typically readings taken every single minute to try and better catch the spikes.

I also do CO2 decay tests to work out an infiltration rate, 2/3 of the time the result is repeatable, but 1/3 of the time the result is out to lunch and this is because of the outdoor CO2 level changes. I still think this is more accurate than a blower door test. The only way to improve accuracy is to trace a gas decaying inside the home that is not found in the atmosphere.

I have a few teleaires but you put two of them side by side and you will be lucky to be within 50 PPM on the two readings. In the case of indoor levels being around 600 PPM, 50 PPM means a difference of 34%, so no point in tracking what is indoor and outdoor at the same time, the devices are not accurate enough. All I can do when I run this test is to record outdoor level at the beginning of the test with the telaire that I am doing the logging from and if I have the time right after I finish the indoor test with the same meter, and average the two values.

There was an article linked to on IEQ by Lew Harriman, was a report based on testing a lot of the name brand CO2 meters including telaire and vaisala along with about 10 others. Moral of story, just use the same meter for both indoor and outdoor measurements. Sort of like if you had a thermometer that was off by 5 degrees-- if you were measuring a temperature differential just use the same themometer twice and the error cancels out on the differential value.

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13. why the log scale on vertical axis

if it is just you in there then 800 PPM is like 25 to 30 CFM of outdoor air. Hard to tell with a log scale making everything look like a flat line

if the CO2 numbers do not drop much more in winter it can explain your window condensation
Last edited by Carnak; 10-15-2009 at 06:25 AM.

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