# Thread: Procedure for determining ACH based on CO2 levels

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## Procedure for determining ACH based on CO2 levels

I have access to a good CO2 sensor (but without logging) and would like to take measurements to determine if the CFM/run time for an installed ERV is correct. What would the recommended procedure be? Sample locations, intervals, etc?

Thanks

2. The ideal air change rate when you are in your home is an air change in 3-4 hours. The CO2 levels resulting from a steady state of cfm of fresh air ventilation and number of occupants in the home. Mixing of both fresh air and CO2 is critical for accuracy. With the occupants in the open part of the home and air handler heating or cooling, cfm of fresh air per person can be determined. I do not have the chart at home but will post it Monday. Which meter do you have. To check the meter, place the meter outside for 30 min. Typical outside CO2 levels are 400-450 ppm.
Using the total ft.^3 volume of the home including utility areas, calculate the number cfm of air needed to change the air in four hours.
3,000 sqft home X 9 ft. ceiling = 27,000 ft^3 ttl volume
4 hours= 240 min.
27,000 / 240 min. = 112 cfm during occupancy provides an air change in 4 hours. More to follow later.
Regards TB

3. Excerpt From ASHRAE/IESNA Standard 90.1-1999 Users Manual Regarding Ventilation Control with CO2 (pg. 6-27)

Ventilation Controls for High¬ Occupancy Areas (§ 6.2 3 9) Spaces with high design occupant densities offer an excellent opportunity for clemand controlled ventilation PCV) systems since these spaces are seldom occupied at their design occupancy. DCV systems modulate the amount of outdoor air supplied to a space as a function of the number of people present, providing significant energy savings when spaces are only partially occupied. The Standard requires DCV for all ventilation systems with design outside air capacities greater than 3000 cfin serving areas having an average design occupancy density exceeding 100 people per 1000 ft². This typically includes assembly spaces such as theaters, meeting rooms, ballrooms, etc. As an alternative to DCV, systems may be provided with air to air heat recovery systems complying with § 6.3.6.1.
DCV systems must maintain ventilation rates in accordance with ANSI/ASHRAE Standard 62 and local

Table 6 B Typical Met Levels for Various Activities

Activity met
Seated, quiet 1.0
Typing 1.1
Filing, seated 1.2
Filing, standing 1.4
Walking, at 0.89 m/s 2.0
House cleaning 2.0 3.4
Exercise 3.0 4.0

standards. Unfortunately, neither Standard 62 nor local codes address in detail exactly how to design and operate DCV systems. 'Me following procedure, which is consistent with ASHRAE Standard 62 and Interpretation IC 62 1989 27 of Standard 62, is recommended.
Since most ventilation codes prescribe outdoor air rates proportional to the number of people in a space, it follows that a DCV system should modulate outdoor air as a function of die number of people. The most commonly used indicator of the number of people present in a space is carbon dioxide (CO2) concentration. People give off CO2 and other bioeffluents (including body odor) at a rate proportional to their activity level. Hence, CO2 concentration is a good indicator of bioeffluent concentration and thus is ideal for DCV.

To properly use CO2 for DCV, the system should be designed as follows:
1. Determine the design outdoor air rate based on the maximum (design) number of people expected to occupy the space. No diversity is allowed in the design population assumption. Some standards, such as Standard 62 (§ 6.1.3.4), allow a designer to reduce the design occupant count by some percentage because people are not expected to be present for a period long enough to reach steady state. A good example is a pre-function area of an assembly space; it can have a very high peak occupancy, such as during the intermission of a performance, but the peak seldom lasts more than a half hour or so. Credit for such occupant diversity cannot be taken when DCV systems are used because the outdoor air rate tracks the occupant load; the space is not overventilated before and after the occupant peak the way it would be if no outdoor air controls were specified.
2. CO2 sensors must be located within the occupied zone of the space served.
Alternatively, the sensor may be located in the space return air grille provided the setpoint or sensor reading is adjusted to reflect any short circuiting that may occur in the air distribution system. Fortunately, most densely occupied spaces are always in a cooling mode (the temperature of the air being supplied to die space is less than the space temperature), which results in nearly perfect mixing even with less than ideal ceiling supply and return systems. In this case, die CO2 sensor may be located almost anywhere in the room or in the return air duct from the room. If located in the return air duct, the sensor should be located as close to the room as possible so that return air duct leakage does not distort the concentration reading.
For systems serving more than one room, locating the sensor in the common return from all zones is not acceptable since it would indicate only average CO2 concentration, possibly allowing some spaces to be underventilated while others were overventilated. Doing so is analogous to controlling room temperartire, in multiple rooms with a return air temperature sensor. For systems serving multiple rooms, CO2 sensors must be installed either in all rooms, or possibly only in those rooms that are judged to be "critical." Critical rooms are those requiring the highest percentage of outdoor air. If CO2 is not measured for all rooms, spaces for which CO2 concentration is not measured should be assumed to be occupied at peak occupancy conditions at all times the system is operating.
3. Determine the CO2 concentration setpoint using the following equation:

CR = COA + 8400 m/Rp

where CR is the room CO2 concentration, COA is the outdoor air CO2 concentration, m is the metabolic rate (1 met = 58.2 W/m², see Table 6 B for typical values), and RP is the rate of outdoor air per person. This equation assumes that the air change effectiveness of the air distribution system is near unity (a good assumption for systems in the cooling mode). An outdoor air CO2 sensor is not required if the outdoor air CO2 concentration in the equation is set to a conservatively low value (e.g., 350 ppm). As an example, a movie theater CO2 concentration at 15 cfm/p, 350 ppm outdoor air CO2 concentration, and a metabolic rate of 1.0 would be 910 ppm.
4. If separate CO2 sensors are used for
room and outdoor air concentration
measurement, they each should have an
accuracy of ±50 ppm. in the range 300
ppm to 2000 ppm. If a single sensor is
used to measure both points, or only an
indoor sensor is used, the sensor should
have an accuracy of ±100 ppm in the
range 300 ppm to 2000 ppm.
5. The system must be designed to
ensure that a minimum outdoor air intake
is maintained regardless Of CO2
concentrat . ton to account for contaminant
sources from building materials,
furnishings, etc. Unfortunately, Standard
62 1989 and most building codes do not
provide any data on what this minimum
rate should be. An addendum to Standard
62 to provide building related air flow
requirements is being developed. Contact
ASHRAE for current information on the
6. Outdoor air rates must be controlled to maintain the measured space CO2 concentration at or below the setpoint determined above. Any type of control logic is acceptable that meets this criterion, including on/off and modulating control. The latter is preferred for system stability and to avoid rapidly changing space temperatures when outdoor air conditions are extreme.

4. some meters will give an estimate on how many CFM of fresh air per person is occurring.

The meters assume that the outdoor CO2 is 400 PPM and they also assume an 'average metabollic rate'. The busier you are, the more CO2 you exhale.

Now to try an use CO2 to estimate how many air changes you are getting, you first need to know when it has stabalized and it is hard to tell if you are not data logging.

But for a quick estimate try this.

early in the morning when everyone is up take some readings in the house, average them.

then read what it says outside.

If your average indoor reading is not much more than 700 PPM higher than what it says outside, then you are probably getting about 15 CFM per person.

You can go through the info genesis has posted, you can also try posts 172,to 174 here http://hvac-talk.com/vbb/showthread....250842&page=14

the outdoor levels can fluctuate up and down depending on time of day, traffic, which way the wind is blowing.

the amount of CO2 produced inside goes up and done depending on how active you are, if you are cooking, burning candles, smoking.

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I have an Industrial Scientific CDU440, should be accurate to +-10ppm. It has been calibrated to 1000ppm, but zero'd to outdoor air (I don't have access to a "zero concentration" sample) basically my 2 acre backyard surrounded by 80 acres of farm. I'll assume that is about as close to average (400ppm) as I can get. Just some quick info, the main area of the house seems to average about 170ppm over outdoor when occupied. Master bedroom this morning was 200ppm over. Areas of the home that are unoccupied for a period of time move down to 130ppm over. Measurement at a supply vent with ERV running (and main air handler) is 120ppm.

6. I have brush where I live, it means organic matter

when it rains CO2 goes up outside, it stirs up organic matter laying around.

Why assume what it is outside - measure it

Do not measure the vent outlet, measure where your mouth and nose are, that is where you are inhaling.

It you are in fact only 200 PPM higher than outside it would mean your home through ventilation and infiltration would be a good 40 PLUS CFM per person a lot more air than normally required.

If I go into a home with chronic humidity problems and I read low CO2 in there like under 600, I know the problem is too much outdoor air moving through the building. it then becomes a matter of figuring out if it is because of infiltration, excessive ventilation or un-intentional mechanical means such as duct leaks etc.
Last edited by Carnak; 08-15-2009 at 11:58 AM.

7. if your master bedroom was only 200 over, I doubt you got lucky last night

8. Originally Posted by teddy bear
The ideal air change rate when you are in your home is an air change in 3-4 hours.
you like every 5 to 6 hours in other posts, you thinking about selling ERVs again?

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Originally Posted by Carnak
if your master bedroom was only 200 over, I doubt you got lucky last night
My 2 year old woke up at 11 and didn't want to sleep in her room anymore, so no...

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Originally Posted by Carnak
I have brush where I live, it means organic matter

when it rains CO2 goes up outside, it stirs up organic matter laying around.

Why assume what it is outside - measure it

Do not measure the vent outlet, measure where your mouth and nose are, that is where you are inhaling.

It you are in fact only 200 PPM higher than outside it would mean your home through ventilation and infiltration would be a good 40 PLUS CFM per person a lot more air than normally required.

If I go into a home with chronic humidity problems and I read low CO2 in there like under 600, I know the problem is too much outdoor air moving through the building. it then becomes a matter of figuring out if it is because of infiltration, excessive ventilation or un-intentional mechanical means such as duct leaks etc.
I have retired thoroughbreds and plenty of green grass surrounding me. All of the room measurements were 5 feet off the floor, arms length with the meter. The measurement at the vent was just for reference during an ERV run.

Also, we left the house for about 2 hours this morning. I set the ERV to run full time for 100 minutes as we walked out the door. Reading upon returning was 80 ppm over outdoor.

I'd love to read an "actual" measurement, but again no zero gas available right now.

We seem to have humidity under control (I have another post with data from my zone controller). I need to sit down and figure out if all of the numbers are adding up. ERV is rated 210CFM, running about 30%. I have not measured actual condensate for a whole day, but calculating about 50 pints per day with the light cooling load we've been having. House is 45-50% RH at 75-76 degrees.

11. sounds like you have quite an abundance of fresh air

perhaps shut the ERV off as an experiment to see how high the CO2 goes. If it does not start getting upwards to 700 above ambient, could mean a lot of air on its own moves through your home naturally, OR your meter reads way low.

12. This is about using a CO2 to determine the actual air change rate of a home. Most IAQ experts recommend an air change in 3-4 hours when occupied. I suggest a little less mechanical fresh air ventilation because you will get some natural ventilation especially with ERV/HRS. Big homes, figure a little less air change rate and small homes, a little more air change.

CO2 ppm = fresh air cfm/person
450 = (if 450 ppm outside)no occupants
600 = 70
800 = 30
1,000 = 19
Three (?) occupants with 40 cfm fresh air per occupant += 120 cfm
120 cfm X 60 min X 4 hours = an air change in 4 hours in 3,200 ttl sqft home.

You need steady state with good mixing of fresh air and house air.
Regards TB
Last edited by teddy bear; 08-16-2009 at 05:07 PM.

13. 450 would be inifinity not zero

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