Pros and cons of HRV vs Fresh Air Ventilation in a dry heating/cooling climate.

[teddy bear]

On a mild Saturday afternoon and with little wind and everyone home I had 1280PPM. It stayed above 1200 well into the evening.

Here is a dilution table assuming 450 PPM CO2 outside air, good mixing of the inside air, CO2 from the occupants, and the fresh air infiltrating the space.

550 ppm = 106 cfm of fresh air per occupant in space
650 ppm = 53 cfm
750 ppm = 35 cfm
850 ppm = 26 cfm
950 ppm = 21 cfm
1050 ppm = 18 cfm
1150 ppm = 15 cfm
1250 ppm = 13 cfm

If you have two occupants at 1200 ppm CO2 inside and 450 ppm outside and with good mixing, I would estimate 26 cfm of fresh air infiltration. With a 2,000 sqft. home/ 9 ft ceilings, you should have 60 cfm as minimal adequate infiltration. If we are going to use the CO2 levels as a signal to activate fresh air because of single occupancy and a lack of natural fresh air, activate fresh air at 650 PPm.

Wait for the CO2 level to stabilize. With poor mixing, expect higher levels when the occupants are near the meter. Cold windy weather forces more fresh into the home. Calm moderate temps decrease the air change rate.
The objective is to get an fresh air change of the air in the home in 4-5 hours to purge indoor pollutants. Exhaust or makeup air ventilation decreases natural air change at 50% rate. An example is a home getting 30 cfm of natural, activating a 50 cfm exhaust fan will increase the infiltration from 30 cfm to 58 cfm. The exhaust is 50 cfm and the exfiltration is reduced from 30 cfm to 8 fcm.
Balanced flow ventilation, like an ERV, adds directly to the natural air change rate.
Regards TB

[garya505]

TB, thanks for the table, that's good reference info.

While we were out Sunday for about 4 hours, it dropped from about 1100 to 700. I was disappointed that it didn't drop to near outside levels, but then I realized that this is partially due to being so high when we left the house. If it was being kept down to 800 with ventilation, it may have dropped to something much closer to that of outside air, and the sensor could do it's ABC thing.

So, I'm going to watch this longer, but at this point I would say that:
1. We need ventilation in the house for healthy air.
2. CO2-based DCV would work in this case.

[teddy bear]

TB, thanks for the table, that's good reference info.

While we were out Sunday for about 4 hours, it dropped from about 1100 to 700. I was disappointed that it didn't drop to near outside levels, but then I realized that this is partially due to being so high when we left the house. If it was being kept down to 800 with ventilation, it may have dropped to something much closer to that of outside air, and the sensor could do it's ABC thing.

So, I'm going to watch this longer, but at this point I would say that:
1. We need ventilation in the house for healthy air.
2. CO2-based DCV would work in this case.

Sorry, I forgot the number in your home and the size.
Expect a 70% reduction in CO2 levels with one air change. 1100 to 700 is about a 60% reduction of CO2 levels. Did you leave the dog in the house? Big or small dog? You get the idea of how this works.
By the way what were the winds outside during this time? Also did you place your meter outside for an hour?
Regards TB

[garya505]

Sorry, I forgot the number in your home and the size.
Expect a 70% reduction in CO2 levels with one air change. 1100 to 700 is about a 60% reduction of CO2 levels. Did you leave the dog in the house? Big or small dog? You get the idea of how this works.
By the way what were the winds outside during this time? Also did you place your meter outside for an hour?
Regards TB

No dog, very light winds. The SupCo IAQ50 has ABC and manual calibration. Not knowing if indoor CO2 would get low enough for the ABC to work, I turned OFF the ABC and manually calibrated it outside. When manually calibrating, I think the IAQ50 assumes outside air is 400PPM. I suspect the ABC might work better than manual calibration, IF indoor CO2 ever got low enough.

On a sensor like the Honeywell C7232A that only uses ABC, I suspect that even if the calibration wasn't "perfect" because you never dropped to 400 indoors, it would still work. For example, if the lowest period for the week was 500PPM, I would expect the C7232A to open the vent at 900 instead of 800. In other words, when it is set to switch at 800, I would think the sensor would switch at an offset of 400 relative to what it took as the outdoor baseline during ABC (500 + 400 = 900).

BTW, last night only my wife and 2-year-old slept there, and it was 620PPM at 7AM. It was cold last night so the furnace ran a lot. It was not windy.

[garya505]

It's cold and windy today, CO2 at 640PPM. Yesterday we were out for a while but it only went down to 600. I don't think a sensor that requires ABC would work in this house. It would work fine in a business where it's empty for several hours every day.

[teddy bear]

It's cold and windy today, CO2 at 640PPM. Yesterday we were out for a while but it only went down to 600. I don't think a sensor that requires ABC would work in this house. It would work fine in a business where it's empty for several hours every day.

Keep in mind with active ventilation, the CO2 level with decrease faster.
What ever the low is over the recalibration time will be assumed to be 450 ppm. The meter will adjust to 450 ppm while it is 600ppm. This results in a 150 ppm under read. Setting the control to activate at a lower level will compensate for the lack of no occupancy. Activating the ventilation to operate continuously for several hours once a month will replace lack of occupancy. Also occasionaly opening a window for an hour will also recalibrate the sensor. I have used the GE sensors which have the ABC set for accepting the lowest reading once per week as 450 PPM and have not needed to recalibrate. They are +-75 PPM. Will you be able to keep your meter in place for a time to experience calm moderater weather?
Regards TB

[garya505]

Keep in mind with active ventilation, the CO2 level with decrease faster.
What ever the low is over the recalibration time will be assumed to be 450 ppm. The meter will adjust to 450 ppm while it is 600ppm. This results in a 150 ppm under read. Setting the control to activate at a lower level will compensate for the lack of no occupancy. Activating the ventilation to operate continuously for several hours once a month will replace lack of occupancy. Also occasionaly opening a window for an hour will also recalibrate the sensor. I have used the GE sensors which have the ABC set for accepting the lowest reading once per week as 450 PPM and have not needed to recalibrate. They are +-75 PPM. Will you be able to keep your meter in place for a time to experience calm moderater weather?
Regards TB

Thanks for the tips. I see what you mean about the automatic calibration. If I had a sensor that assumed a 400 calibration level, and was set to switch 800, and the inside CO2 only got down to 600, I would have a 200 PPM under-read. That would result in it opening at 1000 instead of 800, which would still work.

Yes, I can watch the CO2 meter for a while to get more input.

[teddy bear]

Thanks for the tips. I see what you mean about the automatic calibration. If I had a sensor that assumed a 400 calibration level, and was set to switch 800, and the inside CO2 only got down to 600, I would have a 200 PPM under-read. That would result in it opening at 1000 instead of 800, which would still work.

Yes, I can watch the CO2 meter for a while to get more input.

Here is a graph on a large home in Naples FL. This home has 5 a/cs with whole ventilating dehumidifier using timer controlled fresh air ventilation. The doors are open to the outside occasionally. It is difficult to determine the fresh air venitlation levels without knowing the activities and number of occupants. But it is interesting. You will need fresh air ventilation during calm, moderate weather in most types of homes. Keep us posted on the CO2 levels during the coldest windest weather and the mildest calm weather.

CO2 Naples FL RH Dew Point.pdf
Regards TB

[garya505]

Interesting data. From what I've been seeing, our levels are considerable higher. We never leave the windows or doors open though.

So, given a house that never gets down to 400PPM ...

What about running a vent on a timer continuously, but at a lower rate, with CO2 switch in parallel, to open it at the threshold? In other words, let's say the house needs 60CFM (continuous) to bring the CO2 from a maximum of 1300PPM down to 700PPM, and the CO2 never drops below 600PPM even when the house is unoccupied for a few hours. OK, then if a vent timer opened the vent 20 minutes per hour, that's 20CFM which on it's own might only bring the CO2 down from it's max of 1300PPM to 1100PPM (not enough) when occupied, but would bring it down to 400PPM when unoccupied. Then the ABC on the CO2 sensor would work properly, and it would switch at the correct level. Of course, the timer would still open the vent when the CO2 sensor had reduced it to the threshold, but if this threshold was set to 1000PPM, then it would only go down to about 800PPM anyway.

In essence, the low continuous rate provided by the timer makes the house "leakier" and lets the ABC on the CO2 sensor work like it should.

Just a thought I had today.

Of course, just opening the vent continuously for a few hours per month would do it too. I wonder which idea would be more reliable and cost effective.

[garya505]

I been reading some stuff on this and found that my idea is nothing new. They call this "non-zero minimum ventilation" or "base ventilation" which is usually used to account for non-occupant sources of CO2, to force the unoccupied CO2 levels down for the Automatic Baseline Calibration to work better. Seems to me the same concept would apply to a building that is not leaky enough to get down to ambient CO2 levels quickly enough.

[teddy bear]

You can also judge the amount of air change that a occupied space has by the inside dew point verses the outside dew point. Keep in mind that the occupants are adding moisture to the air in the home. When the outside air is passing through the home home at an unknown rate. The inside dew point will be higher than the outside dew point depending on the amount of air passing through the home. An adult add about .5 lbs. of moisture per hour when in the home. 100 cfm of air passing through a home with a l lb. per hour of humidification from any source will have an increase of approx. 13^F dew point. You are adding more moisture more moisture.
I am attaching data from a 3,800 sqft. WI foamed home that has the ventilation off for most of the time data was collected. The natural infiltration is inadequate for purging indoor pollutants with current weather conditions. Based on the outside/inside dew points, I would estimate that the home is getting 30-40 cfm of fresh air infiltration. That is why the indoor dew point is much higher than the outside dew points. This home needs 100 cfm of fresh air when occupied to purge indoor pollutants.
Ideally, the inside dew point would be 10-15^F above outside based on the size of the home and the number of occupants.
I am doing a follow-up on the home with the ventilation settings corrected to get better fresh air ventilation when occupied.
Hope you find this constructive. Not many want this much information. Will post the new data next week.
Regards TB

Nov WI dew point inside outside-789833-55-114033.pdf

[garya505]

Thanks for the info on inside dew point. I hadn't thought about that.

[Chuck]

Great thread! I will, hopefully soon, be getting a CO2 monitor to measure my house. I have an ERV with a percent timer. I bet I am over ventilating a lot of the time.

[teddy bear]

Great thread! I will, hopefully soon, be getting a CO2 monitor to measure my house. I have an ERV with a percent timer. I bet I am over ventilating a lot of the time.

Keep us posted. My guess is that you are over-ventilating during cold windy weather, about right during moderate weather, and under ventilating during warm calm. What part of the country are you in? How is your indoor dew point verses outside dew point or indoor %RH? How many occupants and sqft. of home?
Keep us posted.
Regards TB

[garya505]

I read an article somewhere that said they checked a bunch of homes and found that 77% were overventilated.

From what I've read so far. I think that without using some measure like CO2, the only way to insure enough ventilation is to overventilate part of the time. Seems kinda whacky to me, but that's what it appears.

[Chuck]

Keep us posted. My guess is that you are over-ventilating during cold windy weather, about right during moderate weather, and under ventilating during warm calm. What part of the country are you in? How is your indoor dew point verses outside dew point or indoor %RH? How many occupants and sqft. of home?
Keep us posted.
Regards TB

NW Wisconsin.
2800 sq ft including conditioned basement.
Mom, dad, a 2 year old and 4 year old.

[tedkidd]

Great thread! I will, hopefully soon, be getting a CO2 monitor to measure my house. I have an ERV with a percent timer. I bet I am over ventilating a lot of the time.

+1, really really good stuff.

Maybe I'll get me a co2 logger and stop running my erv continuous.

[teddy bear]

+1, really really good stuff.

Maybe I'll get me a co2 logger and stop running my erv continuous.

From what I am finding, you will save energy, moisture, and wear by not operating your erv when not occupied throughout the year. Also operating full time when occupied and calm winds/moderate temps provides better indoor air quality. The % timers under ventilate when you need the ventilation. You over-ventilate when unoccoupied and windy, cold weather. Better moisture conservation during extreme low outdoor dew points is the biggest benefit. 3-4 occupants are needed to keep a small home at comfortable moisture levels during low winter outdoor dew points. CO2 ventilation control does have benefit for automating the on/off control of any ventilation device. Most occupants will not deal with the daily adjustment needs for ideal ventilation schedules.
Regards TB

[tedkidd]

Keep repeating it. All those concepts are sinking in, making ever more sense.

[garya505]

Getting back to my idea about "base ventilation". This is used in commercial buildings where there are non-occupant sources of CO2 that prevent the indoor CO2 level from returning to outside levels (~400PPM). In other words, the building always needs that much ventilation to return to "fresh air" CO2 levels even when not occupied. This allows sensors that use ABC (automatic background calibration) such as the Honeywell C7232A to calibrate properly. Using ABC, the sensor never needs manual calibration for the life of the sensor (~15 years).

OK, so that's for commercial, but we're talking about residential so why am I bring this up? Sensor ABC algorithms need the indoor CO2 to drop to outdoor levels at least once every 7 or 14 days, depending on the sensor. Checking my indoor CO2 levels, I see that when the house is unoccupied for a few hours, I never get to 400PPM. It does get to 600 or 700 occasionally, either when we're out for a few hours or when we're sleeping, or when it's really windy. So, I was thinking, why not use base ventilation to bring in a small amount of ventilation continually, to get the base level down closer to that of outdoor levels. In commercial, I have seen numbers like 20% or 25%.

So how would we do that in a home. What about controlling ventilation with a timer to provide 20-25% of the ASHRAE requirement, in parallel with a CO2 sensor that would fully open the vent? So, in other words, the house always gets 20% to 25%, but gets full ventilation if the sensor reaches the threshold (800, 1000, or 1200).

Any thoughts or ideas?

Here's an interesting paper on the subject. Though written for commercial, some of it still applies.
https://www.airtest.com/support/reference/paper1.pdf