# Thread: CO2-based On-Demand Ventilation

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## CO2-based On-Demand Ventilation

Hi everyone,

I searched the threads and only found this old one talking about CO2-based On-Demand Ventilation: http://hvac-talk.com/vbb/showthread.php?t=169032

My thought is - is there any manufacturer who sells a thermostat that can adjust the ventilation rate based on combined logic using CO2 and humidity? That is - to only run when there is a demand for CO2 or humidity adjustments?

I read an excellent article investigating the idea at
http://nlcpr.com/OnDemandVentilation.php

The guy is an electrical engineer. His article states that, based on demand, he found that his on-demand HRV would run, reactively,:
9% of the time just sampling
25% of the time purging CO2/humidity
The remaining 66% of the time it is off because it isn't needed.
Obviously if you used a timer to turn it on 25% of the time you would exhaust the right quantity of air, but there is no way it would be timed right. Furthermore, without the controller, I would never have known that 25% was ...average... This of course is the whole problem with rules-of-thumb and sizing units based on the suggested number of air exchanges per hour. There are too many variables, so unless you have some way of measuring the pollutants in the air, you are just taking a wild guess.
When I wrote him he said this:
"Other than the one I made for myself, I am not sure they exist anywhere for sale. I will make a printed circuit board for it later this year (the current one I have is hand soldered together using perf-board and the display is inserted into a box carved out of a piece of leftover 2x4
spruce). When that happens, I can manufacture a bunch of them easily for anyone that wants one. I am currently gathering data and will probably use
it as an Engineering Masters degree project and work out the math for the optimal control algorithm."
If I can have my HVAC contractor do something off the shelf, even better.

Thanks for your thoughts,
Danny
Last edited by scriptures4life; 04-05-2011 at 11:16 PM. Reason: formatting

2. Regarding humidity control; are you wanting to ventilate to reduce humidity? Do you live in a dry climate, year round? Is your house built tightly?

3. Originally Posted by scriptures4life
Hi everyone,

I searched the threads and only found this old one talking about CO2-based On-Demand Ventilation: http://hvac-talk.com/vbb/showthread.php?t=169032

My thought is - is there any manufacturer who sells a thermostat that can adjust the ventilation rate based on combined logic using CO2 and humidity? That is - to only run when there is a demand for CO2 or humidity adjustments?

I read an excellent article investigating the idea at
http://nlcpr.com/OnDemandVentilation.php

The guy is an electrical engineer. His article states that, based on demand, he found that his on-demand HRV would run, reactively,:

When I wrote him he said this:

If I can have my HVAC contractor do something off the shelf, even better.

Thanks for your thoughts,
Danny
There are CO2 controls available that operate ventilation equipment. I also have similar data that says CO2 control dramatically reduces the need for mechanical fresh air ventilation. During cold windy weather most homes get adequate air change rates to purge indoor pollutants and renew oxygen without mechanical ventilation.
The CO2 meter does an excellent job of detecting occupancy and the lack of adequate fresh air. There are several available that are adjustable and affordable. Most are not variable but simple "on/off" controllers. For most application, simple is good.
I suggest activating the fresh air when the CO2 levels are at minimal level based on the square footage of the home and a single occupant. Most suggest an air change in 5-6 hours at a minimum when occupied. A 3,000 sqft. home needs 80-100 cfm of fresh air when occupied. Roughly, operating the 90 cfm of fresh air when the CO2 is +700 ppm would do the job.
Technically, the milder calmer weather will require more mechanical fresh air. Outdoor moisture levels should not interrupt minmal fresh air ventilation. With proper minimal fresh air ventilation, you will quickly discover that some supplemental dehumidification is required when the outside dew points are +60^F to maintain <50%RH in the home. A free standing whole house dehumidifier is required to maintain <50%RH, to supplement the a/c during the mild cool weather. Basements and cool slabs need <55^F indoor dew points to avoid mold growth and dust mites.
As stated by your research, mechanical fresh air was only needed <25% of the time during cold weather. You will find during mild weather, home need much more mechanical fresh air.
As a result, I suggest the whole ventilating dehumidifier may be a better choice for most green grass climates for fresh air ventilation and humidity control. A CO2 contoller is a good choice for activating fresh air with either HRV/ERV/VWHD.

Cost is the only problem.
Lowest cost is a new product.
http://www.co2meter.com/collections/...FUvd4AodWmNKsA

Honeywell recently purchased a CA company that make a good adjustable sensor. Keep us posted.
REgards TB

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Originally Posted by shophound
Regarding humidity control; are you wanting to ventilate to reduce humidity? Do you live in a dry climate, year round? Is your house built tightly?
Thanks shophound and teddy bear,

I live in Winnipeg, Manitoba, Canada. It is extremely cold and dry in the winter and VERY humid in the summer, according to this page:
http://atlas.nrcan.gc.ca/auth/englis...976&mag=0.0625

I want to ventilate to reduce humidity, CO2, and VOCs in the air.
The house will be built very tightly- not quite Passivehaus, but Canada's R-2000 style house. R-24 walls including dense cellulose and drywall caulked to the studs, R-60 ceiling, blown cellulose and airtight drywall method with ICI / Glidden Prep & Prime Vapor Barrier primer. We'll seal around all the recessed pot lights with foam, etc.

I'm looking forward to seeing what Honeywell and Peter at http://nlcpr.com/ come up with.

Cheers & thanks,
Danny

5. Originally Posted by scriptures4life
Thanks shophound and teddy bear,

I live in Winnipeg, Manitoba, Canada. It is extremely cold and dry in the winter and VERY humid in the summer, according to this page:
http://atlas.nrcan.gc.ca/auth/englis...976&mag=0.0625

I want to ventilate to reduce humidity, CO2, and VOCs in the air.
The house will be built very tightly- not quite Passivehaus, but Canada's R-2000 style house. R-24 walls including dense cellulose and drywall caulked to the studs, R-60 ceiling, blown cellulose and airtight drywall method with ICI / Glidden Prep & Prime Vapor Barrier primer. We'll seal around all the recessed pot lights with foam, etc.

I'm looking forward to seeing what Honeywell and Peter at http://nlcpr.com/ come up with.

Cheers & thanks,
Danny
Okay, so we're talking about a house yet to be built. And by what you describe, if it is built correctly it will be a tight house. Tight homes can require dehumidification year round, along with ventilation. In winter you can ventilate through the dehumidifier with outdoor air to purge indoor pollutants and control humidity, carefully modulated. In shoulder seasons and summer your dehumidifier can alternate between ventilating with dehumidification to dehumidifying via recirculating indoor air.

Main concern I might have is the wintertime scenario if you're controlling by CO2...depending on how quickly the ventilator could lower CO2 to acceptable levels, if your outdoor air is very dry it might overdry the house. That would require careful observation to see if that's happening once you get it all set up and running. Being your home will be new construction it will run higher indoor humidity levels for about the first year or so as the construction materials dry out. So this problem might not creep up at first, if ever. In later years it might, however. Just throwing it out there.

6. Honeywell Analytics Gas sensor, and a dehumidistat.

7. The problem that I see with all of the CO2 monitors is they are geared toward commercial building automation systems. There outputs are all 0-10vdc or 4-20mA. Something that can read that output and then do something with it is still requires.

8. Originally Posted by cartercrew
The problem that I see with all of the CO2 monitors is they are geared toward commercial building automation systems. There outputs are all 0-10vdc or 4-20mA. Something that can read that output and then do something with it is still requires.
The E3point is a 24v system, with built in relays. It's an all in one system, not counting the fan/ventilator.

Edit:
whoops. E3point does alot of gases, but not CO2.

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