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CO2, VOC, RH, and CO ventilation controls for IAQ

Irishjake | Posted in General Questions on

I will have a Zehnder HRV in my house, and I’m installing an exhaust fan in my garage. I want the HRV to operate and provide ventilation based on what is required for air exchange. I would also like it and the garage vent fan to run automatically based on the level of airborne gases and humidity.

For the garage, are there exhaust fans that will operate based on CO levels, and not just when the garage door opens and closes?

For the house, can a Zehnder HRV be set up to operate based on levels of CO2 (parties, large get-togethers), VOC, CO (cooking), and RH? Can they be remotely managed, and can the data be logged? Are these whole house IAQ systems?

Are there recommendations for products?

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  1. user-2310254 | | #1

    Zehnder has a brochure that indicates its Comfortsense controller can serve as a bridge to CO2 and other sensors. I think you should contact the company with your questions--especially since you are spending big bucks for its HRV.

  2. GBA Editor
    Martin Holladay | | #2

    I don't know how far along you are with your project. If you are still in the planning stage, and you haven't yet purchased the Zehnder equipment, you may want to consider purchasing and installing a Conditioning Energy Recovery Ventilator, or CERV.

    Here is a link to an article with more information on the CERV: A Balanced Ventilation System With a Built-In Heat Pump.

  3. Irishjake | | #3


    Very interesting. Thanks for leading me to that article. I have not gotten to the point of purchasing the equipment yet. In fact I only have the first floor deck on the house, so there is time for a change to be made. (I have been focused on a garage/workshop/apartment above first, but that is nearing completion.)

    My concern (which is addressed in the article) is ducting. The Zehnder has home run seamless ducts (which I have to assume improve duct performance and losses). The other concern is the complexity of the unit.

    I may be going with a Sanden Heat Pump Split Water heater, instead of a HPWH to heat my water for domestic use. I wish there was a heat pump that could heat domestic water, space heating, and also conditioning/ventilating.

  4. SwitchgrassFarmer | | #4

    Brad, I am slowly adding "Demand Controlled Ventilation" capability to my HRV; it sounds like this is what you are after. Within the last month I added such controls to toggle my Honeywell HRV into high when CO2 levels exceed a threshold. (My HRV pulls air from the four bathrooms on a continuous basis, trips to high and more fully opens a balancing damper on a call from a particular bathroom.)

    I have a Honeywell IAQPoint2 CO2 monitor directly connected to a sample port on the stale air discharge out of the HRV. A proportional DCV voltage out of the IAQPoint2 drives a Honeywell JADE economizer controller. I have the system currently set to toggle my HRV into high once the CO2 levels exceed 700 ppm.

    So far this works pretty well, a few extra people in the house and/or running our propane range will send the CO2 levels above the threshold and trip the HRV to high. From what I can tell, ASHRAE says anything 600 - 700 ppm above outside CO2 levels is considered acceptable, haven't approached anywhere near that so far, about 850 ppm total is highest I have seen in this lash up so far. What is interesting is how long it takes for the CO2 levels to come back down to my normal "baseline", overnight.

    Another thing that I can't yet explain, is that when I run the range hood fan to flush cooking odor out of our kitchen I still get some CO2 increase in the house, even if I open (supply/fresh air) windows elsewhere. It's almost as if the hot cooking gases go out the range hood but the CO2 doesn't.

    I may hook up some monitoring/logging for the JADE later this year, once the house gets closed up again for winter. In particular I am curious to see if we are getting any CO2 spillage from our masonry heater.

    (The reading in the picture is from today, most of the windows in the house were open all day. Typically I see around a 550 ppm baseline with the windows closed.)

  5. Irishjake | | #5


    Yes, Demand Controlled Ventilation is what I'm after to manage my IAQ. I will not have any fossil fuel appliances in the home, it is an all electric house and property, outside of our grill and the generator.

    I had looked at the SiteSage by Powerwise to monitor and collect the IAQ data, elctrical and PV production. It was about $300/year for a two year subscription, and $2700 for the IAQ sensors and various equipment. I want to not only harness the info, but manage IAQ, electricity and other stuff via the controls. The Powerwise controls, aren't sampling duct air, but air within the rooms. This will give me a more accurate reading and better management I think via an HRV if I can link the systems together. Not sure this is the best approach though??? I was also hoping for remote control while on vacation and such.

    We routinely have lots of people at our house for get-togethers. (We had over 60 folks and their kiddos for Thanksgiving dinner two years ago), so managing that CO2 and IAQ is important for me. Being new construction, I'm very interested in capturing VOC data too.

    I wonder if why you aren't dropping your CO2 level because you are sampling whole house air and not by the room or floor?

  6. nvman | | #6

    Brad and Andrew, in your quest for Demand Controlled Ventilation to reduce indoor pollutants, have you investigated/utilized any additional filtering system for your HRV so that you are not introducing outdoor pollutants?


  7. SwitchgrassFarmer | | #7

    Additonal/replacement filtering is on my list. Right now I only have the washable filters that came with the HRV and a coarse screen over the intake to same.

    I wiped out the HRV cabinet and washed the core and filters a few weekends ago; I find that needs to be done about every six months. The most significant contaminant I have here at the farm is dust and pollen. That and small spiders that seem to have no problem going from one side of the core to the other.

    I ordered an extra set of Honeywell HRV filters the other day. Once they are in hand I may see if I can get someone to make matching filters with a better MERV rating. What came with the HRV are about MERV 0.1, enough to stop a baseball.

    I am considering some sort of standalone filter box ahead of the HRV, something like this one: Even better if this box could also filter the air that feeds the small intake relief air damper that opens when I run our dryer or range hood exhaust. What has stymied me to date is figuring out how to insulate such a box so it doesn't have a condensation problem in the winter. My last thoughts on this were for an outside filter housing as found on a truck:

  8. charlie_sullivan | | #8

    A solution to the problem of condensation on the filter housing is to put the final filter after the HRV rather than before. The filter before, usually built into the HRV, prevents clogging of the heat exchanger; the one after is to reduce whatever pollutant you are concerned about. That's particularly important with a carbon filter, which works better with lower relative humidity. (The HRV lowers the relative humidity of the incoming air when it warms it.

    As for the controls, there's a range of options from commercial grade expensive systems meant to control ventilation based on CO2 levels in spaces such as auditoriums and classrooms where occupancy varies widely, to DIY hacker approaches using basic sensor boards and writing code for Arduino controllers. I would tend towards a simple CO2 based control of the ventilation, and a sensor system that monitors more things and logs them. One option for a simple CO2 based control is this:

  9. SwitchgrassFarmer | | #9

    Yes, the extra filtering can go after the HRV; that is an excellent approach particularly for temperature sensitive media. In my case I am trying to cut down on HRV cleaning cycles. Having to wash the core, wipe the cabinet, and clean the dust off fan blades and motor at 6 month intervals is not optimal. It would be far better to replace one disposable filter every so often.

    As I think about it, I am not surprised that my HRV gets as dirty as it does considering that it is often altering the relative humidity of the inbound air. Dampness and dirt stick together!

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