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Community and Q&A

ERV Sizing and Interior CO2 Levels

Tim S | Posted in Mechanicals on

I recently out of a tightly sealed 1980s passive solar apartment with no ventilation and into a new-built SIPs “pretty good” (1.7 ACH50) house with an Broan ERV 140 and minisplits. The HVAC contractor was pretty new to ERV’s, but seems to have done a conscientious and thorough job based on what I can see as a layperson.

For the past couple of years I’ve been monitoring my indoor air quality with an iqAir monitor. At the old place, without windows open we would routinely hit indoor CO2 levels over 1,000 ppm, and even exceeding 2,000 ppm, especially overnight. Luckily the south-facing exposure to solar cranked out enough heat in the winter that we could mostly leave windows open, so we just got into the habit of doing that.

At the new house, with all the windows closed and ERV running, I’m seeing CO2 levels of 700-900 ppm, with the ERV running at it’s next-to-highest CFM range. The house is 1330 square feet, with two occupants, so based on the ASHRAE standard it would seem like the 65 CFM that the ERV is supposedly pumping out would be more than enough.

I’m confused about why the CO2 levels are still so high indoors — is this an indication that the ERV isn’t performing up to specifications? Or, is that actually just a normal CO2 range for a tightly sealed house with an ERV? Anything I can do on this end?

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  1. Jon R | | #1

    > CO2 levels of 700-900 ppm

    This is a reasonable range for sleeping or thinking.

  2. Eric Habegger | | #2

    I think the range is OK but far from ideal. The outside atmosphere averages about 400 ppm so your average is double that, so not ideal. Because you are two people it would a little more than you might think. I don't know if you have pets.

    I still think your ERV should be operating at a lower range just from my own experience with a Panasonic ERV. It runs at 60 CFM and 450 ppm with 1 individual. There are only two things that could account for it if I assume its not defective.

    1 The air filter is too restrictive either because it needs a filter change or just because it is too small. Many ERV filters are really small and restrictive.

    2. The ducting is too restrictive either due to either small diameter (4""), long runs, or too many bends and/or kinks.

    I'd attack number 1 first by replacing the filter. If it's clean then I'd try a larger inline filter box and see if that cuts down the restriction. It did on my panasonic and its only using 30 watts measuring with a kill-o-watt. Mine has ECM motors, as does yours, so I'd expect something similar allowing for double the occupancy.

  3. Expert Member
    Akos | | #3

    Your 800ppm is in the ballpark for 2 people and 65CFM ventilation. If you want lower than that, you need to increase the flow rate. No way around that.

    Humans breathe around 0.4 CFM at rest, breath is 4% CO2 so 40000 PPM. For two people with 65CFM of ventilation:

    (0.8*40000+ 65*400)/66=878 PPM.

    1. Eric Habegger | | #4

      Not saying this isn't correct but there is another way of looking at the problem. Both the Panasonic and Broan ERVs have ECM motors. That means they "should" have the same relative efficiency. If what Akos is saying is correct and it is running as it should with normal airflow then the wattage used should be very similar and the doubling in CO2 that he is experiencing is just due to double the occupancy of mine. His unit shouldn't be working harder than mine.

      If his unit is drawing significantly more power then something is off and that formula isn't quite right. I'm always suspicious of formulas that don't quite model reality as closely as they are supposed to. But I can always be proved wrong by just purchasing a Kill-o-Watt and finding a similar power draw as mine, 30 watts, at 450 ppm CO2 single occupancy, or ~ 900 ppm CO2 double occupancy.

    2. Eric Habegger | | #7

      BTW, I just looked up the specs on my ERV. It says about 40 watts for the cfm I selected to produce around 500 ppm CO2. It used to actually be 50watts. When I retrofitted a new much larger inline filter box and filter the cfm required went down to get to 500. The power draw also went down to 30 watts. That's a pretty impressive improvement in my mind and much better than specification for the that ERV. It was quite a nice surprise and makes me think doing the same "experiment" with the filter on other types of ERVs might not be a bad idea.

      1. Tim S | | #10

        Eric, my watt meter is apparently still packed up in a box somewhere so I may not get to try that experiment after all anytime soon :D. Cleaning the filter didn't seem to have much impact on output.

        All things considered, 700-800 with all the windows closed is a big improvement over the old place, but I'm surprised to learn how much more CFMs I would actually need in order to have lower CO2 levels. Maybe someday can put in an extra Lumos HRV in the bedrooms..

  4. Tim S | | #5

    Thx for the insights! I've removed the air filters -- the exhaust filter had a decent amount of dust in it, and they're currently drying. Will measure the wattage while I'm at it and report back...

    Akos -- how does that formula work?

    1. Expert Member
      Akos | | #8

      It simply doing the ratio of the two air flows mixing. The only source of fresh air in your house (beside the bit form air leaks) is your ERV. So your ERV is brining in 65cfm of 400ppm CO2 outside air. This in turn mixes with the breath of the people, two people so 0.8CFM with 40000ppm CO2.

      HouseCO2PPM=(NumberOfPeople * 0.4 * 40000 + ErvFlow * 400)/(NumberOfPeople * 0.4 + ErvFlow)

      In a leaky house you would add the ACH(nat) to the ErvFlow rate.

      In steady state the size of the house doesn't effect the equation, so the air indoor ends up at the ratio of the two air streams.

      1. Tim S | | #9

        Thanks -- makes sense!

        So now I'm just curious, do the ASHRAE standards just assume an indoor CO2 level in the 700-1000 range?

        1. Jon R | | #11

          Hard to believe, but ASHRAE standards ignore the health effects (poor sleep, decreased cognitive function) and focus on acceptable levels of body odors. Don't go less than 25 CFM per person.

  5. Trevor Lambert | | #6

    The C02 level seems a little on the high side based on the numbers you provide, but not a lot. I sometimes see numbers creep up over 700ppm with 100cfm, two adults, two children and a dog. Have you measured the CFM? If you're just going by the specs of the ERV, it's probably not that close.

  6. John Clark | | #12

    CO2 of outdoor air is 400 ppm. You have nothing to worry about.

    1,000 ppm to 2,000 ppm. is in that yellow zone.

    Over 2,000 ppm is when you should worry.

  7. Erik_Brewster | | #13

    I'm having almost the identical experience. I've got a mid century modern that I have tightened up and added a Broan 80 CFM HRV to. I've been doing extensive CO2 monitoring and found that with two adults, two younger kids, a dog and cat, we almost never see < 800 PPM when everyone is home and all windows are closed. 1200 PPM is falrly common. I had 1800+ PPM with the HRV off (turned it off during wildfires). Crack a window, turn the kitchen fan on high and we can get to 500 PPM in 1/2 hour or so. This tells me that mechanical ventilation can do the job, but my setup isn't ideal, yet.

    While I agree with the others that you are safe with your current CO2 numbers (you won't keel over and die), I disagree that they are good enough. Here is a paper that might get you to consider more mechanical ventilation (expensive, inconvenient, admittedly). It's worth the read, even if just looking at the figures.

    My take on ASHRAE 62.1 is that it's a good starting point and will certainly keep you from any severe sickness, but why stop there? When I went from 1800 PPM to 800 PPM in a day, I could feel the effects very clearly. When I open a window and turn on more fans and get to ~500 PPM, I feel better, yet. Open a window and turn on a fan. If you feel better, you will know the answer about if you have enough fresh air. Use your CO2 meter to quantify the level that make you feel a certain way. Then you will know what number to design / work to.

    Also consider retrofitting a large MERV 13 or better filter to your HRV intake. This takes you firmly into science project land, as you will need to deal with rebalancing, ensuring the filters don't lower your CFM too much, etc. But then you can leave your HRV on full blast without too much concern for outdoor air quality.

    I'm finding great IAQ isn't super simple, but it's worth the work to get there.

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