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trying to understand spike in CO2

Trevor_Lambert | Posted in General Questions on

Fairly new, airtight house with Zehnder ERV. The ERV is linked to two CO2 monitors, and programmed to ramp up if the level of either sensor reads >800ppm. Under normal conditions, our levels are in the 420-650 range.

Went to bed last night and everything was normal. Woke up this morning to the ERV running at full. At first I thought maybe someone was up before me and activated the boost in the bathroom. Went downstairs and saw that the controller said “sensor”, indicating that it’s boosting because of CO2 levels. I’ve been trying to figure out why, and have NO clue. The living room sensor was reading 980, despite there being no people in there. The master bedroom was reading 1060, despite having the balcony door open all night. We have an all-electric house, and no source of CO2 besides respiration. I opened up lots of doors and windows, and though it didn’t seem to do anything for the first 10 minutes or so, by the time I left for work it was down to around 840.

Any ideas? Sensors went south, both at the exact same time in the exact same way? Something inside the house malfunctioning and giving off CO2? Can’t think of what. An outside source? Can’t think of what would do that either, plus it started to drop after opening all the windows.

This has me spooked.

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  1. charlie_sullivan | | #1

    That's odd. The only thing I can think of is that maybe something came loose in the ducts and you are actually drawing the supply air from inside the house? But still 1060 with the door open seems pretty high. What kind of sensors? I wonder if they could be sensitive to some other gas and fooled by that?

  2. Trevor_Lambert | | #2

    They are Dwyer CDT series, with a single beam dual wavelength nondispersive infrared (NDIR) sensor. Based on a quick read of the theory of operation, the other gas hypothesis doesn't seem plausible.

    "For CO2 detection there are two wavelengths of interest which are 3.91 µm (here light red) and 4.26 µm (here dark red). IR radiation with 3.91 µm is not absorbed by either CO2 or other common gases in the atmosphere and is transmitted without loss to the detector. Therefore, it is well suited as reference with no absorption. IR radiation with 4.26 µm wavelength is only absorbed by CO2 molecules and by no other gases in the atmosphere. This means, that the degree of radiation reduction on that channel depends only on the CO2 molecules in the air or in other words on the CO2 concentration."

    I guess the only wiggle room is a gas that is not common in the atmosphere, but it would have to be present in an incredibly high concentration to throw the readings off so much.

    1. JC72 | | #4

      Factory reset?

  3. Jon_Lawrence | | #3

    Do you have a portable CO2 sensor that you can place outside for 15 minutes to check the background level of CO2? Any wildfires in your area? I am 3,000 miles from the West Coast and the smoke has made it here, but too far to increase the CO2 levels.

  4. ranson | | #5

    If not a fire, any chance that that the wind last night would have blown a neighbors chimney smoke to your air intake?

  5. AlexPoi | | #6

    Do you have an attached garage? Otherwise, my guess would be wildfire or smog.

  6. Mark_Nagel | | #7

    Have to ask just to make sure (install instructions mention this, but sometimes...): Are your intakes and exhausts separated as per instructions?

    Seems unlikely that drawing exhausted CO back into the fresh inlet could result in such high numbers (with no other sources found) other than if they were nearly connected to one-another!

  7. Trevor_Lambert | | #8

    I don't have a portable sensor. Interior readings were back down to 490 by noon, according to my wife.

    No wildfires. No chimneys close to us, and no one in their right mind would be using a stove or fireplace at this time.

    No garage. Intake and exhaust well apart.

  8. bigred | | #9

    Be careful confusing CO (carbon monoxide from incomplete combustion) and CO2 (carbon dioxide from respiration). Could you have a big household full of plants? Plants take in oxygen and give off CO2 during the nighttime hours so if your home is full of house plants, this could be a possible factor. Ambient levels of CO2 are now 420 ppm, so levels below this are suspect.

    1. user-5946022 | | #10

      ...don't plants take in CO2, and then release about 1/2 of the CO2 they take in, and convert the other half to oxygen through photosynthesis?

      1. bigred | | #11

        During daylight hours they take in CO2 and release O2. Once sunlight ceases, photosynthesis ceases and they respire by taking in O2 and releasing CO2.

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