I’m concerned the unit allows the mixing of the stale and fresh air. My supply air into the rooms is about 800ppm CO2 when it should be just above 400 if coming direct from outside. I think air is getting mixed in the unit.

Opening the filter flaps allows air to get sucked in and mixed with the supply. So in principle it can happen I think.

I see these a gap on the bottom left between the black rubber on the heat exchanger and the unit. I think stale air can get sucked in through there and then pumped into the rooms. Do you think that’s possible?

Would I be correct in saying the fresh air always goes though the heat exchanger, and it’s the stale air that ‘bypasses’ the heat exchanger when the bypass is turned on , typically in summer?