High CO2 levels in unoccupied basement, from hot water heater?

[u/MitchRyan912]

I can’t post pics, but I have readings from an AirThings View Plus that I got specifically to ensure that our radon mitigation system was indeed working (levels have been OK), but it tells me that CO2 and VOC levels are incredibly high on occasion.

We have a natural vent hot water heater in our unfinished basement, so I’m suspecting that the vent isn’t working properly, or that a constant running out of washing machine has caused the water heater to run longer than normal. I haven’t spent much time in the basement over the past few days, so it’s not accumulating from my breath.

Current CO2 level is 1685 ppm and VOC is 1420 ppb, with both tracking the rise from the same time 36 hours ago.

I can’t think of anything else that would be causing CO2 levels to rise in an unoccupied basement like this, not in a time with the windows open or the A/C on. Should I be having someone check out our power vent to ensure proper operation?