CO and Tankless Water Heaters

I was wondering if anyone else checks CO at the vent (interior side) of a tankless water heater? I have run across a few in the last few weeks and I get elevated CO during the functional test at the interior side of the vent.



I have one in my house. I check it every now and then for CO but have never measured any!

A zero reading is not normal, CO is almost allways part of the combustion process. If you are not reading anything, I’d be suspicous of the meter or where you are taking the reading.

Im not an expert or anything but if you are taking the measurements from the proper areas you should not have any indication of CO.


I take the CO reading outside of the combustion area and the flue like 2 inches from the water heater. There should be 0 CO!

There may be some CO spillage upon initial firing of the appliance which is not a concern. Sustained elevated levels of CO while the appliance is operating would be indicative of a need for service.

Jonathan can you post a photo of one? There is a situation where you can detect CO by a tankless water but I don’t want to insult your intelligence by even suggesting it.

Thank you all for the feedback.

There were other reasons I felt an HVAC contractor should come to the houses, other than the tankless heater.

I advised my client to have both the water heater and furnace serviced