CO testing of gas fired furnaces

I’m curious if other inspectors use CO detectors to measure CO at the heat registers during a home inspection. I do, and find that it can verify my other observations concerning flame and heat exchanger conditions. My question to other inspectors is…If you are, how are you interpreting the results? I have asked HVAC technicians this question and get a variety of answers…i.e. 1-5 ppm=normal…6-10=needs service…11+ possible cracked heat exchanger. But, there is not always consensus on this. Anyone have any thoughts on this?


I always take my CO measurements in the center of every room. Make sure you open at least 1 window in the room 5 minutes prior to testing. Using this method, I’ve never had a reading above 1 ppm.

2 questions: Why open one window?
Are there generally industry accepted standards how to test and what the magic number should be?

He was being a typicall Chicago A**

So, as an alleged HVAC Expert… what is your advice to the OP?

Please provide a reference to industry standards that you use for this method of testing.
Thank you,
Typical Chicago A**

Level of CO
Health Effects, and Other Information0 PPM Normal, fresh air.9 PPM Maximum recommended indoor CO level (ASHRAE).10-24 PPM Possible health effects with long-term exposure.25 PPM Max TWA Exposure for 8 hour work-day (ACGIH). Pocket CO TWA warning sounds each hour.

This is interesting, you test for carbon monoxide but you don’t know how to test or what the test results is?

Exactly what does this have to do with home inspection? Why are you doing it?

There are industry standards for this testing but is generally for weatherization purposes. Not a home inspection.

It’s my advice for home inspectors to carry a personal CO monitor on their person at all times. If the thing alarms then you have a hazardous condition that you should then report and get the hell out of the house.

A high carbon monoxide level in a house measured at the HVAC system does not necessarily mean the HVAC system is at fault. In most cases a broken heat exchanger, which is a deficiency on its own, does not produce any increase levels of carbon monoxide in the house due to the system design of the heat exchanger.

So again, what are you testing and why.