Mystery CO Leak & Personal CO Detectors During an Inspection

During a recent inspection I had the homeowners plug in CO monitor go off about halfway into the inspection. Realtor was like: “Whad You Doo!!” - (Tommy Boy)

Home: Bungalow 1200sf, NE Ohio, light rain, warm
Built 1924
AC - Running at the time
Forced Air with single point ducted cold air return (Retrofit)
Gas Heat - Off at the time
Gas Hot Water Heater, Newer, Natural vent - Running at the time. Nice blue flame
Gas stove - off at the time
No make up air accomodations in the house/basement
Furnace is a type 1 Mid efficiency With induced natural draft venting - No backdrafting/rollout
Furnace and HWH vents are connected with a Y just before entering the chimney. All vent connections and the chimney top looked good.

The CO Alarm did not sound for most of the inspection and while I was testing the furnace. It wasn’t until shortly after filling the tubs that the CO alarm sounded. Seems the HWH is the source but what might the cause be?? I know I do not need this for the report however I think it is a good exercise to at least try to determine the cause/source.

I was hoping some of you HVAC buffs could steer me in the right direction.

I had no way to cross check the homeowners CO detector so it got me thinking about buying some new toys. Does anyone here carry a personal CO detector during an inspection?

I do not, and please excuse the short reply here but: Monoxide. O

Hi Arthur, I believe you mean CO (carbon monoxide). Yes I carry a portable CO detector. I believe I purchased it from inspector outlet. There may be some blockage in the flue/chimney.

1 Like

LOL Thanks for the proof read!

I was thinking it might be a blockage also, There was a sealed rain cap so I couldn’t see down the flue without disassembly

LOL Heavy mouth breathing by the HI causing buildup up of CO2 in the home. Got that fixed thanks for the proof read!

Was it a low Battery? I had to change one out today, It kept sounding off, It stopped when I changed the battery

If a water heater sits for a while, it can build up hydrogen sulfide. Opening a faucet will then set off CO detectors.

1 Like

That very well may have been it Scott. Batteries are a valuable commodity around here though, wouldn’t catch me putting new batteries in detectors on an inspection.

The house is on a well and it did smell high like high sulfur water

If I can’t determine the age of a home installed CO detector, I recommend replacement. Average life span is 5 to 7 years. Not saying anything was wrong with the one that went off, but further testing should be done to verify the accuracy of the home unit.

Same with smoke detectors. Batteries may work and make it beep, but does the “sniffer” work? Anybody still test with smoke pens or matches?

@aduhaime The battery replacement was at my house, not a clients. I ended up replacing the CO detector.


You have a natural draft furnace and water heater vented to a common chimney. In that configuration, if the home is depressurized to only -3 pascals, you will have back drafting. The depressurization limit of the home from stack effect, bath fans, etc is -3 pascals. That depressurization can be easily obtained in some home configurations.

With basement windows closed, use incense or a smoke source to do a back drafting check at the vent hood of the water heater and the furnace.

I carry a personal low level CO detector around my neck on inspections with atmosphere vented appliances, for exactly this reason. They cost a little over $100 for a low level alert device.

As an edit: Make sure you get a device that allows you to set the threshold alarm limit. I have mine at the ASHRAE limit of 9 ppm.

One other item is that home CO alarms typically alert at fairly high levels. Levels had to be quite high in the home you were inspecting, recommending immediate evaluation AND evacuation. From one alarm; “An alarm’s response time will vary depending on the level of carbon monoxide in the air. For example, an alarm will sound after three and a half hours of continuous exposure at a level of 50 PPM, but after only eight minutes of continuous exposure at a level of 400 PPM.”

Review the CO exposure chart at CO exposure limits

1 Like

Michael thank you so much for this informative response! I think you’re on the right track with the negative pressure. Given that the homes cold air returns are retrofit and the fact that while the AC was running I had a door sucked shut by airflow.
I will will be adding a CO detector and liquid smoke to my arsenal. Thanks again for your input very helpful!

1 Like

Did you do a draft check at The water heater draft diverter ( top hat)? more times then I can count I have found significant spillage at that location extinguishing the match well before you get close to it that rather then pulling the flame in. Just a thought

1 Like

I did not. Though I will be upping my game in that area after this one.

Liability issue: I assumed you warned the buyer about the CO monitor alarm and CO hazard. If not, you should do a follow up. There’s a potential liability issue there if someone dies from CO poisoning. It is not uncommon. If need be, update your report and send a revised copy to all parties involved clearly stating, among other things, that one can die from CO. This is one of those issues you do not want to leave hanging. CYA!

Of course I did that the day of the inspection. Good point to mention though. Thanks Michael.