RE: Combustion testing and CO range


 I use a TPI for checking combustion. What are some possible causes or sources of CO ppm measurement. Most furnaces I wanna see less than 20 ppm. I’m told 50> is acceptable range and 50<  “we can do better”. I know that high gas, not enough O2 and bad heat exchanger (delamination) and so forth can cause high co readings. 

I recently found myself working on a Goodman furnace. It was a warranty call, New construction, single pipe, two years old 92% single stage. I believe 100k BTU w/ 4 ton drive. Customer complain was awful smell that hasn’t been fixed since closing. Only happens in heating season! My co reading outside was 30ppm and indoor at flu natural steady state 30 ppm. I had 9.2% oxygen, 30ppm co, gas was at 3.3″wc. I forgot to write down the Co2 reading. Furnace doesn’t trip limit. 

The smell was a plastic type gas smell. Utility gas company came out to check for gas leak and detected “incomplete” burning of fuel. Never pinpointed a joint leaking gas.

Also add that the client complained about high humidity in home during summer (65% to 70%). 

By the time I tested everything I condemn the gas valve as I thought it was over feeding after being cut off from heat cycle. I also ordered a new evap pan, after inspecting heat exchanger I noticed the pan was warped. I thought maybe the pan being warped may have contributed to high humidity in home during summer. Maybe the condensation flipping out into air stream? 


Sorry was so long, hoping for an educational discussion. Hope I included enough info, and testing out this cool forum. 




Mikayo88 Rookie Asked on April 12, 2018 in Residential HVAC.
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Like trick 1 said check your analyzer because a bad sensor can wreck your day. Also I would commission the furnace again maybe the first guy didn’t check the safeties and the limit has not worked since new. I am only saying this because the drain pan being warped makes me think it is getting to hot or not passing enough air. Did you do a temp rise? or check the coil ? You also have some sort of combustion issue if your analyzer is working properly. Also I would clock the meter and make sure the appliance is burning the correct amount of gas. and possibly up the gas pressure a bit. Also if it was me I think I would install a second pipe for intake so it isn’t robbing air from the house.  If the furnace is located near a  laundry it could be drawing in dirty combustion air.

Rookie Answered on April 14, 2018.
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