Funny CO read

Mitch_6

but seems like you do have to get the pressure into spec then go from there.

Have you checked the inlet pressure is it within spec?

Mitch S.

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Dale

Ran into an interesting co read,comments please

We check co after doing nat gas furnace work. Co was a few hundred parts on a 90% ruud in the exhaust, spiked up to 2000 when the blower came on. Testing showed excessive input from a gas valve giving line pressure 7" to the manifold instead of 3.5", and stuck and not adjustable down. We will replace valve, our thought is that the rapid condensation in the secondary cell slows down the flue products enough to raise the exhaust CO. Anyone ever run into this?

Jim Bergmann_2

CHeck the heat exchanger

I would definitely check the heat exchanger, with the high gas pressure you may have a failure and air disruption of the flame is causing impingement when the blower comes on. Plug the flue and test each of the tubes by using a monometer and a rubber stopper with a hole in the center to check for pressurization of the chamber when the blower starts. If need be you can use a cotton rag to block the burner passages. This is obliviously done with the burners removed and it will save you from removing the heat exchanger. The primary collector box on the back of the exchanger may be breached.

Empire_2

another thought

Was the gas line pressurized to check for leaks? if the valve was exposed to excessive pressure the regulator would be shot.

Mike T.

Dale

fixed it

Thanks to you and the others who responded. The problem was fixed with a new gas valve where the manifold adjustment could be adjusted back to 3.5" wc. We always go outside, on the ground level vents at least, to check CO in the exhaust of the 90percenters. We have gotten use to seeing a spike in CO when the blower starts but it usually goes to normal very quickly. We also have found CO to spike on the multi stage gas input high end furnaces when the valve goes from high to low fire. This makes sense to me since there is a visible flame change for a very short period of time. Thanks again for the response.

S Ebels

My guess

I would be very careful there for this reason. If that gas valve was delivering 7" to the burner manifold for any length of time at all, even a few days or a week, you definitely would have had some impingment going on. As thin as the material is on gas furnaces, it doesn't take very long to perforate the HX. This would explian the spike in CO when your main blower comes on. You may have solved the original problem but not the more serious problem which was brought about by the first one.

Sometimes you have to think through the whole sequence of events that brought about the failure or problem you are actually looking at. Kind of like Reverse Rube Goldberg trouble shooting. Here in front of you is the end result, but in order for that to happen, first this component failed, which caused this failure, which then lead to what you're actually looking at!

I'd be very careful with that one and go back to check your CO again as the blower cycles on. My bet is that you have a failed HX. You may find your CO is down from the initial reading due to the now correct pressure, but I'll bet it still changes. This is about as bulletproof an indication of a failed HX as you can get. Other than laying your eyes on the crack or hole which is often impossible.