Gas Odor after Boiler Shuts Down

Bridgestone · October 12

I have a problem with a lingering gas odor after my boiler shuts down. The model is a Weill McLain CGi-5. There is no gas odor before the boiler starts, or when it’s running. The odor occurs only immediately after the inducer-fan shuts down, lasting 4 to 5 minutes. As the gas odor moves up the stairwell it slowly diminishes.

A technician from the gas company came and used his gas detector. It turned red for 1 second and then went off. His diagnosis was “ likely gas valve bleeding through”. He did not red-tag the boiler. Using his words I called Weil McLain and under warranty they provided a new gas valve. The heating tech left shortly after installing the new valve, both of us assuming the problem was solved. To my dismay the odor problem remains the same.

Before I have the technician come back I would like to get some input on what the problem could be. I should note that the inducer runs for about 14 seconds after the “flame” light” on the panel goes off. During those 14 seconds I don’t smell gas at the boiler, only right after the inducer stops running.

Any thoughts on what could help diagnose the problem would be appreciated.

EdTheHeaterMan · October 12

This is going to be a process of elimination thing. You now know that the replacement gas valve did not solve the problem. Your technician needs to look further. Without actually being there, It is hard to say exactly where the odor is coming from

HVACNUT · October 12

Does the inducer vent into a chimney, or sidewall? The 15 second run time when the cycle is complete is post purge. The very purpose of it is to expell unspent gasses from the system. Start looking for flue or HX obstructions.

EBEBRATT-Ed · October 12

Do you have any exhaust fans running in the house? The house could be under negative pressure compared to the outside. Someone needs to decide if theis smell is unburned natural gas or fumes from combustion

Bridgestone · October 13

The inducer vents out a sidewall. There are no exhaust fans running in the house. Given the strength of the odor, I think its fumes from combustion, but not sure. Since there is no odor when the boiler is running I would think that its venting properly. I wondering if the post purge time can be extended, maybe that would help.

mattmia2 · October 13

watch what happens to the burner as it shuts down, it shloud go out quickly. maybe the gas valve is slow to close.

check the manifold fittings after the burner with soap too, make sure it isn't leaking such that it gets burned off while it is firing but leaks out after it shuts down

Bridgestone · October 23

Thank you for your comments. The tech guy came, tested the boiler and did fall maintenance. He stated that the odor I smell after the burner and inducer shut down is combustion fume, not gas. The post purge of the inducer cannot be extended beyond 15 seconds. There was nothing else he could do to improve the situation. I'm satisfied that there's no danger from the fumes and have decided to live with the odor as the fumes dissipate. The boiler is located in a large basement room so shutting the door to that room and another at the top of the basement stairwell will help.

EBEBRATT-Ed · October 23

@Bridgestone You could easily install a time delay relay to extend the purge regardless of what your technician told you.

You shouldn't be getting that backdraft and if It was my house I would investigate further.

Gas Odor after Boiler Shuts Down

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