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Old 09-05-2020, 12:25 PM   #1
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Odd gas detector alarm - solved (i think)

Yesterday I was working on the MoHo when the gas alarm went off. I immediately went looking for the problem - fridge was on electric, I hadn't bumped a stove control, water heater was off. I shut the supply valve off and it left me scratching my head. Had this happen once before on a trip across Utah while on the road.

The coach has been sitting for nearly a year. Had the airbags blow out on it on the last outing, then a big sewage overflow inside with the required dismantling, cleaning, and drying - especially in some basement compartments below the bath.

This led to a re-build of the wet bay and some valve replacements, and then a re-wiring job and finally pulling the water tank to replace some leaking hydraulic lines. We've been itching to get out in the woods for a while, especially since it's been hot as blazes here for the last month.

it was during the testing of the water system that the alarm went off.

The system had been completely empty for at least nine months. I had the faucets on purging air out of the system and looking for any leaks.

When the alarm went off I sniffed all over the place - no gas odor. Then I figured it out - either methane was being forced into the coach as water flowed into the grey and black tanks - the vents come up under the bathroom sink and the cabinet doors were open - or methane had formed due to organisms in the water heater. As it filled it was blasting air out of the kitchen and the bathroom faucets.

Of course I couldn't smell anything - methane is odorless - unless it's odorized for commercial use...
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Old 09-05-2020, 12:41 PM   #2
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If it was from Grey tank via the 'vent' under bathroom sink then you need to replace the AAV. (Air Admittance Valve)

They are on Grey tanks ONLY and should Open when DRAINING the grey tank. They get pulled open when draining to allow air into sink drain line to prevent 'P' traps from getting sucked dry.
When draining of tank is completed they CLOSE to STOP odors from coming back up from grey tank

Possible bleeding the air out of RV Plumbing lines ---especially hot side.....MIGHT have set LP Detector off

How was battery voltage when this was happening.
Low DC will set the alarm off also
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Old 09-05-2020, 12:50 PM   #3
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Boiling batteries are another cause of alarms.
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Old 09-05-2020, 02:15 PM   #4
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Battery voltage was OK. Batteries are in a compartment open to the atmosphere and a ways from the detector.

Didn't know about the AAVs. Makes sense, and while the water was running there was quite a smell coming from the bathroom. I bet they are stuck open.
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Old 09-05-2020, 07:37 PM   #5
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Both those valves had less than a thread of engagement. Surprised they hadn't just fallen off.
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