Rheem LPG tankless, suddenly starts to fire up when using cold water taps

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Andy D

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WH is about 2 years old, has been installed and working for at least 1.5 years with no issues. This is a small house we have been building ourselves that runs off of a gravity-fed water agency, which we are first in line on so our PSI at the meter is just 18 (we are just barely below the agency's water plant/tanks). I installed a booster pump just downstream from our house's inlet and run the house pressure at 40 psi because that has been working perfectly for our usage.

Yesterday came time for me to finally hook up our bathroom sink, so I had the hot and cold pipes drained and the booster pump off. Last night after finishing the bathroom sink, I noticed that the WH was starting to fire up whenever a cold water supply was opened in the house, which has never happened. Open a cold valve, and the fan in the WH kicks on and what I believe is the gas solenoid opening (single click that always precedes the burner firing). I do not believe the burner actually fires when this happens and the WH shuts down if the cold valve is left open. Again, the WH is not turning on long enough to heat water, just briefly starting up then stopping.

It seems to me that there is probably air trapped in the system after yesterday's draining and maybe a water hammer effect when the cold is turned on is changing the pressure in the WH enough to trick it into thinking that a hot water valve has been opened, and then as pressure equalizes, it shuts off. I don't have any hammer arrestors installed in the system. Looking for advice on how to troubleshoot and fix the issue.
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