Fan cycles on every few minutes.There is no demand and the burner doesn't come on but the exhaust fan cycles on for a few seconds every couple of minutes
You're most likely getting a pressure wave (spike) that is generating momentary flow across the flow sensor. The heater is registering this "instant" flow and beginning its firing sequence. Once the flow dissipates, the unit then goes into a purge cycle. I'd recommend having your water pressure tested to make sure it isn't abnormally high (above 60 psi) or abnormally low (below 30 psi). Assuming the pressure is normal, you can use a pressure reducing valve or pressure regulator to absorb the spikes and stop the unit from cycling.
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You're most likely getting a pressure wave (spike) that is generating momentary flow across the flow sensor. The heater is registering this "instant" flow and beginning its firing sequence. Once the flow dissipates, the unit then goes into a purge cycle. I'd recommend having your water pressure tested to make sure it isn't abnormally high (above 60 psi) or abnormally low (below 30 psi). Assuming the pressure is normal, you can use a pressure reducing valve or pressure regulator to absorb the spikes and stop the unit from cycling.