Navien 240A fan runs when toilet is flushed.

[kpkoren]

LP. 2020 mfg date. Came with the house. PEX inlet and outlet. DIP switches all off. I have all the books and paperwork for it. No external recirc. Recirc valve set to internal. Was not registered with Navien at purchase. Navien won't help. Heater works fine. Lately, flush toilet or for that matter turn on any cold water faucet, the exhaust fan on the WH turns on for a while. Burner doesn't come on. Flushed with 5 gallons of vinegar for 45 minutes 7 days ago and came out clean, clean water inlet, clean air filter, clean condensate cup. Installed new internal check valve, old one looked exactly the same as new one. If I turn off the cold water inlet valve below the WH, the problem goes away. Have ordered a new flow sensor and a check valve for the cold water inlet below the WH. I've searched ad nausem here and elsewhere for answers. Any other ideas?
TIA
Ken Koren
Rockville, IN

 

[kpkoren]

LP. 2020 mfg date. Came with the house. PEX inlet and outlet. DIP switches all off. I have all the books and paperwork for it. No external recirc. Recirc valve set to internal. Was not registered with Navien at purchase. Navien won't help. Heater works fine. Lately, flush toilet or for that matter turn on any cold water faucet, the exhaust fan on the WH turns on for a while. Burner doesn't come on. Flushed with 5 gallons of vinegar for 45 minutes 7 days ago and came out clean, clean water inlet, clean air filter, clean condensate cup. Installed new internal check valve, old one looked exactly the same as new one. If I turn off the cold water inlet valve below the WH, the problem goes away. Have ordered a new flow sensor and a check valve for the cold water inlet below the WH. I've searched ad nausem here and elsewhere for answers. Any other ideas?
TIA
Ken Koren
Rockville, IN

Well today I installed a new Navien flow sensor in the WH and a Sharkbite check valve on the cold water PEX. Issue remains. Am pulling out what little hair I have left!
Ken Koren
Rockville, IN

 

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[kpkoren]

SOLVED!!! On another forum, it was suggested to get the air out of the WH. I turned on Internal Recirculation (DIP Switch #1 to ON), let the holding tank fill, heat up and settle down, turned on all the hot water faucets in the house at once, ran them full out for a minute, turned off the faucets, let the water heater settle down and no more problem! I wasted more than $70 in parts. I hope this helps someone else.
Ken Koren
Rockville, IN

 

[GReynolds929]

Glad that solved the problem. My only other guess is you had a cross connection somewhere, usually it's a shower mixing valve cartridge that is failing and let's hot and cold water flow through it when it's not supposed too. Usually there is temperature issues as well when that happens.