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Gas boiler burner shuts off within a minute for no reason
I have a newish Trio boiler with a Carlin gas EZ burner, indirect DHW tank, HydroStat 3250 plus base control, and a Tekmar 260 outdoor reset control. Working fine for 4 years. Lately, the burner seems to go on a lot with only domestic hot water demand and no space heat baseboard demand. Looking more closely, I see the burner comes on sometimes (not always) for just a minute, then shuts off, only to repeat many times until it stays on. Even when the burner turns off before demand or high limit is met, the Tekmar displays the burner icon indicating the switch is activated for the burner to light (but doesn't). The Tekmar #11 and 12 burner switch terminations correctly indicate continuity when the burner icon is shown.
Nothing to do with the Warm Weather Shutdown or Unoccupied mode. There must be a sensor somewhere doing this?!
What could to causing this? Thanks for any suggestions to check.