Two identical boilers in the mechanical room. Both are for heat. Only one does the DHW. Ongoing issues with the DHW boiler. I'll call this the bad unit. I get there today to find an ignition lock-out. Goes through pre-purge. Switches make. Igniter is sparking. Good ground. 24 volts to gas valve, hear the valve click. 2 seconds later, it drops out. No flame at all. Manometer on outlet of valve never moves from zero after the gas valve opens. Heat exchanger is clean. Intake/exhaust are clean and free of blockage (the previous month had us cleaning rocks and a McDonald's bag out of the exhaust). I even completely disconnected the intake/exhaust from the boiler. You can smell gas when the blower starts, but no flame. I switch the gas valve, and the control board from the working boiler. Same problem. And the good boiler still works. Then I switch the igniter and flame sensor between boilers. Start the bad unit up, and it fires without a hiccup. Unit cycles. DHW is back on for now until we can troubleshoot the bad parts some more. I go to the previously working boiler with the now "bad parts" expecting to see an ignition lock-out due to all of the faulty parts I just swapped into it. Jump it out to call for heat. Fires up. Cycles. Forehead smacked. Questions unanswered. Confusion setting in.
What am I missing?
What am I missing?