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Air Handler Suddenly Wont Shut Off

jeffceo Member Posts: 27
Hi Everyone,
Our problems started after a rare winter thunderstorm came through. There was a lighting strike pretty close by. I noticed the Nest E showed an error "no power on R wire" even though I was getting 27v at R&Y1 and R&W1. Called Nest and they said the base is bad. I also noticed the heat was not shutting off. We have a Laars Endurance propane boiler that feeds a First Co. MBXB air handler in the attic. A second zone is baseboard hot water downstairs but rarely used.

Luckily I had an extra brand new Nest E handy. I swapped out the base and tried both Nests on the new base. Both were reading voltage now. I still could not get the boiler or the air handler to shut off. The 4 tstat wires come from the First Co. air handler in the attic. From there, a wire runs to the boiler. It looks like the tstat controls the air handler which then controls the boiler or AC in the summer.

I had to turn off the circuit breaker to the air handler which then allowed the boiler to power down. It seems that the boiler is still working for domestic hot water. I'm leaning towards the First Co. air handler control board CB201 being the culprit but I'm not an expert. Anyone have any ideas? Thank you!


  • SuperTech
    SuperTech Member Posts: 2,127
    First thing I would try is shutting off the power to the air handler and hooking up the blue thermostat to the common terminal on the thermostat. Connect it to the brown (looks black in the picture) wire sticking out of the First Co. If the problem continues try to disconnect all of the wires from the thermostat. See if the fan shuts down when you switch it back on after power is back on. If it doesn't the board is ok and the Nest is the problem. Either way I'd ditch the Nest thermostat, nothing but problems.
    HVACNUT Member Posts: 5,765
    Yeah I would say theres fused contacts in the board in the A/H.
    You will see 24v at R-Y or R-G when your checking at the sub base. But you should be checking between R and C. But alas, C is not connected. The voltage your seeing is creeping back through the fan relay and A/C contactor coil(s). That's what powers the thermostat (when nothing is running) because theres no Common connected.
    You need to connect the Blue wire to C on the Nest and to the Brown (Common) low volt wire at the A/H. That will power the Nest properly.
    What brings the fan on for heat? The circuit board, aquastat at the hydro coil?
    The two purples are the isolated contacts for the heat relay at the boiler. You could remove a low volt wire from the zone relay at the boiler to shut the heat off if needed. The fan will continue to run though.
  • jeffceo
    jeffceo Member Posts: 27
    Thanks so much guys. Great info, I really appreciate it! I flipped the breaker back on with the thermostat removed. The air handler kept running even without a thermostat hooked up. I would imagine it should be off until a tstat calls for heat.

    I think the tstat is what brings the fan on for heat. It signals the board which then signals the boiler relay. The paperwork for the First Co. says there is a 45 second delay and then the fan turns on.

    I will hook up the blue to common on the tstat and the brown wire on the A/H. I was able to pick up a new CB201V control board this morning so I will probably try swapping that out next.
  • pecmsg
    pecmsg Member Posts: 4,704
    NEST has a huge amount of issues like this.

    Get a different stat! Preferably supplied by a contractor not a big box store!
  • jeffceo
    jeffceo Member Posts: 27
    Solved! Thanks guys for your help. I replaced the CB201 control board on the air handler and replaced the Nest base plate. I also connected the blue tstat wire to the brown common on the A/H to properly power the Nest. It seems that the thunder storm took out the Nest base plate and the control board. I know some of you guys are not crazy about the Nest. Mine ran fine for 5 months until the lightning. I'll keep an eye on it though and switch to a pro grade tstat if needed.