Thermostat Issue or Boiler Issue?

vinteloz

Hi everyone. I've had a Nest thermostat installed for a couple years without issue. The nest controls my gas fired steam boiler and my air conditioning for my first floor. A few weeks back during the "bomb cyclone" on the east coast, I woke up in the middle of the night to a cold house. I checked the nest app on my phone and the nest was "offline". In summary, the Nest was no longer recognizing power power from the Rh wire. I attempted to troubleshoot without any success. I installed a spare honeywell thermostat that I had laying around and boom, boiler kicks on without any problems. Nest has sent me a new base plate, which didn't solve the problem, and then an entire new display, which has not solved the problem either. Meanwhile the honeywell has continued to work fine. The folks at Nest believe that there is some issue with the voltage coming from the boiler, that something went haywire during the night when it was very cold. Before calling an HVAC expert, wanted to see if this is something I could tackle myself...thinking maybe a fuse needs to be replaced on my boiler? (no idea if I'm heading in the right direction or not - this is very far from my area of expertise). Either way, thanks in advance for any help that can be provided.

Jamie Hall

Is the Nest powered from the boiler's 24 VAC circuit? If it is, there might be a bad connection there. Check for 24 volts at the output of the boiler's transformer.

If the nice old Honeywell is controlling the boiler, it's not in the two principal thermostat wires. Nor is it in the boiler controls themselves.

Fred

After you replaced the baseplate and the Display on the Nest, does it still show "Off line"? Have you checked continuity of that rh wire?

unclejohn

This sounds like a two transformer system. One trans. for the A/C and the other from the boiler. Did you use the RC and RH terminals on the nest. Maybe you got lucky for a few years if you had both trans on the same circuit.

vinteloz

Fred said:

After you replaced the baseplate and the Display on the Nest, does it still show "Off line"? Have you checked continuity of that rh wire?

Correct - still shows an e74 error message - it's not detecting the Rh wire. I have not yet checked the continuity of it - but the fact that the Honeywell thermostat works fine is what is confusing the hell out of me.

unclejohn said:

This sounds like a two transformer system. One trans. for the A/C and the other from the boiler. Did you use the RC and RH terminals on the nest. Maybe you got lucky for a few years if you had both trans on the same circuit.

Only used the Rh wire with the nest....

Jamie Hall

The Honeywell thermostat is "stupid". Which simply means that it knows what it's supposed to do (turn on the heat when it's cold) and does it. Reliably, every time. Funny thing...

The Nest is real smart. And not at all reliable.

That said, check all your wiring. The Nest has to be getting common power as well as the two thermostat wires -- and they all have to be off the same transformer in the end.

EBEBRATT-Ed

Does the nest have RH & RC? Do you have 2 transformers or 1? Maybe the AC transformer was providing the 24 volt power and that transformer went bad or the ac power is shut down

John Mills_5

Never try to understand a Nest! Call on Jan 1, 6 month old furnace, older Nest. Just flat out wouldn't call for heat or fan. Soon as I walked in and saw that hockey puck on the wall I guessed I'd be changing it.

But could be something else...

SuperTech

The problem is that you only have two wires from the boiler to the nest. It needs a common wire.

As a service technician I absolutely hate the Nest thermostat. It's the biggest POS ever made.

I have seen this problem a few times. I would either keep using the Honeywell thermostat or buy a new Honeywell thermostat with WiFi capabilities.

You can still use the crappy nest, but you will need to buy and install a common wire adder. Google it and you will see what I'm talking about