Since I installed a Nest-E I've seen my Weil-McLain boiler cut power to the R-wire and C-wire intermittently.

Voltage drops to nearly 0 and the Nest-E errors with E-195 no power. Other times thermostat works fine to control heat.

I'm trying to figure out if the transformer is bad or if the Nest is just not a good fit for this system.

How do I know if the transformer is going kaput?

  • What model is your boiler, or can you post a wiring diagram for it for that matter? – ThreePhaseEel Jan 14 '18 at 19:35
  • Weil McLain, EG-55-PIDN. All I have going from boiler to thermostat is a R, W, and C wire. No cooling in this 100 yr old house :) – Bill Jan 14 '18 at 19:59
  • Can you make sure the thing isn't cycling on a limit switch? (The LIMIT light should come on if all is well during a call for heat. If the LIMIT light on the control turns off when the R wire gets power cut to it, then that's your problem) – ThreePhaseEel Jan 14 '18 at 20:22
  • Also, how is your boiler configured? (Hydronic vs hydronic + DHW coil vs steam?) – ThreePhaseEel Jan 14 '18 at 20:31
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    Your boiler probably lacks an integrated control, which means that the limit switch is downstream of the thermostat most likely (check it anyway though). Check the LWCO as well, too -- it is upstream of the thermostat in the "stock" steam configs on the Weil-McLain EG series. – ThreePhaseEel Jan 14 '18 at 20:52

The voltage drop I saw was due to the triggering of the low water cut off system. When the LWCO kicked on it dropped power to the thermostat. Thank you for the tip @ThreePhaseEel !

Gotta get some of those leaky radiators fixed.

As a side-note, the Ecobee handles the power drop better than the Nest. I tried two different Nest-E units and they both read 5-6 degree lower ambient temps while recovering from the power drop. The Ecobee stayed constant.


If it is severing the R wire in response to a limit situation, that means the furnace's wiring is not compatible with the Nest.

The furnace is designed to have a simple mercury make-or-break 'stat. Interlocks are also make-or-break, so your furnace has simply placed them all in series without caring about sequence. As a result, some of those interlocks are interrupting the R line when they should be interrupting the W line.

Now, if your system also had air conditioning or auxiliary heat stages, they would also be shut off by an interlock that severs the R line. Study the system carefully to see if that is intentional/desired. If it is not, the furnace should be rewired so those R-line interlocks instead interrupt the W line, and nothing interrupts R. That will make it compatible with the Nest.

If C is being interrupted, them you got C from the wrong place.

  • The C is being interrupted as well. Dang, sounds like it is not wired correctly then. I didn't install the C, but when the HVAC guy comes back I will bring this up. Much appreciated @Harper. – Bill Jan 16 '18 at 2:10

The transformer typically will either work or will not. I suspect that you have a loose connection somewhere that is making and breaking intermittently.

  • Ok, appreciate that insight. Will check along the wire path and the board. – Bill Jan 14 '18 at 20:04

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