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Nest Thermostat and Steam Heat
I have a one-pipe steam system and recently purchased a Nest Thermostat e. I had two wires (R / W) from the T-T connection to the thermostat, no common wire. Of course, I awoke one day to what I believe is my gas valve opening and closing (loud clicking).
Understanding this was the thermostat trying to steal energy, I decided to tap off the 24V transformer's common since I had an unused wire. This seemed to work well until the boiler went off on pressure. At that point, it seems all power to the 24V transformer is turned off (I learned this nuance) until the pressure goes down and it safe for the boiler to start making steam again. The Nest thermostat indicated power was lost this time (because of the common wire) but it resets itself in a funny way. Prior to going off on pressure, the thermostat indicated room temperature was 67 degrees with a 68 degree setpoint. When it returned from the no power state, the thermostat said the room was at 64 degrees (which was incorrect based on another thermometer I grabbed).
You can see where this might be going. A cycle of boiler reaching pressure, going off, thermostat resetting, incorrect (low) room temperature reading, build up steam, cycle off... keep heating up the room. When the room reached 72 degrees, I turned the thermostat off.
Two questions. Is it normal for the 24 volt transformer to turn off completely when the pressuretrol upper limit is reached? Second, could I make the Nest Thermostat e work by puchasing a 24 volt transfomer and running a second set of wires - one to the R terminal and one to the C terminal. Essentially having two wires running to the R terminal on the thermostat.