Another “rigging up a smart t-stat” post

ForcedWhaaat

I’ve got the most basic of basic: two wire heat only, old boiler with no board/panel. The system is powered from a 24vac transformer with the thermostat, LWCO, pressuretrol, and gas valve wired in series on the hot side.

I believe, because there isn’t constant power going to the thermostat (i.e., if the pressuretrol cuts out, so does power to the thermostat) I will need to add two wires (C for neutral and I think HC for hot). Is this correct? Can I simply tap each side from the transformer, or will I need a separate transformer?

Edit: Should the hot wire from the transformer go straight up to the thermostat? Should the safety switches be in series on the neutral (W) side, not the hot (RH) side?

Dave T_2

One 24v terminal from the transformer should be in series with all the components you mention JUST BE SURE the thermostat is first in the series. The other 24v terminal can now be considered the 'neutral' and connects to the gas valve AND connect to another (new) wire up to the common on your thermostat. You will have constant power to thermostat because it is straight from transformer before any limits and the additional wire from transformer to thermostat common completes the ac circuit.

Not sure what thermostat you have but the 3 terminals you connect to are probably R(h) = wire from one side of transformer, W = wire back to continue series through limits, and C = new wire to connect to other side of transformer.