Slant Fin GXHA-160 EDPZ Common Wire

blazn420

I have a one pipe steam heating only system, I want to upgrade to a Honeywell T9 mainly for the remote sensors. Currently my thermostat is connected with 2 wires, there are several wires that are not in use, I want to use one of them to connect the C wire. I don't know where to connect the C wire, the only place that looks like I can connect it to is the transformer on the other side of the power cut off switch. Any ideas?

Steamhead

That might work, but then it might not.

The problem is, when one of the safety controls- the pressuretrol and low-water cutoff- trips, it removes the 24V feed to the thermostat circuit. Some thermostats have enough backup battery power to ride this out, others not.

I like to install a completely separate transformer-relay unit such as a "fan center"- Honeywell's R8225 and R8239 are good examples. The versions with SPDT switching are all you need. In a 3-wire cable, Red goes from the transformer to the thermostat and Green goes to the transformer common terminal. The thermostat sends 24V via the White wire to the G terminal on the unit, which operates the relay. The relay switches the boiler's thermostat circuit. So as long as there is power to the transformer, the thermostat receives 24V to power the electronics.