I am trying to install a Honeywell RTH6300B to replace a Honeywell 3-wire thermostat. The wiring on the old thermostat is:

Red wire – R terminal
White wire – Y terminal
Green wire – W terminal

There are no other wires. The system is an oil burner with hot water baseboard heating. There is no air conditioning or heat pump.

When I connect these wires to the same terminals (R,Y,W) on the new thermostat, warm water is passing through the baseboard. This happens even if the thermostat is set to a lower temperature than room temperature or even if the thermostat is set to the “System Off” setting. I’ve had this setup for over a day thinking it might take a little time to figure out what to do.

I don’t want to ruin anything by randomly switching wires. It seems the thermostat doesn’t know how to shut off the heat. I put another one on a different zone in the house and it’s doing the same thing.

Thanks for any suggestions.

Mike

My guess is that your system requires power to the white wire to shut off. The old thermostat was a simple double throw switch, the red wire was always connected to either the green or white wire. The new thermostat will not power the white wire unless switched to cooling. Try it, and see if it shuts down. Addition: If the thermostat has an RC terminal, this won't work unless you jumper the red wire to it. If so, you can make it work by installing a double throw relay. Connect the red wire to the com terminal, green to NOC, and white to NCC going to the boiler. Then connect a second red and green going to the R and W terminals of the thermostat. Connect that green to one of the coil terminals of the relay. Connect the other coil terminal to the commmon side of the transformer.

The convention is to connect the white wire to W terminal and to the gas valve and relays that start the boiler. You may want to go through and switch the green and whites. I would instal the relay at the boiler. If you don't want to try to buy a relay at a HVAC or electrical supply, Radio Shack has them.