Around the House

Control a 3-wire zone valve with a 2-wire thermostat

I always thought heat-only thermostats needed only two wires (red and white), until this week when I decided to switch from my old school round Honeywell T-26 thermostat to digital programmable thermostats. Off to Home Depot to buy some Honeywell RTH2300B Digital Programmable 5-2 Day Thermostats. After installing one I discovered that it called for heat but never shut it off. That's because the thermostats I bought lack support for the third wire (B, blue) on my Honeywell V2008A zone valves, the wire that closes the valve.