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Three wird hydronic thermostats

I occasionally work in a big-box store in the Northeast. Thermostats are quite popular with customers now. Everyone seems to want to install their own. One question that continually arises is "What do I do if I have a three wire hydronic heating system in which most thermostats will not work? Most household thermostats seem to work for most heating systems except the three wire forced hot water system. Is it because a forced hot water system requires a wire for the burner, another for the zone valves and another for the circulator pump? I suggest the customer call a pro. Nevertheless, I am curious about the reason most thermostat will not work on a three wire system.



  • carol_3
    carol_3 Member Posts: 397

    A three-wire stat is needed only if there is a three-wire zone valve. If there is a 3-wire zone valve, it needs switching from the thermostat to open the valve and to close it. So the thermosat has to be a single pole, double throw switch, and that's 3 wires. Most zone valves are two wire, and power only to open. They spring close. So an ordinary thermostat works for them. One model of the Honeywell T87 comes with a 3-wire backplate that converts it to 3-wire.
  • Dan_29
    Dan_29 Member Posts: 111
    three wire stat

    Thanks for your reply.

  • About the only three wire

    zone valves still around are made by White-Rodgers 1311-102 or 103 or 104.
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