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three wire hydronic systems

I was looking at 24 volt thermostats recently and noticed that they work on just about every heating system except for "three wire hydronic systems" and multi-stage heat pump systems. Why the exclusion on three wire hydronic systems? What type of thermostat will work on such systems?

Dan R


  • Most other types of heating systems operate on the simple "thermostat's a switch, turn the heat on when it closes and off when it opens" concept. It doesn't really matter what the equipment is, as long as it can deal with being switched by a 24-volt signal, this standard type of thermostat will work.

    The three-wire hydronic system is different. It involves a zone valve that's powered to open and powered to close. The thermostat can't be a simple normally-open switch anymore, it's got to be a double-pole switch, where the normally-open contact is wired to drive the zone valve open and the normally-closed contact is wired to drive it closed.

    There are thermostats available specifically for this application. You can also use a standard 24-volt thermostat on this type of system as long as you've got a relay between it and the zone valve. The relay plays SPDT switch for the zone valve, and is controlled by the thermostat, converting the three-wire system required by the valve into two-wire. As far as the thermostat's concerned now, you've got a normal two-wire system.

    Multi-stage heat pumps are a different story, and in order to work with them correctly, you need a thermostat capable of driving the stages. Normal thermostats can be thought of as only capable of handling a single stage of heating.
  • carol_3
    carol_3 Member Posts: 397

    You're in luck. Honeywell has stats that will do BOTH of the things you're looking for. Check out the TH6220 and TH8230. You just go into installer set-up and tell the stat what you want it to be. The TH6110 and TH8110 will do the 3-wire, but not multi-stage heat pump. You can go on line and look at the specifications at http://customer.honeywell.com. Scroll down to the bottom right corner and enter one of those numbers at a time in the little white box. Then click on installation instructions when the list of literature comes up.
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