I have a 4 zone system. I want to put 4 new thermostats on the system. I wish I looked at the wires before I removed them all! So dumb. Anyway..

  • There is an air handler in the attic.
  • A compressor outside.
  • A cast iron oil boiler for the 4 loop radiant baseboards.

Would these 2 statements be correct?

Each thermostat will have 2 (18-5) wires coming out of it. One of the wires will lead up to the air handler and one of the wires will lead to the valve that opens for hot water.

Each thermostat will need a "power" wire from either the air handler or the boiler.

Thank you!

1 Answer 1


If you originally had 4 thermostats, each controlling 1 zone, then the normal way for wiring was that a thermostat opened and closed each zone valve and when the zone valve opened an end switch made and allowed the boiler to fire. Zone valves can be either 2 wire or 3 wire hook-up. Most are 2 wire but there are some 3 wire available. AS I said, that is the normal way. How yours were hooked up is anybodies guess. Now for the air-conditioning system; one of the thermostats could control the A/C, or there is a separate thermostat for that. Does the air handler have a hot water coil mounted in it or is it just for air-conditioning? Check each thermostat you removed and determine how many wires were hooked to each thermostat. You may have to trace all the wiring to see where the wiring went and how the connections were made. I would suggest that you consult an HVAC company in your area that has a good reputation to help in case this is above your area of expertise

  • The air handler does not have a hot water coil. Would each of the 4 thermostats have 2 wires coming out. One will trace to the hot water zone valve and the other to air handler? Commented Oct 6, 2017 at 14:36
  • If the thermostat has 2 wires only then the wires would probably go to the zone valve, or the device that powers the zone valve. You probably should trace out each run of wires from each thermostat and see where they go and where they connect... It is impossible for me to trouble shoot your job from 30 miles north of Pittsburgh, Pa. AS I said in my previous statement each thermostat should control 1 zone valve and one of the stats could also control the A/C. If you can't figure it out you need to ask a local expert for help.
    – d.george
    Commented Oct 6, 2017 at 16:39
  • Thank you! I guess what I am trying to ask is if there is ever a case where separate 18-5 wire bundles will go to different areas from 1 thermostat. Example a thermostat has an 18-5 wire coming out of it that runs down to the heat. AND there is a second 18-5 wire going to the AC Commented Oct 6, 2017 at 17:00
  • BTW I don't have the old wires anymore. I removed them during demo. I regret not looking at them first. Commented Oct 6, 2017 at 17:01
  • 1
    You can run the furnace and A/C with 18-4 thermostat wire. So if one of the thermostats has more than 2 wires that one probably ran both the heat and A/C.
    – d.george
    Commented Oct 17, 2017 at 12:10

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