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Is 24 volts coming to thermostat and is Tstat Ok

After checking a zone valve and assuming it was bad I asked members about draining my system to take apart the zone valve since I tried swapping the actuator with a new one to no avail. The problem in the zone started with overheating and ended with no heat. There was 24 volts at the thermostat wall plate but I hadn’t checked the continuity between R and W when the Tstat called for heat and lo and behold there was none so the thermostat for the zone in question was no good. I had two  thermostats  and swapped them and the dead zone now was heated and the former good zone was not. What was puzzling however is why did the bad thermostat start out malfunctioning by causing the zone to overheat? Glad I realized to test the thermostat before I drained the system and rip a zone valve apart. In any case, with heating problems check for 24 volts at the thermostat wall plate and the continuity of the thermostat when it turns on and off first. Can anyone suggest a good simple programmable thermostat for a heat only system or should I ditch the programmable? Smart Tstats might be good but I don’t have wiring for them as yet


  • Jamie Hall
    Jamie Hall Member Posts: 23,635
    A two wire thermostat may be regarded as a simple switch. In fact, that's exactly what the nice old reliable bimetal or mercury ones were. All they do is close the circuit between R and W. In many cases there may be a measurable voltage -- usually 24 volts -- between R and W when the circuit is open (thermostat not calling). There may -- or may not -- be a measurable voltage between either R or W and ground.

    Some modern digital thermostats have a relay which is also a simple switch, but driven by digital circuitry within the thermostat -- some of them battery powered, sometimes otherwise. Others use stricktly solid state switching. In either case there are quite a number of things that can go wrong in the circuitry to cause the circuit to either fail open or fail closed -- of first one or the other and then the other. Programmable thermostats are often in this group. Smart thermostats always need outside power.

    What about the OP's thermostat? Most likely it initially failed closed -- and led to the overheat. Then died completely. They do...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England