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Question about Boiler zone control and thermostats.

So I have an older house with both boiler hot water radiators and forced air (although the forced air is only for the first floor). I've had issues with the upstairs not getting heat, and the zone not having hot water running through it. The part in question is "Honeywell 40003916-048 50/60 Hz Replacement Head for V8043F with End Switch, 24 VAC" which has an auto and manual setting for opening up the zone valve. If I manually open it up hot water flows upstairs to the radiator. However, I noticed that it sometimes works on auto without me manually opening the valve. This morning the upstairs was cold again, so I turned the thermostat up and down twice and the radiators started to get heat again. Think this is a problem with thermostat upstairs (mechanical, I can hear it clicking when the temperature is below the set temp) or the power head on the zone?


  • Jamie Hall
    Jamie Hall Member Posts: 22,952
    A truly mechanical thermostat? It could well be kind of tired. It's not hard to check. On the zone valve, find the two wires from the thermostat and carefully jump between the two terminals to which they are attached. The zone valve should open. Remove the jumper and it should close. If that works, your problem is either the thermostat or the wires, so go upstairs and take the thermostat off the wall -- there should be two wires attached to it. Jump between the two terminals to which the wires are attached, and the zone valve should open (it may help to have a helper to call and tell you!). If that works, it's probably the thermostat. And there are any number of inexpensive, two wire, battery powered, programmable or non-programmable replacements.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Zman
    Zman Member Posts: 7,551
    What is supposed to happen is this:
    The thermostat senses the temp is low and closes the circuit
    Zone valve opens
    When the zone valve is all the way open, it tells the boiler to fire.

    Which part is not happening? If you jump the 2 wires in the t-stat does it work?
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • HomerJSmith
    HomerJSmith Member Posts: 2,408
    Yes, it could be your thermostat. Do as Jamie said. Remove the thermostat and jump the two wires, usually white and red, does that open the valve and fire the boiler? There should be 24V AC between the two wires connected to the thermostat. You can get a cheap volt/ohm meter at your local home depot or harbor freight store.

    If no, leave the jumpers on and go to the valve. It has a lever underneath. Move the lever from auto to manual. Does the lever move easily or is there spring resistance and the lever moves back to auto when released?

    If it moves easily and the boiler doesn't turn on look at the micro end switch inside the box that the lever presses against. Are all the wiring connections good? Take a jumper and jump the end switch connections on the front of the valve. Does the boiler fire? If yes, look for a faulty end switch or the lever not pressing against the end switch. You can manually press the end switch with a small screwdriver to turn on the boiler.

    If no, look for a bad wire connection between the valve and the boiler connection. If you have more than one valve and one works and the other is intermittent that isn't usually the problem as the valves end switches are wired in parallel.

    If the lever moves with spring resistance and the thermostat wires are jumped and there is 24 V AC, then look at the valve motor to see if it is correctly wired to the transformer and valve and the lever not binding.

    Sometimes, the thermostats and valves are wired to the transformer, together, with wire nuts. Check them for a loose connection.