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Thread: troubleshooting the thermostat/relay/circpump triad

  1. #1

    Default troubleshooting the thermostat/relay/circpump triad

    Hi,

    I have a boiler unit in my house that heats a 3 zone baseboard heating system.

    One of the zones is only getting intermittent heat and I'm having trouble
    sorting out whether the culprit is the thermostat, relay or circ pump.

    I checked the other zones' thermostats and it seems that the "command" for heat on is
    to short the leads, so I disconnected one lead from the thermostat and shorted
    the low voltage leads on the relay box. This yielded the satisfying clunk of
    the solenoid engaging, the slight hum of the suspect zone's circ pump and the
    zone's feed pipe getting hot.

    So, I thought "done" - the culprit was the thermostat.

    However, I left the leads shorted for about 1 hour while I cleaned up all the
    periphery and tended to other things. I came back to find the feed pipe to
    that zone again cold. The boiler was running and the solenoid still responded
    to short the low voltage leads, but it was clear (at least to me) that the
    circ pump wasn't moving water.

    So, I see these possibilities to explain the intermittent behavior.

    1) the solenoid is physically engaging but the relay isn't reliably
    providing power to the circ pump,
    or
    2) the circ pump is near dead and intermittent.

    This is an old system, and I don't know what type of voltage is going through
    the relays to the circ pumps.

    Any hints on how to sort this out ?

    Thanks,
    hh

  2. #2
    Rancher
    Guest

    Default

    The thermostat probably doesn't directly turn on the pump and the zone valve, it could the zone valve but if it went directly to the pump then it would be cross connected to the output of the other thermostats and therefore turn on the other zone valves, and to be honest with you I don't remember if the zone valves I use have a separate output to turn on the pump. So look for a relay controlled by the thermostat that controls the pump, or a separate output from the zone valve that would control the pump.

    Good Luck.

    Rancher

  3. #3
    Rancher
    Guest

    Default

    OK to follow up, I went and looked at the instructions for my TACO zone valves and they have 3 terminals, one of them goes to your boiler control box to hook to one of the "T" connections, to turn on the relay to the circulation pump. So it could be a problem with your zone valve...

    Rancher

  4. #4

    Default

    Air in the system will stop water dead in it's tracks

  5. #5
    Retired Defense Industry Engineer jadnashua's Avatar
    Join Date
    Sep 2004
    Location
    New England
    Posts
    21,405

    Default

    I'll second the air. Try bleeding the errant baseboard units and see if that helps.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  6. #6

    Default re circulator pump relay

    I seem to have a similar problem with my oil boiler with white control. The relay rattles but does not stay engaged and therefore the circulator does not stay on. need to know what provides the voltage to the solenoid? There is also a small transformer in the unit.

  7. #7
    DIY Senior Member CHH's Avatar
    Join Date
    May 2007
    Location
    Denver, CO
    Posts
    225

    Default

    Is this an ice cube relay? If it is then change it on general principles.

    A little more seriously, the circulators should be 110v and just about everything else should be 24 vac.

    It sounded like everything was starting up ok (you said you could feel heat in the supply side, could you feel heat in the radiators?). If it's shuting down mid-cycle you'll have to wait for that to happen and then see where either the power stops or a valve closes (if it has zone valves) or a circulator shuts down. If they are Taco circulators the power ends can be swapped with the power end from a good zone to see if that eliminates the problem. Cannibalizing is sort of a last resort but it is an option.

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