troubleshooting the thermostat/relay/circpump triad

Discussion in 'HVAC Heating & Cooling' started by howner133, Feb 6, 2007.

  1. howner133

    howner133 New Member

    Feb 6, 2007

    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,
    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 ?

  2. Rancher

    Rancher Guest

    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.

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  4. Rancher

    Rancher Guest

    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...

  5. Stringer

    Stringer New Member

    Mar 15, 2007
    Air in the system will stop water dead in it's tracks
  6. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Sep 2, 2004
    Retired Systems engineer for defense industry.
    New England
    I'll second the air. Try bleeding the errant baseboard units and see if that helps.
  7. rprint04

    rprint04 New Member

    Oct 31, 2007
    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.
  8. CHH

    CHH New Member

    May 25, 2007
    Denver, CO
    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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