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Relay problem or something else?

One of the three zones in my gas-fired hydronic heating system, which is roughly 8 years old, has begun to work erratically. It will heat properly and cycle when set manually to hold a temperature, but in program mode it now pretty invariably fails to start. When this happens, the 24V mechanical relay in the system becomes very hot.

Components: Taco SR503 3 zone switching relay, Totaline 24V relay, Honeywell RTH6400 series programmable thermostat, First 36 MBXR 3HW heat exchanger/air handler.

Diagnostics already done:

1) replaced Totaline 24V relay with identical component (P2830340)

2) replaced previous programmable Honeywell thermostat with current model (RTH6400D). Also, air handler fan starts immediately when set to run continuously at thermostat.

3) exchanged electronic Taco relay modules between working (zone 1, hot water heater or zone 2, second floor heating) and non-working zones (zone 3, 1st floor heating). There are no obviously fried components in the relay box, though I haven't removed the circuit board to look its backside.

Any suggestions where to go next?


  • Strange

    Measure the voltage at the Totaline relay under both scenarios.
    Often wrong, never in doubt.
  • MartyS
    MartyS Member Posts: 3

    I have made some progress, thanks much to Alan’s very logical suggestion of doing diagnosis with a voltmeter rather than just a screwdriver. But I am still in need of some help.

    1) The (incomprehensible!) difference between manual and programmed call from the thermostat is not consistently reproducible: neither mode reliably starts the furnace blower reliably for more than 12 hours.

    2) The problem zone will hold temperature for 4-12 hours, cycling properly during that time, but eventually fails to run the blower.

    3) Measured voltages at the 24V Totaline (normally open, DPDT) relay are as follows.

    a. With no call for heat, the thermostatically controlled voltage across the coil (terminals 7 and 8) is 0 VAC and the voltage across terminals 1 and 3 (to the SR503 switching relay) is 26.6V.

    b. With a call for heat, the thermostatically controlled voltage across the coil (terminals 7 and 8) is 26.5 VAC and the voltage across terminals 1 and 3 (to the SR503 switching relay) is 0 VAC.

    c. The coil resistance with all leads disconnected is 18.3 ohms (spec is 19 ohm).

    d. With a call for heat, the SR503 relay also properly supplies 120V to the Taco circulating pump for the zone.

    4) There is no difference in these voltages whether the blower is cycling normally or failing to start.

    5) When the thermostat calls for heat but the furnace blower motor fails to start automatically, I can reproducibly (more than 5 consecutive attempts) get a start within 1 minute by transiently disconnecting and reconnecting the leads to the Totaline 24V relay (coil terminals 7 and 8). The system then cycles normally for 4-12 hours before the problem recurs.

    Should I be suspecting a problem within the First furnace/air handler? Is there a possibly problematic relay there? Would a thermal cutout on the blower motor explain this? Would it be a useful diagnostic to set the blower manually to on and see if it stops after several hours?

    Sorry if these are naïve questions, but I’m a simple homeowner and not an experienced HVAC person.
  • Tim McElwain
    Tim McElwain Member Posts: 4,506
    Marty I suggest

    you have a professional look at this system before it costs you more than you want. There may be a problem with the hydro air unit (air handler) communicating with the boiler system.
  • MartyS
    MartyS Member Posts: 3
    edited January 2010

    Thanks very much for reading, I will follow up when there is an answer to this question.
  • geno54
    geno54 Member Posts: 43
    Where to look

    If you are getting hotwater to the coil consistently then I would look at the air handler. Possibly a sticking relay or aqua stat  may not be bringing on the blower motor
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