The diagram below shows the control circuitry of a heating system. I am trying to understand why the wiring depicted here caused the fan relay (PC90-360Q) to 'chatter' constantly when powered on.

Like many HVAC systems the controls operate on a 24 VAC signal which is produced by a transformer stepping down from 120V. Various equipment such as pumps & fans run on 120V also and so relays are used to mediate between the controls & the devices.

The seemingly relevant part of the intended operation was as follows:

  • System is powered on
  • Thermostat detects heat is needed; closes its internal switch
    • Red / green path is energized through the fan relay coil
    • Water pump relay is also energized (via one of the zone sections on the bottom, all actually wired as in the blue example)
  • Fan relay closes its 120V contacts, energizing fan motor

Now what actually happened was that when the thermostat switch would close, the fan relay would 'chatter' seeming to switch on/off fairly rapidly.

The problem was not the thermostat itself - replacing it with a jumper wire produced the same result.


  • The control voltage was actually about 26VAC, but that seems to be within tolerance as far as I am aware.

  • The controller ZVC406 is a self-contained board which normally just operates thermostats, valves, & pumps. Controlling the fan was an additional requirement necessitating the addition of the relay in question.

enter image description here

FYI I have actually solved the issue by instead wiring the relay between the switched output of the thermostat and the return pole of the transformer (i.e. a parallel path instead of series). So this question is really just for my own understanding.


1 Answer 1


it's chattering because the thermostat loop already has a relay in series with the supply so there is not a full 24V when you interrupt that circuit.

Undervoltage makes relays misbehave.


simulate this circuit – Schematic created using CircuitLab

the parallel connection you have used is one solution, another could be to connect the fan relay in parallel with the solenoid output, but then the fan would only run when the heat was active.


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