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I have constructed a simple OR gate using two HVAC fan relays to control a 120VAC/1.5A booster fan for my home HVAC system. I have a zoned system, with multiple thermostats, but only one zone/thermostat controls the booster fan. The thermostat switches 24VAC on one wire when calling for heat, and 24VAC on another wire when calling for cool. The voltage is supplied by the control board of the furnace. When there is no temp call, there is no voltage across either wire to common (ground). The relays are sized correctly; they are typical mechanical HVAC relays, SPST-NO - commonly used in the HVAC industry.

I want the fan to turn on whether calling for heat or cool, hence the OR gate. It seems a simple enough task and circuit. However, I've noticed some strange behavior. Intermittently, when the thermostat ceases to call for heat or cool, the fan does not turn off. So the 120V side of the relay stays closed, even when there is no voltage across the coil. The weird thing is, to get it to open again (and the fan to turn off), all I have to do is touch the metal junction box containing the relays. Note that I did not say 'hit' the relays. I don't have to touch the relays at all. I don't think it is a typical fused contact situation. The relays are new and all I have to do is make contact with the metal box in which the relays are mounted. Everything is well grounded.

My first thought was for a flyback diode (as you would have for any inductive load) but of course this is 24 volts AC, not DC, so a diode wouldn't make any sense. I don't think this is an arcing issue, so the typical MOV solution doesn't seem like the solution, but I may be wrong.

I am not completely certain if the low voltage or high voltage side is 'sticking', but I suspect it is the high voltage side (not sure why I suspect this, though). I know the thermostat is cutting the voltage to the coil. The part that stymies me is that it is intermittent.

Has anyone seen this type of behavior with an AC relay? Can anyone offer any troubleshooting advice? Do I need to add something to the circuit? Are there any suggestions to make this circuit reliably turn the fan on and off depending on the heat and cool inputs?

Here is a quick and dirty schematic, showing only the relevant parts.

enter image description here

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    \$\begingroup\$ install a DPST switch between relay and 24VAC so that you can disconnect the relay coil without touching the box ... also when the relay sticks, flip the breaker at the mains panel and then turn back on to see if removing the power releases the relay \$\endgroup\$
    – jsotola
    Nov 27, 2017 at 2:16
  • \$\begingroup\$ You said the relays are properly sized, but just to be sure.. what are the contacts rated at? ANd the low voltage side can't stick, not in the relay. \$\endgroup\$
    – Trevor_G
    Nov 27, 2017 at 2:53
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    \$\begingroup\$ Are you sure there is no demand to the relay at that time. Many systems make the fan run longer than the heat/cool system to flush heat or cold from the heat exchanger. \$\endgroup\$
    – Trevor_G
    Nov 27, 2017 at 2:59
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    \$\begingroup\$ Yes, the heat and cool inputs are coming directly from the t-stat. The inputs are high if and only if the t-stat is calling for heat or cool, respectively. I knew that should be the case in theory, and I confirmed it with the multimeter. So when the temp is satisfied, the inputs immediately go low. \$\endgroup\$
    – Adam
    Nov 27, 2017 at 5:10
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    \$\begingroup\$ This is the description listed for the relays: Heavy Duty Fan Relay, 24V, 4 Terminal, 50/60 HZ, SPST-NO with multi-positional mounting bracket. This relay has a contact voltage of 125/250/277VAC, coil voltage is 24VAC and the control style is SPST-NO. The amp FLA is 12/8/7, amp LRA is 60/48/42 and the amp RES is 18A \$\endgroup\$
    – Adam
    Nov 27, 2017 at 5:21

3 Answers 3

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I have seen new 16vac electric door strikes come with a copper leaf. The instructions say "If the solenoid sometimes sticks in the actuated position, place the leaf over the pole armature face."
I guess the pole piece holds a bit of magnetism against the return spring. Maybe look at the problem from a magnetism viewpoint.

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    \$\begingroup\$ Copper is not magnetic so your theory can't be correct. It's more likely that the copper introduces a gap to weaken the holding force so that any remnant magetism is proportionally reduced to a value below the holding value. \$\endgroup\$
    – Transistor
    Nov 21, 2018 at 7:17
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    \$\begingroup\$ Copper supports eddy currents and has surprising effects on magnets. Please look at youtube.com/watch?v=sENgdSF8ppA. \$\endgroup\$
    – John Canon
    Nov 21, 2018 at 15:36
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Sounds like residual magnetism. Putting a piece of electrical tape between solenoid and plate should be enough to keep them from sticking.

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Where common wire comes from?

Most thermostats do not have common wire, which forces DIYers to rummage in the furnace box for the other side of 24VAC transformer. Then if you connect relay to those wires it will be in parallel with the motors, which can cause all kinds of weird behaviour and potentially damage the relays when power removed.

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