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Construct a simple OR gate using two HVAC fan relays

Nidale's Avatar

Join Date: Mar 2018
Posts: 1

03-13-18, 02:44 AM   #1  
Construct a simple OR gate using two HVAC fan relays

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?(I've read an article about relay in an electronic blog called kynix blog) 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.

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beelzebob's Avatar

Join Date: Oct 2015
Posts: 447

03-13-18, 03:36 AM   #2  
The 24 vac for heat and cold may not be independent . Try removing the 24 vac to the cool relay and see if the aux blower operates correctly. If yes, repeat for the heat relay. If no, you need to get a schematic of the 24 vac circuit to look for sneak current paths and try to isolate it.

ibpooks's Avatar
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Join Date: Feb 2005
Posts: 10,236

03-13-18, 06:43 AM   #3  
Are you using programmable / smart / wifi thermostats? Sometimes these power themselves by bleeding a small amount of current on the HEAT line, and it's possible there's enough to keep the relay latched. If so, perhaps try the setup with a mechanical thermostat or some rigged up toggle switches to ensure a complete open of the signal circuit.

Good luck with your project!

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