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Old 02-13-2012, 03:05 PM   #1
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Residential Furnace relay module sparking


Hello,

I have an older Lennox heat pump furnace with a circuit board which has inputs from the thermostat and outputs to the run the blower, heatpump, etc. Recently, I have heard clicking from a relay on that circuit board during heat mode. I looked at it and saw the relay lighting up inside (while clicking) I thought I saw a spark from the hot lead on the board going to the heatpump. Is this a symptom of a bad relay, board, or short? The symptom only seems to occur when the thermostat is set over 70 degrees and it forces the heatpump on. Any ideas on how to check the circuit board and/or relay? thanks.

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Old 02-13-2012, 03:46 PM   #2
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Residential Furnace relay module sparking


A relay is just a switch. All switches will spark some when disconnecting a load. If it was functioning properly otherwise, then probably no problem.

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Old 02-13-2012, 04:01 PM   #3
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Residential Furnace relay module sparking


Well, it isn't exactly functioning properly, because I can hear the loud clicking and sparking on and off. Almost like it isn't getting constant voltage. I checked the voltage load from the transformer and that checked ok (~24V). Could bad soldered connection to the board cause this?
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Old 02-13-2012, 04:17 PM   #4
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Residential Furnace relay module sparking


Sparking is normal for relay contacts serving an inductive load but
"a spark from the hot lead on the board going to the heatpump"
is puzzling because it generally takes at least a kilovolt to make a tiny spark through air.
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Old 02-13-2012, 04:28 PM   #5
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Residential Furnace relay module sparking


The spark I saw was at the wire connectin to the terminal strip on the board. At least I think thats what I saw. I killled power to the unit (via a kill switch) after a few seconds of listening the the relay make alot of noise. I then turned the thermostat back down to 65 (in heat mode) and everything ran fine (no clicking of the relay). Later I even heard the heat pump kick on outside, still no loud clicking. The event only seems to occur when you turn up the thermostat to around 70 or above. It is strange.
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Old 02-13-2012, 04:33 PM   #6
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Residential Furnace relay module sparking


Quote:
Originally Posted by bupa94 View Post
The spark I saw was at the wire connectin to the terminal strip on the board. At least I think thats what I saw. I killled power to the unit (via a kill switch) after a few seconds of listening the the relay make alot of noise. I then turned the thermostat back down to 65 (in heat mode) and everything ran fine (no clicking of the relay). Later I even heard the heat pump kick on outside, still no loud clicking. The event only seems to occur when you turn up the thermostat to around 70 or above. It is strange.
I just remembered that a high resistance connection can make a relay oscillate.
It tries to switch on a heavy load, the voltage across the bad connection causes the relay coil voltage to drop, the relay drops out, the voltage across the bad connection drops to zero because the current goes to zero, the control module continues to ask the relay contacts to close, etc.

One analogy is when you try to crank a car with bad battery connections. The starter solenoid oscillates for the same reasons.

Last edited by Yoyizit; 02-13-2012 at 04:36 PM.
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Old 02-13-2012, 08:10 PM   #7
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Residential Furnace relay module sparking


some case with heat pumps there is a second stage sometime try to engerized it and some case with bad thermosat you can have both heat with cooling on ( with electrique back up heaters )

Merci,
Marc
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Old 02-14-2012, 01:40 AM   #8
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Residential Furnace relay module sparking


Clicking and some visible sparking at the contacts is normal for a relay switching a heavy load. Sparking anywhere other that at the internal contacts of the relay is not OK. Chattering of the relay is not OK either. The problem may be loose connections to the relay, or internally in the relay. Can you post pictures, or better yet put a video on youtube and post a link?
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Old 02-14-2012, 07:54 AM   #9
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Residential Furnace relay module sparking


I just tried to attach a picture.
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Residential Furnace relay module sparking-circuit-board.jpg  
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Old 02-14-2012, 08:02 AM   #10
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Residential Furnace relay module sparking


Ok, picture worked. At one time, I traced all the wires from the thermostat to the board and from the board to the air handler and condensor. Last night the heat worked fine 40-50 degrees outside with thermostat set at 68 with no relay chatter. I suspect that if I would have turned the thermostat above 70 that I would have started to hear the chattering.
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Old 02-14-2012, 03:47 PM   #11
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Residential Furnace relay module sparking


After a little more research, I am considering ordering a relay to replace the existing one on the board. I would have to remove the soldered connection and then resolder. If the existing soldered connection doesn't look good then maybe all I need to do is resolder it. Has anyone atempted this?
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Old 02-14-2012, 03:53 PM   #12
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Residential Furnace relay module sparking


Thermostat is only an on off switch. Putting it higher should not make any difference other than it will stay on longer until the room gets warmer.
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Old 02-14-2012, 04:12 PM   #13
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Residential Furnace relay module sparking


I agree about the thermostat. All it does is send a signal to the board. All I know is that so far the problem only occurs when the thermostat is set to heat over 70.. maybe its a bad thermostat. I think I saw where the unit uses first and second stage heating but I am not sure how this functions.
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Old 02-14-2012, 08:09 PM   #14
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Residential Furnace relay module sparking


OK so the problem is with the second stage heating. As long as you only use first stage heating then you probably won't see the problem.

Sounds like the problem could be a poor solder connection. Power off and check the bottom of the board.
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Old 02-15-2012, 02:53 AM   #15
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Residential Furnace relay module sparking


A bad connection to the relay's coil circuit is likely. This could be at the board, or could be inside the relay. Checking the solder connections on the board is a good first step.

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