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#1 Posted : Tuesday, June 16, 2015 4:54:48 AM(UTC)

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I have an ancient Trane furnace/ac unit (inside gas furnace with cooling evaporator above that, outside air conditioner/compressor unit). The inside box(es) have the air handler/blower - that blower's motor is switched by a 24VAC relay driven off a transformer that also sends 24VAC to the thermostat and outside contactor (relay) coil.

Anyway, this all works. Can turn on the fan switch on the thermostat and the blower runs, can turn on cooling and the outside contactor does its thing, the whole nine yards.


The 24VAC relay's coil (standard 90340 style: image search for "90340 relay" to see lots of images of it) gets extremely hot - WAY too hot to touch. So, being naive and figuring there might be some sort of internal short in the relay's coil, I just replaced it.

The replacement relay is getting WAY too hot to touch.

The specs for the relay say it's 10VA for the coil. I believe that (~6W?) suggests the wattage could be enough to heat up the thing, maybe quite a bit.

But still...

Is this extreme temperature normal behavior for an otherwise working fine air handler relay/relay coil?

If not, where could I look to find what's causing the current draw/heating through the relay's coil? (I removed the relay, cleaned its plugs, and replaced it - same heat; I also checked the voltage across the coil and it was ~24 VAC so no short from wall voltage/current)

One of the next steps (to just prove to myself that this IS normal) would be to simply unplug the relay and wire JUST its coil to the transformer to prove that it is indeed the coil that's causing the heat (that is, disconnecting the relay completely from everything other than the coil connection).

But I thought I'd check here first.

Anyone seen this? Anyone have a quick answer? Procedure?

Thanks very much for any feedback.
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#2 Posted : Tuesday, June 16, 2015 4:11:34 PM(UTC)

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Check the voltage output of the low voltage transformer. IF you have an amp meter place on low scale and put in series with one of the relay low voltage (control) terminals and see what it is reading. IF high, in some cases when the transformer gets old or has been replaced with a transformer of less VA (voltamps) than the original it causes relays and other items dependent on the low voltage to also draw more current = more heat. Just my .02 and good luck..
#3 Posted : Tuesday, June 16, 2015 6:07:23 PM(UTC)

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Just a followup that may end the thread, though feedback from anyone who has noted this excessive heat themselves is certainly still appreciated.

Anyway, to answer the above reply, the voltage out of the transformer to the relay's coil is correct - 24-ish VAC.


I unplugged the relay, set it on a nonconductive surface, and attached jumpers from the transformer to the relay's coil connectors. I then turned on the air handler power (though only the transformer then got power because the relay wasn't plugged in, disconnecting everything else from the mains), the relay made a significant clunk as it engaged, and then I let it sit like that for about 15 minutes. When I came back the relay was, again, WAY too hot to touch/handle.

So that's that. The heat is totally being generated from the relay's coil (since all other relay connectors were disconnected since it was unplugged). It didn't matter which of the two relays were checked (ancient or new), they both got too hot to touch. From these two datapoints (huge dataset!), I'm concluding that this is indeed normal behavior for the 90340 relay being fed directly from a 24VAC transformer.

Of course, if you know otherwise or have that experience please correct me.
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