I own a Fahrenheat FUH724 (7500 watts) electric heater for my 2 car, uninsulated garage. I installed this unit 12/2017 along with a Nest thermostat. An Aube rc840t-240 relay was used to step down the 220v to 24v for the Thermostat to properly work and all appeared to work properly until about 3 months later when I began to experience malfunctions and this is where I need the collective help from an informed community found here.


  • 40A dedicated circuit breaker
  • 8awg wire.

The electrical connections to the contactor became the point of failure. The OEM wires within the unit were overheating, failed and ultimately broke off and the internals of the contactor revealed signs of malfunction. I contacted the manufacturer, replaced the defect wires, and contactor also checked for loose connections throughout. All seemed acceptable. 4 weeks later, same failure points.

I contacted the manufacturer again and they sent me a replacement and I have yet to install. Could the Aube relay be causing this much heat buildup within the wiring to cause the malfunction? While trying to diagnose the problem, I took temp readings at various connection points with and without the relay in place and the temp at those points didn't change so I'm not sure that is the root of the problem.

I'm also questioning whether or not I may need supply wire exceeding 90c?

Sorry for the long-winded question but I felt it was important to describe the problem with all detail i've experienced. If i've left out any important info please ask away.

See attached picture for installation.

enter image description here

enter image description here

  • A brief web search for Aube rc840t-240 reveals that it can't handle 7500W, but maximum 22A . – Marko Buršič Oct 11 '18 at 18:45
  • When you refer to the "contactor", are you referring to the Aube relay or something else? – brhans Oct 11 '18 at 18:55
  • Do you own a soldering iron ? You could replace the internal relay with external solid state relay (SSR) – Marko Buršič Oct 11 '18 at 19:17
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    The sole purpose of the Aube relay was to allow connection of a 24v thermostat rather than use the internal line voltage stat and is connected to the OEM contactor which is rated at 35A per pole. – Jamie McGannon Oct 12 '18 at 13:55
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    @MarkoBuršič I'm not using the Aube to power the heating element but rather to energize the contactor. – Jamie McGannon Oct 16 '18 at 16:28

You're overloading the contactor. 7500 watts at 240 volts is 30+ amps, and the contactor you're using is rated for 22 amps.

You have three choices:

  1. Get a different contactor.
  2. Follow the manufacturer instructions to change the wattage of the heater to 5000 watts or less.
  3. Use the internal thermostat of the heater.
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    The OP is using the Aube relay to control the heater's existing internal contactor, not to switch the heater element directly. – brhans Oct 14 '18 at 22:10
  • exactly @brhans. Basically, I need the same functionality as the Aube but capable of handling the high current of the 7500 watts – Jamie McGannon Oct 15 '18 at 13:57
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    @longneck, The Aube is controlling a contactor. – Jamie McGannon Oct 15 '18 at 15:39
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    @longneck red wire is connected to the coil terminal. Sorry for the poor picture. I was wondering about the contactor rating as well but I'm not knowledgeable enough to speak to the design. The heater is built by Marley which if it was a problem I'm sure others would be having issues as well. – Jamie McGannon Oct 16 '18 at 16:33
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    @longneck see attached photo, appears to be rated 35A per pole! – Jamie McGannon Oct 16 '18 at 16:38

Remember your equations Jamie, It helps out quite a bit

enter image description here

Since you have a Double pole (120×2)

120 Volts * 20 Amps = A single 20A breaker is capable of producing 2400 watts of power

120 Volts * 40 Amps = A single 40A breaker is capable of producing 4800 watts of power

240 Volts * 40 Amps = A Double pole 40A Breaker is capable of producing 9600 watts of power

Your contactor says It's rated at 35 Amps per pole

So 120 Volts * 35 per pole = is capable of taking in 4200 watts of power on each pole adding up to 240

So imagine 9600 watts of power being supplied to a contactor that can only take in 8400 watts of power

Better yet try imagining a hose full of water, If one person stands on the end of the hose and the other person behind the house squeezes the hose, The guy at the end is going to get hot and bothered and say what's going on and give up. Your contactor got hot and gave up

  • Although this is interesting, this doesn't answer the original question. – Daniel Griscom Nov 17 '18 at 21:41
  • Long neck already gave an accurate answer, this just helps him understand why it heated up. – user70085 Nov 17 '18 at 22:57
  • The contactor shouldn't be heating up and failing like this -- it's the OEM contactor, rated for 35A per pole (i.e. enough to handle the 7.5kW element) – ThreePhaseEel Nov 17 '18 at 23:47

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