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I'm trying to install a wireless thermostat but i ran into issue putting in an external thermostat. I have a 2 Stage Cool / 1 Stage heat system.

I have the Red 24V from Air Handler connected to R. I have a external 24 Volt Transformer Hooked up to Rc, and C. I get 27 volts between Rc, and C. However when i tell thermostat to turn on fan nothing happens. If i remove external transformer and jump Rc, and R using AHU transformer everything works fine.

What could be causing this?

Photo of Wiring

Brian Weaver
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  • The external transformer probably doesn't have enough power to pull the relay. What is the Volt-Ampere (VA) rating of the transformer? – Tester101 Sep 04 '14 at 23:36
  • Actually, now that I think about it. When you say "external transfirmer", you probably mean something like a wall wart. In that case, it will have an ampere rating not Volt-Ampere. – Tester101 Sep 04 '14 at 23:43
  • Output: AC 24V 300mA. http://galleryplus.ebayimg.com/ws/web/261115137424_1_0_1/1000x1000.jpg – Brian Weaver Sep 04 '14 at 23:47
  • Any idea what the relay is rated? Or could you include the make and model of the air handler? – Tester101 Sep 04 '14 at 23:52
  • Not home right now to get specifics. But its a Rheem 3 Ton 16 Seer. Im just worried its something complex cause i have a $150 thermostat coming tomorrow. – Brian Weaver Sep 04 '14 at 23:52
  • Worst comes to worst I guess. I could hook up as follows: Red (24V from Internal Transformer) to Rc, and External to R (Rh) and C. Thus sacrificing heat capability but im in florida so it doesnt really get that cold. – Brian Weaver Sep 04 '14 at 23:53
  • If it works with the transformer in the air handler (which is 24V 40VA from what I could find), I'd guess the wall wart just doesn't have the power to pull the relay. – Tester101 Sep 05 '14 at 00:01
  • Could I do this then? Leave 24V from Internal Transformer connected to Rc & Jumped to R. And hook up only 1 wire from external transformer to C and cap the other end basically using the neutral? – Brian Weaver Sep 05 '14 at 00:06
  • Does the concept of Leaving Rc hooked up to 40VA* transformer and hooking up external to Rh and C sound like it would work. I have no use for heat. – Brian Weaver Sep 05 '14 at 00:08
  • I've seen people on the Internet use only one wire from a wall wart as a C wire, but in my opinion it's not the best way to go. The correct thing to do, is to run a new cable with an additional wire. – Tester101 Sep 05 '14 at 00:17
  • Will one wire with wall wart work? My airhandler is sealed 100% with metal foil tape even the service access panel and its still under warranty – Brian Weaver Sep 05 '14 at 00:20
  • I've only seen it done on the Internet, so maybe. I wouldn't do it in my house, especially with the price of a service call from an HVAC company. – Tester101 Sep 05 '14 at 00:31
  • Ok. So it sounds like External Transformer to Rh, and C is the safest option. It wont have enough power to close contact for heat but i will never use it. – Brian Weaver Sep 05 '14 at 00:34
  • No. The safest option, is to pull a new cable. The external transformer could damage the system, and/or void the warranty. – Tester101 Sep 05 '14 at 00:38
  • Opening the air handler would void warranty thats what im trying to avoid. – Brian Weaver Sep 05 '14 at 00:39
  • Air Handler Rheem is rhpl-hm3621ja. Where would i find the amp of amps to trigger the relay? – Brian Weaver Sep 05 '14 at 05:42
  • With the model number, you should be able to find replacement parts online. You should be able to find a replacement blower relay, which should list all the specs. You might also be able to find owners, installation, and/or maintenance guides online. – Tester101 Sep 05 '14 at 12:14

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