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enter image description hereI have read most of the posts here and have a specific question about my install. In the picture you can see the 3 blue wires coming off the 24v side of the transformer. One goes to the side and than another blue wire is connected to the metal and goes to the ac unit outside. One goes up to what I believe is the exhaust fan. The last one connects to the circuit board. There is also a 24v HUM screw on the board. My question is should I connect to the 24v Terminal on the board or one of the wires that are screwed into the metal next to the transformer. Thanks for any help.

Transformer Circuit board

  • I am trying to figure this same thing out. Where did you end up connecting the free wire from the thermostat to get it working? A picture would be awesome. – user35291 Apr 4 '15 at 15:27
  • Kyle, I used the blue wire that goes to my ac unit. As was mentioned you can connect to any of the blue wires. I chose the one going to the AC unit because it was already next to the wires to the thermostat and had a wire nut connection to a wire going outside. Hope that helps, Pat – PatM6 Apr 4 '15 at 21:42
  • Pat, are you saying that you just tied the blue in with the blues that went to your account. It look spike I have the exact same system – user48305 Jan 24 '16 at 18:40
  • Yes. I just tied in with those. Just make sure they go to the 24V transformer Like in the picture at the top. – PatM6 Jan 24 '16 at 22:10
  • Pat, et al, A year late, but dealing with the same issue. Please share a pic of final wiring solution if possible. – user60497 Sep 24 '16 at 18:03
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You should be able to connect to any of the blue wires, or to the chassis where the transformer is bonded. I wouldn't use the HUM terminal, unless you've looked at the schematic and determined that it's connected directly to the transformer. If you include the model number, or a photo of the schematic. We'd be able to tell you for sure.

If you're supplying an A/C unit contactor, the furnace circuit board, and an exhaust fan from this transformer. You may find that it doesn't have the power to also supply a thermostat. There may be no problem at all, but it's something to be aware of.

  • The easiest option is to connect to the wire that goes to the ac unit so I will go that path. Is there way I can measure the output to check if it can support the thermostat. If not, is there another option. Thanks again. – PatM6 Feb 10 '15 at 13:19
  • If you put an ammeter on the red wire coming off of the transformer, and took the reading while everything was running. That would give you the load that is on the transformer. With a 24 V, 40 VA transformer. The max draw would be about 1.667 amperes (40 VA / 24 V= 1.66666667 A). You might have to contact the thermostat manufacturer, since I don't think they list the current draw in the documentation. – Tester101 Feb 10 '15 at 14:13
  • Usually it's not a problem, but it can be, so I like to at least mention it. – Tester101 Feb 10 '15 at 14:19

protected by Community Sep 24 '16 at 19:28

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