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WDTC-20 How to test with volt meter?

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    WDTC-20 How to test with volt meter?

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    #2
    Here is a site that has HVAC wiring info with definitions: http://wiki.xtronics.com/index.php/T...als_and_wiring
    And here's a link to the diagram on that site: http://wiki.xtronics.com/index.php/Image:Thermostat.gif

    Unfortunately, there are no standards when it comes to furnace and A/C wiring color codes, but in general, the Red wire (terminal R) is generally considered the 24vac "hot" and the Black wire (terminal B) is 24vac "common." The 'stat works by connecting 24vac hot to the other terminals to call for heat, A/C and/or fan. These command inputs are generally relays and the other sied of each relay is connected to 24vac common inside the furnace and a/c units.

    Where 24vac hot get a little messy is when both the furnace and a/c units each supply their own 24vac hot. This is where the RH and RC terminals come in as they 24vac hot.

    So, to test that you really do have 24vac being supplied to your 'stat, measure the AC voltage between terminals RH and C for the furnace and and then measure between RC and C for the A/C unit. If either or both of these are near 24vac (20-28) then you should be good. If not, the 24vac common line may not be connected back at your furnace or A/C unit.
    Best regards,
    -Mark-

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      #3
      Thanks Mark, I'll give that a shot.

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        #4
        RockLobster,

        The wdtc-20 has a jumper, I forget which one, but is marked if you have both Heat and AC with separate power supplies I think you remove it.
        Then If you want to run the stat with an separate ac supply you would put one connection on the C and the other on the RH. Your heat wires from the furnace would go to RH and W.
        Tech support at WD told me that this isolates the thermostat supply from the individual supplies.

        Stuart

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          #5
          Did you get to the bottom of this?

          I had all sorts of issues with it consuming batteries, it turned out that the problem was not the wiring, but the fact that the hot wire contact was not making contact with the stat, so it was running on batteries the whole time. This was because the wall it was mounted on was a bit rough, so the left side of the plate was a tiny bit deformed, causing the pin to be slightly further away from the stat when it's mounted on the plate. By 'slightly' I mean something like 0.01mm, but, due to the crappy design of the backplate, that was enough to screw things up.

          I've now done away with the backplate completely and hardwired the wired direct into the stat through a custom plate. I like the WDC-20, but it's design is very poor.

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            #6
            Thanks for all the replies... it's not resolved yet, I've got my HVAC company coming in less than a week to check on the wiring. I'll post what the final solution is...

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              #7
              OK, my HVAC guy is here and we found the problem... hopefully this will do it. Using a voltmeter, the Common to RH is only giving 15-16v. The Common to RC is giving the full 24v, but I've learned that the WDTC-20 needs 24v from the *heating system*.

              So the HVAC guy is going to run a jumper from the cooling system to the heating system, we'll then test to ensure we're getting 24v between C and RH.

              Which reminds me... is there any way to tell when the 'stat is running off of batteries? It sure would be nice to know one way or the other before he leaves!

              Thanks again everyone...

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                #8
                Originally posted by RockLobster View Post
                OK, my HVAC guy is here and we found the problem... hopefully this will do it. Using a voltmeter, the Common to RH is only giving 15-16v. The Common to RC is giving the full 24v, but I've learned that the WDTC-20 needs 24v from the *heating system*.

                So the HVAC guy is going to run a jumper from the cooling system to the heating system, we'll then test to ensure we're getting 24v between C and RH.

                Which reminds me... is there any way to tell when the 'stat is running off of batteries? It sure would be nice to know one way or the other before he leaves!

                Thanks again everyone...
                I dont think there is an icon on the display to indicate when it is running on batteries, so you're never really sure

                Try removing the batteries completely - if the display works, then it is working from the 24v AC supply.
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                  #9
                  Try removing the batteries completely - if the display works, then it is working from the 24v AC supply.
                  That's the correct way to verify things, according to the tech support guys at Wayne Dalton. Worked for me.

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                    #10
                    Removing the batteries worked as well... it's been 2 months since I purchased the t-stat and it now appears to be working like a champ. Thanks again everyone...

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