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24V transformer

digger_2digger_2 Member Posts: 39
The thermostat and pump relay on my heating system are powered by a transformer with an output rated at 25VAC and 100VA. It has worked well for 10 years. Also note that the thermostat closes the pump relay on a call for heat. The hot water is standing by in a 50gal gas fired water heater.

Two weeks ago I installed a wirsbo 512 (same as tekmar512). It showed no time display and cannot be programmed to perform any funtions that have anything to do with timing. It will run the pump for the default on cycle about 20 min then shuts down for the off cycle even though the call for heat is not yet satisfied.

In short, it's broke!

Normally I see transformers that are24VAC 40VA. So before I land wire on my new 511 I was trying to find out if this transformer was an issue.

The broken 512 that I installed appeared to have been "out of the box" before I got it. I think it was trashed before I got it. Just trying to cover all bases though


  • JFearJFear Member Posts: 3
    edited November 2009
    Double check the wiring

    First off, I want to make sure we are on the same page.  You are talking about the Uponor/Tekmar 512 programmable thermostat correct?

    I have run into issues with the display and operation acting up before, and it usually has to do with the wiring to the stat. 


    Am I correct in assuming that you have run a 24v supply from this transformer to R(1) and C(2) on the 512, and your TT connection from your relay is run to the end-switch (6 and 7) on the 512? 


    I have run into numerous installations where the contractor has only run 18/02 wire rather than 18/04 wire, and has attempted to use the 24v power from the relay to power the thermostat and make the end switch connection.  This will not supply enough power to the 512 for the display to operate correctly, and may cause interference in the wiring, disrupting operation. 

    The 100va transformer you have should have more than enough juice to power the 512 and a powered pump relay.  If it is wired correctly you should have no issues with utlizing the 512 and a 100va transformer.
  • digger_2digger_2 Member Posts: 39

    It is the Upnor 512. It is wired per the diagram "one stage heat with zone pump and cooling" (using unpowered relay) under the heading "wiring the 512" in the installation guide. In my application the cooling is not used. I junpered the R to terminal 6 and 5 goes to the unpowered relay. Terminal 7 is used for cooling in the diagram. I had pulled the old 18/2 out and replaced it with 18/5. Two of these conductors are not used at this time. I was going to add a floor sensor later.

     There is 30' of wire between the transformer / relay and the T stat
  • Randy BaergRandy Baerg Member Posts: 26
    512 wiring

    As already mentioned many jobs with 'bad' thermostats turn out to be a wiring problem. You did say your stat had already been opened. To test it and also to eliminate a bad wire between the stats and the mech. room try wiring the stats R & C terminals 1 & 2 directly to the transformer. If the screen powers up and looks ok then it is a wiring issue . If it doesn't power up then the stat must have been fried by the first guy that opened it.

    If it powers up then look at your wiring and maybe try a continuity check on all your wires. Lastly these stats need a minimum of three wires. You say you have 24V R and C wired to terminals 1 & 2 and then jumpered the R from term 1 over to terminal 6, that's all good. Now make sure that term 5 (call for heat like a W wire)  is wired back to one side of the relay coil. As long as the other side of the coil is wired back to 24V C on the same transformer you should be good. The relay on the stat (and the external relay contact) should close when the stat calls, It shows an H1 icon on the LCD.

    If you have a powered relay (T-T connections) then you need to wire up 5&6 to the T-T and wire 1&2 (R,C) to a transformer.

    The thermostat uses about 2VA of power.  I have seen quite a few wiring problems on the neutral connection causing blank screens.
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