I've been diagramming my boiler wiring so that I can understand it before installing a Tekmar 256 external reset. What's been throwing me for a loop is that it looks as if the 24v common is being fed to the thermostat instead of the load. This is reversed right? Should I reverse the blue and green wires going into the transformer?

Note that there are 2 other zones not shown here, but they tie in similarly (thermostat gets the green and the taco 2 is attached to the blue).


Edit -- It turns out that the transformer was mislabeled. What's labeled as R/LOAD on the transformer is actually the common.

Boiler System Wiring

  • What make/model are the boiler control and the valve? Can we assume the t-stat is a mechanical type? – ThreePhaseEel Sep 16 '17 at 21:34
  • The controller is an Aquastat L8124. The valves are TACO 571-2. The thermostats are LUX TX9600TS with white going to W and the black going to Rh which is also jumpered to Rc. – VoltRanger Sep 16 '17 at 21:48

Yes, this is backwards -- while leaving it as is is mostly fine, swapping it won't hurt either

This is indeed backwards from the conventional way to wire things -- normally, the switching is all done in the R wire, and C is used as the transformer return. Putting the switching in the C wire, as the installers of your setup did, works, but has the undesirable side effect that grounding out the wiring from the zone valve to the thermostat or boiler control will cause things to turn on, as the C wire is usually grounded at the transformer secondary. Oops!

Swapping R and C at the transformer won't hurt anything either, though, and probably is a good idea because it's far harder to tell with electronic controls whether they want to be finicky about which wire is which -- older electromechanical controls never gave a toss, which is why it worked just fine for all these years. (My suspicion is that your new control won't be particularly picky, but I'm not a tech support guy at Tekmar, so I don't have any insider info.)

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  • Thanks! Before making any changes I took a voltmeter to the transformer to be sure. It turns out that what's labeled on the transformer as load is actually the common. So, the wiring is correct as is (so long as you ignore the labels on the transformer). – VoltRanger Sep 17 '17 at 16:41
  • @VoltRanger --- hahahaha xD no wonder it looked backwards – ThreePhaseEel Sep 17 '17 at 16:46

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