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C or Common wire on WIFI Thermostat

Have three digital HW thermostats (two wires each, R &W) powered by batteries hooked up to Tekmar Zone Control 306V on 3 zones.

One day, just testing to see how it works, bought a Sensi WIFI thermostat and had it hooked up to include C wire connection by a separate 24vac transformer as in Photo# 3 and worked flawlessly. https://sensi.emerson.com/documents/sensi-wi-fi-thermostat-installation-guide-10-17-2-en-us-4278840.pdf

Then, since this Tekmar has an internal transformer, we decided to ditch the separate transformer and have it hooked up as in Photo#2. Operationally, everything works in terms of keeping the temperature fine and fires/stops the boiler fine. And we have heat. I can also use the Sensi cell phone app to do things (eg remote-adjust temp etc.).

But some features on the phone app disappear and Sensi Co. said “your Sensi only has ‘power stealing’ and you need to check with Tekmar”. And Tekmar does not have a support line. Just refer to the “Local Rep”.

I love to have this “Continuous Backlight” feature back for my old dad’s bad eyes. Any suggestion?


  • STEVEusaPA
    STEVEusaPA Member Posts: 5,787
    edited October 2019
    Did it work right with the separate transformer?
    You can email Tekmar, kinda annoying for back and forth.

    I think you may need a resistor across C & W like a T5 thermostat and ditch the C wire.
    Someone else would know for sure, like super genius Bob Boan ( @Ironman )
  • mattmia2
    mattmia2 Member Posts: 5,992
    I'd start with an AC voltmeter to make sure you have 24 v at the control and the t-stat. Look at the manual for the control, I notice you have the wall wart connected to R cool, perhaps it only takes power from one or has a jumper that selects which one it takes power from.
  • HomerJSmith
    HomerJSmith Member Posts: 1,688
    edited October 2019
    In the '90's', Honeywell came out with a current stealing (robbing) thermostat which caused havoc with my installations. The fix was an isolation relay. I still have the Honeywell instructions on that. Just some info to consider.
  • 1sttimeposter
    1sttimeposter Member Posts: 39
    Thanks everyone.
    I jumper-wired RH to RC on the thermostat and started to work fine now.