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Powering a Tekmar 519 Thermostat from a Navien 240e

I have a question about running power from the 24v (CN25) output of the Navien which has a common and hot terminal into a Tekmar 519 thermostat. Navien tell me that it is for a low water pump or something. I am replacing a battery powered honeywell air temp with the 519 to use the slab sensor function. I have the thermostat wired to the TT like the previous thermostat but no power. Should I buy a separate 24Vac transformer and wire that in separately or do you think it is fine pulling the 24Vac from the Navien boiler. Anyone else done this?


    HVACNUT Member Posts: 4,595
    edited November 2017
    I'm not familiar with Navien, but from your description, it seems like they're N.C. dry contacts for a LWCO. Meaning it's not a 24v power source, it's just a switch.
    Is that a combi?
  • ratio
    ratio Member Posts: 3,185
    The manual I found indicates that CN25 is indeed 24VAC, intended to power a probe-style LWCO (page 18). Page 70 indicates that terminal has 24 volta at 0.3 amps available, or about 7 VA. The Tekmar indicates a maximum VA requirement of 56 VA. About 48 VA of this are to account for the load through the relay (2 amps max). The minimum is about 2 VA. It's possible that it will work. The easiest way to check would be to wire it complete, including any remote sensors you plan on using, make it call for heat, and watch the voltage on CN25. If it droops below about 24 volts, it won't last.

    If you are using a LWCO fed from this terminal as well, you aren't going to have enough power available.

  • yourefree
    yourefree Member Posts: 4
    Thanks. I am looking for a separate plug in style 24V transformer and just use that to power the thermostat. I don't want to cause any issues with the Navien. Yes it is a combi boiler. It works well and is very efficient. I do have a transformer with 1A 24v, i will use that to test the thermostat and slab sensor.
  • ratio
    ratio Member Posts: 3,185
    If you are using a separate power supply, follow the schematic on page 10, "Switching Relay". Power the stat on "C" & "R", & tie "Rh" & "W1" to CN24, polarity insensitive. Note the jumper between "R" & "Rh" is not installed.