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Upgrading thermostats and cables with c-wires, L8148J two zones c-wire question :)


Hi all! really thankful for this site as i have learned alot here over past few days!

Ok so I have a L8148J Honeywell aquastat (See images)

which was connected by previous owner in a pretty convoluted way (at least seems to me it was ) to 2 separate old Honeywell thermostats (one for 1st level and one for 2nd level of the house).

I am now upgrading my old thermostats to the ST55 Sensi Smart which requires c-wire.
Because of that I removed the old cables and ran 2 new cables (5 wires including a blue c-wire) from the old thermostat locations (1+2 floors) all the way to the utility room where I have my Aquastat L8148J. The issue is that down in the utility room the whole operations seems a mess :) see image with annotations:

im trying to figure out 2 things:

  1. where on the L8148J so i actually have a c-wire port
  2. since there seems to be multiple splits of the white and red cables where do i connect the 2 new cables from the ST55 (ie exactly the same as before?)

thx alot in advance, really appriciate it!




  • HomerJSmith
    HomerJSmith Member Posts: 2,479
    edited June 9

    Wire it this way. You have Taco 570 type zone valves. Don't expect to wire a third wire to the Honeywell. You need an aux transformer. This is for a Nest but should work for you. The TT Boiler connect is to the boiler aquastat TV & T connections.



  • HomerJSmith
    HomerJSmith Member Posts: 2,479
    edited June 9

    Ethan is my favorite name for a boy.

    The 2 & 3 connections on the Taco ZV are just a switch that connects to the TV & T connections on the Honeywell aquastat and energizes the relay on the aquastat which turns on the pump and the gas valve's safety circuit, opening the gas valve. It's a single throw—double pole relay. You generally don't have enough power to run everything off the aquastat transformer which is why you need an aux transformer to run Taco 570 ZVs and charging thermostats. You do seem to have an aux transformer. Disconnect all the wires and rewire. Follow the wiring diagram. The diagram shows where the circuit connections go but you can make these connections in different ways. For instance, the two C wires from the thermostats can connect together then a wire can go to the #2 screw on the ZVs.

    You need the relays tho. Double check your work before powering up.

  • ethan22
    ethan22 Member Posts: 2

    thx @HomerJSmith!

    i must say im not an expert (just a DIYer) so ill try to follow along :)

    You need the relays tho

    you mean i need to buy 2 relays?

    thx alot

  • HomerJSmith
    HomerJSmith Member Posts: 2,479
    edited June 10

    Yes, two relays. See my link to SupplyHouse.com

    Just connect one wire at a time and make sure it is correctly wired.