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Upgrading 2 wire Tstat wiring question- Honeywell

NoBis Member Posts: 1
edited February 2019 in THE MAIN WALL
Hello. I'm trying to upgrade my oldschool 2 wire thermostat to a Honeywell WiFi unit RTH6500. This C wire eludes me...
This is my heating system. It runs 2 zones but I'm just looking to just do the one. The other thermostat runs to the Taco relay switch that doesn't have cover removed.

The red and white wires coming from the old thermostat plug in to the Aquastat at T and T/3

Honeywell L7224

Where might I add the C wire? I'm new to the home heating game but have a solid background in automotive diagnostics and electronics repair. Trying to research how everything works, I would much rather take apart a motor. Thanks in advance!


  • Alan (California Radiant) Forbes
    Alan (California Radiant) Forbes Member Posts: 4,106
    edited February 2019
    It looks like the Taco SR501 turns on the pump and not the burner - strange. Edit: I see the tankless coil now.
    I'd replace the SR501 with a Caleffi ZSR102 and wire both thermostats and both pumps to it. The Caleffi will give you a "C" terminal for your thermostat.

    Instructions here. See page 14 and on.
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
  • Steamhead
    Steamhead Member Posts: 17,008
    You'll also need to replace the existing 2-wire thermostat cable with one that has at least 3 wires.
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service