I have a boiler for heat/hot water, with multiple zones. It is a backup for a ductless mini-split installation. I'm trying to work something out where the boiler actually does backup the ductless mini-splits automatically, so I'm adding wifi-enabled thermostats where I can.

The boiler control board is a Honeywell Model L7224U. The manual for that is here: https://forwardthinking.resideo.com/related_links/thermalsolutions/l7224u/install/69_1720.pdf

I'm trying to do things properly, so if I can, I'd like to use the common in the boiler control board, but I'm not sure if there is one. I can't find any reference to it in the manual, certainly. I'm attaching a picture of the control board schematic and the current wiring.

Current Wiring Control Board Schematic

The system wiring is a mess, some dating I think from the 60s, and needs to be cleaned up, so I don't have a complete diagram of that, yet. And some zones are already using a separate transformer for power, not hooking up to the boiler control board. But, again, if I can do things properly, and use a common from the boiler control, I'd like to do that.

  • Can you measure to see if there's 24VAC between terminals 2 and 3 with the boiler running? Also, is there a zone controller somewhere in the system, or are the zone thermostats simply wired straight to the zone valves?\ Oct 13, 2020 at 2:31
  • The zone thermostats are wired directly to the zone valves. I’ll have to check on the voltage of terminals 2 and 3. Oct 13, 2020 at 3:10
  • 27 volts between 2 and 3, which is the same as between 1 and 2. Oct 13, 2020 at 16:35
  • Do either of terminals 2 or 3 have continuity to ground with the boiler OFF, for that matter? Oct 13, 2020 at 22:55
  • 1
    Will come back to this. When I was trying to trace out a diagram, I discovered CAT-5 cable being used in some places instead of thermostat wiring, so . . . I have work to do. Dec 1, 2020 at 21:40


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