Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

Honeywell L8124C Relay problem

Donnyndad Member Posts: 2
What causes an oil boiler to continuously run, even with the Aquastat relay in the open position (non energized)? In other words, you can switch the power on/off but relay doesn't move either way, stays in open position, and still, boiler starts and will stay on until you power off? Honeywell L8124C; Wall thermostat completely removed.


  • Jamie Hall
    Jamie Hall Member Posts: 21,569
    There are also relays in the burner control, and it's possible that one of them has welded closed. I've seen it happen.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • STEVEusaPA
    STEVEusaPA Member Posts: 6,269
    edited October 2019
    The relay for running the burner for domestic hot water would close any time the control 'thinks' its below the low limit set point, or welded closed like @Jamie Hall stated.
    If the sensing bulb wasn't sensing temperature, and/or that sensed temperature was below the low limit setting (leaving differentials out of it for this simple explanation) then the B1 terminal would be energized.
    But don't stop there. Turn off the power (confirm) and remove the wire from the B1 terminal. If the burner still fires, the problem is most likely in the primary control.
    Forgot to mention the ZR terminal, wire coming from another control could also signal the boiler to fire.
  • Donnyndad
    Donnyndad Member Posts: 2
    I seem to have fixed it, maybe temporarily? For two years Aquastat 'HI' has been set at 190 deg for more efficient heating, just set it to 180 deg and let boiler run, and lo and behold, it shut off at 180 deg, set it back to 190 deg, and it shut off like it normally does, might be the control is on it way out, best to now order a spare for the near future.
  • STEVEusaPA
    STEVEusaPA Member Posts: 6,269
    Order a HydroStat 3250+, get the proper replacement well. Then you'll add low water cut-off to your boiler, plus other nice features. If you have a separate stand-alone LWCO, get it anyways. 2 couldn't hurt.