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LWCO Wiring..

ChasMan Member Posts: 459
edited December 2017 in Thermostats and Controls
I have read various discussions here on LWCO wiring. The Taco instructions say to break contact to the burner. My French Boiler control has a special contact for it. My original installer wired it to cut power to the boiler but nothing else. My assumption is the most likely cause of an LWCO relay alarm would be for the control to fail leaving a freezing house and causing the pipes to burst. But I guess the boiler could crack or the feed valves could fail and the return pipes could leak. I have no radiation beneath the boiler. I already have the burner wire package laid bare on a set of terminal blocks because of the proving switch on my fan in the can. The biggest electrical safety issue is that Riello Air shutter and I now have a timer on the burner like a priority protection timer to deal with it.

Is there a generally accepted way to wire these LWCO's?


  • Jamie Hall
    Jamie Hall Member Posts: 22,212
    The purpose. The only purpose. Of the LWCO -- low water cutoff -- is to prevent the possibility of dry firing the boiler.


    The only correct way to wire it is so that, one way or another, it kills power to the burner(s) so the boiler cannot fire. There are a number of ways to do that, but it must be positive.

    My own preference for those safeties is in the 120 volt circuits somewhere. However, if the controls permit it, there is something to be said for allowing the post purge to happen.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • ChasMan
    ChasMan Member Posts: 459
    Yes, I just realized after posting, the way I have the pumps rigged, they will never get power if the boilers reset control loses power. So, My only choice is the burner. I will wire it in series with the Fan In The Can.
  • ChasMan
    ChasMan Member Posts: 459
    edited December 2017
    Just in case someone reads this in the future and thinks it is ok to wire a LWCO to a Burner in this fashion, there is an exception in case of Riello where there are two sources of power that may not be immediately obvious. You can not wire a LWCO in series with a Fan In A Can (FIC). That is not safe because the electronic damper fitted to Riello Burners can short the AUX power back to the burner motor under certain failure conditions. A proving control like the FIC can not interrupt the burner in that type of situation. The LWCO must cut the auxiliary power and the Burner power both and therefore needs to be wired as a primary limit and not an operating limit on a Riello burner equipped with, or that may be retrofitted to, use an electronic air shutter. A FIC can not be wired as a primary limit to a Riello burner because the burner needs to have power to shut the damper. The safest thing to do is rip the damper off and disconnect the aux power line and call it a day. Then we would all be much safer.

    HVACNUT Member Posts: 5,490
    @ChasMan, the electric air damper hasn't been offered in years. They don't even use a hydraulic Jack anymore because of frequent piston seal failure. Riello says heat loss through the burner is minimal so they don't use anything.
    What's your French boiler control.
    I dont know how yours is wired but the Aux is a constant hot. The LWCO should only break L1 to the burner and the fan in a can should only prove to power L1.
    Basically, B1 on the aquastat to N.C. on the LWCO to fan in a can to L1 on the burner.
  • STEVEusaPA
    STEVEusaPA Member Posts: 6,393
    The electric damper may be gone, but the jack is back. The seal failure has been resolved.
  • ChasMan
    ChasMan Member Posts: 459
    Electric damper was a disaster. Can you retrofit it to a jack? I want a kit :)

    LWCO must break L1 and AUX if equipped. Every wiring diagram has it that way even Riello's. Because AUX power can keep the burner on when a limit switch breaks in the shutter. Of course, what are the odds of low water and a failed shutter limit. Here is the scenario. Burner fires, limit in shutter fails. TT is met, burner locked on... Temp rises to High limit. High limit fails, block cracks, LWCO turns off L1 burner still running... LWCO needs to wire before my burner power.