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LWCO buzzing and failed, do I have an electrical issue?

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[Reposting from the Controls forum, as recommended] Have a McDonnell-Miller PS-802 LWCO on my steam boiler. Blew out a couple of years ago (a lot of buzzing, then eventually it died). Antique 24V transformer suspected (but it's been a busy couple of years, so no replacement). Buzzing started on replacement LWCO a few days ago. Plumber won't replace without wiring updated, so replaced transformer and cloth wire. Now the LWCO just buzzes and won't do anything. Multimeter tells me 26V at the (new) transformer. Is this within tolerance, or do I have an electrical issue? Is the LWCO just toast from the old transformer, or do I have bigger issues? Obviously will try to get an electrician and a plumber lined up tomorrow (hopefully a plumber tonight), logistically easier if I only have to get in a plumber. Opinions?

Comments

  • Fred
    Fred Member Posts: 8,542
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    Does the probe that goes into the boiler have any teflon tape on it? Those probes have to ground to the boiler block in order to work.
    When you took the 4 wires off, were there any jumpers on any of the terminals?
    If you take the two wires off of the LWCO and hold them together or put a jumper on those 2 wires, will the boiler burner kick in?
  • JStar
    JStar Member Posts: 2,752
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    You need somebody familiar with boilers and controls. Your best bet may be an HVAC tech. Some plumbers and most electricians have a hard time with 24VAC controls.
  • Taylor_4
    Taylor_4 Member Posts: 55
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    Right now I'd settle for a plumber replacing the LWCO.
  • Captain Who
    Captain Who Member Posts: 452
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    So.............the replacement LWCO functioned fine when it was first installed? For how long? Was there anything that happened electrically like a power surge, lightning strike, power failure etc.?
  • Taylor_4
    Taylor_4 Member Posts: 55
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    Functioned fine for a couple of years.

    The only thing that happened recently is I turned off the shut-off switch to clean the sight valve. The buzzing started when I turned it back on. Today I turned off the circuit to replace the wiring. When I turned it back on, only buzzing.
  • Captain Who
    Captain Who Member Posts: 452
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    So I surmise the buzzing is coming from the transformer that is in the LWCO. Maybe something went bad internally and your replacing the 24V transformer and wiring was not able to fix that, no surprise.
  • BobC
    BobC Member Posts: 5,478
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    If there is a relay in the LWCO it could be that buzzing

    Bob
    Smith G8-3 with EZ Gas @ 90,000 BTU, Single pipe steam
    Vaporstat with a 12oz cut-out and 4oz cut-in
    3PSI gauge
  • Jamie Hall
    Jamie Hall Member Posts: 23,324
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    Electrical/wiring problem. With a safety circuit -- be happy that it failed off.

    Trace all your wiring and make sure that all the wires are good; that all your grounds are good -- and that everything is hooked up correctly. I doubt very much that it is a parts problem, although it could be, of course.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Taylor_4
    Taylor_4 Member Posts: 55
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    After replacement, all wires between transformer, LWCO and thermostat are good. I defer to plumbers on the control wiring. I've spoken to electricians who won't touch that stuff.

    What does grounding mean for this setup? The junction box feeding 120V to the transformer is grounded back to the panel, but there is no ground wire for 24V. The LWCO manual (Troubleshooting section) says that grounding of the probe can cause the unit not to function, so I don't understand when people say the system should be grounded.

    My working hypothesis is that the old transformer was underperforming and damaged this unit and its predecessor. Does that sound plausible?
  • dpnewkirk
    dpnewkirk Member Posts: 1
    edited December 2020
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    My McDonnell-Miller PS-801-24 had taken to intermitting buzzing, but imagining that it had an ac relay or solenoid inside I didn't think anything of it -- until it failed this morning, with a major Nor'easter on the way. With the closest service appointment a day into the storm, I figured I had nothing to lose in taking the cover off the PS-801 and having a look. Okay, to cut to the chase: This unit should not buzz, as its control relay and the circuitry that drives it all run on dc that's derived from the 24-V-ac control voltage. Traces of overheating in the vicinity of the dc-power-supply electroytic filter capacitor -- a black cylinder in the upper right corner of the 801's circuit board, marked "470 uF 25 V" -- suggested to me that that part was defective. (That would make sense, as failure of the filter C would cause the power supply's dc output to fall and/or become "ripply", potentially causing the unit's dc-operated control relay to buzz.) Punchline: I replaced the 470-microfarad, 25-V capacitor with an equivalent part from my hobby "junkbox", and now my PS-801-24 works and our heating system is running again. (If you try this fix, be sure to power down your system before starting work, and be sure to put the replacement capacitor in with correct polarity; I recommend marking + and - on the circuit board before you remove the stock part so you don't forget which terminal goes where.)
  • Jamie Hall
    Jamie Hall Member Posts: 23,324
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    or @Taylor_4 -- the grounding requirement means, very simply, that all the metal -- conduits, block, pipes, control boxes, etc. etc. must be securely bonded together and returned, via a single solid grounding connection to the system ground on the main electrical panel (which, in turn, connects or should connect to a building ground).

    In many cases -- though not all -- the 24 VAC output is floating, and intended to be.

    The active probe part of the LWCO probe must not be bonded or grounded -- but the threads of the probe must be.

    The 26 VAC from the transformer at no load is well within specification -- but what is of interest is what is the voltage when it is hooked up to everything? There is a definite limit as to how much current those transformers can provide (specified as "volt-amperes") and stay in tolerance.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • New England SteamWorks
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    26 volts is within tolerance. 
    New England SteamWorks
    Service, Installation, & Restoration of Steam Heating Systems
    newenglandsteamworks.com