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Steam short cycle

unclejohn Member Posts: 1,833
Worked on a Weil Mc eg75pi today. Drained out at least 35 gallons of water to get below the LWCO. That worked, red light came on. The water feed was a 24v hydro level I think. Old control with two sequencer type relays. Feeder came on for maybe a gallon and shut off, and after waiting 20 minutes no more water and still below center of glass and not making LWCO. Filled manually. Had voltage to both relays in the feeder. Boiler fires and makes stem after about 10 minutes of making steam the burner short cycled. 2 mins or so on 30 too 40 seconds off. House temp about 48*. Vapor stat set at 10 oz off 5oz. on. Set up to 14 oz on and 8 off. No water hammer and near boiler piping good. Rest of piping behind walls once out of boiler room which was small. watched for 45 mins and all convectors hot and silient operation, just that short on off cycle. Is that normal. I condemned the feeder and will go back and replace, figured it must have stuck on. The water in the glass bounced maybe 1/2" or less and stayed constant while I was there.


  • Fred
    Fred Member Posts: 8,542
    That's a big boiler. Did you compare connected EDR to boiler output? Did you look at the venting on the mains? How long did the boiler run, from a cold start before it started short cycling? Trying to move the house temp up from 48* to say 66 or 68, I can see the boiler running for an hour, maybe two before it starts to short cycle (especially if it is well over-sized). 10 minutes on, a few seconds off is normal after it gets to a place where the pressure gets to Cut-out.
  • Jamie Hall
    Jamie Hall Member Posts: 22,905
    Well, 10 minutes on, just long enough for post/pre is nice if you can do it. Mine is a bigger boiler relatively, and manages 2 minutes on, post/pre off once it gets well into a call -- say half an hour to 45 minutes. While yours really shouldn't start cycling on pressure in 10 minutes, I suppose it is possible if the boiler is significantly bigger than the radiation.

    If you had a main venting problem, I'd expect it to start to cycle even sooner.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • unclejohn
    unclejohn Member Posts: 1,833
    The boiler has been in since 1990 or 1991 from date codes. There are no vents that I can see in the boiler room. The customer said I was there just over two years ago for the same type issue. Just drained water out of the boiler not as much as this time as I recall. I did not check EDR they are not gonna take a boiler out that's worked for 20+ years, at least I don't think so. If any rads or convectors were remove it would have to be from the basement since the upper floors all have recessed convectors. The convectors all have two pipes. In the boiler room it looks like a one pipe system. Two runs off the header going left and right and one return line coming from the right side only, then all pipe behind wall. I don.t see where the left side return can cross over.
  • Big Ed_4
    Big Ed_4 Member Posts: 2,690
    You need to find out where the circuit is breaking .. thermostat , LWCO ,vapor-stat or even an bad connection ... Always an good voltmeter
    I have enough experience to know , that I dont know it all
  • unclejohn
    unclejohn Member Posts: 1,833
    It's the vapor stat. I watch the bulb swing back and forth.