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Short-cycling boiler in a single-pipe system...with NEW VENTS

Hi Folks -

Back when Click and Clack were on the radio, they used to have a segment called "The Puzzler"...well, in the spirit of the Tappet Brothers, here is a puzzler.

I'm in Massachusetts, so almost by definition I have a small house...about 1100 sq-ft. Built in the 20s, has single-pipe steam heat. I read Dan H's book "We Got Steam Heat" and enjoyed it a lot and got to know my system well. My house has 7 radiators, I have Gorton vents on them ranging from 4s to a C, no main vents, gas-fired Weil-McLain boiler about 10-15 years old, Honeywell Pressurtrol, the standard stuff. The system works great.

A good friend of mine just bought a house down the street. Almost identical - maybe 1500 sq-ft, recent gas-fired Weil-McClain boiler, single-pipe, 7 radiators, Honeywell Pressuretrol, again standard stuff. No main vents either. I visit the house and it's cold and the boiler is short cycling. Runs for about 2 minutes, shuts down, about 3 minutes later, fires up, repeats endlessly. The near boiler piping is hot, a few feet down the line most of the pipes get cold.

I know what you're thinking. It's the same thing I thought.

I walked around the house (it had been sitting empty for a few months, maybe a year) and made sure the valve at each radiator was open. I also saw that there were old, dirty vents on all the radiators. I ran over to the hardware store and bought a couple Maid-o-Mist 5s. They only had 2 in stock so I figured it's better than nothing. I replaced the vents on the two coldest radiators and bingo! The boiler stopped short-cycling, the two cold radiators got hot. The house is now heating "OK" but less than optimal. There were still some really slow, dirty vents so I used the Gorton/MoM chart and ordered the rest of the vents. "C" in the upstairs bathroom - furthest from the boiler, a 4 in the living room with the thermostat, mix of 4s and 5s elsewhere. I installed them a couple days later, turned the thermostat up and stood back to admire my work.

The boiler starts short-cycling again.

I run down the basement - all the piping is hot now. The mains, the risers, everything I can feel before it disappears up into the house. My first thought is either a couple bad vents or the total venting capacity of the MoM vents is less than the total capacity of the dirty old vents even in their dirty state. I ran back upstairs and took 5 of the vents completely OUT of the radiators.

Boiler continues to short-cycle.

I open up the Pressuretrol, differential at 1 and cut-in at 0.5. Correct settings. Just for the heck of it, I crank the differential up to 4.

Boiler continues to short-cycle.

The water level in the boiler is of course fine, and I feel pretty confident it's not shutting down on pressure. And yes the thermostat is still calling for heat.

I'd love some ideas...Thanks folks!


  • Dave0176Dave0176 Member Posts: 1,101
    What Model Weil-McLain boiler do you have, and have you measured your EDR (radiation measurement) and finally please post some pics of your boiler piping?
    DL Mechanical LLC Heating, Cooling and Plumbing 732-266-5386
    NJ Master HVACR Lic# 4630
    Specializing in Steam Heating, Serving the residents of New Jersey

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  • Jamie HallJamie Hall Member Posts: 12,712
    Also -- if it's not shutting off on pressure, what is it shutting off on? Can you figure out?
    Br. Jamie, osb

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • aceboileraceboiler Member Posts: 14
    I would take some pictures of the system. Include pressure on gauge. Boiler piping for sure and a couple of radiators.....i like radiator pictures!!!!
  • gfrbrooklinegfrbrookline Member Posts: 610
    Assuming they are the same boiler and the venting has been addressed, is the boiler shutting down on low water? Could either be clogged returns or a hole in the boiler.

    Since you are in Boston I would have New England Steam Works give it a once over.
  • FredFred Member Posts: 8,418
    It's not likely a hole in the boiler. While you say you don't think it is shutting down on pressure, my money says it probably is. That system has no main vents so you are depending on those small radiator vents to evacuate all the air in the mains, the radiator run-outs and the radiators. That boiler is running so long, it is building pressure before it can even get steam to the radiators. You can not properly balance a system without first putting some main vents on it, then you adjust the radiator vents to the desired flow. I know you say your system has no main vents and it works but I can assure you it doesn't work like it could and, needless to say you are burning/wasting fuel making the boiler push all the air out through the radiator vents.
  • A little more color commentary. The house in question was built in 1930, and has not been altered - at least structurally. The new owner bought it from the estate of the previous owner, who bought it in 1956. I can't guarantee it, but I am 90%+ sure the radiators are original to the house. I saw no marks on the floors from other radiators having been there, no evidence of anything having been moved or removed. The radiators are caked in layers of peeling wall paint. So while I don't know the EDR of the radiators, I am assuming that the "dead men" sized them properly when building the house. I've attached some pics - they are common hot water/steam radiators, Aero brand.

    I will get the model of boiler in the next day or two as well as pics of the piping. It is piped in black iron pipe in what looks to be a "normal" way.

    I was also suspicious of lack of main vents, but removing all the radiator vents AND increasing (temporarily) the differential in the pressuretrol resulted in zero change in the short cycling duration. Pressure can't build with big ol' holes in the sides of the radiators can it? If a clog somewhere else was causing it, wouldn't the increased differential give it a longer burn before shutting down on pressure? And there was that brief shining moment last week where it ran normally and heated all the radiators (admittedly slowly and with sub-optimal timing).

    I've watched the boiler go through several short-cycling cycles and the water in the sight glass is at the right level and stays at the right level - it doesn't drop or surge up and down. The system is also quiet - no hammering (which I would expect if water was pooling somewhere due to a clog).

    The system has a stand-alone gas-fired hot water heater, so the boiler hot water loop isn't being used.

    Excellent ideas from everyone! Thanks!
  • I took those pics prior to changing the vents obviously. The mix of "Dole" and "Vent-Rites" are still present. There are two more radiators of the in-wall variety. I don't have pictures of them.
  • FredFred Member Posts: 8,418
    On the boiler there is a name plate that has some specs on it. It should say "Sq. Ft. Steam". What is that number. That is what you want to compare to the total radiator EDR. How does the size of that boiler compare to the one in your house? If it is massively over sized, it will short cycle. Main vents will help that problem but will not eliminate it.

    Also turning up the Differential will exacerbate the problem, in that steam moves slower, the higher the pressure. I doubt there is a clog anywhere on the steam side of the system. That would be very rare and if there were a clog on the return side, the boiler water would drop and remain low, for some period after the boiler shuts down.

    Are all of the radiator supply valves fully open? Is there any gate valves on the main(s) that may have been closed off? Do all the radiators get hot?

    It would be great to see how the boiler is piped.
  • FredFred Member Posts: 8,418
    Also, are those radiator pitched back towards the supply valve a little. It looks questionable from the pictures. If not, I suppose some of those rads could hold enough water so as to not let steam into them. That supply valve in your first picture looks like it's closed and a couple others don't look fully open either. Shut supply valves off and you reduce the EDR, relative to the boiler capacity. Any partially closed ones may not let water out. Make sure they are all pitched back towards the supply valve.
  • ethicalpaulethicalpaul Member Posts: 1,572
    > I am assuming that the "dead men" sized them properly when building the house.

    Haha! The dead men also liked to sleep with the windows open in February so possibly not!

    But it is a good mystery!
    1 pipe Utica 112 in Cedar Grove, NJ, coal > oil > NG
  • GordoGordo Member Posts: 727
    edited December 2018
    Did you check for a clogged pigtail, perhaps?

    Also, the t-stat itself could cause short-cycling if the cph is set incorrectly.
    All Steamed Up, Inc.
    "Reducing our country's energy consumption, one system at a time"
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
    Baltimore, MD (USA) and consulting anywhere.
  • FredFred Member Posts: 8,418
    Most radiators are well over the size they need to be today, for various reasons, open windows, old coal fired boilers, less tight envelopes, etc. In any case, for steam, the boiler should be sized to the radiation that is there. They may not get hot all the way across but if the thermostat is satisfied, that's ok.
  • JUGHNEJUGHNE Member Posts: 6,793
    Pigtail could have a sludge plug working as a check valve.
    Did you put a voltmeter across the P-troll to confirm that it did not shut the boiler down?
  • FizzFizz Member Posts: 522
    In one of your earlier posts, you replaced 2 vents on cold rads with 2 MOM's and bang, they heated, and boiler stopped recycling. Yet heat wasn't optimal??? Might be worth going back to that set-up and see what is working and what is not. The fact the boiler stopped short cycling is a good thing.
  • Jamie HallJamie Hall Member Posts: 12,712
    Ah... let's go back to square one here. The boiler is short cycling. Before you do anything else, verify what control is turning the boiler off and then back on. Don't guess.

    Then let's try and figure out what's amiss.

    Oh -- just because all the vents are out on the radiators doesn't mean the boiler can't build pressure. If there's no main venting, it will. But you should get steam shooting our eventually...
    Br. Jamie, osb

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • FizzFizz Member Posts: 522
    In P-trol's initial post, he noted his 1 pipe system works perfectly with-out a main vent. Why is that?
  • Mike CascioMike Cascio Member Posts: 140
    As noted in an earlier post. Can you please post pictures of the boiler and near boiler piping.

    Also what is the square ft of the boiler compared to the connected radiation?

    What kind of thermostat are you using?
  • quack24quack24 Member Posts: 62
    you can take off the thermostat and put a jumper on the wires to eliminate that as a problem had a thermostat before that couldn't decide if it was on or off due to low batteries
  • EBEBRATT-EdEBEBRATT-Ed Member Posts: 6,940

    Go back to the basics. As @Jamie Hall mentioned. it's short cycling the question is ON WHAT. Yes you may have other problems causing the short cycling but first you need to find out what it is short cycling on.
    1. Pressure control, remove it clean it and the pigtail.
    2. Low water cut off. The water level may or may not be ok, could be a bad switch, loose wire. float with a pin hole etc
    3. Thermostat, what about the cycles /hour or heat anticipator setting.

    You can jump any one of those controls (if you know what your doin) FOR testing PURPOSES ONLY.


    This is the first thing to do jumping around while trouble shooting will run you in circles
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