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BMKBMK Member Posts: 5
I have installed a Burnham MagaSteam boiler in an old house replacing a Ideal round coal fire converted vapor boiler.  I did not realize what "It" was that I demoed out with the asbestos.  I researched and bought Dan's Lost Art Book to help with the start up of the system.  Now that fall is coming on I seem to have a few problems.  At 8oz the boiler is short cycling every 45 seconds the radiators have a metering ADSCO graduated supply valve and no radiator traps.  some new work was done when the coal boiler was converted to oil burner and at that time 2-wall convectors were installed to replaced two CI radiators and 1 Supply valve was replaced with a standard steam radiator valve.   The radiators are all supplied with top connections.  The steam main starts out 4in and splits two ways 3in around the house. the dry return starts at one side of the split and travel around the house sloped down to the return.  At the end of the two steam mains they drop, with out main vents, to about 18inches off the floor and then rise up to the dry return.  Which carries on to pick up the other radiator return pipes on its way back to the boiler to a main vent 24in before the drop to floor and returning up to the hartford loop.   The water level in the boiler is about 36in  above the floor.  The mains return to the dry return at about 72inches  above the floor. I have tried the vaporstat setting from 6oz  to 14oz and it still short cycles.  Any Ideas <a href="">Reply</a>


  • Dave in QCADave in QCA Member Posts: 1,759
    Cause of short cycling?

    I assume that the vaporstat contacts are opening to cause the short cycling.  But I wanted to confirm that first.  Or, is it coming from the low water cutoff, or an improper thermosat? 

    Does the new boiler match the EDR of the connect load and are all of the radiators getting hot?  Are they fully hot before the boiler starts to short cycle, or are they still heating.  Is the vent hot and closed before the short cycling starts?  If not, is it venting?  What type of vent is it?  Mfg and model.

    You mention that there are no radiator traps, but with the type of supply valves, it must be a two pipe system.  Are there orifices in the supply or outlets?  How are the outets connected to the return piping?  Any special or unusual items?
    Dave in Quad Cities, America
    Weil-McLain 680 with Riello 2-stage burner, December 2012. Firing rate=375MBH Low, 690MBH Hi.
    System = Early Dunham 2-pipe Vacuo-Vapor (inlet and outlet both at bottom of radiators) Traps are Dunham #2 rebuilt w. Barnes-Jones Cage Units, Dunham-Bush 1E, Mepco 1E, and Armstrong TS-2. All valves haveTunstall orifices sized at 8 oz.
    Current connected load EDR= 1,259 sq ft, Original system EDR = 2,100 sq ft Vaporstat, 13 oz cutout, 4 oz cutin - Temp. control Tekmar 279.
  • short-cycling

    this looks like the venting is so inadequate that the air is being forced out by the pressure of the boiler. as the pressure is set to 8 oz, the back pressure causes the short-cycling.

    if all the air is out of the system, and it still short-cycles, then i would suspect an over-sized boiler, whose steam capacity quickly exceedes the condensation capacity of the system.

    can you put on a temporary ball valve where the air vent is, [or should be] to make a test? run the system, with the valve open until the air is out, and see if the short-cycling stops.--nbc
  • RodRod Posts: 2,067
    Vent the Mains

    Hi- Lets see if I have this pictured right.  We have a 4 inch steam feed that splits into two  3 inch steam mains. The steam mains have no vents and end in a loop seal that feeds into the return main. The return main has a main vent.   The radiators have a “metering ADSCO graduated supply valve and no radiator traps” so we assume that this is a two pipe system with some type of adjustable (?)  orifice setup ?  The system now short cycles every 45 seconds on reaching 8 oz. of pressure.   You haven’t mentioned how well the system is working now (other than it is short cycling) ie: Are the radiators getting hot all the way across?

    I agree with NBC that it sounds as though the venting is pretty inadequate. If this was a typical two pipe system with regular valves (no orifices) and traps I’d say just increase the venting capacity of the present vent on the return, but since the system apparently has orifices, I would doubt venting large (3")  steam mains through the orifices is practical.  It may have worked  okay when there was a large coal fired boiler where steam pressure built slowly and steadily over time but with a modern boiler, which quickly produces a high volume of steam, this wouldn’t work very well and short cycling would result.  The solution I expect would be to get the air quickly out of the mains either by giving them each their own main vents or by connecting each main to the return with some sort of trap system in between and then increasing the main venting on the return.  NBC mentioned using a ball valve to test venting possibilities on the present vent. You might also try this out on ends of the steam mains and see whether there is an improvement. As others have said there maybe other factors contributing to the short cycling but I would first address getting the steam mains vented.

    - Rod
  • BMKBMK Member Posts: 5

    Thanks,  I will give the test ball valve a try but I might be thinking of install traps and radiators valves at radiators with steam main vent and trap to bring this system up to low pressure steam system.
  • RodRod Posts: 2,067
    edited September 2010
    Vent the Mains

    Well you can do add traps though the "trend" seems to be to consider going with orifices over traps. You might want to read this post by Boiler Pro who is an expert on steam systems.

    You might also want to look up and article on orifices by Henry Gifford at

      Scroll down the page to the "Technical Information Section" and in the "Two Pipe Steam Heating Systems" click on the "orifice Article"

    Before going to the cost and effort of adding traps, I'd get some large main vents at the end of the steam main and see how that works.

    - Rod
  • BMKBMK Member Posts: 5
    Orifces Vs Traps

    Thank for the link.  Orifices will be quicker and less money then installing the traps.  I might be able to make the orifices myself if I can't find them at the supply store.  I have graduated valves on the radiators now which have a slit opening that the valve handle opens from Off -- 1/4 -- 1/2 -- 3/4 -- On.  these valves are missing on one radiator and two convectors.  Maybe new radiator valves with the orifices??
  • RodRod Posts: 2,067
    Orifice Plates

    I would stay with the original valves and just use new valves where you need them.

    Orifice plates are available at Tunstall

    Until you get some experience with using orifice plates you may have a bit of trial and error with the size of individual apertures. Measure the EDR of each radiator and keep notes of the size orifice you put in each radiator.

    - Rod
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