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2 pipe low pressure steam system cycling once every 2 minutes

ALG Member Posts: 1
I have a 2-pipe low pressure steam system that heats a 5-unit condo building 10,000 sq ft total. The rusty 30 year old WeilMcLain boiler broke down and was replaced with a Weil McLain LGB6 eight years ago.

While the LGB6 (produces around 500,000 Btu/hr) handily heats all units of the entire building, I noticed that the system does a cut in (at 0.5psi), and cut out (at 1.0 psi) cycle once every 2 minutes. So, basically, it stays on for 1 minute until it passes 1.0psi, then stays off for another 1 minute, until it goes below 0.5PSI .... over and over again until the central thermostat turns the whole system off .

I guess one way to reduce this cycling is to lower the cut-in, to, say: 0.2 psi.

Is there another way? For example, disabling some of the burners or adjusting

them so that the vaporstat is not constantly clicking on and off like a clock?


  • gerry gill
    gerry gill Member Posts: 3,078
    The LGB

    boiler usually comes with a low fire gas valve that i seldom, and i mean seldom see anyone install a trigger device for..i suspect thats how yours is set up too. I add a vaporstat and use it to trigger the low fire gas valve..
    Serving Cleveland's eastern suburbs from Cleveland Heights down to Cuyahoga Falls.

  • Short-cycling

    Is this boiler over-sized for the radiation? Using the staged firing would help the over-firing.

    Is the air getting out of the system as it fires without being forced out?--NBC
  • Kevin_in_Denver_2
    Kevin_in_Denver_2 Member Posts: 588
    Short Cycling is more likely with a Vaporstat

    In the general field of heating, short cycling usually causes a drop in efficiency.

    In the case of steam, it doesn't hurt efficiency. Here's why:

    The most efficient steam boiler would fire right at the boiling point, never below, and never above. The boiling point is the lowest possible temperature for the boiler to operate at. If it is below the boiling point, no steam is being delivered to the load. If it isn't delivering heat to the load, it's efficiency is almost zero.

    If the boiler keeps firing and raises the pressure more than necessary, it is operating above the temperature it needs to be at to satisfy the load. If it's hotter than it needs to be, it is losing more heat than it needs to, through the jacket and the flue, and that drags down efficiency.

    Last year I replaced my Pressuretrol with a Vaporstat, and this tighter control saved about $400 worth of gas on my old 1mmbtu Peerless. But it definitely cycled faster.

    One downside: short cycling will wear out the control faster. But the savings more than make up for that.

    Gerry's advice would have it operating even more efficiently and with longer cycles.

    0.2psi might be too low for your building, so it may cause trouble on cold days.
    Superinsulated Passive solar house, Buderus in floor backup heat by Mark Eatherton, 3KW grid-tied PV system, various solar thermal experiments
    EBEBRATT-Ed Member Posts: 15,312
    short cycling

    I would investigate weather the boiler has low-high fire capability and use it if you can. You could also raise the cutout to 2 psi. Thes two things will probably help smooth things out. Also check that the traps and vents are working correctly
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