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Vaporstat Setting

Leaky Member Posts: 16
So I just finally installed a vaporstat (yea!) and now am wondering about setting recommendations.  Initially I set the cut out at 8 oz, sub. diff @ 6 oz.  The boiler would cycle on and off on pressure during a heat call,i.e: acheive pressure and then off for about 1min 20 sec, on for about 1min40 sec.  Thinking this represented short cycling I set the vaporstat to 11oz, sub. diff@ 10 oz.  The cycles are now a bit longer, but I was hoping to run a low pressure.

Our system has only 5 radiators, one-pipe steam.  The boiler is a Peerless 3 section.

What would you recommend setting the vaporstat at and why?  Thank you!


  • Steamhead
    Steamhead Member Posts: 16,734
    Might be time

    to have the burner down-fired slightly. Which Peerless do you have, and what burner is on it? 
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
  • DavidK_2
    DavidK_2 Member Posts: 129
    are you radiators all hot

    when the vapourstat switches on high pressure?

    I'm no expert, just learning, but my understanding is until all the radiator vents have closed there should not be much pressure in the system. Just enough to drive the steam down the pipes (probably only a few ounces). If your presuretol  (vapourstat) is triggering on high pressure before the radiators are hot, you might need more venting.

    If all the radiators are hot, then you are probably making more steam than your radiators can condense and steamhead's suggestion for downfiring makes good sense. Again, I'm just learning, but think in an ideal system the radiators will condense steam as fast as the boiler makes it.
  • Jamie Hall
    Jamie Hall Member Posts: 22,947

    may be no expert -- but he's got it.  That's exactly what is supposed to happen once all the radiators are hot -- if there is enough venting.  So... check your venting first (main vents -- hard to have too much!) and then take a look at Steamhead's suggestion.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Leaky
    Leaky Member Posts: 16
    edited December 2009
    Thanks everyone

    If I understand correctly, the vaporstat should almost never cut out the boiler if all the radiators are condensing at a rate equal to the boilers production of steam.  Is this true?

    Indeed, this would mean I have a need of downfiring.  First I'll take a closer look at the venting situation, then revisit the issue if necessary.  The more venting the better.

    BOILER IS: Peerless WBV-03-110-SPT               Nozzle .75 80*nf #20  (what the oil man wrote)

    What amount of time should exist between boiler "firing" cycles during a heat call, in your humble opinions??
  • Jamie Hall
    Jamie Hall Member Posts: 22,947
    Well, almost

    During a normal, reasonably average day you are quite correct -- the thermostat and the vaporstat will arrive at the conclusion to shut the system down pretty much together.  However, recovering from a deep setback, or a particularly cold day will see the vaporstat controlling the system, even though all the radiators are full.  Remember that the boiler is sized for the radiation plus a pickup factor, so this is reasonable.

    It is hard to give a hard and fast rule about how the on/off times for a system running with full radiators and the vaporstat controlling, but typically a minute or so off time and a somewhat longer on time might be expected.  But it varies so much...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
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