Click here to Find a Contractor in your area.
Welcome! Here are the website rules, as well as some tips for using this forum.

If you've found help here, check back in to let us know how everything worked out.
It's a great way to thank those who helped you.
Need to contact us? Visit

Vaporstat Not Working

At least it does not seem to be working to me. My one pipe steam system runs nicely at 1.5-2oz of pressure. However when it runs from a large set back, say 4-5 degrees the pressure starts to climb and the rads start to hiss. This happens at about 3oz and up. I have fiddled with the vaporstat but nothing seems to shut of the boiler until the tsat is satisfied.

Any thoughts?


  • BobCBobC Member Posts: 5,109
    edited February 2011
    Possiby set wrong?

    Did this vaporstat ever work right on this boiler? It looks like your differential is set higher than the main, that might cause the vaporstat to lock up. Try setting the differential to something less than the main.

    Can you actuate the vaporstat by moving the lever that operates the microswitch? It's possible the contacts are welded shut.

    Is the vaporstat on a pigtail?

    Have you checked that pigtail to be sure it's not plugged?

    Check the vaporstat to make sure it's not plugged.

    Smith G8-3 with EZ Gas @ 90,000 BTU, Single pipe steam
    Vaporstat with a 12oz cut-out and 4oz cut-in
    3PSI gauge
  • crash2009crash2009 Member Posts: 1,484
    Go back

     to the factory default settings.  Main 8 oz, Diff 5 oz.  These settings will give you ON at 3 oz, and OFF at 8 oz.
  • Steve_175Steve_175 Member Posts: 234
    All New

    The boiler was installed this past summer. The vaporstat and gauge were installed recently and have a snubber between them and the pigtail. I know the pigtail is clear because the gauge shows pressure changes.
  • Steve_175Steve_175 Member Posts: 234
    On at 3oz?

    How or what is off below 3oz? I want the boiler to shut down when the pressure gets above 3oz not 8oz. Is there a way to set the vaporstat to do that. Assuming it is functioning properly.
  • nicholas bonham-carternicholas bonham-carter Member Posts: 8,133
    edited February 2011
    non functioning vaporstat

    i think i had that same problem, with a snubber installed, so i took the snubber off, and all seems well. if the rads are hissing, that would indicate a need for more main venting.--nbc
  • crash2009crash2009 Member Posts: 1,484
    The factory settings

     will allow the boiler to come on at anything below 3 oz.  For example:  after a long setback your boiler will come on and run at 2 oz, like you said.  Then eventually all your radiators will become full.  Once the radiators get filled the pressure will begin to rise.  It only takes a minute or two to go from 2 oz to 8 oz, then the vaporstat will shut the burner off.  As the pressure drops from 8, and then finally gets to 3, then the boiler will come back on, run for a minute or two, and then shut off again at 8.  This ON OFF ON OFF will go on until the thermostat is satisfyed.
  • crash2009crash2009 Member Posts: 1,484
    On your advice NBC

     I did not install a snubber on the vaporstat, I just put one on the gauge.  I don't know if it makes a difference but I installed my snubber the opposite way Steve did.  Mine has the filter to the bottom.  Mine worked fine right out of the box.
  • Steve_175Steve_175 Member Posts: 234

    I asked the Valve Store if it made a difference which way the snubber went in this application and I was told no. I also asked on this site where I should put the snubber and that is why I did it this way.

    BTW the snubber is able to handle up to 5000PSI.
  • crash2009crash2009 Member Posts: 1,484
    Must be your settings then

     or you got a dud.
  • vaporstat fried?

    did you try bobc's suggestion of actuating the lever inside to see if the switch will cut the power when firing? you don't have the "make on rise model" do you?--nbc
  • Steve_175Steve_175 Member Posts: 234
    Vaporstat Model

    It is the L408J1009

    I have not yet tried Bobc's suggestion.
  • Jamie HallJamie Hall Member Posts: 12,865
    A few thoughts, here

    First, you do have the correct model vapourstat.  That's a start.

    Second, you never set the differential to a value greater than the cutout, since the differenctial is subtractive -- unless your system is guaranteed, every time, to go into a vacuum.  Which most aren't.  If you do, and the vapourstat trips off, it will never trip on again.  No heat.  Not good.

    In your situation, where you want it to trip at 3 ounces (which is lower than I would recommend, even for the most finicky system), the differential should not be more than 2 ounces.  The 'stat should work at that range, but the interval between switching off and switching back on will be very short -- much less than a minute.  I would prefer to see you set it at 6 ounces cutout, with a 4 ounce differential.

    Third, a snubber is not a filter.  It serves a completely different purpose -- to keep short duration (high frequency) pressure pulses from passing through.  It doesn't make any difference which way it is mounted.  I strongly recommend using them on vapourstats which are set very low, as high frequency pressure pulses are inevitable in a steam system.

    Fourth, does you vapourstat ever shut off the burner?  At any pressure?  If not, it may be defective -- but it is also possible that your wiring is defective.  Make sure that the vapourstat is in series with the other controls (e.g. secondary pressuretrol, LWCO, secondary LWCO, etc. -- they should all be in series).  I've seen them in parallel, and that won't work.  Trace your wiring.
    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
  • Steve_175Steve_175 Member Posts: 234
    I hear you

    I have readjusted the cut out and differential.

    I have never observed the vaporstat shut of the burner, at any pressure.

    I do not have a secondary presuretrol and do not know how to check the wiring to make sure everything is in series.
  • Charlie from wmassCharlie from wmass Member Posts: 4,123
    What terminals are you wired to?

    is it wired in series with the other controls?
    Cost is what you spend , value is what you get.

    cell # 413-841-6726
  • Charlie from wmassCharlie from wmass Member Posts: 4,123
    The photo shows you used the wrong terminals

    The middle and the top I think you should have used.
    Cost is what you spend , value is what you get.

    cell # 413-841-6726
  • Steve_175Steve_175 Member Posts: 234
    I thought it was correct

    Look at MarkS's comment on the subject in an earlier thread of mine.
  • nicholas bonham-carternicholas bonham-carter Member Posts: 8,133
    edited February 2011
    wiring a vaporstat

    my vaporstat is using the upper and lower terminals like yours. i think the middle terminal makes it into a "make on rise".

    if you can take the cover off and actuate the rocking lever at the bottom [with a wood pencil] while firing then you can see if it is cutting off the circuit, and therefore the boiler.

    honeywell should really put a tell-tale light on these controls, so it could be readily seen if:

    there is power to the control,

    if the vaporstat swich is open or closed.

    how expensive would that be? and yet so valuable to the owner!!--nbc
  • crash2009crash2009 Member Posts: 1,484
    Time to read the manual

    Another way to see if it will shut off the burner, is while it is running, just turn down the MAIN screw until it shuts off.  If all else fails, read the manual.

    Do you have an Ohm meter?  If so follow the "checkout" procedure on page 6 of the manual
  • Charlie from wmassCharlie from wmass Member Posts: 4,123
    Open mouth insert foot

    I looked at the diagram not the actual schematic. Yes RB are the terminals to use. I think you are trying for too low of a pressure at 3 oz.
    Cost is what you spend , value is what you get.

    cell # 413-841-6726
This discussion has been closed.


It looks like you're new here. If you want to get involved, click one of these buttons!