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Help with Vaporstat settings

ansky Member Posts: 41
Hi, I need some help getting my Honeywell Vaporstat to the right setting. All my radiators are hot by the time my pressure is up to 0.3psi. However, my heating person set it so that the boiler would turn off at 0.7psi. When I look at the front of the module, I see the Main setting is just below the 1psi, and the Diff setting is right at 10 ozs. Can someone please tell me what I should set this to so that my boiler goes off at 0.3 psi? I tried reading the manual and it doesn't make sense.


  • brian_44
    brian_44 Member Posts: 59

    .3 psi would be approx 5.3 oz.
  • Jamie Hall
    Jamie Hall Member Posts: 22,894
    I would suggest

    the main (cutout) at 6 ounces, and a differential of 4 ounces.  Tip: never set the differential to more than the main minus 2 ounces, as above.  If you do, it may not reset, and you will be chilly...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • jpf321
    jpf321 Member Posts: 1,568
    scale of your vaporstat?

    can you please tell us the numbers on the MAIN scale of your vaporstat? there are 2 different models of devices .. both labelled "Vaporstat" .. and the difference is their MAIN scales. 
    1-pipe Homeowner - Queens, NYC

    NEW: SlantFin Intrepid TR-30 + Tankless + Riello 40-F5 @ 0.85gph | OLD: Fitzgibbons 402 boiler + Beckett "SR" Oil Gun @ 1.75gph

    installed: 0-20oz/si gauge | vaporstat | hour-meter | gortons on all rads | 1pc G#2 + 1pc G#1 on each of 2 mains

    Connected EDR load: 371 sf venting load: 2.95cfm vent capacity: 4.62cfm
    my NEW system pics | my OLD system pics
  • butlermog
    butlermog Member Posts: 50
    Honeywell L408J1900

    Since we're seeing the coldest temperatures in 20 years here, I'm seeing my boiler run much longer than normal. Usually my 0-20oz pressure gauge never leaves 0oz, but last night during a long run I found it at 7oz. Long before it gets to 7oz the radiators are full, so I need to set my vaporstat to cut out earlier.

    I believe that I need to set my Honeywell L408J main=5oz and differential=3oz. Meaning if the system reaches 5oz it will shut down the boiler. Then the boiler will not restart until the system reaches 2oz. Is my understanding correct?

    Attached picture shows my new settings.

    The Honeywell manual is a bit confusing to me (see attached diagram), because the diagram shows a differential setting > main setting. This either would never work, or I don't understand the settings.
  • Jamie Hall
    Jamie Hall Member Posts: 22,894
    Go by the numbers

    on the scales.  if you look at the Honeywell diagram,you will see that the differential is set at 4 ounces and the main scale at 6 -- which is quite reasonable, if you really can operate at that pressure.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • butlermog
    butlermog Member Posts: 50


    Thanks as always for the reply.

    It looks to me that the diagram shows main=6oz and differential=10oz. that is the cause for my confusion as i thought differential should always be less than the main.

    However, after doing some more digging on the wall I have seen some folks who set main=X and differential=X+1 so maybe the diagram isn't so wrong.
  • hoenywell diagram

    I just installed the same vaporstat. And then I set it at 6 oz. main, with a diff of 4. Or so I think, because the diagram in honeywell installation manual seems confusing to me too.

    The diagram seems to have a main=6oz and differential=10oz -- which is confusing me. If the main is 6 oz, then when you subtract 10, it will turn on the burner at minus 4 oz?

    Is the honeywelll diagram showing an impossible setting? Or am I misunderstanding?
  • Fred
    Fred Member Posts: 8,542
    That diagram in the Honeywell instructions is wrong as far as the setting are concerned. I also have the same Vaporstat and I have tested mine after innstallation and watched it work in operation. The Main scale should be set at the highest pressure you want (in your case 5) and the Differential should be set at the # of ounces you want the pressure to drop before the burner cuts back in. (in your case 3) So 5 - 3 = 2 OZ Cut-in.
  • Fred
    Fred Member Posts: 8,542
    Jim_R said:

    Thanks ...not the O.P. .I was just going through some vaporstat threads and I kept coming back to this thinking I was missing something ..I'm running a L408A and figuring the "MAIN" is SET POINT, cut-out is Honeywells "Breaks" and cut-in is their "Makes"..at least with the L408A..OPERATING CONTROLER–L408A BREAKS WHEN PRESSURE RISES TO SET POINT, AND MAKES AGAIN WHEN PRESSURE FALLS TO SET POINT MINUS DIFFERENTIAL.

    That is correct