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pressuretrol settings

I have what I believe to be a subtractive pressuretrol ( Honeywell L404F) and in reading the "Lost Art" on page page 14 Dan says these mix alot of guys up. I think I am one of them! There is a main scale and a diff scale. I set the main at1-1/2 and the diff at 1.  but the boiler seems to be shuting down too quickly or short cycling. Any ideas? this is a one pipe system with a Burnham Independance gas fired boiler. I also just installed 2 new Gorton #2 main vents on an antler. Thanks


  • How do you know

    it's shutting down too quickly?  Is there a reliable pressure gauge on your boiler?
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
  • JamesH
    JamesH Member Posts: 9
    Gauge reading

    The only gauge is the 0-30 gauge and it does read about 2 lbs, so I guess my question is why does the boiler run about 3-4 min. and then shut down and then refire in about 3-4 min. even though the thermostat is still calling for heat? That's why I was just wondering about the pressuretrol setting which may be causing it to cycle to quickly.
  • Jamie Hall
    Jamie Hall Member Posts: 23,275
    And when

    does it do this?  At the beginning of the cycle?  That is, when it just fires up?  Or does it start to cycle towards the end of the cycle -- that is, when it has been firing for a while (like 20 minutes or so) and the radiators are nice and warm?  If it is the former, you may have some venting problems.  If it is the latter, it is more or less normal, although it is possible that your boiler is a little large or is a little over fired.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • JamesH
    JamesH Member Posts: 9
    towards the end of cycle

    The boiler seems to do this once the radiators are hot but the thermostat is still calling for heat if that makes sense. The thermostat is set at 68 but the temp is around 64.  If the boiler is over sized any cures that may help this. Thanks for the replies.
  • Paul Fredricks_3
    Paul Fredricks_3 Member Posts: 1,557

    Sounds like it's running fine. Once the rads are hot the venting basically stops, so the unit builds pressure and shuts off. As things start to cool the vents vent a little bit more, so the unit runs a bit to make up.

    And the settings are fine for that control: It kicks off at 1.5 PSI, back on at .5 PSI
  • JamesH
    JamesH Member Posts: 9
    edited December 2009
    Radiators are barely getting hot

    Thanks for all the responses, I am still new to Steam and have learned a lot from the wall and also  my copy of "the Lost Art". I am still having some issues with this boiler in that now the rads just start getting hot and then the boiler shuts off and then restarts  so its really not running long enough to make steam and will not satisfy the thermostat. This is a brand new boiler that was installed by myself and a plumbing contractor who is a very good friend but admits he does not have a lot of steam knowledge or experience in his 30 years in the trade. So he is a bit at a loss for why this is happenning and so I am trying to get some ideas why. We have not skimmed the boiler yet but plan to do that this weekend. Could oil or some other contaminants cause the pressuretrol to malfunction and possibly cause this? Another thought as I was posting this is and I forgot to mention, this is  a power vent boiler so it could perhaps be a  problem with the vent circuit that is causing this constant cycling.
  • jpf321
    jpf321 Member Posts: 1,568
    insulation and ventilation

    you need to insure that your system is properly balanced in terms of venting capacities both on the mains and the rads. as well, make sure that your exposed piping is well insulated to insure that the steam gets to where it's going before turning back into water.

    here are a few great "off wall" articles that may be worth a read:

    It's all in the venting

    Attaining Maximum Efficiency & Economy from Low-Pressure Steam Heating Systems

    I would also suggest a copy of Gerry Gill's downloadable book:

    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
  • Jamie Hall
    Jamie Hall Member Posts: 23,275
    I'm confused

    in one post, you say that it does the cycle bit towards the end of the cycle and the radiators are hot.  In another you say that the radiators are just barely getting hot.  You also note that the thermostat is calling for 68, but that the temperature is only 64.

    First off, is this one pipe steam or two?  It makes a difference.

    Venting is important.  In either one pipe or two pipe steam, the venting of the mains must be sufficient to allow the air to escape reasonably quickly -- ideally, as fast as the boiler can make steam.  jpf has given you some sources for helping to determine that.  In one pipe steam, and in some (but not all) two pipe steam systems, the mains themselves need to be vented (some two pipe steam systems of the vapour persuasion are vented at the boiler, through the dry returns).  In one pipe steam, the radiators also need vents; in two pipe, they do not have vents.  Either way, a lack of venting can cause pressure to build and cause cycling before the radiators have a chance to warm.

    Insulation is also much more important than many seem to realise.  All the mains must be insulated, and as much of the risers as is reasonable.  Lack of insulation may cause an otherwise adequate boiler to be unable to heat radiators towards the ends of the mains; in any case it will use up a lot of fuel to no good purpose.  However, lack of insulation will not cause cycling on pressure -- to the contrary, lack of insulation may make it nearly impossible to build any pressure at all.

    Your mention of the temperature being 64, but the thermostat calling for 68, taken by itself, suggests that the whole heating system is undersized -- not just the boiler.  The question I would ask relative to that is do you mean that the system cannot raise the temperature past 64?  Or do you mean that it takes forever to get there?  It is not at all unusual for a heating system to take an hour or two to raise the space temperature 4 degrees.

    You also seem to have some question as to what is causing the cycling in the first place.  The 0-30 psi gauge, while required by code, isn't much help in determining what your system pressures really are.  A good 0 - 3 psi gauge added to the system would be needed for that.  However, you should identify whether it's the pressuretrol or something in the burner that is causing the burner to turn off and on.  My guess would be the pressuretrol, performing as advertised -- which goes back to the question of why the pressure builds early, if it really does.

    Ideally, a steam system, whether one pipe or two, should run for quite some time -- as I noted in my first post, perhaps 20 minutes or more -- without building any significant pressure.  During that time, the vents are mostly open, air is leaving the mains and the radiators, and steam is reaching the radiators.  At about that point, the radiators should be hot, although they will not necessarily be hot all the way across and down, and all or most of the vents closed.  When that happens, the demand for steam production drops to where the only demand is from condensation in the radiators, and the pressure should begin to build.  And the pressuretrol should turn the burner off.  As condensation continues, the pressure will drop, and the pressuretrol should turn the burner back on.  This cycling is perfectly normal and expected, and should continue until the thermostat is satisfied.  If you are just maintaining temperature, rather than raising it from a setback, it is likely that the thermostat will be satisfied first, and the system won't get to the everything full point and no pressure build up will be seen -- and no cycling on the pressuretrol -- before the thermostat is satisfied.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • 1dodson
    1dodson Member Posts: 13
    Could be your thermostat

    I had a lot of short cycling on mine cause by an old electromechanical thermostat.  The previous owner had replaced the gas valve and didn't adjust the anticipator setting on the old thermostat.  The result was 10 cycles an hour and marginal heat. 

    I replace the old stat with a digital and it works great.   So my advice is to check to see if the thermostat contacts are really closed and calling for heat.  It just might not be any relation to your pressuretrol.
  • JamesH
    JamesH Member Posts: 9
    sorry for the confusion !

    In my previous posts I did contradict my self as to when the boiler was cutting off, but the first time the system fired it really heated up great ( some of the rads that never fully heated were hot) but then after that  is when it stated shutting off and on. Now it doesn't really seem to get fully hot before it starts cycling. This is a one pipe system, I think I mentioned in an earlier post, and has two #2 gortons on an antler for main venting. The old system had one Dole #4 vent. I am also going to check out the other off the wall articles that were mentioned and see if there may be some solutions there also.

     The thermostat is an older honeywell round heat only mercury thermostat and the anticipator is set at .2 so I may just replace it to get a newer digital one. I am going to try and post some pics of the system later and maybe someone will notice something that may look out of place.
  • Mark N
    Mark N Member Posts: 1,115
    Anticipator setting

    Maybe the anticipator setting is wrong. I think Honeywell recommends 1.2 as a starting place for steam. My Burnham manual recommends .8 setting. Try setting it at 1 see how it works. If your overheating try .8

This discussion has been closed.