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low pressure steam question

berm Member Posts: 14
It seems that the consensus on this forum is that your pressuretrol settings should be as low as possible and that your steam system can run run effectively at under 1 psi. Wouldn't running at very low pressure cause the boiler to continually cycle on and off as the pressuretrol keeps cutting in and out?


  • MotownSteamer
    MotownSteamer Member Posts: 110
    it depends on so many things

    This question, in my opinion, really gets to the heart of what this forum is all about. Yes, operating at a low pressure is ideal, but getting a steam system to do that depends on so many things working as designed. Simply turning down the pressurtrol without correcting piping issues (if any), venting properly, etc. will likely result in short cycling just like you said. Like the books say, you need to think about the entire system, not just one component.

    If you are having issues like short cycling, start posting photos of the boiler and piping, give us a description of your system, etc. and lot's of advice will pour into your boiler room
  • Hap_Hazzard
    Hap_Hazzard Member Posts: 2,846
    edited February 2012
    Good question.

    That's a great question because it gets right to the heart of what's really important about this subject: the way to reduce pressure is by improving venting, so the steam can displace the air in the supply system with minimal effort and fill all radiators as quickly as possible. Once that happens, and the vents are all shut, the steam should be condensing at a rate that will approximate the rate at which the boiler is producing it, so you have a lot of movement but very little pressure. You can artificially keep the pressure low by setting a lower limit with a Vaporstat, but that will just make the boiler short cycle, which isn't desirable at all.
    Just another DIYer | King of Prussia, PA
    1983(?) Peerless G-561-W-S | 3" drop header, CG400-1090, VXT-24
  • berm
    berm Member Posts: 14

    what pressuretrol settings do you recommend?
  • Steamhead
    Steamhead Member Posts: 16,901
    Crank it down

    as far as it will go. That's a cut-in of 0.5 and a differential of 1. If it short-cycles at these settings, we have another problem- don't raise the pressure to mask it.
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
  • Long Beach Ed
    Long Beach Ed Member Posts: 1,212
    edited February 2012
    That's the Point

    Hazzard touches on a point almost never mentioned with the "crank it down" mantra. 

    If the pressure control has to shut the burner on pressure, the system's design isn't optimal.  In an ideal system, and there aren't too many ideal systems, pressure should virtually never build to a point that would activate a control.

    A properly designed and vented system would not be building pressure during circulation because air would freely vent from it.   On 95% of the days it operates (less than design temperature), the radiators would never completely fill with steam and the vents would never close.  So the burner would cycle on temperature only.  

    On the days when the radiators are heating at full capacity and all the vents close, an optimally designed system would be condensing steam at the same rate it is producing it -- again not allowing for a pressure increase.  The burner would again shut down on temperature, if it shout down at all.  

    The only times the boiler shut on pressure was when very hot radiators failed to condense steam as quickly as at startup and some pressure built.  ANd this occurred maybe a handful of times each year -- hardly enough to justify a very costly control.

    When we began installing boilers, and designing systems we installed a Vaporstat  on each one -- adding maybe $200 in cost to each job.  We found out that on a properly designed system, we didn't need it.  It would never shut off. 

    Vaporstats do have their place -- but placed on jobs with oversized boilers, undersized piping or poor design, they simply short cycle, or must be adjusted to a pressure that negates their sensitive design.  

    What you have to shoot for is a system that never builds the pressure required to activate a pressure control.
  • Jamie Hall
    Jamie Hall Member Posts: 23,443

    Long Beach Ed describes the situation exactly.  As an example, the museum/house whatever that I care for is a Hoffman equipped vapour system, with a Hoffman Differential Loop as many of those systems were.  The boiler is sized for the system, and very carefully installed and calibrated by Charles Garrity, who does a nice job and really cares about how a system operates.

    It does have a vapourstat, set at 6 ounces cutout.  The only time the vapourstat shuts down the burner is on recovery from a setback, or on a day very very close to design (in this case, 0 Fahrenheit and wind).  There were two reasons for having it: first, the Differential Loop is rather sensitive, and will trigger at abut 10 to 12 ounces.  This does exactly what it is supposed to do -- equalize the pressure in the dry returns with the pressure in the mains, thus a) getting water back into the boiler and b) protecting the traps (I have one or two which are weak, but... otherwise they are 80 years old and going strong).  The down side, of course, is that with the pressure equalized, until enough condensation occurs to drop the pressure to where the main vent opens again, you aren't getting heat (doesn't take that long, but if the burner didn't shut off it takes longer and you're wasting fuel).  The second is that again, Hoffman equipped, and the real control on the radiators isn't the traps, it's the calibrated valves -- which don't work all that well at higher pressures.  So on the whole, on a system like mine it is worth it.

    In many cases -- I have just said on another thread -- it's a nice to have, rather than a must have.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
This discussion has been closed.