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What to Expect Using a Vaporstat?

My house has a Burnham IN-6 Boiler, intermittent pilot, and single pipe steam.  The boiler came with a subtractive Pressuretrol.  It is set at 2 PSI with the minimum 1.5 PSI differential.  The boiler turns off at 2 PSI. When the pressure hits .5 PSI, it's dropping pretty fast.  By the time the boiler fires back up and starts making steam, the system is right on the verge of hitting vacuum and sucking in air . . . so I don't think I want my "on" setting any lower.

I keep reading about the benefits of running lower pressure using a Vaporstat.  What might I expect to be able to accomplish with one? 

If my "off" set point was lower, and the system operated at a lower

pressure, would I notice a difference in my comfort or my gas bill? . .

. or would the boiler just cycle more often and wear out the controls faster?

Would I still need to keep my "on" set point at 8 ounces?  Or. . . .if the boiler turned off at 16 ounces instead of 32, would the pressure be dropping slower as it reached 8, so I could set it lower?

Bottom line: Other than bragging rights, what would I achieve running lower pressure?  Is it worth the expense of trying it?


  • nicholas bonham-carter
    low pressure advantages

    my theory is that low pressure equals lower radiator surface temperature, which is more comfortable, gentle heat. this was more pronounced with the old vacuum systems apparently.

    in my 1,050,000 BTW 55 rad system, the air is out at 3 oz. and  maxes out at 8 oz. i have many vents on each of 6 dry returns.. if you get a lot of pressure cycling, then it could indicate a need for more  venting, or an over sized boiler. there is a pressure drop during cycling, but i never hit zero, unless the thermostat is satisfied. if you get more venting, then you could drop your high to 1.5 psi, or just bite the bullet and get a vapor-stat. 

    is your thermostat set for steam?--NBC
  • Jim Pompetti
    Jim Pompetti Member Posts: 552
    Theres more

    You will need to look a venting , are your mains vent correctly , along with the radiators .Steam is an art ,there is not just one answer to a quiet , even heating system.
  • Big-Al_2
    Big-Al_2 Member Posts: 263
    edited September 2009
    More data

    Thanks for the replies!

    Boiler is about 143,500 BTU output.  The main and branches  in the basement are well insulated . . . right to the boiler. Risers to the second floor are not insulated, but they are routed through living space, not walls.  Radiation is about 75,000 BTU, so, yes, even with a generous pickup factor, it is oversized.  It was in the house when I moved in. Once the system is up to pressure, recovery from .5 PSI to 2 PSI takes about 5 minutes.  The boiler is only 8 years old, so I don't want to replace it.  There are four rooms without radiators, so when I feel rich enough to hire a fitter or have the time to borrow a big threader and do the job myself, I may add some radiators . . . so  the extra boiler capacity might eventually come in handy.

    Venting is OK, I think.  The main has a very large old Trane vent that still works, and I added a Hoffman 75 right after I bought the place in 2007.  (No room for a big Gorton.) The radiator furthest from the boiler has a VariValve on it too, wide-open (undersized radiator in a cold room) so that should help vent the whole system.  The boiler has the usual 15 PSI gage, which is fairly useless at low pressures, but the vents close long before the needle moves off zero.   I'm tempted to add more main venting, but I'm not sure what I'd gain.  The rest of the radiators have fairly new Gorton or Watts vents, working OK and sized for where they are.  I experimented with different venting rates for a while to try to even out heat from room to room.  It helped a little.

    Thermostat is electronic.  I think I have it set up for one cycle per hour, maybe two..  (At work and can't check.) I typically get about a two-or-three degree swing and the boiler isn't short cycling off the stat.

    When the Pressuretrol closes the switch, it takes about 30 seconds for the burner to fire, and maybe another 30 seconds to make steam again.  The Pressuretrol I have is as low as it can go. 

    Looking at my chart, steam at 2 PSI is 6.5 degrees warmer than steam at 0 PSI.  I suppose that boiling the water at a cooler temperature would reduce the stack temperature by the same amount, and extract a little more heat before it goes up the chimney.  Is this where the added efficiency comes from?
  • Jamie Hall
    Jamie Hall Member Posts: 23,442
    Very honestly

    this looks like an ain't broke, don't fix it to me.  The biggest gain from the vaporstat would be that the boiler would be a lot hotter when the burner came back on, which is going to raise your efficiency -- some.  Enough to worry about?  Probably not.  There would be very little change in the number of pressure cycles and hence burner on/off cycles; your biggest problem there is the oversized boiler and not much you can do about that.

    Myself?  I'd leave it until I had to change either the 'stat or the boiler; then I'd get the vaporstat.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • nicholas bonham-carter
    another thought

    with an over-sized boiler, you could think about a staged "hi-lo-hi" burner control valve [if gas]. it needs 2 pressure controls. the first one is wired to open the gas valve full, and the second one will cut the valve down to the 75% point after the air is out [3 oz in my case]. why not do a search here for the "staged burner", or "hi-lo"

    remember you can always mount a taller main vent on the vertical drop from dry to wet return, if headroom is a problem. most of the problems reported here are due to bad main venting, and high pressure. the hoffman 75 is very slow compared to the gorton #2. the trane could be working as well as it once did, or not;  but how can you be sure? why not add to them and see how much improvement there is. in my case, the gas co. came out to replace the gas meter, as i am sure their billing dept. reports lower consumption, [worried about gas theft, i suppose].  the cure is pretty simple, and installable by most DIY homeowners! --nbc
  • Paul Fredricks_3
    Paul Fredricks_3 Member Posts: 1,557

    Steam is a gas. The more pressure you put on it, the smaller area it takes up. Lower pressure steam moves faster than higher pressure steam. For example: 2 cubic feet of steam takes up a longer piece of pipe that 1 1/2 cubic feet of steam. That means the steam gets to the radiators faster, which means your thermostat satisfies sooner, so the burner runs less.

    One problem you may run into is short cycling with a vaporstat. An oversized boiler will make steam too quickly for the closer on/off setting of the vaporstat. Is this gas or oil? Either way, if it can be downfired a little that would help.
  • gerry gill
    gerry gill Member Posts: 3,078

    and if you actually do go off on pressure now, you will most certainly short cycle with a vaporstat..thats not saying don't put it on..you should..but you really need to control the firing rate with two stage firing..one can't control the pressure without first controlling the fire.
    Serving Cleveland's eastern suburbs from Cleveland Heights down to Cuyahoga Falls.

  • Big-Al_2
    Big-Al_2 Member Posts: 263
    On the level.

    1) I did talk with the local industrial steam guy about fitting the boiler with a two-stage gas valve and staging a low/high fire situation with the burner.  He advised that if the burner wasn't specifically made for it, the efficiency could actually be worse that way.

    2) Ever since I moved into the house, it has bothered me that the Pressuretrol was not installed level.  Having a mercury switch, I figured that had to throw things off.  Last night, I took a big wrench and gently re-shaped the pigtail until the Pressuretrol was level.  I turned the stat way up and ran the boiler for a while.  The control seemed to behave much more consistently, and I actually had some usable adjustment at the lower end of the scale. The system has one more radiator connected than it did last year, and the steam pressure didn't seem to rise as quickly either.  It isn't that cold out, and when the house hit 80 degrees, I called it a night.  When heating season actually arrives, I'll experiment some more and see if I can't lower the pressure a little more than last year, using the same equipment.
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