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Pressuretrol - how long should pressure hold for?

Steven Todd
Steven Todd Member Posts: 13
Hello all. I've read the Lost Art but can't find an answer for this:

Assuming the thermostat's target temperture is not yet reached, and the pressuretrol turns the boiler off (because 1.5# of pressure was made), how long typically should the boiler stay off (for a one-family house, one-pipe steam)?

One minute? Two minutes? Ten? The radiators are hot and it seems like the system should 'coast' for a while (but it's not - the boiler kicks back on 90 seconds later).

If it comes back on 'too soon', what might be the cause? Thank you very much. A guy from Find-A-Professional will be tweaking other things next week but I'm not sure if this is a problem to fix. Many thanks for you help.


  • Fred Harwood
    Fred Harwood Member Posts: 261

    Once the pressure switch shuts off the fire, the rate of pressure drop is mostly a function of how much radiation is condensing steam. Ideally, a match between the fire and the radiation would mean that the fire would burn until the thermostat was satisfied, all without significant pressure. However, because most fires are larger than connected radiation, the pressure will build until the pressure switch opens. The interval between on and on again is highly variable, depending on the mismatch between the boiler and the radiation.

    To prevent very rapid cycling of the pressure switch, a differential of perhaps 0.5 psi is built into the switch, so that once the fire is off, the pressure must fall at least that amount before it turns on again.

    In sum, once the fire goes out, pressure rapidly drops because the radiation continues to condense steam. Differing configurations of boiler and radiation make for highly variable on-off cycle times.
  • Drod
    Drod Member Posts: 59
    Mine cycles in a similar fashion,

    Though most of the time, my system shuts down due to satisfying the thermostat, usually long before my pressure gauge even registers any pressure.
  • Fred Harwood
    Fred Harwood Member Posts: 261

    Is the boiler cycling on pressure, or on the thermostat's anticipator? If the latter, you might reduce anticipation by -slightly- moving the brass-colored anticipator slider down (higher amperage). Do this carefully, note the starting position, and watch for temperature overshoot at the thermostat. If the overshoot exceeds one degree, return the slider to the original position.
  • tombig
    tombig Member Posts: 291
    Short Cycling Steam

    Are the rads. hot all the way across when it cycles off on pressure? With the stat not satisfied? It could be venting issues or inordinate condensing in the mains. Is the boiler oversized? It might be building pressure before all the air is out. Quick pressure drop may be due to continued air venting after cycle off.
  • Fred P
    Fred P Member Posts: 77
    are all boilers oversized once theyve heated

    all the piping and rads? I think Noel posted that a while ago, once youve heated all the piping plus rads your right sized boiler is now a bit oversized because the pipes are hot and provide less resistance to the steam (or something like that)

    My boiler does the same thing, shuts off on pressure and restarts a few minutes later when the pressure drops to 0 (or 1/2psi but my pressure gauge isnt that sensitive).

    It does this until the tstat temp I set is satisified or until I manually turn down the stat because it gets to hot!

    Actually from a cold start it runs the longest and after that its a pretty steady few minutes on and off cycle.

    I have two rads out of seven shut off temporarily so I would guess that if they were brought back into the picture the boiler would run a little bit longer since pressure would build a little slower with all the rads open.

  • Steven Todd
    Steven Todd Member Posts: 13
    without significant pressure?

    Thank you Fred. I see that the many variables can make the kick-off, kick-back-on time vary a lot system to system.

    I'm curious about "Ideally, a match between the fire and the radiation would mean that the fire would burn until the thermostat was satisfied, all without significant pressure."

    In the ideal system, the radiator vents would still close soon after the steam hits them, right? For example, while the boiler is bringing the house from 62F to 67F, do the rad vents start open, close when hot, and then stay closed (till 67F)? Or do they close and open, close and open? Mostly just curious. Thanks again.
  • Steven Todd
    Steven Todd Member Posts: 13
    might be the venting...

    Thanks tombig.
    Yes, the rads are hot all the way across when it cycles off on pressure (with the stat not satisfied).
    The vents do seem to be venting a bit more/longer than I'd expect (little that I know); but most of them were replaced six months ago.
    Is there any way to determine if a vent is still good?
  • Fred Harwood
    Fred Harwood Member Posts: 261
    Ideal system

    In an ideal system, the rads would not need vents that close against steam, because the boiler's ability to make steam would be matched by the radiators' ability to condense steam, all without significant pressure. In the real world, boilers should make a little more steam than the rads can condense to ensure that they get enough on cold days, and to initally warm up cold pipe.
    The vents ensure that steam stays in the radiator after the air is gone. As the rad warms, air comes out in burps as the vent closes, opens, and closes as air warms and wants out. The vent usually clicks as that happens, and, if the boiler builds significant pressure, the pressure switch cycles the boiler on and off.
    Also, the thermostat is a delicate instrument with more than one function. The first function is to see the house temperature and to turn the boiler on and off as needed. It usually also has a second function called anticipation, which prevents overshooting the desired temperature. Properly set, the anticipator function cycles the boiler at the correct pace to keep all rads heating evenly and the house temperature more even than without anticipation.
    The anticipator works by supplying a bit of internal heat to the thermostat. The amount of internal heat can be varied in most thermostats, and initially is set to the current flow in the thermostat. By observing your system in operation over several days and conditions, you can slighty change the anticipator setting to tune your system, which then will cycle as the thermostat anticipats house temperature changes
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