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One-pipe vacuum system

Suppose a one-pipe system had no radiator, or main venting, but instead had a strong vacuum pump. On a call for heat, the vacuum pressuretrol would check the vacuum, run the pump, and then fire the burner.
There would be a check valve to prevent the air from reentering the system.—NBC


  • AMservicesAMservices Posts: 408Member
    Unless you can get the system 100% sealed, it doesn't work. If any air can get back into the system during operation it will throw off the balance.
    At 15 hg there is still 50% air left in the piping and radiators. So the rest of the air would be pushed into the radiators.
    For this to work, the system would need to indefinitely hold 27-28 hg and then you could at least heat 90% of the system on every cycle.
  • PMJPMJ Posts: 782Member
    There is always some air in the system no matter what. As you increase the level of vacuum with a pump you do reduce the total amount of air in the system but it can't go to absolute zero anyway. Air and steam don't mix so any space in the system (one-pipe or two pipe) that isn't steam is still air... just less of it per unit volume than at atmospheric pressure. I think it is important to view vacuum operation as just the total system is at a different(lower) pressure. Nothing about the operation goes upside down. Steam still flows from areas of higher pressure to lower pressure. It is just that the the absolute measure of those pressures relative to the outside atmosphere is lower. Gravity still returns condensate to the boiler just the same. There is no dramatic change in operation.

    However, having said that we all know that where steam goes is about very subtle differences in pressure. This is where vacuum operation is different when the burner is off. It is important to think about what happens in the radiators every time the burner goes off. The point of lowest pressure in the system is always is in the rads where the steam is condensing the fastest - actually a slight vacuum always in any radiator only partially full. To actually fill a radiator to a higher level of fill in an open system the pressure in the main must be slightly higher than outside atmospheric so the void being created by the condensation is replaced by new steam from the main. But in the open vented radiator when the burner goes off that void is then very quickly replaced by air from the room. Steam collapses so quickly(even in the mains) that open vented the room quickly becomes the highest pressure area. Air from the room fills the void being created in the radiator and then quickly starts to fill the mains too as all condensation in the rads quickly stops. But if you simply instead don't let that air back in then the void being created will continue to be filled by steam from the main even as the pressure in the entire system drops below atmospheric. Within the system in the closed case, the boiler(still making new steam as the pressure drops) will still be the highest pressure area relative to all others and steam will still flow from it to the rads. And the rads in the coldest areas will get the most steam from the mains which evens things out. On next fire it will take several minutes for all the vacuum to disappear so new steam during this period also flows more to rads in colder areas. With vacuum, even relatively small amounts, you take advantage not only of the reduced resistance to getting the steam to the radiators because there is next to no air to remove each time, but also the balancing effects of more steam going always to the colder areas ....wherever they are today.

    To me it seems people think about vacuum too absolutely, too all or nothing. Clearly more is better, but significant benefits are to be had from even small amounts of it. This is illustrated best I think by the Hoffman text posted on the other thread stating the benefits of putting one of their vacuum/vents on just one radiator in a one-pipe system. Clearly there would be no great system vacuum level in that case yet one radiator could be made to draw a higher percentage of the steam available in the main than it was letting air back into it every cycle.

  • jumperjumper Posts: 1,302Member
    In a customary one pipe the steam certainly doesn't push all the air out either. I remember (or believe that I remember) systems that were evacuated manually. Building operators did so only periodically so air leaked in slowly.

    Of course, one can't have a mixed system in which air can enter anywhere. So the method to feed boiler has to be thought through. Now we're really talking about gravity.
  • PMJPMJ Posts: 782Member
    But air does leak in everywhere ... very slowly. No big deal.
  • FizzFizz Posts: 494Member
    Didn't Hoffman or B&J make a vacuum vent solely for single pipe system? That being said, I had a Richardson System, which I put in vacuum by swithcing=out main vents with MEPCO vacuum quick vents 3 yr ago, and the system registers 4-10 hg, with thermostat on 3cph. The heat is steady, no lock-outs, and I haven't blown-down or added water to system. Site glass water crystal clear. Could this be done with single pipe parallel flow by removing rad vents, and replacing mains with vacuum vents?
  • coelcanthcoelcanth Posts: 62Member
    Hoffman made the #2 vacuum vent but i don't think they have been available for a while..

    i would love to try to run my one pipe system in vacuum like this but i don't think there are any similar valves made nowadays..
  • mikeg2015mikeg2015 Posts: 761Member
    coelcanth said:

    Hoffman made the #2 vacuum vent but i don't think they have been available for a while..

    i would love to try to run my one pipe system in vacuum like this but i don't think there are any similar valves made nowadays..

    Yup. I still had 2 of them on my system. But they vented too slowly and my system isn’t a modulating coal system and not perfectly sealed so won’t work anyhow. Plus it’s hard to balance the system and the Hoffman 75 vacuum main vent is pretty small. But a modulating coal system, that wasn’t a big deal.

    They had a long tube that inserted. I think it helped drain more condensate back into the radiator rather than spitting when early steam first hits it, but a lot of air is still left to vent. It was a float style but had a small disc on top to act as a check valve.
  • mikeg2015mikeg2015 Posts: 761Member
    You probably have to repack all of the valve stems and even the smallest leaks between radiator sections will become big at over 10psi vacuum.
  • mikeg2015mikeg2015 Posts: 761Member
    I did contemplate trying ot figure out how to install threaded mini check valve in a Maid O Mist since it has a threaded office. I asked if they were insterested in trying ot make a special vacuum vent.
  • mikeg2015mikeg2015 Posts: 761Member
    edited November 2018
    If I remember correctly, 10-32 threads.

    Only downsize is the 0.5psi cracking pressure. So peak vacuum will be reduced,
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