Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

Vacuum in steam riser

tjs Member Posts: 2
I have a problem at a customers house where 3 radiators are not getting hot at all.The radiators are on the second and 3rd floors far from the boiler, but not the furthest. The house is approx 7000 sq. ft.with oil fired steam boiler replaced approx. 10 years ago. Original insulation on most piping.  House is very unique and consists of convectors, cast iron radiators, under floor radiators with heat ducts for 1st floor. One pipe system. Pressuretrol is set correctly at 1/2 > 2. The 3 radiators that are not heating are all on one separate main. The main is getting hot and the risers to each radiator are getting hot,but radiators are ice cold.Steam is not making it to radiators.  I disconnected each radiator to check the valves with boiler making steam and found that these 3 pipes are pulling vacuum. If you put a match at the valve it sucks the flame inwards.The main with these 3 radiators does have a Hoffman #75 vent in the proper location according to Huffman's Steam manual. Thanks for any advice.


  • Jamie Hall
    Jamie Hall Member Posts: 22,891
    The only way...

    you can pull a vacuum with one pipe steam (or most two pipe, for that matter!) with the boiler running is to be in an area where the steam is condensing faster than the system can feed it.  Are there any other radiators/convectors/whiz bangs etc. connected to this particular riser?  Is ths particular riser uninsulated whereas others are (that shouldn't be enough, but...).

    Is there an obstruction somewhere on the riser?  A partly closed valve, for instance (I have seen valves in the oddest spots...)?

    Just some thoughts for starters...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Paul48
    Paul48 Member Posts: 4,469
    As in

    a venting main, that someone introduces a gush of cool air?
  • Long Beach Ed
    Long Beach Ed Member Posts: 1,185
    edited January 2012
    Smaller Vents

    The other radiators in the system are venting too fast, causing them to set up a vacuum in the system before your cold radiators begin to heat and condense steam. 

    This is an occasion where you must slow the venting of the rest of the system. 

    Put some slower vents (like a Hoffman #40)  on the radiators that tend to heat first when the boiler begins steaming.  You have to hold back the steam from wherever it's going first -- you want them to heat later, which will force steam to find the path which it's avoiding. 

    It's all in the venting -- a game of pressure differential.  The steam rushes to the condensing radiator which is drawing a vacuum.  It likes it so much that it ignores the riser that's at atmospheric pressure.

    Of course the boiler may be too small for the connected load, or be underfired, or piped so poorly that it isn't producing enough dry steam, or the water's oily.   In those cases, you'll play hopscotch with radiators not heating, as you move vents around.  But it's rare that boilers are too small -- they are more often over sized. 

    Good luck with this one.  I know guys who have snaked out 2" pipes trying to solve this one!  Steam's fun, eh? 
  • nicholas bonham-carter
    nicholas bonham-carter Member Posts: 8,576
    vacuum in riser

    how long was the radiator disconnected while steaming. the air must have been pushed out somewhere in the system for a vacuum to suck in the flame. i don't see where the air went and how the system can be in a vacuum longer than a few seconds. is there a horizontal element in these risers which trap water enough to prevent the air from getting out, but empty when vacuum forms? can a fish-tape be put down the riser through the bonnet the same distance as the height above the main?--nbc
  • nugs
    nugs Member Posts: 77

    Long Beach Ed is on the right track here.  Change or adjust the first off radiators so that they heat slower than the far runs.  
  • tjs
    tjs Member Posts: 2
    Additional info

    Thanks to all for advice. On this 2" main,just before the risers for these non-working radiators, there is another riser going into a ceiling hung radiator that is enclosed in a metal ( insulated ) box that feeds hot air thru ducts and rises to first floor room via floor registers. Each one of these boxes are fed with steam at the top of the radiators with a return on the lower side. They are vented on the return side with Gortons #1 main vents. Each return line comes out horizontally with a swing check valve before they drop down into a wet return. There are one of these boxes on this main before the said risers. Now, the risers for the non-working rads run horizontally for about 8 feet before they go up thru an interior wall.They are getting very hot at this point.Pipes are all pitched the right way.The #75 was installed on the dry return 15" before it drops to the wet. I disconnected the radiator on the third floor along with the return in basement for this loop and ran water down the pipe to see if there was a possible blockage and that was negative.I left the bonnet off the valve while the boiler was steaming and still nothing, but I did hear a slight gurgling noise coming from the pipe. All the vents in the house are brand new vari-valves and I tried closing them half way with no difference to these 3 radiators.Keep in mind that this house is probably 80-90 years old and I really don't know when these radiators last worked.Thanks Again for your advice.
  • nicholas bonham-carter
    nicholas bonham-carter Member Posts: 8,576
    open pipe-no steam-vacuum

    what would happen if you closed all the adjoining radiator valves and leave the bonnet open again?

    it is true that an under-sized boiler will not be able to fill all the rads with enough steam to warm them sufficiently, but i don't think the open pipe plus vacuum comes from that.--nbc
This discussion has been closed.