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Changing Maid o' Mist valves

Hello all!  After a lengthy renovation that included changing the old boiler for a new one, rerouting several steam pipes, and adding a hydronic hot water radiant system to the basement level, I've been dealing with very loud clanking for a year. 

Thanks to this site and others, I've been educating myself and I'm starting to balance my system.  I have seven radiators and first took the time to make sure they're all sloped correctly.  Two of them were cemented to the floor in a negative slope! 

I discovered that the previous owner or last plumber to deal with my system installed ALL D valves on the radiators.  I've started to replace them with better sizing (for instance, all C on the second floor, a mix of #5 and #6 on the first floor.  I have to test more, but I think I've quieted a lot of the clanking.  But I need to keep experimenting.  My question is: can I simply change the screw-on cap w/ the hole in it on the valve in order to change its size?  Or must I change the entire valve?  I have a couple spare valves to give me flexibility, but hate the thought of screwing and unscrewing valves into the radiator - it can't be good for the threads.  But a cap change is easy and if that is all that's needed, I can balance this system better.


Cris in Brooklyn


  • Boiler Talk
    Boiler Talk Member Posts: 136
    Banging has nothing to do with venting

    I walked into one store and lots of Maid-o-mist boxes were torn open.  I wonder why that happened?  Your banging is due primarily to installation or maintenance.  Please correct me if I'm wrong. 

    Your piping could be incorrect.  That is, are the pipes pitched properly just like your radiators?

    What is the water quality?  Did your installer skim your steam boiler?   How dirty is the water in your boiler? 

    At this time, without further information, that's my guess.  How about some photos of this and that? 
  • More info; pictures to follow

    I'll try to post pics Wednesday, when I'll have access to the boiler room again (it's through my tenant's apartment).  As far as I could tell, everything is pitched correctly, except MAYBE the main steam line coming from the boiler, which seems to slope downward as it heads away from the boiler.  I'll need to check with a level.  But this may be intentional as the condensate is supposed to return via other lines and dumping condensate directly into the boiler via the steam pipe may be what that negative slope is avoiding. 

    The boiler is a year old.  Water is clear.  About half-way up the sight tube.  Pressure is set at lowest on/off settings.  New equipment all around.
  • Hap_Hazzard
    Hap_Hazzard Member Posts: 2,849
    Most mains do slope down from the boiler.

    If they slope towards the boiler there should be drip lines near the boiler. The usual arrangement is to go the other way, so steam and condensate go in the same direction through the main, and the drip lines are near the ends of the mains, connecting to the wet return, which runs back to the boiler.

    If smaller vents quieted things down you might be looking at a runout that's not pitched right or is undersize or both. Reducing the venting might have just kept the steam from forcing its way through the water--that's what causes water hammer--but it might also keep the connected radiator from getting any heat.

    When you get around to balancing, you can swap the vent ports around. If the floats go bad or the valve stops working you need to replace the whole vent, but while you're experimenting it's a lot easier to swap ports than the whole vents, especially in recessed radiators where there's barely enough room to unscrew them.
    Just another DIYer | King of Prussia, PA
    1983(?) Peerless G-561-W-S | 3" drop header, CG400-1090, VXT-24
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