Vacuum Foe ??

StudyingHard

I have a 2-pipe steam system, dry returns drain into bottom of Hartford Loop.  Certainly I am a newbie, but have read all of "we got steam", and a good bit of Lost Art.  I started this because of high fuel bills and unbalanced radiators, several were hot, several were too cold.



My Question:  When the system is cool (an hour after cutting it back for the night) it develops minus-8 psi.   Is this normal?   No, it is not a vacuum system (no pump).   The main vents do vent on fire-up;  are they sticking shut and not allowing air back in as the system cools down?   During burn cycles (4 or 5 minutes long, with 5 to 9 minute rests)  the ends of the Mains are getting hot, so they are probably continuing to vent, although I don't know for sure (no thermometer logs taken yet).



<span style="text-decoration:underline;">More Details, if you care to read them before answering:</span>

 

I suspect that I have some traps that may be sticking open;  one radiator is blazing hot and so is its return.  The traps are 1/2" Trane pots (B3 maybe) with no model markings on them; probably 87 years old.  The house is 1923 vintage and the traps look as tho no maintenance has ever been done.  I took one apart (shut down that radiator), and will be installing the B&J cages as I find the bad ones.



Pressuretrol is set at 1/2 psi cut-in and 2 psi differential (I have already set it back from 3 psi, and will be taking it down lower as I <em><span style="text-decoration:underline;">slowly</span></em> make changes to the status quo.



No insulation on the pipes, but will be soon (now on a UPS truck coming to me).



As you can see, I am "Studying Hard", proceeding slowly, trying to find the details on my own,  but can't seem to locate anything on Vacuum in the system, except  references to that being a good thing in the coal-fired days.  Which this system was originally.  It  now has an 8 year old Slant-Fin.  Don't know of the intervening years.



Oh yeah, there is no header.   Straight up to an elbow into the Main steam supply.  No water hammer,    And, no F&P traps between the steam return ends (two loops) and the dry returns.  But these are other problems for next summer.   ..... Unless, they somehow begin to demand more attention.



Strangely, the Pressuretrol seems to work even with negative psi in the system.  It still cycles the boiler on and off, although the range seems to be a little wider:  like -3 psi to +1 psi.

Steamhead

How does the air

get out of that system? Does it leave thru vents that can hold vacuum?

StudyingHard

I'm stumped

about that.   I know they vent on cold fire-up.  I placed rubber gloves over them and watched them poof in about three minutes.



However, I can't seem to hear anything venting during regular cycling.  



So, maybe they stick closed  for a long, long time?  and then open?  The vacuum is still there after several hours, although I have not given it days to release.



Nor have I pulled one yet for inspection.  They are Hoffman 76 vents. 

Steamhead

Take a picture

of the vents, and post here. Let's have a look at them.

StudyingHard

Shiver-me-timbers

Matey!   Dern, if right underneath the  Hoffman 76 it says "Vacuum Valve"



Wazzat?   Back to books for me.   Gonna go do some research right now.



Meanwhile I will try to figure out how to post a pic here, although you may not need one now.



Thanks

nicholas bonham-carter

hoffman 76 vacuum valve modification

unscrew the top of that vent to expose a small disk in a housing. this is the one-way valve which makes it a vacuum valve. remove it and you have a plain hoffman 75.--nbc