How many atmosphere's to set vacuum breakers?

Al Roethlisberger

I will be adding some vacuum breakers to the top of my overhead gravity hot water system for drain/fill functions and picked up a pair of B&G #26(which I think are rebadged Hoffmans) that I believe have a default setting of 2 atmospheres.  They are adjustable, although the adjustment is just a pair of locked nuts on a spring loaded threaded stem that doesn't indicate "atmospheres" or any other scale.



I was wondering if there was any standard/typical setting of this value or if it varies upon some function I am unaware of?  Or are they fine at the factory default setting?



Thanks,

Al

JohnNY

Your terminology

Is/are 2 atmospheres the equivalent of 14.7 pounds per square inch twice?

Al Roethlisberger

Not sure..... factory set at 4" mercury

Good question, and I would assume that is true.  I ran across a spec sheet somewhere that said it was set by default to "2 atmosphere", but I also just found this:



http://www.bellgossett.com/ProductPages/Parts-No-26-Vacuum-Breaker.asp



[quote]



Designed to

protect closed vessels and piping systems against collapse when the induced vacuum exceeds

design conditions. When used on steam heating systems, the No. 26 Vacuum Breaker controls

induced vacuum, permitting normal return of condensate to the boiler. Adjustable range

1/4" to 20" (mercury) vacuum. Factory set to 4".



[\quote]



The question remains, is the factory setting sufficient?



Thanks,

Al

Al Roethlisberger

Anyone using these?

My plan is to install one or two of these at the top of my old gravity hot water system so that draining the system is easier than having to climb up in the attic and open my vent which is waaaay in the back corner.



In the case of a low water condition where I am not draining the system, I just don't want these to open before the fill/make-up valve starts to fill the system and in doing so introduce air.  So they obviously should be set to stay closed to allow some safety margin that would keep them closed except for when there is a significant vacuum from actually draining the system with the fill/make-up off.



Any thoughts on if the factory setting is sufficient or not, and if not, any recommendations on what to set them at?



Thanks,

Al

JohnNY

I don't think you need vacuum breakers...

...you should be purchasing air vents, like the Taco Hy-Vent, which should open up when you drop your system pressure:

http://www.taco-hvac.com/uploads/FileLibrary/100-7.1.pdf



According to Google:4 inches of mercury = 1.96439112 pounds per square inchI guess I should've known this from the days of mercury manometers.

Al Roethlisberger

Thanks, I have vents.

Thanks John, I have vents, which vent air out of the system.



However, I wanted to install vacuum breakers for those rare times when I want to drain down the system and don't feel like climbing up in the attic to open my main top "vent".  The vacuum breakers do the opposite of the vents, which is to allow air *in* to the system to avoid things like collapsing compression tanks or sucking in air through valve packings and damaging them.



I just want to set the vacuum breakers so that they open well after the fill valve would normally kick in so that in normal operation the vacuum breakers shouldn't open on a minor low water/pressure scenario.  I'd only want the breakers to open well past when the make up valve would open, which I would have turned off during an attempt to drain the system.



My cold water pressure is about 19lbs.



Al

Jean-David Beyer

Taco Hy-Vent...

Does not something like a Taco Hy-Vent allow air to enter if the pressure goes too low?

That is what the contractor relied upon to drain some water from my indirect at one point.

JohnNY

It does.

That was my point.

Al Roethlisberger

Interesting, wasn't aware.....

I have a couple hy-vents and didn't realize they would let air back into the system as well under this circumstance.  I have a couple on some air-separators and just went and tested this(by blowing) and sure enough....



Interestingly the documentation for the hy-vent doesn't mention this, and my B&G high capacity air vent(which is at the top of the system) explicitly has a check valve to prohibit this.



Thanks,

Al