Proper place for vacuum breaker

[Mike Garrod]

The way I was taught was to always put a vacuum breaker above the hot water tank, on the cold supply, theory being if a vacuum was formed it would be broken before siphoning the tank dry. That's my understanding anyways. But I bring this up because I am with a large mechanical company working on a 50 story highrise, and our 'veteran' guy working on the rooftop mechanical room has the vacuum breakers on the four hot water tanks on the hot side of the tanks. Just not sure if the situation of the tanks being above all the fixtures they serve has something to do with it? Or is it just one of those things that plumbers argue about?

 

[Jadnashua]

Watts installation instructions call for them to be installed on the cold supply line
http://media.wattswater.com/1910930.pdf

 

[Sponsor]

 

[hj]

They have absolutely no function on the hot side.

 

[Reach4]

If there is no checkvalve on the hot side, the water pressure/vacuum will be the same as on the cold. So while the hot vacuum breaker may be "wrong", I expect it would do its job completely. The temperature rating on the vacuum breaker would be important to know to determine how wrong things are. I am not a pro.

 

[Mike Garrod]

They have absolutely no function on the hot side.

Is my understanding right? You want the vacuum "broken" before it reaches the tanks, correct?

 

[Reach4]

Is my understanding right? You want the vacuum "broken" before it reaches the tanks, correct?

I was thinking the purpose of the vacuum breaker is to prevent the tank from collapsing. If air can pass either way, then there will not be sufficient vacuum to collapse the tank.

However http://www.watts.com/pages/_products_details.asp?pid=815 says that another purpose is to prevent siphoning. Since the dip tube is on the cold side, I think the vacuum breaker could only stop siphoning if it were on the cold side. So I have changed my thinking, and now think that the cold side would be the only acceptable side if stopping siphoning is an objective. I don't see the problem with such siphoning, but I guess there may be one.

 

[hj]

The "dip tube" in the cold side CAN create a negative pressure INSIDE the can that can cause an implosion. A vacuum relief valve on the cold prevents the suction, and other problems such as "dry firing" an empty tank.

 

[Terry]

The "dip tube" in the cold side CAN create a negative pressure INSIDE the can that can cause an implosion. A vacuum relief valve on the cold prevents the suction, and other problems such as "dry firing" an empty tank.

I agree with hj.

 

[Mike Garrod]

Is a siphon considered a vacuum?

 

[Jadnashua]

If there's no way for air to get into the tank, siphoning it (not all that much will get out like a finger over the end of a straw) will create a vacuum.

 

[Smooky]

You cannot siphon without forming a vacuum

 

[hj]

siphoning from a closed vessel will create a vacuum, otherwise it just creates a low pressure area so the liquid starts flowing.

 

[Mike Garrod]

siphoning from a closed vessel will create a vacuum, otherwise it just creates a low pressure area so the liquid starts flowing.

So, if I understand you correctly, having the vacuum breaker on cold side will allow air to enter under back flow conditions, thus preventing a vacuum from collapsing the tank, or siphoning the water from the tank as well? If it was placed on hot side would it protect the tank from collapsing but not prevent it from possibly being sucked dry?