Why Vacuum Relief Valves?
Jeff Wisnia
I was advised to install a new vacuum relief valve when I recently
had to replace our home's (leaking) electric water heater. Since the
part was cheap enough and I'd never thought to replace it in the 23
years (and two previous water heater replacements by me.) since its
original installation, I went ahead and put in a new valve.
Then I started wondering just what conditions would cause enough
negative pressure to damage a water heater.
*******
The description of Watts' vacuum relief valve reads:
Series N36 Watts Water Service Vacuum Relief Valves are used in water
heater/tank applications to automatically allow air to enter into the
piping system to prevent vacuum conditions that could siphon the water
from the system and damage water heater/tank equipment.
*******
Since our home is at the crest of hill, I suppose that if the city
pumping system went bad, or a water main below us burst there could be a
long enough column of water "hanging down" to pull up to one atmosphere
of vacuum in our home's supply, which could be enough to
collapse a water heater tank?
A plumber I asked said that a fire engine pumping out of a nearby
hydrant could also create a damaging negative pressure in the main.
And, I suppose that even if the vacuum wasn't great enough to collapse
the tank if someone opened a hot water faucet under those conditions and
didn't close it promptly the water in the heater could be sucked out
through the dip tube, leading to burned out electric heater elements
and/or a melted plastic dip tube?
Are those the expected kind of things a vacuum relief valve protects
against?
Are there others as well?
Thanks guys, and Happy New Year,
Jeff
--
Jeffry Wisnia
(W1BSV + Brass Rat '57 EE)
The speed of light is 1.8*10^12 furlongs per fortnight.
Jay
the tank if someone opened a hot water faucet under those conditions and
didn't close it promptly the water in the heater could be sucked out
through the dip tube, leading to burned out electric heater elements
and/or a melted plastic dip tube?
I've never heard of doing this. But there are many times I've shut off the
house water to do repairs and then had to shut off the w/h supply valve to
get the water to stop. This includes single story homes. And yes, having the
power on to an electric w/h with no water in the tank will fry the elements.
Mark M.
I was working on recently finished hillside home that we did the plumbing
on. Some other company did the radiant heat which used a large stainless
water heater as a boiler. I was called out to reroute the water supply
feeding the house. The key to the house was supposed to be in a certain
spot but it wasn't there. I looked over the job and figured I could do it
all from the outside. I shut off the water at the meter and cut into the 1"
line feeding the house. There is always some water that comes back but
this was really a lot of water coming out of the house. I remembered the
big stainless water heater for the radiant. About that time I heard the
power burner start up for the water heater. Now I was really worried. I
rushed around and couldn't find the gas shut off so I found the electric
meter and shut down the main breaker. Ten seconds later I heard the
emergency generator start up, so I had to figure out how to shut that thing
down too.
After I finally got the key to the house I discovered a vacuum breaker on
the hot side of the water heater. The breaker had admitted air and allowed
the entire water heater to empty back out the cold side and down the hill
where I cut the pipe.
If the vacuum breaker had been on the cold side it would have prevented
siphoning.
Mark M.
ken...@adelphia.net
top fed heater has an anti syphon hole in the dip tube to prevent the
tank from syphoning out under certian conditions.
kenny b
Jeff Wisnia
Thanks for that piece of info, I wasn't aware of that.