What is this device?

[Mark Underwood]

This is installed on the cold water supply to the water heater where an expansion tank would normally be installed. The black cap just spins around, so I'm thinking it's an air gap or vent of some kind. I saw no label. Anyone know what it is?

Mark

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[rkenney]

Water hammer arrester.

[Marc]

Water hammer arrester.

That doesn't explain the device atop the pipe.

I'll take a guess...air bleeder.

Marc

[Chad Fabry]

Good lord- you didn't really turn it, did you?

[John Kogel]

It is a vacuum relief valve, boys. We are required to have them here. The theory is that if water pressure fails, that valve will prevent stinky tank water from siphoning out and entering the potable water supply. I am told this is never going to happen, but the plumbers are obliged to install them anyway.

The plastic knob is just a cap. You can spin it all day if you feel so inclined. Chad is just jerkin your chain. []

BTW, it only sucks, whereas a pressure relief valve only blows. []

[Rob Amaral]

Can prevent a water heater storage tank from collapsing if Mr. Fire Truck starts sucking nearby on the public system... They are required here.. Vacuum-breaker

[Jim Katen]

If you have stinky water in your water heater, what difference does it make if it backs up into the cold water supply? It's just going to mix with the cold water supply at the fixtures anyway.

Regarding the fire pump thing, the physics don't make much sense. Vacuum breakers aren't present in this location across the vast majority of the country, why aren't those tanks collapsing when the fire trucks do their thing?

[Rob Amaral]

"Welcome to MA" folks.. Its what I was told.. I don't know of any occurring either!!

[mlparham]

I do smell a lot of stinky water heaters in my market now a days. Years ago it was only the old ones, in some parts of the city. Now, even ones that are only one or two years old and in homes that have never been occupied stink, once the heat to the unit has been activated.

[John Kogel]

If you have stinky water in your water heater, what difference does it make if it backs up into the cold water supply? It's just going to mix with the cold water supply at the fixtures anyway.

Regarding the fire pump thing, the physics don't make much sense. Vacuum breakers aren't present in this location across the vast majority of the country, why aren't those tanks collapsing when the fire trucks do their thing?

There is another scenario, where the tank could siphon itself out if a water main broke, or if pressure dropped for any reason like a fire down the street. Then an electric water heater might burn out an element and a gas unit could overheat itself. Anyway, they have been mandatory here forever, except in manufactured homes maybe not.

Sure I see water heaters on occasion that never had one installed with no apparent ill effect, but that would be out in the sticks where folks are kind of reckless. Like Oregon. []

[Jim Katen]

There is another scenario, where the tank could siphon itself out if a water main broke, or if pressure dropped for any reason like a fire down the street. Then an electric water heater might burn out an element and a gas unit could overheat itself. Anyway, they have been mandatory here forever, except in manufactured homes maybe not.

Sure I see water heaters on occasion that never had one installed with no apparent ill effect, but that would be out in the sticks where folks are kind of reckless. Like Oregon. []

The vacuum breaker is what would make it possible for a water heater to drain itself dry. Without the vacuum breaker, the water could never leave the tank unless someone opened a faucet or other fixture to let air in. I know this because I live on a hill, 100-feet above my water meter. Our water co-op is a rather loosely maintained affair and we lose all water pressure a few times a year. When this happens, everything is just fine unless I open a faucet. In that case, the faucet sucks air and water starts to drain out of the hot water tank. Close the faucet, and it stops draining.

[John Kogel]

There is another scenario, where the tank could siphon itself out if a water main broke, or if pressure dropped for any reason like a fire down the street. Then an electric water heater might burn out an element and a gas unit could overheat itself. Anyway, they have been mandatory here forever, except in manufactured homes maybe not.

Sure I see water heaters on occasion that never had one installed with no apparent ill effect, but that would be out in the sticks where folks are kind of reckless. Like Oregon. []

The vacuum breaker is what would make it possible for a water heater to drain itself dry. Without the vacuum breaker, the water could never leave the tank unless someone opened a faucet or other fixture to let air in. I know this because I live on a hill, 100-feet above my water meter. Our water co-op is a rather loosely maintained affair and we lose all water pressure a few times a year. When this happens, everything is just fine unless I open a faucet. In that case, the faucet sucks air and water starts to drain out of the hot water tank. Close the faucet, and it stops draining.

The vacuum breaker is supposed to break the suction on the cold line at the top of the tank. Sounds like you need one. [] Except you'd get 100 feet of air in your line every time.

How about a check valve down by the meter? That's OK, I hate working on plumbing too.

The standard here for new homes is for a check valve and an expansion tank. And they still put the vacuum relief valve in. I think the check valve makes an expansion device necessary. But I'm no plumber.

[Douglas Hansen]

Our plumbing code (UPC section 608.7) requires them whenever a water heater is elevated above the level of the fixtures. An example would be a commercial building with a small water heater above the ceiling of the restroom baths. We don't often find attic water heaters in houses, though if we do, they are supposed to have these vacuum relief valves.

Here is what Watts says: "Series N36 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."

It still doesn't make sense to me.

[Marc]

Makes sense to me. Under zero pressure conditions, the valve allows air to enter the cold water inlet at the top of the water heater and relieve any vacuum that might develop. Without a vacuum, siphoning via the street main can't drain it unless a cross connection of some sort exists.

Marc