Draining system to eliminate water hammer.

I have received instructions on how to drain my domestic system and then re-charge in order to eliminate water hammer in the system.
The procedure calls for shutting off the main supply and then turning on all faucets, water feeds etc. and then shutting off the water supply to the hot water tank. (gas-fired).
My question is, will this not empty the tank, and if so, won't it be damaged by the gas heating system trying to heat the empty tank?

Way back when (and unfortuneately still today) people put a capped vertical stub of copper pipe near valves thinking that they would stop water hammer. Well, they work for a short while, but then fill up with water as the air is absorbed in the water. Basically, they are a short-term solution. The thing that does work is an engineered hammer arrestor. These either are a chamber with a bladder filled with air, or a piston with a seal and air on the other side. In either of those, the water never touches the trapped air, and those can work for years before they wear out. So, if you have water hammer rather than some other problem you've misdiagnosed what you are planning won't work for long.

But, if you want to drain the system to introduce some air to be trapped in those short pipe stubs, first, turn the water heater off. then, if it is drained, you won't ruin it. Don't turn it back on until you've turned the water back on then opened each faucet (hot and cold) so you get a full flow of water without air...then you can relight the water heater without fear that it is going to burn up because there is no water in it.

Don't bother! The draining will not work for long. Install hammer arrestors where needed.

Have you checked your water pressure with a gauge? You should!

air chambers

Don't even bother trying to drain them. The air chambers are a capped riser. Therefore,
1. You will NEVER drain all the water out of the system so that there will be air at the entry of every air chamber.
2. Because it is a sealed riser, there is no way for air to rise to the top of it so the trapped water will drain down. This is a fact of life whether it is an air chamber, a water heater, or a pop bottle.

And unless you drain the water heater with its drain valve, there is no way to get the water out of it once you shut off your water to it, or the house, and open a faucet to relieve the water pressure, even if the heater is in an attic; it won't siphon out.

even if the heater is in an attic; it won't siphon out.

Click to expand...

Well, actually then can.
In the city of Bellevue new electric water heaters are required to be plumbed with vacuum relief valves so that they won't siphon.

Once the top element hits air, it burns out in seconds.



http://www.watts.com/pro/_productsFull_tree.asp?catId=64&parCat=331&pid=815&ref=2

If you have water chambers previously installed, draining down the home will replenish the air in them for a little amount of time.
So little that now mechanical hammer arrestors have become code.
Those won't waterlog.
http://www.siouxchief.com/B_WaterHammer_Fall.cfm

heater

The heater can "siphon" out under the right/wrong conditions, but it will not drain out unless the drain valve is opened, AND air is admitted into the top of the heater.

Most fairly new water heaters have the vaccuum relief built into the dip tube in the form of a small hole drilled about 2" down.

Hey Terry, thanks, I didn't know about the new water heaters having a vacuum breaker/relief valve. But you have a typo, it should be so they will siphon, not so they won't siphon.

Hey Terry, thanks, I didn't know about the new water heaters having a vacuum breaker/relief valve. But you have a typo, it should be so they will siphon, not so they won't siphon.

Click to expand...

hj and Gary,
The water heaters are the same, we just install a tee on the incoming cold side and put the vacuum relief on it to prevent waster from the dip tube from going backwards up into the cold supply line when the main is shut off.

If the water heater doesn't have the vacuum relief, I sometimes loosen the incoming cold flex connector at the top of the tank to achieve the same purpose. That will also prevent the water heater from draining through the cold supply.

Most fairly new water heaters have the vaccuum relief built into the dip tube in the form of a small hole drilled about 2" down. NHmaster

Click to expand...

They do, good.
Now I need to check with Bellevue and see if that works for the inspectors.
That would be a lot quicker for me.
It also now makes sense why some heaters siphon and some don't.

Still confused

Thanks for all the replies, but I'm still confused.

Simply put, is it safe to drain the house system without shutting off the gas to the hot water tank?

Terfer.

Simply put, is it safe to drain the house system without shutting off the gas to the hot water tank?

Click to expand...

Gas?
It should be fine.
The only reason I worry a bit about the electric ones is the elements.
And most of the time, that isn't an issue either.
If you are just shutting down, and bringing back up, the tank never really drains.

heater

Water heaters had the opening in the dip tubes back in 1955, and have had them since then also. It minimizes the possibility of siphonage, but is not a perfect cure since it can scale over and become ineffective. I do not see a typo, since the vacuum relief valve is to PREVENT it from siphoning, not LET it siphon.