Why is a vent pipe necessary?

[Richard Tobin]

Why does a hot water tank fed from the cold water tank require a vent pipe? If the water overheats, why is it not just forced back along the pipe to the cold water tank? Surely the pressure can't be greater than the head of water (plus a bit of friction I suppose).

I'm asking because we have a leaking hot water tank, and the plumber who came to replace it found that it didn't have an "expansion pipe" (which really means "vent pipe" according to the D-I-Y plumbing book I consulted). So we apparently need to have a pipe installed running up through the house from the tank.

-- Richard

[Aidan]

Richard Tobin wrote: If the water overheats, why is it not just forced back along

It can be, but (and it is a big but):

1) the cold feed should have an isolating valve. If this is shut, the expansion route is shut off. Result: burst cylinder. The OV should not have any valves.

2) The cold feed could possibly have a stop-c*ck fitted (it shouldn't, but it happens). Some of these have a loose jumper which acts as a non-return valve. The expansion path is closed. Result as above.

3) If the immersion heater fails in the on position (it happens), the water will boil. The steam will collect at the top of the cylinder and propel the water downwards and up the cold feed into the storage tank, which will overflow. Result; flood of hot water. Unless the expansion path is closed as above, in which case the cylinder bursts explosively. Result; rebuild large section of house. Assuming you survive.

4) The OV also allows air, that comes out of solution in the water when it's heated, to escape.

The plumber's right.

[Christian McArdle]

Yes, normal expansion is handled in the manner you describe, up the feed pipe. The vent pipe allows steam to escape, which wants to rise. It also allows expansion when the supply pipe is blocked for whatever reason.

In any case, I would regard a hot water system without a vent pipe (or not complying with the regulations for sealed pressurised cylinders) to be of the "immediate danger" category. I would physically disable it until it could be fixed or replaced, including removing fuses, especially from any immersion heaters.

Christian.

[Doctor Evil]

The cylinder can implode if the water is forced back up the cold feed. If the cold feed valve is off, it can explode.

He should have cut off the boiler or taken the fuse out of the Consumer Unit, or disabled the immersion in some way.

If he didn't do any of that don't hire him.

_________________________________________ Usenet Zone Free Binaries Usenet Server More than 120,000 groups Unlimited download

to open account

[Dave Plowman (News)]

The cold feed usually has a stop c*ck to allow washer changing etc. Close that with the water heating on, no expansion pipe and bang.

[Aidan]

It should have an isolating valve,i.e., a gate valve or a full-bore ball valve. A stop-c*ck is a globe valve, with about 5x the resistance to water flow of a gate valve.

[Dave Plowman (News)]

Yup. Should have used the correct term. ;-)

A ball valve is better - gate valves rarely completely stop the water flow after a few years.

[Christian McArdle]

Or even when new.

Christian.

[Dave Plowman (News)]

I'd sort of guessed they got bunged up in a hard water area. The ones I changed to full flow ball valves (from my header tank) certainly were.

[Doctor Evil]

Gate vales are no to totally stop water. They are to isolate flow in a circuit.

_________________________________________ Usenet Zone Free Binaries Usenet Server More than 120,000 groups Unlimited download

to open account

[Aidan]

It would'nt be an issue if a stopcock had been installed in the cold feed; unless you didn't have an open vent pipe.

Yeah, love 'em.

[Kaiser]

Gate valves are to control the flow. They must also be capable of totally stopping the flow, if they don't they need replacing. We use gate valves up to 250mm (10") for controlling flow test lines, and they must be capable of completely stopping the flow of water.

[Doctor Evil]

Gate valve stop flow "in a circuit". Have one on an open high pressure pipe that empites to outside and turn it off. Most "will" pass water. They are meant to stop, or control, flow in a circuit, there are no rubber seals to ensure complete "stopping" of the flow. Very different to a stop c*ck, which is meant to completely stop flow on an open pipe (could a pipe with an open tap).

_________________________________________ Usenet Zone Free Binaries Usenet Server More than 120,000 groups Unlimited download

to open account

[Doctor Evil]

Not at all. Read it again.

_________________________________________ Usenet Zone Free Binaries Usenet Server More than 120,000 groups Unlimited download

to open account

[Kaiser]

You've just contradicted yourself.

[Kaiser]

I have and you've just contradicted yourself. So maybe you should read it again.

[Doctor Evil]

It is to stop flow and create water tight seal. Got it?

_________________________________________ Usenet Zone Free Binaries Usenet Server More than 120,000 groups Unlimited download

to open account

[Kaiser]

I've got it, but apparently you haven't. This is now the second time you've contradicted yourself, but carry on, I'm enjoying watching you make a fool of yourself..

[Doctor Evil]

in

Either you have a poor comprehension or a just a total fool.

_________________________________________ Usenet Zone Free Binaries Usenet Server More than 120,000 groups Unlimited download

to open account

[Kaiser]

If I'm a fool there's not much hope for you. Anyway Dr Evil/IMM you can have the last word if you wish as I can't be bothered replying anymore.