Using a pressure equalising valve.

[Carl Barker]

Hi,

I am looking to install a shower with a booster pump, but am wondering about the best way to do it.

The set up is as follows: I have a system boiler, so all the cold water in the house is mains pressure, but the hot water is held in a cylinder in my airing cupboard and is supplied from a header tank directly about it in the attic.

I wish to fit a shower in my bathroom, which contains the airing cupboard. The shower is a exposed valve thermostatic and needs higher hot water pressure.

The obvious solution is to fit a double-ended pump, supplied with hot water from the cylinder and cold water from the header tank.

However, I am considering an alternative, which is to use a single-ended pump to take hot water from the cylinder to the shower valve and take the cold water from the mains to the shower valve, but put a pressure equalising valve before the shower valve.

The rationale for doing the above is that it should mean that the header tank only empties half as quickly, because it is only supplying the hot water, and so allowing for a longer shower.

I asked the question elsewhere and got the following reply:

"In theory this is possible but you will probably find that joining the cold after the pump means that the increased rate of the hot may "suck" cold out of the tank faster than it wants to go and cause all sorts of air problems."

I'm not sure why this might happen, but I don't understand how pressure equalising valves work.

Thanks for any advice.

Carl

[Christian McArdle]

Works fine. I did this in my previous house. Your shower valve might already incorporate a pressure equalising valve (most, but by no means all do). Check before installing another one.

Well, not quite. If your cold mains are marginal, then the flow of cold through the shower will cause the header tank to be refilled slower, so the effect may be reduced.

Eh? Whoever wrote that was confused. However, there is a problem that they might have been trying to state.

That is that if you try to suck out too much water from the hot water cylinder, it may start drawing air, which is bad. Some things that help reduce this are:

1. Large bore pipe connecting the bottom of the cylinder with the cold tank, with few bends and only full bore valves.
2. Use of a "flange" to take off the hot water, rather than using the vent pipe from the top.
3. Reduce maximum flow rate by turning down the pump.

In my case, the 22mm connecting the tank was fine, and there was no need for a flange.

Choose a good single impeller pump and run the bath tap off it, too, for rapid bath filling. If possible, mount the pump near the cylinder, so it pushes, rather than pulls. Finally, if possible, run the kitchen off the unpumped section, so it doesn't run the noisy pump.

Christian.

[Owain]

Either way, be aware that failure of the mains supply could leave you with unmixed and unexpectedly hot water at the shower. If you take a direct feed from the cold tank *below* the tank outlet for the hot water, then if the tank empties (or the mains water fails) the hot water will dry up before the cold.

Owain