Shower Supply - 15mm or 22mm hot feed?

[Richard Conway]

Hi all,

We are in the middle of revamping the bathroom. We currently have a Bristan Pisa shower which is gravity fed, but despite this it gives quite a good shower. There is a fair bit of head as we have a combination HWC in the loft and the bathroom ceiling is pretty high. I was just wondering really if there would be any noticeable effect if I were to replace the current 15mm hot feed (about 3-4 metres horizontal in the loft and then about 1-2 metres vertical down to the shower) with

22mm. It wouldn't be a big job but I won't bother if the difference would be negligible.

Any comments appreciated.

Richard.

[dom]

IME - the difference is substantial.

[adder1969]

Will it take longer for the hot to come through?

[Christian McArdle]

That depends on the increase in flow rate, for which there are too many variables to calculate. Typically, replacing 15mm with 22mm on a gravity shower system will increase flow rate by about 50%, but it will take 50% longer for hot water to become available.

However, this is only a rough average and will be affected by many factors, such as the pipework routing, shower mixer type, available head and shower head type.

Christian.

[dom]

Yes - you've roughly doubled the volume in the dead leg.

I've got a trevi boost (gravity with hot flow boost from cold pressure/venturi effect) and gettting rid of a short section of 15mm considerably improved flow. Head is about 1.5-2m.

[The Natural Philosopher]

Bur doubled the flow rate?

[dom]

Maybe. Depends if the shower is operated at the maximum flowrate available.

(As Christian says though, a higher flowrate will likely be available - unlikely to be double though).

[Richard Conway]

Ah, but although there is more cold water to clear, if the flow rate has increased then wouldn't that mean the cold water would be cleared more quickly? Might end up taking around the same time in that case, or is my logic flawed?

Shower is a thermostatic mixer, the 15mm pipework is T'd from the 22mm outlet from the HWC just after it leaves the cylinder. It then curves down to loft floor level (plastic piping clipped to rafters) and crossed the loft floor (about 3-4 metres in total) then there is an elbow that takes the feed down to the shower - the leg to the shower is probably between 1 and 2 metres. I presume that if I were to replace the whole run it would be worth substituting the elbow for a gradual curve to get maximum flow.

[adder1969]

The shower head is a restriction to flow so I can't see that the increase in flow rate out of the shower would be all that much greater.

[Christian McArdle]

No. Your logic isn't quite flawed. Had the flow rate doubled, then the wait time would be unchanged. Had the flow rate tripled, the wait time would actually be less. However, the flow rate simply won't be doubling or tripling, so the wait time will just about always increase.

You could. Personally, I would just fit a pump and be done with it. But then, I don't like low pressure showers, whatever the flow rate.

Christian.

[David Hansen]

On Fri, 20 Oct 2006 16:29:34 +0100 someone who may be Richard Conway wrote this:-

Probably.

Unless you wish to do a lot of calculations then there is probably no substitute for either leaving it as it is, or doing it in the hope that it makes a noticeable difference but accepting it might not.

A venturi shower has already been mentioned. That would be one way to boost the shower performance without a pump.

[Richard Conway]

The point is that we're not unhappy with the shower - I just thought that if it made a noticeable difference it would be worth doing while I'm in a position to do it.

[Christian McArdle]

If you are happy with the shower as is, then leave it.

Although the proposed changes might (or might not) improve the shower, they will definitely result in increased water and heating usage, even if the flow rate doesn't improve significantly.

Christian.

[adder1969]

Maybe try it and see?

[The Natural Philosopher]

Then not much point in replacing the 15mm to start with is there?