Power shower with combi boiler no-no?

heppy23

We are getting the bathroom done in the next few months.

The original plan was to have the bath filled from the overflow with a wall mounted tap so no taps on bath, but there is not space in the wall for the tap gubbins.

Our 2nd best option is then normal tap but we'd like a power shower but we have a combi boiler and for some reason you can't have a power shower when you have a combi boiler.

Can anyone explain why? I have a good understanding of physics but I can't work it out.

My brain says combi boiler takes cold water and turns into hot, power shower does the same. Does running a power shower do something to the boiler by means of altering the water pressure or something?

Avoriaz

If you have a combination boiler you probably don’t need a power shower as the hot water will be at mains pressure.

http://www.diyfaq.org.uk/plumbing/combi.html

flang

I have one of these in my bathroom which runs off a combi boiler, its not very powerful however its not that bad
I have a seperate smaller shower attachment on the taps which is very powerful. (same as in the photo but connected to the rear of the taps)

heppy23

Avoriaz wrote:

If you have a combination boiler you probably don’t need a power shower as the hot water will be at mains pressure.

http://www.diyfaq.org.uk/plumbing/combi.html

Thanks. Yes there is plenty of pressure but we are trying to get round the fluctuations in temp and pressure when someone flushes a toilet or when the washing machine/dishwasher is on.

Looks like a thermostatic system may be the way to go.

SmartRRRR

There is a specific type of shower that works off a combi - it has a pressure balancing device inside and from experience does work very well.

Ours is here- http://www.plumbworld.co.uk/1409-15228
but there are others.

Most, but not all, thermostatic showers work well with combi boilers.

Avoriaz

heppy23 wrote:

Thanks. Yes there is plenty of pressure but we are trying to get round the fluctuations in temp and pressure when someone flushes a toilet or when the washing machine/dishwasher is on.

Looks like a thermostatic system may be the way to go.

Yes, a good thermostatic shower is very useful to avoid those nasty shocks and a power shower is so much better than a gravity fed shower. I would hate to go back to a shower that dribbles water at suddenly varying temperatures and flow rates.:eek:

BobProperty

heppy23 wrote:

..... but we'd like a power shower but we have a combi boiler and for some reason you can't have a power shower when you have a combi boiler.

Can anyone explain why? I have a good understanding of physics but I can't work it out.

My brain says combi boiler takes cold water and turns into hot, power shower does the same......

The problems (and why you can't do it) are that a power shower pumps its water from a header tank, which gives it a reserve of water. If you were to pump a combi output you are relying on the mains input being sufficient to feed the pump (which in some places mains pressure is less than even the smallest pump can manage) along with any other water need at the same time. (Usually, you are not allowed to pump the mains water, I think because of the supply problems and backflow problems it may cause.)
The other side of the scenario also gives rise to difficulties. If the combi is heating up the water but it is being "pulled" through the combi "too quickly" then it won't heat up enough.
There is also the risk of "cavitation" within the water supply which would damage the pump and the boiler in a relatively short space of time.
As has been said already, you don't need a pump as the combi should give hot water at a reasonable pressure and flow, which is the same end result as a shower pump.

ozskin

A thorny area, define power shower. when the govt thought about legislating a few years ago they found it was too difficult and gave up. Literally it is a shower with power but everyones perceptions will differ. Usually people are refering to a shower with an integral pump, not an electric shower though. these are a big no no with a combi, you can never ever pump in any way downline from a combi. just dangerous. exactly what do you want to achieve. If you fill the bath from a bath manifold filler as you seem to suggest there are bsm valves that will control both shower and bath fill. let me know the details and i will recommend. usually a combi will deliver a 'power' shower but a weak bath fill but this is down to your boiler and little to do with the shower.