DIY Doctor UK DIY Forums

I am considering getting a combi boiler fitted and I hope to take out my electric shower and make use of a shower bath tap mixer. Will any standard combi boiler offer enough water pressure to operate my shower mixer safely and satisfactorily?

See answer in your other thread on the same subject.

As long as it is thermostatic mixer - for safety reasons - it should provide a really good/powerful shower (mine does)


htg

Its not advisable to run a combi boiler with combination bath and shower mixers taps, as they mix in the body of the shower and could send the hot water from the comb back to the boiler as the pressures will be different on the hot an cold and that will keep shutting the boiler down,,, you can get a pressure equalizing valve fitted and non return valves fitted, but by the time you have done all this you might as well put in the correct high pressure type thermostatic wall mixer in at least they are made for the purpose.

Hi all,

Rosebery said this in the other thread
“Electric showers run off mains pressure.
Combi boilers run off mains pressure.
For pressure, therefore, you should be OK. “

Gasmick
Are you saying the above is not correct?

Sorry to ask but there seems to be two trains of thought on the same thing!

Cheers

Hi,

Also would a Shower Check Valve from Screw fix £1.50 (it screws in to the top of the mixer tap) stop the back flow Gasmick has mentioned?

Cheers

hi as i have mentioned before bath shower mixers do not work properly on combi boilers i know what you are saying about pressure
but the hot water flow rate on the tapas will be a lot different to the cold that is why you will need a double pressure equalizing valve fitted so the cold pressure does not force the water back up the hot pipe and then into the boiler shutting it down all the while,
if you want to go ahead and do it as you think with a check valve only fitted, go ahead, but it will cause you problems, so my advice is to do it correctly in the first place and use a thermostatic wall mixer

But you can get a 'thermostatic wall mixer' that fits onto the bath - this is the one I have in my house

https://supreme1.sslpowered.com/cgi-bin ... item=00025

Gives a very goos shower (if your boiler is up to it).

Also you shouldn't be using any type of mixer shower with a combi - that does not have thermostatic controls, as well as the cheaper one's not having NRV's I think this is what mick is trying to say, it doesn't matter if it fits on the wall or on the bath, standard bath shower mixers are not suitable.


htg

Thanks all for the advice and thanks Gasmick what you have said does makes complete sense. I’ll stick with my electric shower for now.

Sorry to have to disagree with Gasmick, but you won't see a back flow problem when a combi is installed. If you think about it, if the flow is forced back through the boiler, it only can go back into the cold main which is also supplying the cold tap that is supposedly forcing the water back. The Oozlum effect I think?

Backflow which is only ever seen on gravity fed systems (where header tanks are fitted) can be stopped easily by fitting a non return valve, but the water pressure can be so low, it is unable to open the valve so the hot flow totally stops. Also, because of the unequal pressures, it is impossible to get a balanced temperature from the shower head.

I fit bath shower mixers very often, and they do work well with a combi as Htg says. The only downside is BSM's are only really designed for washing hair etc whilst having a bath consequently, not all are supplied with a wall bracket or a long enough hose. If you intend to use one as a proper shower I recommend following Htg's advice and choose a purpose designed thermostatic mixer. The Mira Excel is a superb bit of kit!

"Sorry to ask but there seems to be two trains of thought on the same thing!"

No the two answers are not two traisn of thought on the same thing. I was answering your question specifically as it related to pressure and stoppped there. Like PB I don't entirely agree with gasmick either but don't forget eveyone is entitled to their opinions. If anything the pressure on the hot will be ever so slightly less than the cold but not to the extent that it will make any appreciable difference.

However I do agree with GM, HE and PB. If you are going to use a BSM then use a thermostatic one.

Cheers

"Also would a Shower Check Valve from Screw fix £1.50 (it screws in to the top of the mixer tap) stop the back flow Gasmick has mentioned?"

The purpose of this device is (as PB says) primarily for use on low pressure systems to stop the "contaminated" bathwater being syphoned back into the water supply when you drop the shower head into a bath full of water.

Cheers

Nearly forgot on the safety front. HE rightly commented "provided your boiler is up to it" or something very similar. A combi boiler heats the water as you demand it. If the demand is greater than the ability of the boiler to heat it the temperature will fall. Chances are the user will then alter the divertor towards the "hot" setting to increase the temperature and the next pulse from the boiler could be very hot indeed and a scalding could result. Hence use a thermostatic valve of some description.

Cheers