Plumbing advise reqd

[Jeff]

Combi boiler fitted feeding mira excel thermostatic shower - according to the mira book ideally i need a pressure reducing valve (PRV) (drop tight) on the cold feed and if i do so then I need a 'maintained expansion vessel' inbetween the PRV and the shower. I understand the need for the expansion vessel but is there a way round it ? I have read that if the PRV is before a toilet cistern that the ball valve acts as an expansion relief ? Where should I put the PRV ? on the incoming mains or local to the shower ?

Regards Jeff

[Andrew Gabriel]

I would investigate replacing with a thermostatic shower designed for use with a combi or multipoint. These are designed to cope with high cold water pressure and widely varying hot water pressure (which it sounds like yours was not). I have a suspicion you might spend some effort fiddling around with your shower, only to find it still won't handle the output from a combi to your satisfaction.

[Jeff]

The mira excel is designed for combi's etc but ideally wants the same pressure for hot and cold. Hot is from the combi at 2.5 bar whilst the cold is at unregulated mains pressure ( dunno what that is ) and should not be above 10 bar. It will probably work as is but as I believe water meters prevent backflow (as would a PRV) how is the cold water expansion taken care of ?

Regards Jeff

[John Rumm]

Does your combi include pressure regulation on the HW feed then? Most don't and the static hot water pressure should be comparable to the mains cold water pressure anyway.

Is this an actual problem with the shower in use, or just a theoretical one and you have not yet tried it?

[Andrew Gabriel]

Hum, that doesn't make sense. Showers designed for combis/multipoints specifically allow for wide pressure variations between the two sides. IIRC, mine allows for 1:1 through to 20:1 variation whilst in use.

Usually it's the hot which is less controlled, either by pressure or by temperature as other demands are made of it. A combi/multipoint thermostatic show also needs to be aware that you can't necessarily reduce the temperature mix just by reducing the hot flow as many other thermostatic shows do, because the combi/multipoint may well simply increase the temperature to exactly cancel out the effect.

Are you talking about inside the boiler? There's usually a minimum stated pipework distance between the boiler and any reverse flow prevention device in the supply feed.

[Jeff]

Pressure regulation for the combi is via a pressure reducing valve on the cold water inlet set at 2.5 bar ..... so i presume the hot will come out at

2.5 bar. Yes it is a theoretical problem, but one that will be easier to correct now than later due to ongoing work.

Regards Jeff

[Jeff]

Hmm - I'm not putting this across very well, lets try again....

If I put a pressure reducing valve on the main water supply to my house, what consideration do I have to give to thermal expansion given that it cannot now expand back into the mains ?

Regards Jeff

[John Rumm]

Does the valve also act as a non return valve then?

[Jeff]

What little info I can find suggests that they do - its not a problem anymore as the shower is superb :-)

2.5 bar hot and 3.5 bar cold

would still like to find out the answer though, just as a matter of interest

Regards Jeff