Bristan Shower Runs Cold

12 Oct 2009
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United Kingdom
Can anyone suggest why my Bristan Prism Thermostatic shower runs cold?
I am on my third cartridge in 2 years. Each time I replace the cartridge it is fine and boiling hot for 3-4 months then it gradually becomes colder but only at certain times of the day. You can have a hot shower at 4pm but a warm one at 7pm.

When this happens there is always hot water available from the other taps in the house and the hot water tank is full of hot water. The boiler is new and 1 year old.

Each time we renew a cartridge we check the filters in the elbows for limescale build up - there is none. The old cartridges are free of limescale as well.

I am just about to contact Bristan again and ask for my 4th cartridge.
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Where is cold supply to the shower coming from - mains or a tank?

Is the hot water tank vented or unvented?
Hi - I'm in the same boat. Every 6 months approx. our prism goes cold. Bristan originally suggested limescale was the problem but when we pull it apart no limescale is visible but the moving 'washer' end has stuck hard against the main body of the cartridge. When we free it up the shower works properly again.. until the next time!

I spoke to Bristan again today, who told me that nobody else was having these problems! The chap also said limescale should not be a problem "in so short a time". He offered to send a new cartridge, or to replace the valve with an Artisan which is the only bar valve they do with a different cartridge.

So - two things :

1/ 'iamnotaplumber' - you didn't say, but are you also finding the moving end piece stuck against the body of the cartridge when the shower runs cold? Freeing it will be a much cheaper fix than replacing the cartridge when your warranty runs out!

2/ are 'iamnotaplumber' and I the only people with this problem? I can't believe we are. How long do other folks find their prism runs before it goes cold? This valve does not seem fit for purpose to me.

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I have the same problem on my Prism thermostatic valve with diverter. Very hot water at all other taps in the house, yet warmish shower & bath.

I'm on my second thermostatic valve and just about to ask for another one. The last one was free and I hope this one will be too!

Is there a chance that my hot water is too hot? Perhaps causing a fail safe? I have a thermal store Boilermate set to 80 degrees. As I said the water is very hot elsewhere in the house.

It seems there might be a few of us out there.
I've had the same problem with a Bristan Regency dual control shower which I suppose uses the same thermostat.
I'll probably cut my loses and buy another shower as mine is just out of warranty and I don't want to stump £100 for a fix that, from what I read, might not work. Also I have other Bristan fittings (taps) and the washers in both failed early. Hence time for another brand I think.
Bristan what model ?

You should of ran it on equal pressures. You have probably knackered the cartridge.
@ Imnotaplumber

Unequal pressures are not the issue.

Bristan make their showers to work on a pressure ratio of 5:1 (like most companies) so unequal pressure problems will only normally pop up if your gravity hot cold mains.

The biggest problem you have is that you've not given us what shower model you have, as I know from their website the prism range is huge.

The problem is normally the grease around the oring on the "shuttle" that moves has worn off an small amount of scale has build up, once this happens the whole mech' jams and scale builds up on the "worm screw" and "shuttle" and you get one temperature.

I'm guessing as your shower is two years old you've not had the updated plastic cartridge Bristan is doing? These are far better, :) as they cut down on the amount of lime scale that can stick to them and the silicone grease (which you need to replace when you service the cartridge) lasts longer.

Have you got any product codes? As this will help us all to find out which shower you have and offer some real advice.

Hope your able to fix your shower

@ Markp42

How old was your shower? I've spoken with Bristan Technical department and the Regency shower is only two years old, in which case yours just needed some TLC.

If you wanted you could of taken it to them, for a fix, they have some good guys working their trade counter I've had loads of good advice and fixes from them over the years.

Tap washers fail, it is one on those things, it can happen to Deva and Armitage too, so giving up on Bristan after one failure is, well foolish in my mind but hay each to their own.
Same thing hete, Bristan coming out to fit second replacement. Less than a year old.

Pipes are hot but water tepid. It was s corroded seal last time letting too much cold into the mixer.

Will be the same again.

The off position off the shower is curious- it points to 11 o'clock rather than 12 and engineer said not to over rotate. Human nature wants to turn it back to an upright position. We have been uber careful not to over rotate but the same thing has happened.

I'd like just to swap the mixer fir s different model.


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