Hot water scalding from some taps, tepid from others

[Softus]

Any ideas please?

Hm. Thinking hard at this end...

While I'm doing that, here are a couple of snippets from the Powermax MI:

[code:1]It is recommended that a 22mm draw-off is provided from which 15mm or smaller pipes can then be used to supply hot water services to individual terminations to give a balanced distribution system.[/code:1]

[code:1]Use in Hard Water Areas

In areas where temporary hardness exceeds 200mg/L, treatment of the mains water supplied to the appliance is recommended to maintain its performance. The Powermax electronic water conditioner P3237 is available factory fitted or can be retro-fitted. Any scale reducer, or ion-exchange softener, will be most effective when installed immediately upstream of the boiler i.e. in the inlet pipework.

Good quality polyphosphate dosing devices can also inhibit scaling but generally should not be fitted where heat could impair their performance. Always follow the manufacturer's instructions.

Record the type of conditioner being used in logbook.[/code:1]

And then there's Agile's comment:

There is a blending valve which aims to give a constant output temperature. Sometimes these do not work very well. It could well be involved in your problem.

My limited experience of these valves leads me to the same conclusion as Agile.

I believe that there are two versions of the 155x - one uses mains-fed cold and the other is tank-fed. Which is yours?

Also, I wonder what the flow rate is from your kitchen tap and the basin tap.

 

[Aviator]

Think I am getting somewhere. I capped off the feed pipes to the Bristan thermostatic mixer shower and the problem was solved. The unit is by-passing cold water into the hot inlet pipe when a hot water demand is made from an adjacent tap. This is happening with the mixer off. With the shower operating, the water temperature is reduced due to cold water crossing over to the hot water side. If I run a tap in the basin, the mixer gets ice cold very quickly.

I wonder if the mixer can be fixed, or does it have to be replaced?