confused and unwashed

just had a combi boiler installed, at the moment still running elec shower from cold supply, want to install mixer but should i go for thermostat, manual, or pressure compensating valve ?

The advice I usually give here is to keep an electric shower fitted somewhere in the house if you have a combi. When a combi breaks down which sooner or later it will, the parts and engineer to fit the parts are not always readily available. If this was to happen on Christmas eve you could be 'smellybrating' christmas instead of celebrating it.
Assuming you have another electric shower then any shower you fit must be at least thermostatic to avoid scalding. Also the boiler must be of the modulating type which most if not all modern combis are now. Pressure compensating valve bar mixers like this
http://www.plumbworld.co.uk/0-740 are the modern option designed for use with combis and are ideally suited. Dont forget to fit the filters supplied or your guarantee could be void!

Whats a multi pint boiler ?

cheap combi

thats the hot water

It came as a minor shock the other day to learn than a LOT of combis are not modulating on hot water. NONE of the Vaillants for example. I'm still getting round to finding out about others, it is often NOT clear from the blurb- or if it doesn't say it IS, it's because it isn't.

The Turbomax 824 in question was burning hi-lo every few seconds, and the o/p H water temerature was varying, but not much, just a few degrees. At the manual mixer output the temp was constant enough to shower under.
The PRIMARY water temp was varying by 6 or so on the digi readout. HW temp knob was on max.

Thinking about it the burner should be on 100% all the time on HW tap being open full, when you consider the flowrates.

The problem was sludged primary side of the dhwhe.

Reality is that most combis DO run flat-out whenever hot water is demanded because the temp is ALWAYS set to max and users are in habit of using the flowrate through the tap to adjust temperature of HW!

I guess Vaillant and others noticed this and decided not to bother with the extra cost and complexity of a fully-modulating burner.

Anyway - it's presumably a moot point. I assume that ALL condensing combis DO have full modulation.....

My supplier was telling me about a case where a someone was sued because the shower fitted specified that the combi must be modulating and it wasn't. I didn't get the exact details but I think it was to do with temperature fluctuations when another tap was opened. My opinion after further thought is that a thermostatic or compensating valve should allow for this but I just covered my @rse when I made the post. I will now suffice to say that it is something to watch out for in any case and especially if a basic mixing valve is to be fitted