3 or 4 port mixing valve

[ExpatinFrance]

Apologies if this has been asked before, I did have a search but couldn't find anything.

I am living in France and fitting an oil fired boiler for central heating only, ie not water heating.
It is pressured off the mains ( so expansion tank and relief valve fitted, not a header tank)
There are two systems in friends' houses I have been looking at (to copy!) however one pumps the water around the system and back into the boiler and the other has a 4 port mixing valve.
I cannot find any information as to why this would be fitted, or indeed whether one should be fitted.
Thanks in advance
Yours confusedly
Steve

 

[ecowarm]

Hi Steve

The mixing valve will control the temp of the flow to the rads/ underfloor and also the temp of the return back to the boiler, it is probably weather compensated.

I have been working with this product recently http://www.radiantcontrol.co.uk/wp-content/uploads/2010/05/d360e.pdf

The controller above will controll 3 or 4 port mixing valves.