Only Warm Radiators downstairs?

[Kettle 3]

I am having a problem with only warm radiators downstairs, i have a y-plan system with the boiler in the loft, approx 25 years old, have just flushed system again. I have the lockshield valves fully open downstairs, and the upstairs ones restricted, to try and increase the pressure downstairs. the system is fitted with a Grundfoss 15/50 pump and i've got it on 3. What i did notice this week was that i had turned the water off on the programmer and yet the water has never got cold, is it possible that the 3 way valve is not closing fully when the water comes to temp? thus pressure sharing?
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[MANDATE]

It could be the 3 port valve is not moving to the CH only position or if it is then a faulty microswitch would stop the boiler from flashing up.
This means you would get heating, but this is shared with hot water and the valve would be in the mid position. This also means that when the hot water is satisfied the power to boiler is cut off from the cylinder stat and replaced via the microswitch and if it cannot function then you don't get CH only.
If you call for HW & CH shared and boiler funcitons ok, try turning HW off and listen if valve motor moves. The three main problems as far as I know are; faulty motor , faulty microswitch and stiff spindle on valve.
If you look up some of my past posts, you'll find this problem has been covered many times.

 

[bones001]

also put the heating on by itself and feel the pipe to your cylinder to see if it is getting hot ( it shouldn't be) it may be the pump is labouring and hasnt got the umphh to push the water back upstairs, close the bypass valve for a bit more force