advice/help req please

[kingfisher1548]

Hi all,
I have independant HW and CH controlls/timer.
This can be set to HW only or HW and CH together.
The problem is that when the HW only is selected, whether this is on timed or constant, the upstairs rads get red hot but the downstairs rads remain cold.
If HW and CH are selected, upstairs and downstairs rads all get hot.
Also when CH only selected they all get hot.
Would I be correct in saying the 3 port mid position valve is knackered or could this be something else, pump perhaps??
What would cause this?
Any help/advise will be greatly received.
Thanks in advance,
Dave

 

[twgas]

3 port is the most likely suspect, although it may be piped wrongly, especially if it's always been like that.

 

[bengasman]

Could be the valve itself faulty, or just sticking due to corrosion build up, or both