central heating problem

We need to know a lot more about the design and layout of your system.

Is it fully pumped, or gravity hot water and pumped central heating? [Does the pump run when just heating the HW, or only when the CH is on?]

If fully pumped, it's likely that you've got one or more motorised valves. How many, and what type (2-port or 3-port)?

How is the central heating distributed? Do the main pipes go between the floors, with pipes coming down from above to each downstairs rad - or do you have separate upstairs and downstairs circuits with just 2 largish (22mm or

It will be much easier to suggest the likely cause if you answer the above questions.

Roger Thanks for the help. Its gravity HW and pumped CH - pump only runs when heating is on. Problem is, I cant find any motorized valves........duh.....Ive looked along every piece of the pipes run from boiler to loft and no joy. AS far as pipework is concerned, there is a separate run going upstairs....the 2 largish pipes you suggested, going upstairs. Many thanks again Maskie

If it's gravity HW, there almost certainly won't be any motorised valves.

My best guess is that you have a problem with the pump. The shaft may be turning, but perhaps the impellor isn't.

Is the domestic hot water getting hot ok?

When the central heating is divided into upstairs and downstairs circuits, just joined by 2 vertical pipes, it's not uncommon for the upstairs rads to get hot by gravity (convection) circulation, without the pump running - but the downstairs rads won't get hot.