Downstairs Radiators get hot when Central Heating 'Only' on.

[nrw6570]

I have a C.H system that is 10yrs old - with boiler in garage and hot water cylinder in 1st floor airing cupboard.
Heating and Hot Water run via a '3 Way Motorised Value' in airing cupboard.
After recently turning system off to replace an upstairs tap - ever since i've had the following problem:-
If central heating only on - all radiators work fine
but if both Central Heating and Hot Water on - upstairs radiators work fine but the downstairs radiators get hot at the top but are only luke warm at the bottom - (but then when the hot water turns off - the radiators then fully heat up).
Any suggestions, whats causing this?

 

[bengasman]

Lack of flow or lack of capacity. Solution is simple, set the ch and dhw for separate times, or check and replace pump if needed, or clean and balance the system. And close the bypass if you have a manual one, and leave one rad fixed to open.

 

[kevplumb]

After recently turning system off to replace an upstairs tap - ever since i've had the following problem:-

check the valves if any you may have disturbed the cylinder balancing valve

or as ben says the bypass