Radiators cold downstairs

[key]

Using laymens terms so apologies if anything is described wrong.

I have an 2 year old Ideal Icos HE18 condensing boiler and 11 radiators without any TRV's. The pump is a grundfos Alpha+ 15/60.

The central heating works fine until the HW is turned on at which point the radiators downstairs do not heat up. The upstairs are fine and the HW heats up. I have bled all of the radiators and clear water comes out.

The pump seems okay with changes in speed when changing between the 6 settings but I'm no expert.

Any ideas whats causing the problem?

 

[twgas]

sounds as though there is too much flow through the coil of the cylinder. If you have a balancing valve on the hot water circuit try turning it down a little.