I have a Potterton gas fired boiler which on a CH only setting on the programmer starts up but then cuts out before any water can be pumped elsewhere. However, with both HW and CH on it works fine. When only CH is on the pipe coming out and upwards from the boiler (downstairs) gets very hot and the boiler cuts out before any radiators get hot. However, with both HW+CH on the water gets split between heating and hot water due to mid-point valve to the extent that the water takes ages to heat up. If I turn off a few of the radiators the remaining 3 radiators get hot and the water gets hot as quickly as I would expect. Is there a reason why the system only seems to be able to cope with half of the radiators on?