I have oil fired central heating and some of the radiators upstairs and downstairs are hot and some are cold, the pipe leading up to some are warm but the radiator is still cold.
Hot water is also only mildly hot unless I turn up the thermostat on the immersion heater to 75 degrees.

Paul, December 2004

i'll have to agree with SBR plumbing. There seems to be sludge build up. You can add sentinel X300 sludge buster to system . Run it for the duration indicated on the instructions, no more. As it can eat through some old pipework if system is extremely old.. Flush all evidence out from system then add sentinel x100 inhibitor, when satisfied. As SBR said, you may need the system balanced too.

lee S. Wales, January 2007

SYSTEM IN NEED OF FLUSHING AND THEN RE BALANCING

SBRPLUMBING, February 2005

You may recall that hot water output of the heating system of the home go to the radiators and completes its circuit and turns back to the combi. If turn-back filter is closed by dirty water, then the amount of hot water is limited. Consequently some radiators are hot and some are cold. Remedy is to clean that filter.

ali dinc, December 2004

sounds like you have got air in rads you need to bleed them

helzender, December 2004