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help anybody!my hot water and radiators come from the fire which is on 16hours every day.radiators are very hot ,water has got cooler and is now just warm.It used to be scalding.our system is gravity fed to upstairs rads and a pump for downstairs rads which we switch on when water boils.have looked at our cylinder and understand all the pipes (Ithink).the inflow pipe is very hot but the one below it which carries water back to the fire is barely warm.if the coil is blocked how are the rads still boiling hot and wouldn`t the hot water in the coil be heating my domestic water even better.does anyone know how the water circulates e.g downstairs rads then upstairs rads then into the cylinder then back to the fire.Iknow that is wrong but Iwant to understand.Is sludge the problem or blocked coil or something else.cylinder is on 1st floor.Thanks for your patience! ::

There'll be 4 pipes coming from the boiler,

Primary flow
Central Heating Flow
Central Heating Return
Primary Return

The primaries will be 1" pipe - the central heating will be 22mm.

As the water is heated when in hot water only mode the heat rises up the largest pipe (primary flow) up through the cylinder where the heat is lost to the water - then the warm water will go back to the boiler.

The central heating will only work when the pump is switched on, water will always take the easiest route - the primaries - that's why they're a larger diameter pipe, there's less resistance. The water has to be pumped to get it to go down the 22mm pipe.