Radiators That Never Get Warm

One radiator in a house is always the cold one. The position of the cold area on the panel narrows the cause down quickly, because heat, air and sludge each settle in different places.

Cold at the top means trapped air

Air enters a sealed heating system through small leaks, through fresh water added during topping up, and through gases produced by corrosion inside the pipework.

Because air is lighter than water it collects at the highest point of the radiator, insulating the top third of the panel while the bottom stays hot.

Bleeding releases it through a small valve at one upper corner. The system pressure drops slightly afterwards, which is why bleeding several radiators usually means topping the boiler back up.

Cold at the bottom means sludge

Iron components corrode slowly in contact with water and oxygen, producing a fine black magnetite sediment that circulates until the flow slows enough for it to settle.

It settles in the bottom of radiator panels, where the water moves least. Over years it builds into a layer that blocks flow across the lower half of the panel.

Bleeding does nothing for this. The remedy is chemical cleaning or a power flush, and afterwards an inhibitor to slow the corrosion that produced the sludge in the first place.

Cold at one end means a balancing problem

Water takes the path of least resistance. Radiators nearest the boiler are easier to supply, so they can absorb most of the flow and leave distant ones starved.

Balancing corrects this by partially closing the lockshield valve on the easy radiators, forcing more water around the circuit to the ones that were being bypassed.

It is fiddly rather than difficult, and it is done with the system running so the temperature difference across each radiator can be compared.

A whole cold radiator points at a valve

Thermostatic valves contain a pin that is pushed down by an expanding capsule. If the radiator has been off all summer the pin can seize in the closed position.

Removing the head reveals the pin, and a seized one can often be freed by working it up and down gently with pliers rather than replacing the valve.

If the pin moves freely and the radiator is still cold while others are hot, the blockage is elsewhere in that branch of the pipework.

Where the problem is not the radiator

Low system pressure, a failed circulation pump or a stuck diverter valve produce whole-house symptoms rather than one cold panel, so several cold radiators point away from the radiators themselves.

Furniture and long curtains also matter more than people expect. A radiator behind a sofa still emits heat, but much of it is absorbed before it reaches the room.

Anything involving the boiler, the gas supply or the sealed system pressure belongs to a qualified engineer, and in most places that work is legally restricted.