Central Heating Problems: How To Work Out What's Actually Wrong

Usually trapped air, a closed or seized lockshield valve, or sludge settled at the bottom of that specific radiator. Bleeding it is worth trying first; if it stays cold at the bottom after bleeding, that points to sludge rather than air.

Often a system that’s never been balanced, so water isn’t distributing correctly between radiators, or a boiler undersized or de-rated for the property. It can also be a flow temperature set too low on a modern condensing boiler.

Known as short-cycling, this is commonly caused by low system water pressure, a faulty thermostat sending the wrong signal, or the boiler overheating internally due to restricted flow — often from sludge.

Not usually dangerous in itself, but it’s a warning sign. Kettling (a rumbling, kettle-like noise) means limescale or sludge is restricting water flow around the heat exchanger, and left unaddressed it accelerates wear and can eventually cause a leak or breakdown.

Cold at the top with hot at the bottom is trapped air — bleed the radiator. Cold at the bottom with hot at the top is the opposite: sludge has settled in the bottom and needs a powerflush or chemical clean rather than bleeding.

On a system or regular boiler sharing a cylinder, yes — a faulty diverter or motorised valve can affect both. On a combi the two are more independent, so a heating fault is less likely to touch your hot water.

Often pump or pressure related, since upstairs radiators need more force to fill properly. It can also mean upstairs zone valves have failed or the system was never balanced to push water high enough after work on the property.

Compare the temperature the thermostat reports against a separate room thermometer. If they disagree significantly, or the heating won’t respond at all to changes on the thermostat, that points to the thermostat or its wiring rather than the boiler.

Most heating faults are fixable properly rather than something to manage indefinitely — the exception is a system so old or corroded that repairs become a rolling list of failures, at which point replacement genuinely works out cheaper than repeated call-outs.

Recurring air even after bleeding usually means air is being drawn into the system somewhere — commonly a failing automatic air vent, a slow leak, or a faulty expansion vessel not holding pressure. If it recurs within days rather than months, it’s worth having the underlying cause diagnosed rather than bleeding indefinitely.