Central heating repair means tracing why heat is not reaching the rooms that need it and fixing that cause — typically a failed pump, a stuck zone or diverter valve, a faulty thermostat or programmer, sludge restricting flow, air in the system or a boiler fault. The first step is always to establish which part of the system is at fault, because the symptom you notice, such as one cold room, often starts somewhere else entirely.
Before booking anyone, a few safe checks can rule out the simple causes. After that, the table below shows how symptoms usually map to faults.
Checks you can do first
- 1
Controls and power
Is the programmer calling for heat, the room thermostat turned up and the boiler switched on at the fused spur? Batteries in wireless thermostats are a common culprit.
- 2
Boiler pressure
On a sealed system the gauge usually sits around 1 to 1.5 bar cold. Well below that and many boilers refuse to fire.
- 3
Fault code
Note any code or flashing light. It narrows the fault down enormously for whoever you call.
- 4
Radiator valves
Make sure the TRV head is turned up and the valve at the other end has not been closed during decorating.
- 5
Bleed the cold ones
A radiator warm at the bottom and cold at the top usually has air in it. Bleed it, then recheck the pressure.
Symptoms and their usual causes
| What you notice | Likely cause | Typical repair |
|---|---|---|
| No heating, hot water fine | Diverter valve, zone valve or a control not calling | Valve or actuator replaced, controls tested |
| Radiators cold at the bottom | Sludge settling in the panels | System cleaned, filter fitted, inhibitor restored |
| Radiators cold at the top | Air in the system | Bleeding, then checking why air is getting in |
| Upstairs warm, downstairs cold | Pump weak or system unbalanced | Pump checked or replaced, radiators balanced |
| Humming or grinding noise | Pump bearing wear or air in the pump | Pump vented or replaced |
| Heating comes on when not wanted | Stuck zone valve or faulty programmer | Valve or controls replaced |
| Pressure keeps dropping | Leak, or a failing pressure relief valve or expansion vessel | Leak traced, vessel recharged or valve replaced |
Repair, clean or replace?
Most heating repairs are single parts: a pump, a valve, a thermostat. Where the same system keeps producing faults, though, the underlying condition matters. Black water when a radiator is bled, cold patches across several radiators and a boiler that kettles all point to sludge, and replacing parts one by one will not stop them failing.
In that case a clean — a power flush or chemical flush, followed by fresh inhibitor and a magnetic filter — is often the repair that actually lasts. And where a boiler is old, parts are becoming scarce and faults are frequent, it is fair to weigh the next repair against replacement. A good engineer will say which situation you are in rather than assuming.
Safety first
- When should I not attempt anything myself?
- Anything inside the boiler casing, anything involving the gas supply and anything with water near electrics. Leave those to a Gas Safe registered engineer.
- What if I smell gas?
- Do not use switches or flames, open windows, turn off the gas at the meter if you can, leave the property and call the National Gas Emergency line on 0800 111 999.
Note what works and what does not — heating, hot water, which rooms, and any code on the boiler. That short list usually points straight to the fault.
