A boiler or hydronic heating system may have an airlock if radiators heat unevenly, especially when they are cold at the top, and you hear gurgling or bubbling in the radiators or pipes. These signs suggest trapped air is restricting circulation, but they do not confirm it. Low pressure, sludge, valve faults and pump problems can cause similar symptoms.
Signs of trapped air
- One radiator is cold at the top but warm below: trapped air may be collecting in that radiator.
- Upstairs or high-point radiators stay cold: air naturally collects at high points, so this pattern supports an airlock.
- Radiators or pipes gurgle or bubble: this can indicate air moving through the heating water. Banging has other possible causes and needs assessment.
- Heating is slow, intermittent or uneven: several affected radiators may indicate restricted circulation, although not necessarily an airlock.
- The boiler shows low pressure or a fault code: this may accompany a circulation problem but is not proof of trapped air. Record the reading or code and check the model’s manual. Do not change valves or controls unless the manufacturer’s instructions identify them; heating-system pressure adjustments involve model-specific procedures and risks.
The pattern helps narrow down the cause. A single top-cold radiator suggests local trapped air. Cold radiators concentrated upstairs suggest air at a higher point in the circuit. If the whole system stops heating, a wider circulation, control or boiler fault is more likely.
Airlock or another fault?
Air commonly causes cold areas at the top of a radiator. Sludge more often settles lower, causing cold areas near the bottom. This is only a guide, not a diagnosis.
Before seeking help, note which radiators are affected, where the cold spots occur, any noises, the displayed pressure and any fault code. Do not remove the boiler casing or work on gas, electrical or pressurised components.
The right repairer depends on whether the fault involves gasfitting, electrical work or the hydronic circuit. Check which licensed trade can repair central heating in NSW rather than assuming every plumber can complete every part of the job.
Stop using the appliance if it is leaking, overheating, smells burnt or its manual directs you to stop after a fault. If you smell gas, avoid flames and electrical switches, leave the area and call Triple Zero (000) if there is immediate danger or contact your gas distributor’s emergency service from a safe place.
If the symptoms affect several radiators, keep returning or involve a boiler fault, contact our hot water system team for assessment by an appropriately licensed tradesperson.