When your heating first kicks in, the last thing you want to hear is hissing, screeching or whistling radiators. These sounds can be unsettling, but in most homes the underlying cause is easy to pin down and often simple to resolve. This guide walks you through the most common reasons, practical checks you can do safely, and when to call an engineer. Homeowners across Haslemere, Bordon, Liphook, Alton, Farnham and the surrounding GU35, GU34 and GU9 areas frequently report the same symptoms, so you’re not alone.
Quick checks you can do now
Before diving deeper, here are a few simple things you can safely look at yourself.
- Feel each radiator for cold spots that might indicate trapped air.
- Check that radiator valves are fully open if the room feels cold.
- Listen to whether the hissing is constant or only at startup.
- Look at the boiler pressure gauge; most systems run best around 1.0–1.5 bar.
Understanding why radiators hiss when heating
Hissing or screeching at startup typically comes from air movement, water restriction, or pressure imbalances within the system. When your boiler fires up, water rapidly circulates through pipework and radiators, and anything that obstructs or redirects that flow can create unexpected noise.
Air trapped inside the radiators
One of the most common reasons for noise is trapped air. Air pockets reduce efficiency and often cause gurgling, hissing or fizzing sounds. Homes in areas like Haslemere and Bordon with older pipework are particularly prone to this.
Signs of trapped air include:
- The top of the radiator feels noticeably cooler than the bottom.
- Intermittent hissing or bubbling as the system warms up.
- Rooms taking longer than usual to heat.
The usual fix is to bleed the radiator. Use a radiator key, turn the screw slowly until air escapes, then close it once water begins to flow steadily. Always check the boiler pressure afterwards.
High speed water flow or partially closed valves
If water rushes too quickly through pipework or radiators, it can create a sharp hiss or screech similar to a kettle boiling. This often happens when a lockshield valve is too tightly closed or when TRVs are stuck between open and shut.
A simple homeowner-friendly check:
- Ensure TRVs are fully open when testing.
- Turn lockshields no more than a quarter turn at a time to see if the sound changes.
If you notice that adjusting valves helps but doesn’t completely resolve the noise, a system balance might be needed. Balancing radiators ensures even water distribution and can dramatically reduce noise.
Boiler pressure or circulation pump issues
Boiler pressure that is too high or too low can cause radiators to hiss when heating starts. If the pressure is outside the recommended range, your system may struggle to circulate water smoothly, causing turbulence and noise.
Pump settings may also be responsible. If your circulation pump is set too high, water can be forced through pipework too quickly, producing loud hissing or screeching. Homes in Whitehill and Alton with extended heating systems or loft conversions may be more sensitive to incorrect pump speeds.
Homeowners can safely check the pressure gauge, but adjusting pump settings should be left to a Gas Safe engineer.
Expansion noises from pipework
As metal pipework heats up, it expands. If pipes are tightly clipped or passing through tight spaces in floors or walls, the friction can cause tapping, hissing and creaking. These noises often disappear once everything reaches a steady temperature.
While these sounds are usually harmless, persistent loud screeching may indicate that a pipe is rubbing hard against timber or metal surfaces. This often requires the area to be accessed and insulation or pipe clips adjusted.
When to call a professional
Most minor hissing issues can be resolved through checks like bleeding radiators or adjusting valves. However, professional help is recommended if you notice any of the following:
- Hissing combined with visible leaks.
- Boiler pressure dropping repeatedly.
- Radiators staying cold despite all valves being open.
- Noises coming directly from the boiler rather than radiators.
An engineer can check for pump faults, system blockages, scaling, incorrect pressure settings or failing components.
Next steps
If you’d like a Gas Safe engineer to look at persistent radiator noise, you can book a visit at https://www.embassygas.com/book