Why a Radiator Hisses
Steam radiator air vent allowing trapped air to escape during heating
Radiators work by allowing hot water or steam to circulate through the system and release heat into the room. When something interrupts that normal flow, noise can occur.
Common Reasons a Radiator Makes a Hissing Sound
• Trapped air inside the radiator, which blocks proper water circulation
• Incorrect radiator pitch, causing water to collect instead of draining
• A faulty or improperly sized radiator vent, releasing air or steam too aggressively
• High boiler pressure, forcing steam or hot water through vents
• Loose or worn radiator valves, allowing small leaks
• Minor system leaks, where air escapes and creates a hissing noise
In many homes, more than one of these issues can be present at the same time.
How to Fix a Hissing Radiator
Most radiator hissing problems can be solved with basic checks and simple adjustments.
1. Bleed the Radiator
Trapped air is the most common cause of radiator noise.
• Turn the heating system off
• Allow the radiator to cool
• Use a radiator key to slowly open the bleed valve
• Release air until water begins to flow
• Close the valve securely
Bleeding the radiator restores proper circulation and often stops the hissing immediately.
2. Check the Radiator Pitch
Radiators should be slightly angled so water can drain correctly.
• Use a level to check the radiator
• The side opposite the valve should be slightly higher
• Add thin shims under the radiator feet if needed
Even a small pitch adjustment can reduce noise significantly.
3. Inspect the Radiator Vent
A faulty or incorrect vent can release air or steam too quickly.
• Look for water dripping or steam escaping from the vent
• Make sure the vent is installed upright
• Replace vents that are damaged, clogged, or undersized
Using the correct vent size helps balance air release and reduce noise.
4. Check Boiler Pressure
Excess pressure can force steam or hot water through vents.
• Check the boiler pressure gauge
• Compare it to the manufacturer’s recommended range
• Reduce pressure if it is too high
Proper pressure keeps the system quiet and efficient.
5. Inspect Valves and Connections
Loose or worn valves can allow air or water to escape.
• Look for moisture around valve connections
• Tighten fittings if loose
• Replace worn valve seals if necessary
Small leaks can create hissing sounds even when heat output seems normal.
When to Call a Professional
You should contact a heating professional if:
• The hissing continues after bleeding and adjustments
• You notice frequent pressure changes
• There are visible leaks or corrosion
• Steam systems show water hammer or loud banging
Persistent noise may indicate internal system issues that require professional diagnosis.
Final Thoughts
A hissing radiator is often caused by air, pressure, or venting problems and can usually be fixed with simple DIY steps. Addressing the issue early can improve heating efficiency, reduce noise, and prevent long-term damage to your system.