Why is My Radiator Hissing, And How to Fix It

A hissing radiator is almost always harmless. The sound usually means air or steam is escaping through a faulty bleed valve, or trapped air is venting while the system warms up. Radiators themselves do not carry gas, so a hiss is not a leak risk in the way it would be in a gas pipe. In most cases, the fix takes no more than ten minutes with a radiator key.

There are a few exceptions. If the hissing is actually coming from your boiler, or your boiler pressure keeps dropping or rising unexpectedly, the issue may involve the pressure relief valve or another component that should be inspected by a qualified heating engineer.

If you're unsure whether you're hearing a hiss, a bang, a whistle or another sound altogether, our guide to other radiator noises can help you identify the problem first.

Try these five fixes in order, from easiest to most involved:

1. Check the bleed valve is fully closed.
2. Bleed the radiator properly.
3. Replace a worn bleed valve washer or bleed valve.
4. Check your boiler pressure (1 to 1.5 bar).
5. Check whether the hissing is actually coming from the boiler.

Table of contents

• What causes a radiator to hiss?
• Is a hissing radiator dangerous?
• Why is my radiator hissing when the heating comes on?
• How to fix a hissing radiator step by step
• When to call a heating engineer
• Frequently asked questions

What causes a radiator to hiss?

A radiator making a hissing noise is usually caused by pressurised air or a small amount of steam escaping through a tiny opening. In most homes, that opening is the bleed valve at the top of the radiator. If the valve is not fully closed after bleeding, or the internal washer or valve seat has become worn with age, air can escape slowly and create a soft but continuous hissing sound.

Another common cause is trapped air moving through the heating system as it warms up. As hot water expands, air naturally rises to the highest point inside the radiator. If a small amount escapes through a slightly worn bleed valve, you'll hear a brief hiss before the system settles.

Less commonly, the hissing could be related to excessive boiler pressure causing the pressure relief valve (PRV) to discharge. In very rare cases, a hairline crack in a valve body or soldered joint can also allow water or steam to escape. These situations should be investigated by a heating engineer.

If your radiator is making a sustained high-pitched squeal rather than a gentle hiss, the cause is likely to be different.

• Further reading: How to stop a whistling radiator?

If the sound is clearly coming from the heating pipes rather than the radiator itself, see our article How to Stop Your Heating Pipes From Making Noises.

Is a hissing radiator dangerous?

Usually not. Although the sound can be worrying, radiators do not carry gas, so a hissing radiator is not a sign of a gas leak. In most cases, the noise simply means air or a small amount of steam is escaping through a bleed valve.

Left unresolved, a faulty bleed valve can reduce your heating system's efficiency and may eventually allow small drips of water to appear around the valve. While this isn't normally an emergency, it is worth fixing before the problem worsens.

The main exception is if your boiler pressure keeps falling and needs frequent topping up. That suggests water may be escaping somewhere within the heating system. If this happens, arrange for a qualified heating engineer to inspect the system.

Why is my radiator hissing when the heating comes on?

A radiator hissing when the heating comes on is very common and usually nothing to worry about.

As the water inside your heating system warms up, it expands and pushes trapped air towards the top of each radiator. If the bleed valve is slightly loose or beginning to wear, that trapped air may escape with a short hissing sound.

If the hissing stops after around 10 to 15 minutes, it was almost certainly caused by trapped air venting during warm-up.

If the noise continues after the heating has reached its normal operating temperature, work through the steps below. A bleed valve that needs tightening or replacing is the most likely cause.

If you also hear trickling or running water inside the radiator, trapped air is probably contributing to the problem.

How to fix a hissing radiator step by step

Step 1: Check the bleed valve is fully closed

The simplest fix is often the correct one.

If you've recently bled the radiator, the bleed valve may not have been tightened fully afterwards. Using a radiator key, carefully turn the bleed valve clockwise until it is firmly closed. Do not overtighten it, as this can damage the valve.

Many cases of radiator hissing are resolved by this quick check alone.

Step 2: Bleed the radiator properly

If trapped air is causing the hissing, bleeding the radiator should remove it.

Turn your heating off and allow the system to cool before opening the bleed valve with a radiator key. Let the trapped air escape until water begins to flow steadily, then close the valve securely.

• Further reading: The Complete Guide to Bleeding a Radiator

Step 3: Replace a worn bleed valve washer or bleed valve

If the hissing continues after bleeding, the bleed valve itself is the next thing to inspect.

Over time, the internal washer or valve seat can wear, preventing the valve from sealing properly. Replacing the washer is an inexpensive DIY repair, while replacing the complete bleed valve is also achievable for confident homeowners.

Replacement bleed valves are inexpensive and compatible replacement parts are available on our website.

If you're replacing other valves at the same time, then be sure to read our Complete Guide to Radiator Valves

Step 4: Check your boiler pressure

If the bleed valve appears to be working correctly, check your boiler pressure.

When the heating system is cold, most boilers should read between 1 and 1.5 bar. If the pressure is significantly higher than this, your boiler's pressure relief valve may begin releasing pressure, creating a hissing sound.

• Further reading: How to Refill or Repressurise a Combi Boiler

Step 5: Check whether the hissing is actually coming from the boiler

Sometimes what sounds like a radiator making a hissing noise is actually coming from the boiler itself.

If the hissing is coming from the boiler rather than the radiator, the pressure relief valve may be discharging or another boiler component may require attention. This is not a DIY repair and should be investigated by a Gas Safe registered engineer.

Likewise, if you're hearing intermittent knocking rather than hissing, the issue is likely to have a different cause.

• Further reading: How to Fix a Banging Radiator

When to call a heating engineer?

While most hissing radiators can be fixed at home, some situations need professional attention.

Arrange for a heating engineer if:

• Your boiler pressure repeatedly drops after being topped up.
• You can see water leaking from the radiator, valve or pipe joints.
• The hissing is coming from the boiler rather than the radiator.
• You've worked through the DIY steps above but the noise continues.
• Multiple radiators are hissing throughout an older heating system.

A qualified engineer can pressure test the system, identify hidden leaks and replace worn components safely.

Frequently asked questions

Why is my radiator making a hissing noise?

A radiator making a hissing noise is usually caused by trapped air or steam escaping through the bleed valve. The valve may not be fully closed after bleeding, or the internal washer may have worn out. Both problems are normally straightforward to fix.

Is a hissing radiator dangerous?

No, in most cases it isn't. Radiators do not carry gas, so a hissing sound is rarely a safety concern. If the hissing is accompanied by repeated pressure loss, visible leaks or the sound is coming from the boiler, arrange for a heating engineer to inspect the system.

What causes a radiator to hiss when the heating comes on?

As the heating warms up, expanding water pushes trapped air towards the top of the radiator. That air may briefly escape through the bleed valve, producing a short hissing sound. If it stops after 10 to 15 minutes, it is usually normal.

Does bleeding a radiator stop hissing?

Yes, if trapped air is causing the problem. Bleeding the radiator removes the trapped air before the valve is tightened again. If the hissing continues afterwards, the bleed valve may need tightening or replacing.

When should I call a plumber about a hissing radiator?

Call a heating engineer if you notice visible leaks, your boiler pressure keeps changing, the hissing comes from the boiler rather than the radiator, or you've already tried tightening and bleeding the radiator without success.

A worn bleed valve is the most common reason a radiator starts hissing, and fortunately it's also one of the easiest and least expensive problems to fix. If tightening or bleeding the radiator doesn't solve the issue, replacing the bleed valve is often the next logical step. Browse our range of radiator air vents and bleed valves, or if you're updating other components at the same time, explore our full selection of radiator valves.