Radiators are humble devices that serve our heating needs year after year, but sometimes, they might let out a little whine, hiss or gurgle. This might be nothing to worry about, but it might also be the sound of protest!
Most hissing noises aren’t particularly problematic, but fixing hissing radiators and any associated issues before they develop further is wise. While most noises are benign or easy to fix, there’s always a chance that sounds emanating from your radiator indicate a deeper problem.
This is a guide to hissing radiators and how to fix them.
Hissing noises are often caused by the normal movement of air and water through the heating system.
For example, if your radiators hiss when you turn them on but stop after they’ve heated up, there’s usually nothing to worry about. A faint hissing noise might also signify that your thermostatic valves are doing their job by moderating water flow into the radiator.
Hissing sounds accompanied by banging, thumping, or other louder sounds are more alarming and should be checked by a heating engineer. Amongst other radiator noises, hissing on its own is less alarming than hissing accompanied by banging or loud creaking.
Here are the most common explanations for hissing radiators:
You’re probably most likely to hear a more severe hissing noise after you’ve fitted new radiators, a new pump, or adjusted some settings in your heating system (e.g., the thermostatic valve).
If you’ve fitted new radiators, it’s essential to check that the valves are set up correctly and pointing in the right direction. For example, the lock valve might be too tight, restricting water flow and causing a hissing sound around the valve.
It’s also possible that the pump is set too high, but this is only likely in the case of conventional heating systems where the pump is separate from the boiler.
The bleed valve (usually positioned in the top corner of the radiator) is used to bleed the radiator of excess air. Most radiators should be bled every year or so.
Sometimes, the bleed valve will loosen itself, or someone might not have shut it tightly enough the last time they used it. Bleeding the radiator and closing the valve tightly should stop the hiss.
Above: Bleed valve and key
If you find yourself bleeding radiators frequently, or they’re hissing and gurgling or banging, this could mean that the system is pulling in excess air from somewhere else.
Open-vented systems can pull in excess air through the feed and expansion tank in the loft, or there might be a leak somewhere in your radiators or plumbing.
If you find your radiator hissing and leaking, the hissing noise likely comes from the leak itself. Tiny pinhole leaks are more common in older radiators and copper piping. Pinholes leaks can be hard to detect and are frequently caused by limescale hotspots and internal corrosion.
A leaky radiator might produce a damp patch or drip, and you might find yourself bleeding the radiators more than usual (once a year or so).
Sometimes, the hole can be patched and filled, but it’s sometimes better to just replace the radiator. For example, if multiple radiators suffer from hissing and leaks, your system might contain corrosive sludge. In this situation, a heating engineer might recommend a power flush or even a total revamp of your central heating system.
Hissing noises can be caused by too much water rushing through a radiator. This may indicate a balancing issue, which means radiators closer to the boiler aren’t properly balanced with those further away.
In this scenario, some of your radiators may be much cooler than others or may not heat up properly. Consider rebalancing your radiators by adjusting the lockshield valve. Find out more about rebalancing here.
Hisses in water-driven radiators are not usually dangerous. However, if you have steam radiators (only likely in older houses), then hissing is potentially dangerous and should be assessed as soon as possible.
The advice in this guide applies to the vast, vast majority of central heating systems in the UK. Over 77% of UK central heating systems are gas heating systems, and the majority of those use water-driven radiators instead of steam radiators. Only a tiny percentage of radiators in the UK are steam-driven.
Water-driven radiators are under low pressure, which is generally very safe. So while the noise might be symptomatic of an existing or imminent fault, it’s probably not dangerous!
Hissing noises aren’t always the sign of a serious issue, but it’s still sensible to troubleshoot the problem, especially if the hissing sound is new or getting louder.
The first thing to do is to locate the hissing radiators. If the hissing noise isn’t coming from a radiator, it could be coming from the piping between the radiators, which likely indicates a leak. If you encounter a hissing noise coming from the boiler, then you should get a heating engineer to take a look as soon as possible.
See if you can detect the hiss near the thermostatic and lockshield valves positioned on the pipes heading in and out of the radiator.
The thermostatic valve might produce a faint hiss by heavily restricting water flow into the radiator. Adjust the valve and see if that rectifies the problem. The lockshield valve may also produce a hiss if it’s too tight or loose.
Above: Thermostatic (TRV) Valve
The bleed valve is usually positioned in the top corner of the radiator. If the bleed valve is hissing, fully bleed the radiator and close it tightly.
If you locate a leak in the radiator, valves or piping, this may be repairable with epoxy putty or a pipe repair patch. Leaks emanating from within the valves themselves will be tougher to repair.
Switch the heating off before conducting any sort of repair - it may also be necessary to isolate the radiator.
If your hissing is accompanied by an imbalance in how quickly or thoroughly your radiators heat up, this might signal a balancing issue. For example, some radiators may take a long time to warm up or fail to warm up at all.
You can balance your system by adjusting the lockshield valves to regulate the flow of water through the system. Find out more about balancing radiators here.
Some radiator hissing noises are normal and aren’t a sign of any real issue. Water and air moving through pipes and radiators will likely create some noise during use, but it’s still sensible to properly diagnose a new hissing sound or a hissing sound that seems to be increasing in volume.
If you encounter a hissing noise, try and find exactly where the noise is coming from. The bleed valve is a common culprit, and it might just be a case of bleeding the radiator properly and closing the valve tightly. If not, check the other valves, pump and cold water feed in the loft.
If you’re still unsure of the source of the hiss or suspect you’ve got a bigger problem on your hands, consult a plumber or a heating engineer.