Why Is My Radiator Hissing and How Do I Make It Stop?

You’re settling into bed on a chilly winter night, but suddenly, you notice your radiator hissing. The hissing sound coming from the radiator threatens to keep you up all night, both annoyed by the sound and worrying whether something serious is wrong with your home’s heating system. 

So why is your radiator hissing? As it turns out, there are many different potential causes behind this pesky problem, and some of them are fixable with DIY methods.

How Does a Radiator Work?

Before you dive into the different parts and problems that can lead to a hissing noise in the radiator, it’s helpful to understand the basic mechanics of how the whole thing works. If you’re wondering how radiators work, it all comes down to convection. 

The radiator is connected to a boiler that produces steam or hot water, which travels through pipes to the radiator. The radiator is typically made with metal for better conduction, and the metal heats up and begins warming the air around it. But sometimes, the steam doesn’t vent properly, or there’s a leak in the system. These issues, and several other potential culprits, can lead to a hissing radiator.

1. The Vent Is Stuck

The system air vent, also called the air valve, needs to open fully to either take in ambient air when the radiator is not heating the room or release air during the heating cycle. If the air vent gets stuck, it may prevent the air from properly releasing, leading to a hissing sound.

Next Steps

Check the control valve of the radiator, which should be near the floor, and make sure it’s all the way open. Only close it when you want to turn the radiator off. If the control valve isn’t the issue, you may need a new air vent. 

2. The Air Valve Is Damaged or Worn Out

When you’ve confirmed that the control knob is set fully open and the radiator hissing is still a problem, you may need to replace the air valve entirely. The air valve may be old or damaged, or you could have a valve that’s too small for the radiator.

Next Steps

Check the size of the air valve before heading to the home improvement store for a new valve. You’ll need to turn off the radiator and let it cool fully, then carefully turn the old valve counterclockwise to remove it. Then, wrap the connection with thread sealing tape before screwing the air valve, venting side up, back onto the radiator.

If you aren’t sure how to find the right size or how to properly install the new valve, you should hire a boiler repair person to replace the air valve for you.

3. The Vent Is Obstructed

If you notice your radiator making hissing noises, it’s often a sign that the vent is obstructed and needs some care. The air vent may be clogged with rust, calcium buildup, or other debris, preventing it from functioning properly. Fortunately, this is an issue you can easily handle at home. 

Next Steps

To address the problem, turn off the radiator and let it cool. Then, remove the air valve and soak it in vinegar for at least 30 minutes. After soaking, rinse the valve with cool water, allow it to dry completely, and then replace it. This simple maintenance step can help restore your radiator’s efficiency.

4. There’s a Radiator Leak

With all that hot water or steam moving through the system, a radiator may hiss when these elements begin to leak. If the vent opening is facing sideways or down instead of up, water or steam can leak out. The air valve can also leak if it isn’t properly connected, which is why it’s important to wrap the connection with thread sealing tape before screwing a cleaned air valve or new air valve onto the radiator.

Next Steps

If the issue isn’t the air valve, it’s time to call in a repair person. A radiator leak could mean the entire radiator needs a replacement, and the longer you wait, the more water damage you could be dealing with. You can expect to spend about $100 to $2,500 on the cost of a radiator replacement, while water damage restoration costs anywhere from $450 to $14,700.

5. Air Is Trapped in the Radiator

Trapped air is one of the most common reasons for a hissing radiator. When air gets trapped inside the radiator, it can prevent steam from properly circulating, causing water to block the steam from entering the radiator. This blockage creates pressure that results in the hissing sound. Left unchecked, this issue can reduce your HVAC system’s efficiency, making it harder to heat your space effectively. Fortunately, bleeding the radiator to release the trapped air is a simple fix that can restore optimal performance and eliminate the noise.

Next Steps

Bleeding the radiator involves turning off the radiator and allowing it to cool, placing a towel near the radiator, and opening the radiator bleed valve located toward the top of the radiator. You’ll open it just enough to let the air hiss out, but carefully shut it again when you notice a bit of water start to leak out.

Add radiator bleeding to your annual maintenance checklist for your home’s heating system, as doing this about once per year can help manage any hissing or other unpleasant noises coming from the radiator.

What to Do When the Radiator Is Hissing

If your radiator is hissing or making other strange sounds, like banging or gurgling, it’s usually safest to hire a local boiler repair company to come take a look. There are many different issues that may cause radiator hissing, and if you aren’t confident in what you’re doing, you could be burned by the hot water or steam in a radiator, break off one of the important knobs or valves, or be stuck with a watery mess. Instead, skip the hassle and let the pros handle these repairs so that you can be cozy and warm at home again with minimal radiator downtime.