Building and buying

Vent the radiators and save on heating costs: here’s how

Arne Schätzle

By regularly venting your radiators, you can make sure your entire home remains lovely and warm and save energy at the same time. We explain how to proceed, step by step.

A woman is looking at her smartphone while standing in front of a radiator in a living room.
© Getty Images

In this article

Why do I need to vent my radiators?

Maybe this has already happened to you: you turn up the heating, the radiator gurgles, but it doesn’t really get warm. Often this is because air has entered the heating circuit. Although the heating system is still working and water is circulating, the radiator doesn’t warm up properly due to the air. This is because the air is taking the place of the water – and since air is a poor conductor of heat, the radiator doesn’t become warm.

This is exactly the problem that the venting process resolves. Venting a heating system is simple; you don’t need a plumber to do it. With a little skill and a radiator key, you'll have the problem fixed yourself in no time. Then your house will be warm and cozy again, and you’ll save energy too.

What happens if I don’t vent the radiators?

As well as the actual radiators, the heating circuit also includes the boiler, hot water tank and circulation pump. This heating system is connected by pipes containing water, and it is this water that transports the heat. The circulation pump keeps the circuit going. Air often gets into the system, especially in older heating systems. But the same problem can also occur with new heating systems.

Heating systems operate with hot water because this conducts heat well, while air is a much poorer conductor. If air enters the heating circuit, the water is distributed unevenly in the radiator. This affects the efficiency of the entire system, and energy is lost. As a result, your heating system is no longer able to work properly.

How can I tell if I need to vent a radiator?

Gurgling or banging, hissing, spluttering, even crying or screeching: if the radiator is making strange noises, it’s a sure sign that there is air in the circuit. In addition, the radiator won’t warm up properly. The air preventing the water from circulating is always located in the upper part of the radiator. So if you notice that the radiator is colder at the top than at the bottom, and find that the radiator only gets lukewarm despite the thermostat being turned up, this is another indication that it’s time to vent it.

Two ribbed radiators are fitted under two large mullioned windows with a view of a park.

Whether your radiators are old or new, you should always vent them.

What do I need for venting radiators?

You only need a few tools to vent radiators, but you won’t get very far at all without a radiator key or vent key – a type of square wrench. If you don’t have a vent key at home, you can usually get one at any hardware store. You will also need a small container to catch any water that may escape. There are also vent keys with an integrated drip tray.

The vent valve is usually located on the opposite side of the radiator to the thermostat. The vent key fits all valves, unless your house still has a very old heating system installed. In this case, you can open the valves with a pipe wrench.

Instructions: venting radiators step by step

Basically, you can vent your radiators at any time. Since the heating system will not be out of service for long, there’s no reason not to carry out the procedure, even in the colder months of the year. Still, you probably won’t choose to do it in double-digit subzero temperatures. If you want to vent your radiators, the best way to do so is as follows:

Step 1: turn off the circulation pump

First, turn off the circulation pump, which can be found directly on the boiler. This will cause the flow of heating water to stop, so that air will no longer circulate in the system. Wait 30 to 60 minutes before moving on to the next step. This is how long it takes for the air to collect in the upper part of the radiators.

Step 2: turn up the thermostat

Before venting the radiators, turn all the thermostats to the highest setting. Now the air and water in the heating system will heat up and expand. This will allow the air to escape more easily later.

Step 3: open the vent valve

Start venting on the lowest floor of the house and work your way up, story by story. The drain valve is usually located on the side of the radiator at the top. It’s best to have a rag handy, as it may get a little damp. Using the vent key, open the valve carefully by slowly turning the key to the left, i.e. counterclockwise. At the same time, hold the drip tray under the valve. The air under pressure will now escape from the radiator with a hiss. Don’t turn the valve any further open, but leave it in this position. You need to be careful at this point, as hot water can escape along with the air – that’s why you need the drip tray.

Step 4: close the vent valve

If there is no more air in the radiator, the hissing will stop and only water will escape. Close the valve by turning it clockwise. Proceed quickly, because if too much water escapes, you will have to top up the water in the whole heating system.

Water can be seen leaking from a radiator valve.

By the end of the venting process, if not sooner, hot water will come out via the radiator valve. You should therefore have a drip tray ready.

Step 5: switch on the circulation pump

Once you have worked your way up through the floors, radiator by radiator, you can turn the circulation pump back on to get the heating water flowing again.

Step 6: check the water pressure

Now check in the boiler room whether the water pressure is sufficient. To do this, read the pressure indicator, also known as the manometer. This usually has two pointers: a black arrow indicating the current water pressure and a static red arrow indicating the upper limit. The target range is usually marked in green. Ideally, the black pointer on the scale should always remain below the red pointer, and never exceed it.

Step 7: refill the system with water

When you turn on your heating, the black pointer will move toward the red pointer and the radiators will fill up with hot water. It’s important that the black pointer remains below the red one. If the reading is still too low after venting the radiators, you will have to top up the water in the system. This is because heat will not be distributed evenly if the water pressure is too low when the heating is switched on.

Added comfort: an automatic vent valve

As you can see, venting radiators doesn’t require much effort. If you want to reduce the work involved in venting your radiators even more, then automatic vent valves are a good choice. They will save you time, especially if you have a large heating system with a lot of radiators.

An automatic vent valve uses a float to detect if air is in the radiator and opens the valve to let it escape. When the sensor comes into contact with water, it closes again all by itself. You can change the radiator valves yourself without much effort. Inexpensive valves are available for about five francs each. Alternatively, you can also call in a specialist company to fit them.

The most important questions and answers at a glance

Why do I need to vent the radiators?

You need to vent the radiators if there is air in the heating circuit because this will prevent the heat from being transported efficiently, and the heating system won’t get warm properly.

How can I tell that I should vent my radiators?

If the heating stays cold or you can hear gurgling in the radiators, there is probably air in the heating circuit and you need to vent it

What happens if I don’t vent my radiators?

If you don’t vent your radiators, the amount of air will continue to increase, your heating system will heat less and less, and the room will remain cold.

When should I vent my radiators?

It’s best to vent the radiators before the start of the heating season in October or November. If your heating system gurgles during the heating period, it’s advisable to vent it again.

Why do I have to switch off the circulation pump when venting?

Switching off the circulation pump means that neither water nor air will be distributed in the heating circuit. Air will gather above the water level and will be able to escape more easily via the heating valve.

Should the radiator be warm or cold when venting?

Before venting, turn up the thermostat on the radiator to allow the water and air to expand. This will help you to vent the air.

What happens when venting a radiator?

When a radiator is vented, hot air escapes from the radiator and makes space for hot water. Without this air, the heating cycle can work properly and the heating system is more efficient – because water conducts heat much better than air.

Why does water also escape during venting?

When the valve is opened, pressure escapes from the water circuit. Since the air is above the water, it is released first. Water only starts coming out once there is no more air.

Venting radiators: little effort, big impact

Air in a radiator usually not only causes annoying gurgling noises, but also leads to higher costs because the water circuit isn’t able to work properly. Venting, on the other hand, is uncomplicated and quickly done. Thanks to our instructions, venting will allow you to eliminate annoying noises, optimize heating performance and save money.

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