There are two basic types of boiler/radiator systems. One type is the steam boiler, where the boiler actually boils the water in the system and that steam then flies through the pipes to the radiators, where it gives off its heat, condenses back into liquid water, and then drains back to the boiler. With these systems the radiators and all the pipes are normally empty – filled only with air. Steam systems are most common in larger multi-unit buildings, and fairly rare in single family homes.
The other system, most common in single family homes, is the hot water system. With this type of heating system the “boiler” heats the water (but doesn’t boil it), and a circulator pumps the water through the system. In this system everything is completely full of water all the time – the boiler, all the pipes, and all the radiators should be completely full of water. So the circulator isn’t really pumping water anywhere, it’s just causing that water to circulate through the system.
But sometimes air can get into the system. This can cause problems, and so we’ve taken steps over the last several decades to prevent air from getting in and to remove it when it does. We use better expansion tanks to keep air out and we use devices like air scoops to remove the air that does get in. So this isn’t nearly as much of a problem today as it was for our grandparent’s generation.
Still, no system is perfect and air can still get in to the boiler system. Sometimes this air winds up in the radiators, where it rises to the top of the radiator and displaces the water. When this happens the radiator isn’t full of water and so it won’t put out as much heat as it should. That can leave the room too cold. You can tell if this is happening if the top of the radiator is only barely warm or even cool to the touch, while the bottom of the radiator is hot.
Here are visible and infrared images of a radiator that’s only about half full of water. You can see in the infrared image that the top of the radiator is quite cool (darker colors are cooler), while the bottom is nice and hot. And you can see pretty much exactly where the water in the radiator stops. This radiator isn’t putting out as much heat as it should and this room might be a little chilly.
So this radiator needs to be bled of air. This is pretty easy, but if you’ve never done it before I recommend having a good HVAC technician with experience with boiler systems do it the first time. First make sure that the system is off. Turn it off at the thermostat or just turn the thermostat temperature all the way down. There’s a valve at the top on one side of the radiator.
You’ll get a bucket and hold it under this valve, then open it. Sometimes a special tool is needed to open the valve, and sometimes just a flat bladed screwdriver will do the trick. Here’s a typical tool for the job. It has a square socket and a screwdriver blade and one of these two will work on most radiator valves.
Once you’ve opened the valve air will bleed out of the radiator and water will move in to fill the radiator. Once all the air has bled out water will start to spurt out of the valve – that’s what the bucket is for. Then you close the valve.
Once you’ve done this you might need to add more water to the boiler system. This is where the expertise of a good HVAC technician is extremely helpful.
There is one big caveat about bleeding a radiator, especially in an older house. The house has probably changed a lot since it was first built and since the radiators were first installed. There might be more insulation, better windows, or even walls that have been added or removed. So if you discover that a radiator has air in it, it might be a good idea to wait and see if you think the room is too chilly. If it is too chilly then you can easily bleed the radiator and improve your comfort, but if you bleed the radiator and then decide that the room is too warm it’s a lot harder to go back.