How to Bleed a Boiler

home boiler for heatingBleeding a boiler involves getting air out of the whole water and central heating system. Hot-water, or hydronic, heating systems consist of:

  • Boiler- Gas, oil or electric power- supplies energy to heat the water
  • Water Pump, known as the Circulator- located near the boiler on the return water line
  • Expansion Tank- installed near boiler to allow for excess water volume created by thermal expansion
  • Convectors or Radiators
  • Pipes- either in a one-pipe or two-pipe system
  • Air Vents and Valves

Normally, you would have to bleed a boiler if air were to get trapped in the lines. You usually know this is happening when the pipes get noisy or if your radiators are not heating evenly or at all. It is also part of regular maintenance of a stem heating system. Bleeding a boiler system isn’t easy to do thoroughly. Allow yourself enough time to do it properly, and you will be successful with it. When your home is cold and you can’t get a contractor until next week, it’s a skill that comes in handy.

Air gets into the system when you add water to it. The circulator pump creates a certain amount of small air bubbles from the impeller’s rotation as well. This can also be caused by leaks anywhere in the system. The air then collects and rises to the highest locations, usually in upper stories of the house at the top of the radiators. You can verify if this is the case by feeling the top and bottom of the radiator, with the system on. Notice the difference in temperature.

Bleeding Radiators

Once you have determined that you have air in your system, try bleeding it first from each room’s radiator/convector unit. Make sure the heating system is off first, with both the boiler and pump shut off, or you may introduce more air than you bleed out. Start at the high point of your house and work your way down, upper floor first, etc.

Remove any protective fronts from the radiators and baseboard heaters. Each radiator has a bleeder screw right on the end elbow or on one of the pipes coming up into the radiator. It’s a chrome or brass piece with a screw in the middle of it. You need a hexagonal bleed key to turn the screw on some bleeders, others accept a regular screwdriver.

    1. Place a bowl under this fitting or loosely wrap it with a cloth to absorb the water..

    2. Open the bleed valve 1/2 turn counter clockwise. You will hear some hissing as the air is being expelled.

    3. When water begins to trickle out of the valve, close it by turning 1/2 turn clockwise. Do not overtighten!

    4. Certain types of boilers, such as condensing combi boilers or boilers in a sealed system, will need to be topped up after bleeding radiators. This is because you have just reduced the pressure in the system, and it needs a certain minum pressure level to operate. Consult the boiler manual for instructions on how to re-pressurise the system. Generally, boilers require a little over 1 Bar of pressure when off, but some need more. Usually re-pressurizing involves opening the fill valve on the water supply line until the pressure gauge on the boiler is over the min pressure mark.

    5. Repeat this process on all radiators/baseboard heaters.

    6. Turn the boiler and pump on and turn the thermostat down so that the system will call for heat.

Bleeding the Boiler

Most modern convector heating systems do not have individual bleed valves on each room’s unit. You need to bleed the entire system at once at the boiler.
The procedure will vary depending on the type of boiler and heating system you have.

The three types of boiler heating systems are: combi, sealed system, and standard. Also, if you have a zoned heating system, there will be two or more zones worth of piping to deal with, each can be isolated from the system by shutoff valves. I recommend you consult the manual for your boiler for the specific steps to follow.

    1. Shut off boiler.

    2. On the boiler will be a drain valve that a hose can attached to; it will look like a standard hose bib like you see on the outside of a house. Close the gate valve between the boiler and the bib. Connect a hose to it, and run the hose to a bucket.

    3. Open the valve on the bib, while adding water to the boiler with the fill valve. The fill valve is located on the supply pipe or filling loop, usually the smallest diameter pipe in the system.

    4. Flush the system, letting the fill water run until no air bubbles come out from the hose in the bucket.

    5. Open the gate valve again, this will bleed any remaining air.

    6. Close the hose bib valve.

    7. Restart boiler

It is easy to see that it is best to keep air out of the boiler to start with. The process of bleeding a boiler and the pipes and radiators connected to it is labor-intensive, time-consuming and tiring, but you can do it, given a few instructions and safety precautions.