How to Bleed a Boiler
Bleeding a boiler becomes necessary when your radiators are not heating as well as before, or have stopped heating altogether. Another sign is noisy pipes. In both cases the usual cause is air in your heating system's lines. Having to bleed a boiler is a normal occurrance and you can do this yourself.
Air can enter your heating lines for several reasons; the rotation in the circulator pump can produce air bubbles, adding water introduces a certain amount of air, and leaks can all cause air to build up to the point that it affects the performance of your boiler.
Bleed the Radiators
The first step in bleeding a boiler is to start with your radiators. Your heating system including the boiler and pump, must be shut off during this process or you may end up with more air in the system than when you started. Start with the upmost floor in your house and work down.
Your radiators should have a bleeder screw on the end elbow or on one of the other pipes coming into the radiator, usually it's a screw in the center of a brass or chrome piece.
- Put a bowl or other container underneath this bleed valve fitting to catch the water.
- Open the valve a half turn counterclockwise. You should hear the air escaping. When you start to see water coming out of the valve, close it by turning it clockwise a half turn.
- Repeat these steps with all of your baseboard heaters and/or radiators.
- After bleeding the radiators, you may need to repressurize the system; if you have a boiler in a sealed system or certain condensing combi boilers as some boilers must have a minimum level of pressure to operate. You should be able to find this information in the manual for your boiler. In most cases, repressurizing the system involves opening the fill valve on the water supply line until the boiler's pressure gauge is past the minimum pressure mark.
- Turn the pump and boiler back on and turn down the thermostat so the system will turn on the heat.
Bleed the Boiler
If your system is more modern, chances are that your radiators or baseboard heaters don't have their own bleeder valves. In this case, you must bleed the whole system at the boiler. Depending on the type of heating system you have, the process may change. Zoned heating systems have multiple zones of piping to work with. You should be able to isolate each zone from the rest of the system using the shutoff valves.
The first step, no matter what type of system you have, is to shut off the boiler. Next, look for the the boiler's drain valve. It should look like your standard threaded valve that you would attach a garden hose to on the outside of your house. Once you've found it, make sure the gate valve is closed between the boiler and this drain valve. Then attach a hose to the drain valve and put the other end in a 5-gallon bucket.
Next, find the boiler fill valve; it should be located on the filling loop or supply pipe. While adding water to the boiler with the fill valve, open the drain valve.
Let the water run through and flush the entire system. Let the water run until there are no air bubbles exiting the hose into the bucket. When you've noticed no more bubbles, bleed the remaining air by opening the gate valve again.
Close the drain valve and restart the boiler.
That's it. Your boiler and heating system should be functioning normally without the excess air.
