For a hot water boiler system to work properly hot water must circulate through each zone on demand to provide heat. If you have air in anyone of the heat zones you will not get a flow of hot water through the zone. If no hot water is flowing through the zone you will not have any heat in the appliance (baseboard or radiator) on zone.

How the boiler heating system works.

When you turn on or turn up a thermostat, a signal goes to the boiler controller telling the boiler to start. When the boiler water temperature reaches approximately 120F the controller sends a signal to the zone valve for it to open and hot water circulates through the zone loop to warm the baseboard or whatever appliance you have on the zone for heating. The hot water continues to circulate until the room heats to the thermostat setting and then shuts off. The boiler shuts off first and when the water temperature drops below 120 F the zone valve will close.

Note

A hot water boiler system may use zone valves or individual zone circulator pumps to control the water trough the zones. Most home heating systems use zone valves.

Unfortunately air may get into the system or one of the zone circuits. This can occur when maintenance is performed on the system whereby water is partially drained from a zone allowing air to enter. The air causes an airlock which prevents the hot water from circulating through the zone.

Remedying this problem is relatively an easy DIY task by simply bleeding the air from the circuit that is effective.

All boiler circuits have three manual valves in each circuit, one on the supply line to the zone just after the zone valve (or circulate pump) and another on the return line just before the zone return where the water enters the return manifold going back to the boiler and another (usually with a blue handle) for draining the system just above the return valve.

How to Bleed the Hot Water Boiler Zone:

Usually there are several zones on most home heating systems. To begin the bleeding process turn all the zone supply valves and return valve off except for the supply valve going to the zone you are bleeding. See pic.
Attach a hose to the return manual drain valve that is located just before the return manifold valve. Each of the zones has a separate drain valve for each zone.
Place the end of the hose into a bucket to catch the water.
Turn the thermostat for the effective zone on so the boiler will heat water and open the zone valve to circulate water through the zone. You can tell when the zone valve opens by either looking at the controller open light or by observing the manual valve that is attached to the zone valve.
When the zone valve is open (allowing hot water to flow through the zone) open the manual drain valve about half way. Hot water will circulate through the zone and out the manual drain valve into the bucket. Any air in the zone will be pushed out by the hot water. You may be able to hear a disruption (gurgle) in the drain water flow as the air is removed. Drain approximately one gallon of water through the drain valve or until the drain water is hot.
Turn the drain valve off and turn on the return valve on the effective zone. Let the boiler continue to run until you have heat to the baseboard or appliance. If you do not have heat almost immediately you may have to repeat this process.
When the zone is heating properly, turn all the other zone supply valves and return valves to the on position. This will return the system to normal.

Note

Most all home hot water boilers have an automatic water supply valve to feed water to the boiler when the system is low. If your system has a manual feed supply valve make sure that it is on during the draining process.

That is how to bleed a hot water boiler. This is an easy homeowner DIY task that most handymen can perform with little difficulty. Be sure to wear proper safety equipment such as gloves and safety goggles as the hot water can cause burns.

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