How to Bleed a Hot Water Boiler
How to Remove Air From Your
Hot Water Heating System
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.
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
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
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
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
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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