HVAC523

Filling, purging and air removal

Filling and Purging

For a hydronic system to operate correctly and deliver silent comfort, it must be free of air.

A system that has air in it will have occasional gurgling sounds in the pipes or a complete loss of heat output

Filling and Purging At some point every hydronic system will

contain a mixture of air and water.

This is especially true on a new installation.

Most properly designed systems, installed with an air separator, will rid themselves of small amounts of air in a couple of days of the initial start up.

Filling and Purging

Watts air separator Watts air scoop

Filling and Purging

Even with air removal devices installed on a system, it is always recommended to purge as much air from the system prior to the initial start up.

After the boiler has been installed, the preferred method to remove large amounts of air is called Forced-water Purging.

Filling and Purging

The amount of air that can be expelled from the system during filling is substantially increased when the entering fluid has sufficient velocity to entrain (grab) air bubbles and carry them to an outlet.

In other words, the faster the water is moving, more air can be pushed out of the system.

Filling and Purging

The preferred method of forced water purging is to install a boiler drain and full port ball valve in the return line of the boiler.

Boiler drain and ball valve

Filling and Purging

To fill and purge the system, the fast fill lever at the top of the water feed valve is lifted to admit water at a high flow rate.

Whenever the fast fill valve is opened, you must continually monitor the system water pressure as to not exceed the pressure relief setting.

Filling and Purging

Fast fill lever Triadicator

AND: :                                                                                                                                               

Filling and Purging

Watts brand water feed/backflow preventer.

Filling and Purging

Because the ball valve on the return line is closed, the entering water begins to fill the boiler, and then as the boiler is filled, is forced out through the distribution piping.

Filling and Purging

Boiler drain

Ball valve

Water feed

Expansion tank

Filling and Purging

The purging flow continues through the system and eventually reaches and exits the boiler drain just upstream of the closed ball valve.

A hose can be used to route the exiting mixture of air and water to a drain.

Filling and Purging

Once the entire system is purged of air the fast fill valve should be returned to its normal operating position and the boiler drain is then closed.

The ball valve near the boiler drain is also opened at this time.

Filling and Purging

One last check of the system water pressure should be made at this time prior to starting the burner.

An average system will operate between 12 -15 PSI.

Filling and Purging

Another purge procedure for removing air from a system is called Gravity Purging.

When we used the gravity purge method, air vents must be installed at highest points in the system.

Filling and Purging

As the fill valve is opened water floods the lower parts of the system and air rises to the top of the system and is bled off through the venting devices.

This method, although simple, takes a much longer time and is not recommended.

Filling and Purging

Problems created by entrapped air include accelerated corrosion, loss of heat output, reduced circulator head, improper lubrication of wet rotor circulators and reduced heat transfer within the heat source.

Filling and Purging

In hydronic systems, air is found in three forms:

Stationary air pockets

Entrained air bubbles

Air dissolved within the fluid

Stationary Air pockets

Stationary air pockets can form in the top of heat emitters even those located at the low part of the structure.

Since air is lighter then water, it tends to migrate towards the high points of the system.

Entrained Air Bubbles

Entrained air bubbles are nothing more than air, existing as bubbles, which get carried along with the fluid.

Since this type of air is moving with the fluid, at some point it will have to pass through the air separator device.

Filling and Purging

Air dissolved in the fluid

Perhaps the most misunderstood form of air in a hydronic system is dissolved air.

Molecules of gasses that make up air including oxygen and nitrogen can exist in solution with water molecules.

The best way to remove this type of air is high temperature and low pressure.

Air removal devices

Air removal devices used in hydronic systems can be classified as either high point vents or central de-aerators.

High point vents

High point vents

High point vents are intended to release air from one or more high points in the system piping where it in tends to accumulate in stationary pockets.

Typical locations for these types of vents are at the top of the heat emitters, at the top of the distribution piping or where ever piping makes a downward turn following an upward or horizontal run.

High point vents

Central Deaerators.

A central deaerator is a device intended to remove the entrained air from the flowing fluid a well as to maintain the system at the lowest possible air content.

Central Deaerator

Central Deaerator

Central Deaerator

Manual air vents

The simplest type of high point venting device is a manual air vent.

They are sometimes called coin vents or bleeders

When their central screw is rotated, air can move up through the valve seat and exit through a small side opening

Manual air vents

Float Type Air Vents

The need for fully automatic (unattended) venting a different type of device.

A float type air vent contains an air chamber, a float and an air valve.

Float Type Air Vents

When sufficient air accumulates in the chamber, the float drops down and opens the valve at the top of the unit.

As air is expelled, water enters the chamber and lifts the float to close the air valve.

Float Type Air Vents

Automatic Air Vents

Another type of small high point venting device is called the Automatic Air Vent.

When the upper cap of this device is open, air can pass through and be expelled.

When water reaches the vent, an internal washer or disc swells up and closes off the vent.

Automatic Air Vents

As more air accumulates beneath the vent and the washer or disc dries out and reopens the vent to expel the air.

The time required for the washer to close off the vent isn’t fast enough to stop all water loss, therefore these types of vents may experience a slight water loss from time to time.