Irrigation Winterization (Blow Out)

By: Franz Witte

What is irrigation winterization?
Irrigation winterization (often called a “blow out”) is the process of making an irrigation system safe from freeze damage over the winter. This is accomplished by removing most of the water from the irrigation pipes, valves, sprinkler heads, and other components of the system. Compressed air is used to push the water out of the system: a compressor is connected to the irrigation mainline and then the valves are operated, allowing the air to push the water through the mainline to the valves and lateral lines and out of the heads.

Why is it important to winterize an irrigation system?
Freeze damage to irrigation system components can be very destructive and expensive to repair. It can destroy dozens of feet of PVC or poly pipe, crack solenoid valves, and even split metal pipes.

​Here’s how it happens: When air temperatures drop below freezing for extended periods of time, the ground begins to freeze from the surface downward. The longer and colder the freeze, the deeper into the ground the frost line will reach. If the frost line reaches irrigation components that are full of water, the water will freeze and expand pushing outward on the component surrounding it. This can cause the component to crack or shatter.
However, if most of the water has been blown out of the system, any remaining water will simply freeze and expand into the air inside the pipe or valve.

More about the process
Professionals will use a high-volume air compressor (usually a tow-behind diesel powered unit) to quickly winterize a home irrigation system. This allows sufficient pressure and air flow to be maintained while running one or more sprinkler zones. 

If the system is fed by a domestic (city) water connection or a pressurized HOA connection, the main valve will be closed (and the drain opened in the case of a domestic connection) to isolate the home system mainline from the source.
An adapter is used to connect the compressor hose to the system mainline through a tie in fitting. This is usually a threaded fitting or hose bib near the main valve or backflow prevention device. 
Once the compressed air is provided, the valves can be operated from the controller or manually at each valve. This allows air to flow through the system (just like water normally does) pushing the water in the lines out of the pipes and components. Depending on the system and compressor, several zones may be able to run at the same time and still have enough air flow to effectively clear the water.

When all the heads on a zone are blowing only air or light mist, that zone can be shut off and the next zone can be opened to repeat the process until all zones have been cleared.
Any hose bibs or other hand valves that are connected to the mainline should also be opened and cleared of water.

Some things to keep in mind:
Above-ground components (like RP backflow preventers or above-ground filters) will be much more susceptible to freeze damage since they are not insulated by soil or the air in a valve box. If temperatures below freezing are expected before the system is to be blown out, it’s a good idea to drain or insulate these components. 
After the winterization, it’s worth double checking the main filter. Sometimes remaining water will settle into the filter housing. If this happens, the filter can be disassembled and drained.

Most hose bibs that are attached to a house are self-draining anti-siphon bibs and are less susceptible to freezing. However, hoses should be disconnected before freezing temps to allow the bib to drain properly. Any hose bibs that are not self-draining should be fitted with an insulating cover.

Any ball valves (especially the ones on an above-ground backflow device) should be left half open (in the 45-degree position). This prevents water from being trapped between the ball and the valve housing. An exception to this would be if the main valve on a pressurized connection is the only way to isolate the system from the HOA mainline. In this case, it should remain closed if the HOA lines have not yet been shut down or blown out.

Examples of domestic and pressurized irrigation points-of-connection