Blowing out an irrigation system involves using an air compressor to force pressurized air through the underground piping network, removing residual water before cold weather. This winterization process is necessary in regions where temperatures drop below freezing for extended periods. Water expands by approximately nine percent when it turns into ice, and this expansion inside pipes and sensitive components like backflow preventers can lead to burst lines, cracked fittings, and costly damage. Removing this water safeguards the entire system against the physical stress of ice formation.
Gathering Tools and Preparing the System
Successfully preparing the system requires specific tools, primarily a suitable air compressor. For typical residential systems, a machine delivering between 20 and 50 CFM at 50 PSI is recommended to move the necessary volume of air efficiently. You will also need a specialized quick-connect fitting or adapter to attach the air hose to the designated blowout port, usually located downstream of the backflow preventer.
Before connecting equipment, locate and turn off the main water supply shutoff valve for the irrigation system, often found near the water meter or in a basement. Turning this valve off ensures no new water enters the system during the blowout procedure. Essential safety gear, including sturdy gloves and safety goggles, must be worn before the compressor is activated.
The Step-by-Step Air Blowout Procedure
With the water supply secured, connect the air compressor hose to the blowout port, ensuring a tight seal. Set the compressor’s pressure regulator to the maximum safe level for the piping material, generally 40 to 60 PSI for most residential systems. Starting with a lower pressure setting is advised to prevent overstressing the system. Once connected and regulated, turn on the compressor to build up air volume.
Activate the zones one at a time using the irrigation controller, starting with the zone highest in elevation or farthest away from the compressor connection. Allow the pressurized air to push the water out through the sprinkler heads. The zone must run until the water spray changes to a fine mist or fog, indicating the bulk of the liquid has been expelled. Do not continue running the air once the system is dry, as high-velocity air can melt the internal plastic components of the sprinkler heads and valves.
After the first zone is clear, shut off the air supply from the compressor before closing the zone valve at the controller. This sequence prevents a sudden pressure surge that could damage pipes or fittings. Repeat this process for every zone in the system. Running each zone two or three times for short durations is often more effective at clearing residual moisture without overheating components.
Essential Safety Guidelines and Pressure Limits
Operating an air compressor on an irrigation system carries inherent risks, primarily related to over-pressurization and component failure. The maximum safe pressure is strictly defined by the material of the pipes. Rigid PVC lines have a ceiling of approximately 80 PSI, while more flexible polyethylene pipe should not exceed 50 PSI. Exceeding these limits can cause nozzles to blow off, sprinkler heads to break, or the pipes themselves to rupture. For this reason, the compressor must be equipped with a functional pressure regulator that is actively monitored throughout the procedure.
Personal protective equipment is a non-negotiable requirement, particularly the use of safety goggles to shield the eyes from flying debris or unexpected bursts of water. Never stand directly over any component of the irrigation system, especially the backflow preventer or sprinkler heads, while air is being introduced. High-pressure air can turn small fragments into projectiles. A significant safety rule is to ensure the air is always shut off before the zone valve is deactivated, as closing the valve while the air is still flowing can cause a damaging pressure spike within the closed section of the pipe.
Securing the System for Winter
Once all zones have been successfully cleared of water, the final steps involve securing the backflow preventer and preparing the control unit for winter storage. It is strongly advised against blowing compressed air directly through a backflow prevention device. The seals and internal components, especially those in Pressure Vacuum Breaker (PVB) or Reduced Pressure (RP) assemblies, can be damaged by the heat and force of the air. Instead, the backflow device must be physically drained.
To drain the backflow device, the shutoff valves on the upstream and downstream side should be turned to a position that is neither fully open nor fully closed, often a 45-degree angle. This half-open position is intended to allow space for any residual water inside the ball valve mechanism to expand without cracking the fixture. The small test cocks on the device must be opened by a quarter turn to allow the water trapped inside the assembly to drain completely. Finally, the air compressor is disconnected, and the irrigation controller is set to the “off” or “rain” position to prevent it from cycling valves during the winter months.