When cold temperatures arrive, water left inside an irrigation system poses a serious threat. As water freezes, it expands with immense force, easily cracking pipes, valves, and sprinkler heads. Proper preparation, known as winterization, is the only reliable way to ensure all components are dry and protected from this expansion force. This process involves isolating the system, removing residual water, and securing vulnerable above-ground hardware.
Shutting Down the Water Supply
The first step in preparing the system is to completely isolate it from the main water source. Locate the main irrigation shutoff valve, usually found underground in a valve box or inside a basement or utility room, and turn it off. This valve is distinct from the main house shutoff and must be fully closed to prevent water from entering the system during draining.
Once the water flow is stopped, the irrigation controller or timer must be powered down or set to the “off” position. This prevents the system from attempting a scheduled cycle, which could damage the pump or electronic valves. If your system has manual drain valves at low points, open them now to allow gravity to remove a significant portion of the water. This prepares the lines for the compressed air stage, which removes the remaining water from the underground lines.
Clearing Lines with Compressed Air
Removing the remaining water from the underground piping requires using an air compressor in a process often called a “blowout.” For a typical residential system, the compressor should have a flow rate (CFM) of 20 to 50 to effectively push the water out. The connection point for the compressor is usually a designated port located near the backflow preventer or the main shutoff valve.
Extreme caution must be exercised when using compressed air, as excessive pressure can rupture the piping. Always wear safety goggles and connect a regulator to the compressor to control the pressure entering the irrigation lines. For rigid PVC piping, the pressure should not exceed 80 PSI, with 50-60 PSI being a safer operating range. If your system uses flexible polyethylene (poly pipe), the maximum safe pressure is significantly lower, ideally staying below 50 PSI to avoid stress fractures.
The clearing procedure involves working through the irrigation zones sequentially. Begin with the zone highest in elevation or farthest from the compressor connection. Use the controller to activate one zone valve at a time, allowing the air to push the column of water toward the sprinkler heads. Running the zones individually ensures the concentrated force of the air is directed to a smaller section of pipe.
A zone is considered cleared once the water spraying from the heads changes from a steady stream to a fine mist. Once only mist appears, immediately disconnect the compressor from that zone. Compressed air running through dry pipes generates friction and heat, which can soften or warp plastic fittings and seals if allowed to continue.
Securing the Backflow Preventer
The backflow prevention device is highly vulnerable because it is usually located above ground and fully exposed to freezing temperatures. This device prevents irrigation water from siphoning back into the potable water supply. Water must be fully removed from its internal chambers to prevent freeze damage to its housing and internal seals.
The process involves draining the backflow preventer after the main lines have been cleared. Use a flathead screwdriver or specialized tool to open the small test cocks or valves on the device’s body, allowing trapped water to escape. These test cocks should be left open or set at a 45-degree angle to prevent residual moisture from pooling and allow for air circulation.
After draining, the main inlet and outlet ball valves on the backflow preventer should be set to a 45-degree angle (half-open position). This prevents water from becoming trapped within the valve’s ball socket, which can freeze and crack the metal body. Once drained, wrap the device with insulating material, such as a specialized backflow blanket or foam cover, for thermal protection throughout the winter.