A sprinkler blowout is the process of winterizing an irrigation system by using compressed air to force all water out of the pipes, valves, and sprinkler heads. This procedure is necessary in climates where the ground freezes. Water left in the lines will expand as ice, causing pipes to crack, fittings to separate, or system components to rupture. The pressure of the air, measured in pounds per square inch (PSI), is the most important factor, as uncontrolled pressure carries a significant risk of damaging the system.
Determining the Maximum Safe PSI
Setting the correct air pressure is the most important factor for a safe sprinkler blowout. For most residential irrigation systems, the recommended operating pressure range is between 40 and 60 PSI. This range is sufficient to push water through the main lines and out of the farthest sprinkler heads without causing component damage. Residential systems utilize delicate components like seals, valve diaphragms, and plastic gears in the sprinkler heads, which are not designed to withstand high-pressure air.
The absolute maximum pressure should never exceed 80 PSI, regardless of the pipe material. Exceeding 80 PSI can rupture pipes, damage plastic fittings, or cause seals to blow out, leading to costly repairs. It is safer to operate at the lower end of the recommended range, typically around 50 PSI for systems with rigid PVC piping. Systems using flexible polyethylene pipe should be kept lower, with 50 PSI being the accepted maximum limit.
Using a properly regulated air supply is mandatory; relying on the compressor’s maximum tank output is a common and destructive mistake. The goal is to push the water out gently, not to blast it with force. Applying pressure that is too high can generate heat inside the pipes due to air friction, which may melt internal plastic components of the heads and valves.
The Role of CFM and Compressor Volume
While pressure (PSI) prevents system damage, volume, or Cubic Feet per Minute (CFM), determines the effectiveness of the blowout. CFM measures the rate at which the compressor can deliver air, which is the necessary force to push a column of water through the pipes. A compressor with high PSI but low CFM will quickly pressurize the system yet lack the sustained volume required to move the water out of the lines. The water will be pushed a short distance and then settle back into low spots, where it can freeze.
For a standard residential system, a compressor capable of delivering between 10 to 20 CFM is typically required, though larger systems may need up to 40-50 CFM. The required CFM is often estimated by dividing the maximum water flow rate of the largest zone, measured in gallons per minute (GPM), by 7.5. Since many homeowner-grade compressors only produce 5-8 CFM, renting a commercial-grade tow-behind compressor is often necessary to achieve the required flow rate.
A low-PSI, high-CFM compressor is the ideal tool because it provides the volume of air needed to clear the lines completely without subjecting the system to destructive pressure. The air volume ensures that the water is continuously pushed through the pipe until only air remains. If the CFM is too low, the air will simply travel over the top of the water, leaving a damaging layer of moisture at the pipe’s bottom.
How System Materials Affect Pressure Tolerance
The material used in the underground piping establishes the upper boundary for safe air pressure. Rigid PVC (polyvinyl chloride), particularly Schedule 40, possesses a high inherent pressure rating for water pressure. However, the mechanical joints and attached components like valves and sprinkler heads are the weak links in a PVC system. Therefore, the blowout pressure must be kept low.
Black polyethylene (PE) tubing, a common flexible pipe option, has a lower pressure tolerance than PVC. For systems constructed with polyethylene, the maximum safe air pressure should be reduced to 50 PSI to prevent damage to the pipe or its barbed fittings. PEX (cross-linked polyethylene) is a more modern, durable option with higher stability. However, even with PEX, the overall system pressure is still dictated by the lowest-rated component: the sprinkler heads and valves.
Regardless of the main line material, the sprinkler heads are often the most sensitive part of the system. Their internal seals and rotational mechanisms are engineered for water pressure, not for the high velocity and friction of compressed air. The pressure limits are set to protect these fragile plastic components from overheating or mechanical failure, since the pipes are generally more robust.
Step-by-Step Safe Blowout Technique
The process for safely clearing the system begins with turning off the main water supply. Next, the water in the pipes should be bled off by opening the manual drain valves to remove standing water before introducing air. The air compressor should then be connected to the designated blowout port using the proper adapter, and the regulator must be set to the safe pressure range, typically 40-50 PSI.
The blowout should proceed one zone at a time, starting with the zone farthest from the compressor connection point. Air should be introduced in short, controlled bursts, or pulses, rather than a continuous flow. This pulsing technique minimizes frictional heat buildup inside the pipes and reduces stress on the system components. Once the zone is activated, the water is expelled, and the flow transitions from a heavy spray to a fine mist or fog.
The air flow should be stopped immediately once only mist is visible coming from the heads, which indicates the line is clear. Over-blowing a dry line with continuous air can create friction that damages the seals or plastic components inside the heads. After the first clearing, each zone should be blown out a second time to ensure all residual moisture is removed. Finally, the compressor must be disconnected, and any backflow prevention device should be drained or positioned correctly to prevent freezing.