The choice between using a three-quarter inch or one-inch pipe for a sprinkler system directly influences the system’s efficiency and performance. Choosing the wrong size can lead to inadequate water coverage, resulting in dry spots or excessive wear on components. The correct pipe size ensures that the water volume and pressure required by the sprinkler heads are delivered consistently. This choice is primarily determined by the volume of water available from the source and the physical principles governing water movement through pipes.
Calculating Your System’s Water Availability
Before selecting any pipe, determine the maximum amount of water the home’s supply can provide. This involves finding two specific measurements: the flow rate in gallons per minute (GPM) and the static pressure in pounds per square inch (PSI). The available flow rate sets the absolute limit for water delivered to any single sprinkler zone.
To find the GPM, perform a simple “bucket test” by timing how long it takes to fill a five-gallon container from the outdoor spigot. The calculation involves dividing the volume (five gallons) by the time in seconds, then multiplying the result by sixty to convert the rate to GPM. For example, if it takes twenty seconds to fill the bucket, the maximum flow rate is fifteen GPM (5 รท 20 x 60 = 15).
The static pressure, or the water pressure when no water is flowing, is measured by attaching a pressure gauge directly to the spigot. This reading provides the baseline force available to push water through the system. Note that this static number will decrease once water begins flowing due to friction and system demands. Knowing both the maximum GPM and the static PSI establishes the boundary conditions for the entire irrigation design.
The Impact of Pipe Size on Water Flow
The physical principle that dictates pipe size is friction loss, which is the pressure drop occurring as water moves against the inner walls of the pipe. As water volume increases, the speed, or velocity, of the water inside the pipe also increases. When water velocity rises, friction loss increases significantly, meaning less pressure is delivered to the sprinkler heads.
A smaller three-quarter inch pipe requires water to move at a much higher velocity to deliver the same volume (GPM) as a larger one-inch pipe. This higher velocity creates substantially more friction loss per foot of run. For instance, delivering ten GPM through one hundred feet of three-quarter inch PVC pipe results in a pressure loss three to four times greater than running the same ten GPM through a one-inch pipe.
Higher water velocity also carries the risk of water hammer, a damaging pressure surge that occurs when a control valve closes rapidly. Industry standards recommend keeping water velocity below five feet per second (FPS) to mitigate this effect and limit excessive friction loss. The larger internal diameter of the one-inch pipe slows the water down, allowing higher flow rates while staying within the acceptable velocity limit.
Guidelines for Choosing the Right Size
The choice between pipe sizes depends entirely on the flow rate (GPM) required by the sprinkler heads on any single circuit, known as a zone. The three-quarter inch pipe is suitable for smaller zones with modest water demand. For common residential PVC materials, the recommended maximum flow rate for three-quarter inch pipe is seven to nine GPM to maintain acceptable pressure loss and water velocity.
When the total flow demand for a zone exceeds this threshold, the system should be upgraded to one-inch piping. A one-inch pipe can comfortably handle flow rates up to twelve GPM while keeping water velocity below the recommended five FPS limit. Using the larger pipe minimizes pressure loss, allowing the sprinklers to operate near their designed pressure for proper coverage.
Mainline vs. Lateral Lines
It is important to consider the role of the mainline versus the lateral lines. The mainline, which runs from the water source to the zone valves, should often be the larger one-inch size, especially on longer runs, as it carries the full flow capacity required for the largest zone. The smaller three-quarter inch pipe may be used for the lateral lines that branch out from the zone valves to individual sprinkler heads, provided the flow rate to those specific branches is low enough. Selecting the one-inch pipe is a conservative choice that builds in a buffer against unexpected pressure drops or future system expansion.