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A drain valve is used to allow you to drain either the pipes before the control valves (using a manual drain valve), or to allow you to drain the piping after the control valves (using manual or automatic drain valves).

Automatic drain valves close automatically when the control valve opens and fills the downstream piping. When you are finished watering an area of your yard and the control valve closes, the pressure in the pipe after the control valve lowers to close to atm pressure and the drain valve opens again to drain the ramaining water from the pipes after the control valve.

The water does not pass through the drain valve during operation of the sprinkler system. The drain valve is located at a low spot in the piping and in installed by screwing it into a tee fitting and then installing the tee into the pipe. The water flows through the pipe when the control valve is open and the water exerts an outward pressure which makes the flapper on the drain valve close. Once the control valve is closed, the pressure is relieved in the piping and the spring activated flapper on the drain valve opens and drains the water as if you are pocking a hole in the side of the pipe.
The drain valve will not cause any pressure loss since the drain valve is closed during watering and the water does not pass through it.

<b><u>More On Drain Valves:</u></b>

<b>Manual Drain Valve:</b> The manual drain valve should be installed on the sprinkler system mainline at the lowest point of the system. Additional manual drain valves must be installed if there are multiple low points along the run of pipe where water collection might occur. A ball valve, gate valve, "Stop and Waste" valve, or a simple threaded pipe with a cap may be used to provide drainage. Piping should be sloped properly to allow water to drain out. If the valve is located outdoors, it should be installed over a "dry-well" (underground pit filled with gravel to drain water away from the piping) to allow the amount of water that is drained to percolate into the soil. If the valve is installed indoors, make sure the volume of water can be collected or drained without the risk of overflow or flooding.

<b>Automatic Drain Valve:</b> Caution! Do not install automatic drain valves on the sprinkler system mainline! The automatic drain valve, model 16A-FDV, is a spring loaded device which is installed on the sprinkler pipes or heads. It is a convenient and efficient product for removing water from the lateral pipe network running from the sprinkler system control valves to the heads. Automatic drains should be installed after or downstream of the sprinkler control valves. They are not designed or engineered for use on mainlines. The drain valve will open every time the system is shut off. This will drain all the water out of the pipe providing the valve is installed at the proper location, which is the low point on the line. When the system is pressurized, the water shuts the valve off by pressing against the sealing mechanism, allowing water to flow through the pipe and on to the sprinklers in proper fashion.

Install automatic drain valves at the low points in the sprinkler lines. Generally speaking, one or two drains per line are adequate to do the job. The drain valves have either 1/2"or 3/4" male pipe threads. Use two or three wraps of teflon tape around the threads to ensure a proper seal. Simply screw the drain valve into a threaded fitting positioned downward. If you have a soil condition which drains poorly, we suggest digging a small hole directly beneath the drain and filling it with gravel to assist drainage.

Automatic drain valves remove water from the system every time it is shut off. No manual intervention should be required. Check for excessive puddling on the soil surface should one of the drains become stuck open during sprinkler operation.

For more information on how to use drain valves or to review this information WITH PICTURES, click on the link below:
http://www.sprinklerwarehouse.com/LawnCare/Winterization.asp

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