If you are fortunate, a water stub-out was provided for the irrigation as part of the original construction. All you need to do is connect your piping to the stub out and install a backflow preventer. If not, you will most likely be cutting and tapping into the water supply line leading to the house. In warm winter climates you will tap this pipe outside of the house. In cold winter climates you may need to make the tap in the basement or heated crawl space. If outside, the tap may be made by cutting into the supply pipe and installing a special tee called a "compression tee" (more on that later), or the home builder may have provided a capped pipe on the house for you to tap into. A word of warning here. I do not recommend that you tap into a pipe or use a stub-out where the water has to pass through the house (other than the basement.) In other words, you do not want the water to go through pipes inside the house walls to get to the irrigation system. There are a couple of reasons for this. First, there may be restrictions in the piping as it goes through the house that you can't see and don't know about. This includes multiple turns, small size pipe, water softeners, and all kinds of things. The second reason is that it is likely that the irrigation will be running at night. Often the water will make a lot of noise as it moves through the house pipes and it sounds a lot louder during the quiet of night. So I recommend that if you need to get water from one side of the house to the other, that you install a new pipe around the house. If you decide to use an outlet on the side of the house, perform a simple test. If there is a hose on the outlet remove it (the hose will restrict the flow.) Wait until late at night and then open up that faucet all the way. Then go inside and see how loud it sounds inside the house. The noise will likely be louder when the irrigation system is installed.
Never use water that has gone through a water softener for irrigation. The sodium that most water softeners add to the water will damage both your soil and your plants! Even if you don't use sodium-based salt in your water softener, the next owner of your home may. If your water contains minerals that stain walls, don't put sprinklers next to the walls. Plant a border of low shrubs next to the wall and water them using bubblers or drip irrigation. Irrigation water will damage walls, may cause wood rot, and may lead to mold problems, including toxic mold. Keep sprinklers a minimum of 18 inches away from house walls. If you don't want water stains on walls, they need to be at least 5 feet away from walls, more if you are in a windy area.
Its a good idea to install an emergency isolation valve as close as possible to the point where you tap into the water supply. That way you can easily and quickly shut-down the irrigation system for emergencies or repairs. In areas where it freezes in winter this valve should be protected from freezing. I recommend that you use a ball valve or high quality gate valve for the isolation valve. Note that inexpensive gate valves have a tendency to fail after just a few uses. Water will start to drip out of the valve under the handle, and the valve will not shut the water off completely no matter how hard you turn the handle. This can make repairs extremely difficult.
You must have a backflow preventer on your irrigation system. In most places this is the law, everywhere else its just the smart thing to do. Your landscape has all kinds of nasty things in it that will make you sick or worse if you drink them. This includes toxic chemicals (fertilizers, pesticides, etc.) and animal waste. These things can and WILL come back up your irrigation pipes and into your drinking water if you don't stop them. If you have a well they can go down your well and into everyone else's drinking water. If you are on a community water system you will be poisoning your neighbors. THE MANUAL OR AUTOMATIC VALVES ARE NOT SUFFICIENT TO STOP BACKFLOW. The purpose of the backflow preventer is to protect you when the valve leaks, which all valves will do eventually. Saving a little money by skipping the backflow preventer will not seem so smart after you spend a small fortune on hospital bills (or funeral expenses) for a poisoned family member! Anti-Siphon valves have a built-in backflow preventer, so if you are planning to use that type of valve you are covered. If not you will need to install a backflow preventer between the point where you tap into the water supply and the sprinkler system control valves. For more detailed information on backflow preventers, including selection guidelines, see my page on Backflow Preventers in the Sprinkler Design Tutorial.
A compression tee is a special fitting designed for tapping into existing metal and PVC pipes. Other types of compression fittings, such as couplings and threaded adapters, are also available for situations where something other than a tee is needed. Compression couplings are also used for pipe repairs. A compression fitting works by using a rubber gasket. This gasket is compressed against the existing pipe when you tighten a nut on the end of the fitting. This makes a leak-free seal without the need to thread, glue, or solder the pipe connection. Use metal body compression fittings above ground level, PVC may be used below ground but must be installed below the frost level. Compression fittings may leak if used on heavily corroded, rusty, or pitted pipe. For this reason some professionals do not recommend that they be used on older steel or galvanized steel pipe.
Note: some drip irrigation fittings are also called compression tees, compression ells, etc. They are not the same thing as these tees and ells! Also compression fittings are used for small tubes, they are similar to these, but not the same thing either.
A PVC compression coupling. A compression tee looks similar, but has an outlet on the side.
To use a compression tee to tap into an existing pipe:
Installation of a compression tee.
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