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Yes I will surely get a backflow prevention device, if necessary, although, if the water spigot is closed when fertilizing from stock tank, how would the water flow back into the house system?
Luckily, I only use organic fertilizer, no chemicals, but I still don't want any of that running into my pipes.
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Backflow is one of those things lots of people have a hard time understanding how it can happen. I know of people who have (foolishly) installed an irrigation system with NO backflow, reasoning that water only flows out of the system, and the water can't flow backwards through a "CLOSED" valve. Unfortunately they are wrong. Things can happen, especially with city water, where something causes a sudden drop in system pressure (think line break) and suddenly the pressure of the water in your pipes is greater than the pressure from the system. That will cause water to flow back towards the city system. When this occurs, even water on the far side of an irrigation valve can get sucked back into the system. After all, these valves are designed to hold back water flowing towards the yard. When back pressure like this occurs, it's like the valves are installed backwards, and water can and will get siphoned back into the system.
At
www.irrigationtutorials.com, there is a pretty lengthy discussion on various arguments for various forms of backflow. The simplest summary of those arguments I would like to highlite for the situation in hand is this... If your irrigation system is feeding water ONLY to your lawn, then any backflow preventer (your local building codes allow) are fine. If you are adding something to that water, then ONLY the RPZ (Reduced Pressure Zone) Backflow preventers are considered acceptable. The main reason is becaue the way they are designed, they will prevent backflow even if the internal check valves fail. (The only exception is if the entire RPZ becomes submergged, hence they must be installed above ground). A duel check valve backflow preventer (by contrast) offers no backflow protection if the internal check valves fail. All it takes is some trash in the line, and even city water will get trash in the line (I've got a 200# filter on my city water supplied irrigation system, and I've seen some of the stuff that has gotten filtered out of city water)
Bottom line, when you start getting into fetilizing with an irrigation system, "I will surely get a backflow prevention device, if necessary" doesn't cut it. An RPZ is required to insure drinking water doesn't become contaminated (and they are not cheap, especailly if local building codes require a certified plumber install it, as some do).
As to HOW it can happen, the bottom line is if all the parts are connected, backflow and contamination can happen. Even if you have manual valves to only allow fertilizer or city water to be the source, it only takes human error to have both open at the same time when something that causes backflow to occur. Plus, if fertilizer is flowwing through irrigation pipes, residue from the fertilizer will get left in the pipes. So even when the fertilization system is disconnected from the system, backflow can occur sucking in that residue.
Since you said you were looking for a "cheap" solution, these are the reasons why Jeff suggested a system where the fertilizer is the only source for the irrigation. Let me restate his suggestion to make sure I know what you are both saying...
You want an irrigation system sometimes feed from city water, sometimes feed via a pump from a fertilizer tank. Jeff's suggestion was to ALWAYS feed the system via a pump from a fertilizer tank. To avoid the cost of a backflow preventer, you in effect, fill the tank with water with a hose or pipe suspended above the tank. As long as there is a gap 3 times as high as the source pipe is wide, that air gap makes for a backflow preventer with the same level of protection as the RPZ. Basically, you could design a system where city water feeds the tank with an automatic valve that turns on when water levels in the tank are too low and turn off when the level is high enough (think of the water suppl for your toilet). So as long as the pipe feeding water to the tank is about 3"-6" above the tank, worst case senario, it is physically impossible for fertilizer to get back into the water supply regardless of human error of mechanical breakdown (and no RPZ).