Q. Is it possible to have two valves on at the same time or to run two irrigation valves at once?
A. Yes, it is often possible to run two valves at once. However there are several problems that can occur.
You must have a sufficient water supply for both valves to run at once. If the performance of the sprinklers suffers and you start seeing dry spots in the landscape, you obviously don’t have enough water. You may need to do some adjusting of the sprinklers as the water pressure operating them is likely to be less when two valves are on.
Both valves running at the same time may require more water than the pipe supplying them can reasonably handle. This can result in water hammer, or premature pipe wear/failure, due to high water velocity.
Water Hammer: Listen for a loud water hammer “thump” or “bang” noise when the valves close. A gentle thump is fine, but if the pipes reverberate from it that is not good. Run just one valve and listen to the sound when it closes. Assuming the irrigation is properly designed, that should be the “normal” closing sound. Now listen to the sound when both valves are closed together to see if it is significantly louder. If it is significantly louder, that is not good. You can possibly reduce or eliminate the water hammer problem by closing the valves separately, one at a time.
High Velocity: Premature wear due to velocity is harder to figure out. It generally isn’t a problem unless the water is really flowing fast through the pipe, like 8 feet per second or higher. The only way to determine if it is a problem is to do a couple of calculations. Start with the sprinklers. On top of each sprinkler is an identifying names and part numbers that tell you the brand, model, and hopefully the nozzle size. Write down that information for each sprinkler, then look up the water use (GPM value) for that sprinkler and nozzle at the sprinkler company’s website. (You may need to call the company’s help line to assist you, each brand and model is different so I can’t give exact instructions.) Now add together the GPM values for all the sprinklers that are running at the same time when two valves are turned on. This will tell you how much water the two valves require when running together. Next find the size and type of the water pipe that leads to the valves. (For example it might be a 3/4″ copper tube, or maybe a 1″ PVC pipe. It may be several different sizes and types of pipe, in which case you would use the smallest pipe size and type.) Using that information you can calculate the velocity of the flow in the pipe using the Friction Loss Calculator at https://www.irrigationtutorials.com/formulas.htm#sec8. Just enter the pipe type, size, and GPM into the calculator and it will give you the velocity.
If you decide to use a controller to operate the valves the controller must be a brand that provides sufficient amperage to run two valves at the same time (most do.) If you want the controller to run the valves at the same time, but start and stop them about one minute apart to reduce water hammer, you will need a controller that allows you to run two separate valve zones at the same time. Most controllers have a “stacking feature” that prevents them from doing this. You will need a controller that allows you to turn off the stacking feature. Most controllers can’t do this. You will probably need to enlist a knowledgeable controller salesman at a professional irrigation supply store to assist you in finding a controller that will work for this unique situation.