I have a small cabin in a 12 cabin community which is spring fed (spring flow is about 5 gpm). The water flows from our spring box into a 5000 gallon storage tank. The community was built in the 50's and the water system was originally designed to work off of gravity flow alone. In the upper one third of the community, the water line is 3" diameter. For the middle elevation of the community, the diameter is 2", and for the lower third of the water line, it is 1" diameter, which all seems quite logical to me so everyone probably got about the same flow rate when their taps were open, although at different pressures, when only gravity flow is used. My cabin is about 60' below the storage tank, and I have a pressure of about 27 psig under static conditions. There is one other cabin below me, about 25' lower in elevation. They are water hogs. They installed a booster pump, and whenever their pump comes on, if I have a faucet open, my water flow stops and I hear air being sucked into the faucet. If no faucets are open in my cabin, and their booster pump comes on, I hear a bubble gurgle noise ("glug, glug, glug", like a 5 gal sparklets cold water dispenser) coming from my water heater like water is being sucked out if it and air is being sucked in to replace it. I am quite happy with my 27 psig when the neighbor's booster pump is not on, but having air and/or water sucked out of my cabin is a bit much when they put their pump on. This family has quite a few children/grandchildren visitors who will leave hose spigots on so they can play in the water. They also decided to build a pond which they sometimes fill from the community water, even though they are not suppose to. So having water and air sucked out of my cabin is a rather common occurrence. When they leave their booster pump off and just gravity flow through one of their taps (which some members of the extended family of potential visitors will do at my request), my water pressure is at about 15 psig, which I can live with. I have thought about installing a backflow preventer, but with my already low water pressure, I am afraid the additional resistance to flow that a backflow preventer would create, would lower my flow and pressure even more (maybe another 4 to 5 psig loss?). I have a couple of questions: 1) Can my water heater be damaged if water is sucked out of it? 2) Can a source of contamination be created if I happened to have a hose running in my yard and water got sucked back into the main line through it? 3) Is there something other than a backflow preventer that I could use that would open up and let air into the line if their pump creates a negative pressure in the line, something that would not be a resistance to my water flow? Sorry about such a complicated question for my first one on the forum. Any suggestions would be greatly appreciated. Thanks, Wayne