Water hammer happens when a pump shuts off because the pump is pumping 10 or 20 gallons per minute, depending on the size of the pump, so the check valve is wide open. Wide open for this size check is only about an inch. Water flowing through the check valve is filling the pressure tank at 10 or 20 GPM until the pressure reaches 60 PSI and the pressure switch shuts off the pump. Water in the pipe is flowing several feet per second into the tank when the pump shuts off. Then the 60 PSI of air in the tank acts like a spring and reverses the direction of the flow in the pipe. The water can only move backwards for one inch before the check valve closes, unless the check valve is worn out and slow to operate. The water can be moving very fast even if it only moves an inch but, the further backwards it goes before the check closes, the faster it closes the check valve. The check valve slamming closed causes the water hammer, which is actually a shock wave bouncing off the closed check valve and rippling through the entire system at 3,000 to 8,000 feet per second. This shock wave can also happen from the starting of the pump, as it instantly starts cramming 10 or 20 GPM into the system before the water in the pipe has time to start moving. The larger the water system, longer the pipe, the further this shock wave can travel, and the more places it can cause damage. This is why a single house system does usually not have as much problem with water hammer as larger systems like city water supplies. However even the smallest system can have these problems. The Cycle Stop Valve eliminates water hammer in a couple of ways. First the pump usually only causes water hammer when it is started or stopped. The CSV keeps the pump running continuously as long as at least 1 GPM is being used. So in systems like one I am discussing with a home owner who has a heat pump causing his pump to cycle on and off 19 times per hour, keeping the pump running continuously doesn’t let that check valve close and open 19 times per hour. This not only keeps the check valve from wearing out but, also eliminates 38 water hammer events per hour. The second way a Cycle Stop Valve eliminates water hammer is by restricting the flow from the pump to only 1 GPM when filling the pressure tank When the heat pump does shut off and no one is using water anywhere in the system, the CSV allows the tank to fill at 1 GPM until the pressure switch shuts off the pump. While only pumping 1 GPM, the flow is so low that the check valve is only open the width of a piece of paper when the pump shuts off. Again the flow reverses and comes back from the pressure tank but, is only able to move as far as the width of a piece of paper before the check valve is closed. The reversed flow cannot build up any speed and the check valve closes as light as a feather. The CSV can also eliminate water hammer when the pump starts. As long as the check valve on the pump is the only check valve in the system, the flow from the pump starting will be held back to 1 GPM because the CSV is still closed to the 1 GPM position. The check valve lightly opens as the flow starts gently moving at 1 GPM. Within a second of the pump starting, the CSV senses that more water is required and quickly opens enough to supply as much water as is needed The CSV starts the system at 1 GPM instead of 10 or 20, then rapidly opens up to keep up with the amount of water being used. This eliminates the pressure surge that causes water hammer as the pump starts. A second check valve before the pressure tank, with or without a CSV, can cause the drop pipe to be at zero or even a negative pressure like holding your finger over a straw of water. Then when the pump starts it sees no pressure in the lines which allows it to start flow wide open at 10 or 20 GPM. The water may only move up an inch before it hits the second check valve that is still closed. Even in an inch the water gets to moving so fast that it pops the second check valve open causing a shock wave that we hear as water hammer. Using only one check valve right at the pump, makes sure that the pump starts with pressure already against it.