A 20 gallon pressure tank only holds 5 gallons of water when completely full. So it will feel like it is empty if you shake it or tap on it. The air in the bladder needs to be “pre-charged” like the spare tire on your car. Using a 40/60 pressure switch, the tank should have 38 PSI of air when you take it out of the box, or before it is even hooked up to the pipe. Make sure to turn the pump off, drain all water from the tank, and leave a faucet open while checking or adding air to the tank.
With a 38 PSI air charge in the tank, the pump doesn’t start putting any water in the tank until it builds up to 39 PSI on the water pressure side. Then it puts about 5 gallons in that tank as the pressure builds to 60 PSI and the pump shuts off. With the pump off, when you open a faucet, you should get about 5 gallons out before the pressure drops to 40 PSI and the pump is restarted.
Many things about pump systems are “counter-intuitive”. The pressure tank actually delivers more water with the proper 38 PSI of air charge than it does with a 10 PSI air charge. It is better for the pump to run 24/7 than to cycle on/off part of the time. Restricting the flow from a pump with a valve actually makes the pumps work easier, not harder as you might think.
The amount of water the pressure tank holds is not as important as how many times the pump cycles on and off while you take a shower or run a sprinkler. A 20 gallon tank holds 5 gallons of water. When you take a 30 gallon shower, (3 GPM for 10 minutes) the pump cycles on and off about 6 times, as the tank fills and drains.
With the Cycle Stop Valve system, the 4.5 gallon tank only holds 1 gallon of water. The CSV will make the pump exactly match the amount the shower or sprinkler is using. The water goes right past the pressure tank, and the pump only cycles on once, even if you are in the shower for a month.
A Cycle Stop Valve will work with any size tank. But water systems are better with a Cycle Stop Valve and a small tank than without a Cycle Stop Valve, even if you have the largest tank you could fit through the door.
