I agree with VA. A 20 gallon size tank only holds 5 gallons of water. I am guessing you have a 10 GPM series pump. With a 10 GPM pump, I would have put in a tank that holds at least 10 gallons of water (44 gal size tank).
You have already gotten twice the 7 year average life from that submersible pump, so you are doing something right, or your just lucky. Normally a 10 GPM pump would cycle itself to death quickly with only a 5 gallon tank. But it has a lot to do with how you use the water.
Houses don’t normally use much water. About 300 gallons per day per house is common. So even if the pump cycles on/off for every 5 gallons used, it would only cycle 120 times per day to supply 600 gallons to the houses. That size pump has a maximum of 300 cycles per day, for which it should last about 7 years. So it doesn’t surprise me that it has lasted 13 years doing 120 cycles per day or less.
It is the outside the house watering that usually cycles a pump to death prematurely. If you water the 40 cows with a full open garden hose (10 GPM), the pump should not cycle much, if at all. But if you have float valves on multiple watering tubs, that pump could cycle a lot. The same thing for irrigation demands, if you use 10 GPM the pump won’t cycle. But if you use one 3 GPM sprinkler for hours at a time, the pump/motor would quickly be damaged from the excessive cycling.
Yes the fewer times the pump cycles the longer it will last. But increasing the size of the pressure tank is not always the best way to eliminate cycling. As you have seen, even with the small tank you have, pump cycling for just two houses is not that much of a problem. But if you ran a 3 GPM sprinkler 24/7, the pump would be cycling about 500 times per day, and it wouldn’t last long. Doubling the size of pressure tank would make it cycle 250 times per day, which is still not good.
Even with a very small tank, if you used a Cycle Stop Valve the pump would run continuously 24/7 when using that 3 GPM sprinkler. Pumps are made for continuous duty. It is the cycling on and off that destroys them. Also with the small tank and a CSV, the pump would only cycle about 40 times per day for the houses, instead of 120 times per day as it is now.
So if you want to stop the pump cycling, use a Cycle Stop Valve. A larger pressure tank would only reduce the number of cycles, which is good, but not as good as eliminating excessive cycling as when using a CSV.
