The jockey pump pumps water. If the jockey pump is turning on regularly every night, it's replacing water that has leaked out of the sprinkler system. Because the water isn't spilling out onto the floor of the building, the water has to be leaking somewhere else.
Pressure Tank
Think of the sprinkler system as an air pressure tank. In order for the sprinkler company to perform service on the sprinkler system, they sometimes have to drain the water out of the sprinkler system. There is a valve for draining the whole system.
In order to let the water out, they have to let air in. When the service work is done, they refill the sprinkler system with water. When refilling the system, though, they refill the system with water from a low point. The air doesn't leave the system, the air is just compressed by the water.
In other words, the combination of air in the system and water pressure makes the sprinkler system into one, long pressure tank.
Commonly, there are two places that water could be leaking: the drain-valve and the check-valve.
Sprinkler System Drain Valve Leaks
If the drain valve leaks, it slowly lets water out, and the pressure reduces. Then the jockey pump has to push more water into the sprinkler system to increase pressure again.
That's one common leak that regularly turns on the jockey pump.
Check-Valve Leaks
Sometimes, one or more check valves leak. A check-valve is a one-way valve that lets water flow easily from the city into the sprinkler system, but won't let water backwards into the city water system. With a fire pump, there can be two or more other check valves.
Water leaking backwards through a check valve reduces the pressure in the sprinkler system. The jockey pump then turns on to push water past the check valves, increasing the pressure again.
Remember, even the jockey pump has some sort of a back-flow preventer. Check there also.
Nighttime Jockey Pump Running
At night, the temperature of the air outside continually goes down, and the temperature of the air inside the sprinkler pipes continually goes down. When the sprinkler system air temperature goes down, the sprinkler system pressure goes down.
When the air temperature goes down continuously all night, the air pressure inside the sprinkler system continuously goes down at night. The jockey pump then has to keep turning on and off all night, just to keep the pressure up.
Sprinkler System Service
I'm not sure whether it's a drain valve leak, or a check valve leak. To solve the problems, though, the sprinkler system needs service to find and fix the leakage.
There should be a tag, somewhere on the sprinkler system, showing who does service on the sprinkler system. Talk to them.
Douglas Krantz
