Unless you live in a tower and your rainwater cistern is mounted directly above your roof, gravity will not provide sufficient water pressure to operate a modern washing machine or rapidly fill a toilet. To integrate rainwater indoors, you need to mechanically mimic municipal water pressure (usually between 40 and 60 PSI). To do this efficiently without destroying your equipment, you need two things: a pump and a pressure vessel.
The Limits of "On-Demand" Pumps
Many complete rainwater pump kits are sold as "on-demand" systems. They feature an electric pump attached to a small electronic pressure switch. When you flush a toilet, the switch senses the drop in pressure in the pipes and instantly turns the pump on. When the toilet is full and the valve closes, the pressure spikes, and the switch turns the pump off.
While conceptually elegant, this system has a massive flaw: short-cycling. An electric motor draws a massive surge of electricity (the "inrush current") in the first fraction of a second when it turns on. Furthermore, the physical start-up torque is the hardest wear-event on the motor's bearings. If you have a slow leak in a toilet flapper or turn on a faucet to wash your hands for three seconds, the pump turns on, then immediately turns off. Thousands of these micro-cycles will burn out an expensive pump prematurely.
Enter the Pressure Vessel (Accumulator Tank)
To protect the pump, professional plumbers install a pressure vessel (also known as an accumulator or expansion tank) inline after the pump. This is a metal tank containing a heavy-duty rubber bladder surrounded by pressurized air.
When the pump turns on, it doesn't just fill your toilet; it forcefully pushes water into the pressure vessel, expanding the rubber bladder against the cushion of air until the system reaches its maximum pressure (e.g., 60 PSI), at which point the pump shuts off.
How the Vessel Protects the Pump
With a pressure vessel installed, here is an example of what happens when you flush a toilet:
- The toilet valve opens.
- Instead of the pump turning on immediately, the compressed air inside the pressure vessel squeezes the rubber bladder, pushing its stored water out into the pipes and filling the toilet.
- The toilet finishes filling, and the pump never turned on—saving an entire cycle.
- Only after you flush the toilet two or three more times does the pressure in the vessel finally drop to the minimum threshold (e.g., 40 PSI).
- The pump turns on, smoothly and continuously running for a full minute to refill the pressure vessel completely from 40 back up to 60 PSI, then shuts off.
Sizing the Vessel
The larger the pressure vessel, the fewer times your pump will turn on in a given day. A small 2-gallon vessel might hold enough drawdown capacity to wash your hands. A massive 44-gallon vessel will hold enough pressurized water to flush four toilets and run a load of laundry without the pump ever kicking on once. For a standard residential rainwater system, a 20-gallon pressure vessel offers the perfect balance of cost and pump protection.