Most RV water supply systems rely on an on-demand water pump. These pumps have a built-in pressure switch that triggers on/off operation. When the pressure dips – such as when you open a faucet – then the pump turns on. When the pressure rises – such as when you close the tap – the pump shuts off. 

Sounds great in theory. Open tap; pumps turns on. Close tap; pump shuts off. But let’s consider the physics a little closer, shall we?

The Obnoxious Problem of Pump Short Cycling

What happens if water is just trickling out of the faucet?

• A bypass pressure pump, like the Pentair Shurflo Revolution 4008, doesn’t require an accumulator tank. A bypass pump uses a variable bypass valve to recirculate water from the output back to the input in low-flow conditions (kind of like an EGR valve). Check the pump manufacturer’s recommendations; some allow an accumulator tank alongside a bypass pump; some don’t.
• A variable-speed pump, like the Remco AquaJet or Shurflo Smart Sensor, eliminates short cycling throughout the entire normal range. However, these pumps are not inexpensive, especially compared to a $30-$50 accumulator tank! Again, check the pump manufacturer’s recommendations regarding accumulator tanks.

Things to Know About Accumulator Tank Maintenance and Installation

• The smaller accumulator tanks can be installed just about anywhere, whereas the larger expansion tanks will take up part of a storage compartment.
• Some accumulator tanks, like pumps, should be attached with flex hoses to the PEX tubing.
• Most tanks have a Schrader valve which can be pressurized with a tire inflator, bike pump or hand pump.
• Accumulator tanks should not be stored at high water pressure for long periods.
• Accumulator tanks will be damaged by momentary high pressure. Your plumbing system should have a pressure relief valve (usually either 100 or 125 psi).
• Pressure precharged should be checked seasonally. For best performance, precharge should normally match (or be 2-3 psi less than) the pump cut-in or “turn on” pressure. Check your manual.
• If you don’t want to winterize your accumulator tank, install a shut-off valve between it and the main water line.

Key Takeaways About RV Accumulator Tanks

• Accumulator tanks (aka expansion tanks) store pressure to reduce water hammer, decrease pump cycling, and smooth water flow.
• Most RVs don’t come with accumulator tanks because they aren’t deemed essential.
• If your RV has a bypass water pump or variable-speed pump, you may not need an accumulator tank.
• For best performance, the accumulator tank pressure should usually match or be just below the pump cut-in pressure. Check your Owner’s Manual.