We just bought a house and there is a well on the property. Of course, there is a pump to get the water out of the well and into the house but there is also this red thing in the “well house.” Reverse image search tells me it’s an expansion tank (the brand is Elbi) but I don’t understand exactly what purpose it serves. Can you explain it?
In: Physics
The expansion tank acts as a pressure leveler and pressure sustainer.
When the well pump is pumping, it is pumping water up into the piping system. If there are no faucets open, the pressure rises quickly in the pipes and the pump shuts off. If someone now opens a faucet, the pressure drops quickly (because the pipes don’t have much water) and the well pump restarts to build pressure again. In some situations that causes the well pump to cycle on and off repeatedly and this reduces the life of the water pump. The expansion tank holds some water (like a buffer) in a container that gradually pressurizes so this can help reduce the rapid pump cycling.
In reverse there is also an issue if some faucets are turned off quickly while the water pump is running. This causes a pressure spike from the faucet end to travel back towards the pump. This is known as a water hammer – the pump suddenly sees a high pressure feedback from the pipe system potentially damaging it in the long term. The expansion tank can absorb this pressure spike and not allow most of it to hit the pump.
The tank has a bunch of air in it (probably inside a flexible bladder), as well as water. When the pump forces water into the expansion tank, the air compresses until it reaches a predetermined cut-out setting of a certain psi and toggles a switch to stop the pump. This creates pressure in the water lines. When you open a faucet, water flows and reduces the pressure until a cut-in pressure point is reached, and the pumps starts up again. This makes it easier on the pump by ensuring it doesn’t have to run every time a tap is turned on for a few seconds because starting a pump when the system is under pressure is not an easy job. If there was no expansion tank, even a dripping faucet could make life very hard on the pump. The larger the expansion tank, the better, in most cases.
The tank acts as a replacement for a water tower, so you have water pressure without running the pump continuously. There’s a one-way valve that keeps the water from flowing backwards.
The tank itself has a section with air in it, separated from the water by a rubber diaphragm. And there’s a pressure switch that turns the pump on when the pressure drops enough.
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