Pressure tank and pressure switch problems

A surprising share of "bad pump" calls turn out to be the pressure tank. The tank is what lets a well pump rest. It stores a reserve of water under air pressure, so a glass of water or a toilet flush comes out of the tank instead of starting the motor. When the tank stops doing that job, the pump starts and stops with every draw, and that constant cycling is hard on any motor.

How the pieces work together

The pressure switch watches the line. When pressure falls to its cut-in setting, it starts the pump; when pressure climbs to the cut-out setting, it stops it. University of Nebraska extension material lists typical settings as a shut-off between 40 and 60 psi and a restart between 20 and 40 psi. Most homes today have a bladder or diaphragm tank, where a rubber membrane keeps air and water apart. Those tanks leave the factory charged to around 20 psi, and the same guide says the air charge should sit about 2 psi below the pump's cut-in pressure.

Older galvanized tanks have no membrane. Air and water touch, the water slowly absorbs the air, and the tank gradually fills up with water unless air is added back.

Signs the tank is the problem

• The pump clicks on within seconds of opening a faucet and shuts off seconds after closing it.
• The gauge swings quickly between its low and high numbers.
• Water pulses at the shower or kitchen sink.
• Pressing the air valve on top of a bladder tank lets out water instead of air, which usually means the bladder has failed.

Extension guidance describes a waterlogged tank as one that makes the pump run nearly every time water is used. The opposite fault, an air-bound tank, has too much air and too little room for water.

Sizing a replacement tank

The Nebraska guide gives a rule of thumb: a bladder or diaphragm tank about four times the pump's flow rate in gallons per minute, so a 9 gpm pump pairs with roughly a 36-gallon tank, and a galvanized tank about ten times the flow rate. Bigger tanks mean fewer pump starts. Variable-speed constant-pressure systems are the exception: they can run with a 1 to 2 gallon tank because the controller slows the motor instead of switching it on and off.

Pressure the house actually needs

Most fixtures need at least 10 psi to work, while softeners, clothes washers, dishwashers and other water treatment equipment need about 30 psi, according to the same extension publication. If the tank and switch can't hold the system above those numbers, appliances act up long before the faucets go dry.

Repair or replace?

A tank that has simply lost its air charge can sometimes be drained and recharged. A ruptured bladder, a rusted shell or a leaking fitting usually means a new tank. Pressure switches are inexpensive, but their contacts pit and burn over time, and a switch with bad contacts can leave the pump stuck on or off. A contractor will usually check the tank charge with the system drained, test the switch, and look at the check valve, since a leaking one can mimic tank trouble.

If the tank was the cause, fixing it soon protects the pump. If the pump has already been short-cycling for months, it may be worth having the contractor check its amp draw while they're there. Pump replacement is covered on the replacement page, and the repair page lists other no-water causes.

One caution on chlorinating

K-State Research and Extension's well chlorination guide notes that most pressure tanks are captive-air bladder tanks and that high chlorine concentrations can damage the bladder. If your well is due for shock chlorination after a repair, ask the contractor whether to isolate the tank first.

Questions people ask

What should my pressure tank's air charge be?

Extension guidance puts it about 2 psi below the pump's cut-in pressure, measured with the tank drained of water. Your switch's settings are usually printed inside the cover or on the label.

Why is the top of my tank cold and wet?

Condensation can form on any tank holding cold well water. A tank that is cold all the way to the top and makes the pump cycle fast is more likely waterlogged.

Can a bad pressure tank ruin the pump?

It can shorten its life. Every start puts heat and wear on the motor, and a waterlogged tank multiplies the number of starts.