Freshwater Pump Troubleshooting: Cycling, Won't Shut Off or No Water

Make sure the pump switch and breaker are on (or reset them once), then listen at the pump when you open a faucet. Silence means skip ahead to step 10 (power). If the pump runs, keep going in order. Have a towel handy every time you open anything; fittings hold water.

An empty tank is the most common reason a pump won't shut off: the pump runs, sucks air and never builds enough pressure to trip the switch. Tank gauges on boats are often wrong, so look through the fill or inspection port if you can. Also check that the tank's outlet valve (if it has one) is open, and that you're drawing from the right tank on boats with a selector valve. Turn the pump off until you refill; diaphragm pumps shouldn't run dry for long.

The water tank needs to breathe in air as water is pumped out. If its vent is blocked, the pump struggles and flow fades. Practical Sailor notes that many water tank vents end at an unscreened through-hull or inside the cabin, where insects love to nest. Find the vent fitting, clear it, and consider an inline screen. While you're there, make sure the vent hose has a high loop so seawater can't get into your drinking water. Xylem suggests following the vent line with a flashlight from the top of the tank all the way to the outside fitting, especially where it bends through bulkheads: a kink can choke it just like a clog. Ease the tension on the hose or fit an elbow at a hard bend. (BoatUS adds that a vent that isn't higher than the deck fill will overflow when you fill the tank, and a deck fill cap needs a good O-ring to keep seawater out.)

A leak is the most common reason a pump cycles by itself. With the pump on and every faucet closed, check:

• Galley and head faucets, the shower head and the transom shower for drips
• The water heater, its fittings and its pressure-relief valve
• A freshwater-flush toilet or a deck washdown outlet, if fed from this system
• Every hose fitting and clamp you can reach, including under sinks and behind panels

If your bilge pump runs more than usual, taste a drop of the bilge water: fresh water in the bilge is a strong clue.

If you can't see a leak, split the system in half. Close any shutoff valve on the outlet side of the pump, or pinch the outlet hose just past the pump with hose pinch-off pliers. If the cycling stops, the leak is somewhere downstream in the plumbing or fixtures. If the pump still cycles with its outlet closed off, the problem is inside the pump (a leaking check valve or diaphragm lets pressure bleed back to the inlet) or in the pressure switch. Release the clamp when you're done.

Most installations have a strainer just before the pump; Xylem sells strainers as part of its freshwater systems. Close the tank valve if there is one, switch the pump off, then open the strainer bowl and rinse the screen. Tank sediment and old sealant bits are common. A clogged strainer makes the pump run with weak or pulsing flow. Check the bowl's O-ring as you reassemble: a poorly seated O-ring here lets the pump suck air.

After the tank runs dry, after winterizing, or after any plumbing work, air gets trapped and the pump spits and runs. Open the faucet farthest from the pump (both hot and cold) and let it run until the flow is smooth, then work back toward the pump. If air keeps coming, a loose fitting or cracked hose between the tank and the pump is pulling air in. Xylem's rule of thumb: if air spurts out of the faucet every time you open it, there's a loose connection on that side. A cracked inlet strainer bowl or a loose inlet hose can let air in without dripping any water, so a dry fitting isn't proof it's sealed. Tighten clamps on that side and replace hose that's soft or kinked. BoatUS notes that reinforced potable-water hose is the better choice because it's less likely to collapse on the suction side of a pump.

If one outlet pulses or barely flows while the others are fine, unscrew its aerator or shower head and rinse out the grit. Debris from the tank often collects here after a refill or a strainer cleaning. If the whole system started pulsing after you added a drinking-water filter on the pump's main outlet, the filter's restriction may be the cause, especially as its cartridge clogs. Change the cartridge (Xylem says a water purification filter may need a new cartridge every six months or sooner, depending on use and how much sediment is in the tank; follow the filter maker), and check your pump manual: pump makers commonly want a drinking-water filter on its own faucet line rather than in front of every outlet.

If the pump starts every time you draw a cup of water, or chatters while a faucet runs, the accumulator may have lost its air charge or its internal bladder may have failed. With the pump switched off and a faucet open so the system has no pressure, check the air valve with a tire gauge. Set the air charge to the value in your accumulator or pump manual (it's usually tied to the pump's switch settings). If there's no air, water comes out of the air valve, or the seam is wet, the bladder has failed and the accumulator needs replacing. Not every system has one; some newer variable-speed pumps are designed to work without it.

A pump that's silent, slow or never quite reaches shut-off pressure may be getting low voltage. With a multimeter, measure the voltage at the pump's terminals while it runs, and compare it with the battery voltage. A big difference means a corroded connection, a loose terminal or undersized wire. Check the fuse or breaker, clean and tighten connections, and make sure the battery is charged. See the 12-volt wiring and corrosion guides for how to test and repair connections.

If the tank is full, there are no leaks, the strainer is clean, the accumulator is right and voltage is good, the pressure switch is the next suspect. A switch that's stuck closed lets the pump run forever; one that's stuck open never starts it. Look for corroded terminals on the switch first; cleaning them sometimes fixes it. Some switches can be adjusted, but the pump maker sets them on purpose, so follow the manual or replace the switch with the kit made for your pump. BoatUS's technical editor, writing about common diaphragm pumps, points out that a pump that won't run at all even with good voltage at its wires most often needs a new pressure switch, while one that runs very fast with no water is usually just out of water. Switch kits often include the check valve and switch diaphragm too; replace those while you're in there, keep the gaskets and seating surfaces clean, and drive the small screws back into the plastic pump head gently so you don't strip the threads. Sometimes the only problem is grit in the switch diaphragm or its check valve, and cleaning it fixes the pump for free. Adding a quick-disconnect in the pump's wires makes the next removal easier.

Worn diaphragms and valves, or debris stuck in the valves, cause a pump that runs but won't prime, cycles even with its outlet closed off (from step 5), loses pressure quickly or drips water from the pump body. Many pumps have service kits for valves and diaphragms. BoatUS notes that valves leaking from debris or age-stiffened rubber are a common cause of the night-time chatter, often before the pump stops working altogether. Before buying a kit, open the valve housing (pump off, pressure released) and look: a bit of grit caught between a rubber valve and its seat is sometimes the whole problem. If the pump is old, noisy and leaking, replacing it is often the quicker fix.

Match the new pump to the old one's voltage, flow rating and port size, and check that it suits the number of outlets you use at once (Xylem sizes pumps by system size and how many outlets run at the same time). Xylem also points out that a pump mounted at the stern has to push water the whole length of the boat to a forward head; if pressure is weak only at the farthest outlet, ask the pump maker whether a higher-pressure model fits your system. Then:

• Switch off the breaker, close the tank valve and open a faucet to release pressure.
• Photograph and label the wires and hoses before you disconnect anything; catch the water with a towel.
• Mount the new pump as its manual says (position, rubber feet, strainer on the inlet side).
• Make proper crimped, sealed wire connections and use the fuse or breaker size the manual calls for.
• Open the tank valve, switch on, bleed the air from each faucet (step 7) and check every fitting for drips.