Marine Water Heater Care: No Hot Water, Relief Valve, Anode and Winter Draining

Ask two questions. Is the water cold at the dock on shore power, underway with the engine warm, or both? If only one source fails, the problem is on that side: electrical at the dock, the engine loop underway. If both fail, start with the plumbing (a closed valve, a stuck mixing valve or an empty tank).

At the AC panel, make sure the boat actually has power (shore cord connected, source selector set to shore or generator, main breaker on). Then find the breaker labeled for the water heater. If it is off or tripped, switch it on once. If it trips again right away, leave it off: that points to a shorted element or wiring, which is a job for a marine electrician.

Many owners switch the heater breaker off to save power or when the generator is small, then forget. A tag on the breaker solves that.

Many heaters have a manual-reset safety switch that trips if the tank gets too hot, for example after running with low water or after winter recommissioning. With the breaker off, check your manual for where the reset button is and whether it can be reached without removing a live cover. If the manual shows it under the electrical cover, have a technician do it. If it trips more than once, something is wrong (low water, a failing thermostat or a stuck relay) and a pro needs to find out why.

A heater that was bypassed for winter and never put back will give you cold water from the hot tap. Check that the bypass valves are in the normal position, the inlet and outlet valves are open, and that water flows steadily from a hot faucet with no air spurts. BoatUS reminds owners to never power the element until a steady stream comes from the hot tap.

If the tank feels hot but the hot taps are only lukewarm, the mixing valve may be stuck or set too cool. With the breaker off and the water cooled, look for the adjustment knob (if your model has one) and see your manual for the procedure. A mixing valve that will not adjust usually gets replaced; it is plumbing, not electrical, but use only the part your heater maker specifies.

If the breaker holds, the tank is full and the water still never heats at the dock, the element, thermostat or wiring is the usual cause. Testing them means working near live 120- or 240-volt terminals, and replacing an element means draining the tank and sealing a new gasket. Book a marine electrician. Tell them the breaker status, whether the reset button trips, and the heater make and model so they can bring the right parts.

If dock heating works but engine heating does not, feel or use an infrared thermometer on the two hoses at the heater with the engine fully warm (boat in the water; an inboard or sterndrive never runs without its cooling water supply). Both should be hot. If they are cool:

• Check the engine coolant level (cold engine only; see the closed cooling guide). A low level lets air into the coil.
• Look for closed valves on the heater hoses.
• Make sure the engine actually reaches operating temperature; a stuck-open thermostat keeps coolant lukewarm.

Air trapped in the heater coil is the classic reason for no engine heat, especially after coolant work or when the heater sits higher than the engine. Practical Sailor explains why: the engine's coolant pump is not self-bleeding, so a pocket of air at a high loop can stop circulation through the coil, and makers often want the heater at or below engine level for that reason. Your engine or heater manual should show the bleed screw or procedure. Some installations with a heater above the engine need a remote expansion tank, which Raritan also mentions. If you cannot clear the air, a mechanic can.

The T&P relief valve opens if pressure or temperature gets too high. A valve that drips constantly may have a worn seal, debris on the seat, or it may be doing its job because pressure really is too high (often from thermal expansion when a check valve traps the heated water). Look at the valve and its discharge hose:

• The outlet should be piped to a safe place, such as the bilge, so hot water cannot spray someone.
• Lift the test lever briefly only if your manual says to, standing clear of the outlet; BoatUS suggests checking afterward that it is not dribbling. Raritan's owner's manual asks owners to lift the lever at least once a year so mineral deposits cannot seal it shut, to confirm a discharge hose is connected first, and to switch the heater off and replace the valve right away if no water comes out.
• Drips only while the tank is heating? Raritan explains that a check valve on the cold line traps the water, so as it heats and expands the relief valve lets a little out. Its fix is a small expansion tank (accumulator) in the cold line after the check valve, not a new relief valve.
• If it keeps leaking, it must be replaced. Use the rating and part the heater maker specifies; marine valves are usually rated lower than household ones.

Glass-lined tanks often use a magnesium anode, usually built into the hot water outlet fitting. BoatUS recommends checking it every year at layup and replacing it if it is more than half gone; a smelly or discolored hot water supply can also be a sign. The job: breaker off, run hot water until it is cold, pump off, open faucets to relieve pressure, drain the tank below the outlet, unscrew the fitting, then install the new anode with thread tape. Stainless tanks without an anode skip this step. Raritan adds two practical points: the magnesium rod reaches well into the tank, so you need clear space in front of the fitting to pull it out, and how fast it wears depends on your water, so check it within the first year and set your schedule from what you find.

Electric side first: breaker off and tagged "HEATER EMPTY – DO NOT TURN ON." Let the tank cool. Then:

• Open the drain plug (or disconnect the cold inlet) and drain into the bilge or a bucket.
• Set the bypass valves, or join the inlet and outlet hoses with a bypass hose, so you do not fill the tank with gallons of antifreeze.
• Finish the rest of the freshwater system with the freshwater winterizing guide.

The engine coil is protected when the engine's closed cooling system has the right antifreeze mix. On a raw-water-cooled engine that sends seawater through the coil (uncommon), the coil must be drained or protected as part of engine winterizing; check with your mechanic.

Close the drain, remove the bypass, open a hot faucet and run the pump until the tank fills and water flows with no air. Only then remove the tag and switch the breaker on. Check every fitting, the relief valve and the drain plug for drips over the next hour, and look again after your first engine run.