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Solved problems · Plumbing · Crew answer

Why isn't my boat's water heater heating?

A marine water heater can heat from shore power, the engine, or both. No hot water usually means a tripped breaker or overheat cutout, a failed element, or no engine coolant flow.

Symptom

No hot water, or only lukewarm water, from the tap after running on shore power or after motoring.

Likely cause

On shore power, the heater relies on an electric element, a thermostat and an overheat cutout. A tripped breaker or GFCI, a cutout that has tripped, a burned-out element (often from powering it with the tank empty), or a failed thermostat stops heating. On engine heat, coolant has to circulate through the heater's coil. Air trapped in the coolant hoses, closed valves, or a heater mounted above the engine without bleeding can stop that.

Stop now if

The relief valve is spraying, the tank is bulging, or the breaker trips immediately when you reset it. Turn off the heater's breaker and get it checked. Turn off and lock out the AC breaker, and disconnect shore power, before removing the heater's cover. Expect scalding water, since tank water can be as hot as engine coolant.

The fix, in short

  1. Make sure the tank is full. Never switch on the element with an empty tank. Isotemp warns that the element must not be powered until the tank is filled. Open a hot tap until water runs steadily.

  2. Check the breaker and GFCI. Confirm the water heater breaker and any GFCI feeding it are on. If the breaker trips again right away, stop and get the element checked.

  3. Check the overheat cutout. With power off and the cover removed, find the manual reset on the thermostat or cutout and press it as the manual describes. If it keeps tripping, the thermostat may be faulty, so stop and have it tested.

  4. Test the element. With power isolated, a marine electrician or a careful DIYer can measure the element's resistance with a meter. Open circuit means replace it.

  5. Bleed the engine heating loop. If it only fails on engine heat, check that the coolant valves to the heater are open and bleed air from the heater coil hoses with the engine cold, following the engine manual.

  6. Check the anode and relief valve. Some heaters have an anode that needs periodic replacement. Test the pressure relief valve per the manual, and replace it if it drips constantly.

Written by Sailor Cruiser crew. General guidance, not an inspection of your boat.

Equipment involved

  • Water heater: Marine calorifier with AC element, thermostat, overheat cutout, engine heat-exchanger coil and pressure relief valve

Related guides

Sources

Background reading we checked while writing this answer. The wording above is our own.

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