How do I fix my hot water system?

Dear Boat Doctor,

I am having a problem with the hot water system on my boat. I can make hot water when the engine is running, but it doesn’t work at the dock. Can you give me some ideas on how to troubleshoot this problem? I’d like to fix this myself.                                                         

Linda Reynolds

Baltimore, Maryland

Dear Linda,

Let’s start with a primer on hot water systems. There are two independent heating systems in most water heaters. The water heater is connected to the engine’s cooling system, and a coolant loop is routed through the water heater. The coolant loop transfers heat to the water heater when the engine heats up. There is also a 120 VAC heater in the water heater for dockside use.

The electric side of your water heater is fed from your panel, likely on its own breaker. Power enters the water heater at the thermostat and then feeds to the heating element. The thermostat acts as a switch to turn the heating element on and off based on the water temperature. The thermostat also likely has a high-temperature cut out, protecting the water heater from reaching dangerous temperatures.

We will start with the easiest fix, make sure the water heater breaker is turned on. It may not be obvious if it blown, so turn it off and back on again to ensure it has been reset.

Next, we’ll look at a little trickier fix. Your engine can heat the water far above normal water heater temperatures, which may trip the high-temperature cut out on the thermostat. You will likely see a red button on the thermostat; push that in to reset.

If these first couple of fixes didn’t do it, check to see if power is getting to your water heater. Proceed with these steps only if you are comfortable working with electricity as contact with 120VAC can be deadly. Using a voltmeter, check to see if there is voltage at the thermostat. You should find 120VAC between the black and white wires. If there is no voltage present, you likely have a bad circuit breaker or a bad connection between the panel and the water heater.

Finally, check if there is voltage present at the heating element. If there was voltage at the thermostat but not at the heating element, your thermostat is likely bad. Of course, you need to perform this test with a cold water heater. If the water is hot, the thermostat will open, preventing power from reaching the heating element.

If there is power present at the heating element but it’s not heating, the heating element is likely bad. The heating element is threaded into the side of the water heater tank, and is easily replaced. The number one reason a heating element fails is turning on the water heater when empty. If you do this, the heating element will quickly overheat and will burnout within a minute or two.