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How do I keep my hot water from overheating?

2015 February 1

Dear Boat Doctor, 

I have a potential safety issue with my hot-water system. When I run the engine for a long time the water gets really hot, to the point of being dangerous. It’s not a problem at the dock when the boat is plugged in, the water stays at a reasonable temperature. I am used to it and I try to warn my guests, but it feels like an accident is waiting to happen. 

Ian Sommerstein 

Traverse City, Michigan

Dear Ian, 

You’re right to be concerned about this common problem. Your boat has a water heater with two heat sources: an electric heating element that runs off shore power and a loop of engine cooling water that flows through a heat exchanger in the water heater. This is done so that you will almost always have hot water. 

Of course you wouldn’t be able efficiently to make hot water on battery power alone. At the dock, the shore power heats the element, which in turn heats the water. The electric element is thermostatically controlled and doesn’t allow the water to get above, maybe, 130 degrees. When the engine is running, the loop of the engine cooling water is cycled through the water heater, and cooling water can easily heat to 180 degrees. This loop cycles continuously without a thermostat. As a result, after a number of hours, the water in the heater can get very hot. 

I have never seen a thermostatic system to divert the engine cooling water once the water heater is up to temperature, but there are other solutions. A thermostatic mixing valve can be put in place to limit the temperature of the water that is supplied from the water heater. These valves mix a little cold water with the extra hot water to bring it to an acceptable level on the downstream side of the tank. It can be set to whatever temperature you like. The valves cost about $85 and are pretty easy to plumb in. You simply T the valve into the hot water output of the heater and bring a cold water line to the valve. Once installed, you set the control to the temperature that you want. 

A commonly available valve is the Honeywell AM101C, available at any plumbing supply store. Aside from protecting you from scalds, the valve will also make your hot-water supply last longer and can keep your water safer. The mixing valve automatically adjusts the mix of hot and cold water that is delivered to the hot tap, resulting in less hot water usage, sometimes up to 40%. If you set your water heater to 140 degrees or above, you reduce the risk of bacteria build up in the tank.