In hot water



  • Screwdrivers
  • Wrenches
  • Cordless drill
  • Tube cutter
  • Materials

  • Water heater
  • 5/8″ heavy-duty water hose
  • NPT fittings
  • Fasteners, as needed
  • After upgrading the mostly original fresh-water plumbing system on our 1973 Norlin 34 project boat with new hoses, faucets, cockpit shower, and electric and manual pumps (see “Let There Be Water,” Summer), we thought having pressurized hot water would further increase onboard comfort. There are a couple of ways to achieve this worthy end. One is to install an on-demand propane heater, but that was out of the question for us. I think they’re okay for RVs, but on a boat they’re potentially lethal. Plus, we don’t use propane for cooking. It makes far more sense to plumb a marine water heater into the engine’s cooling system, thereby making use of energy that would otherwise be wasted.

    A marine water heater is a simple device. It’s basically a heavily insulated cylinder containing a heat-exchanger element that is connected via heavy-duty water hose to the engine cooling system. The tank is hooked up to the boat’s fresh-water supply. When the engine is running, the coolant circulates through the water heater, warming the cold water that’s pumped in from the boat’s tank. When you open a hot-water faucet, more cold water is pumped in at the bottom of the heater to replace the warm water that’s being drawn out at the top. Most heaters also have a 115-volt-AC heater coil built in so that you can enjoy hot water in the marina when plugged into shore power; you could also plug it into an inverter, if you have DC energy to spare.

    You can fit a water heater to both raw water- and freshwater-cooled diesels; the latter are far more common, and most come with connection points for heater hoses. The Yanmar 2GM in our project boat had blanking plugs in the cooling system for that very purpose, making it an easy job to install the hose connectors. An older raw water–cooled engine will likely not have these points, and you should seek expert advice before trying to install a water heater.

    The concept of a water heater is simple enough, as is installing one, but there are some things to keep in mind.

    • There must be some kind of pressure-relief valve, in the unlikely event that your engine generates enough heat to make the water boil; most modern water heaters have these built in.

    • You’ll need a mixer valve to control the heat of the water leaving the heater; some heaters come with these.

    • A drain valve is essential for efficiently winterizing the unit.

    • You need to install a check valve on the freshwater inlet side to make sure hot water can’t make its way into the cold-water line.

    • If your heater is wired into shore power, make sure there is water in it before you switch it on.

    The biggest problem with retrofitting a water heater is finding room for it. This is especially true on older boats from the ’60s and ’70s, which tend to have less beam, less waterline length, and hence less interior volume than more modern cruisers. If you think you need a large-capacity heater—upward of, say, 10 gallons—you may have to think again. If you have capacious cockpit lockers, this would be the logical place to accommodate a heater, but if not, you’re in the same predicament as we were.

    There are plenty of good heat-exchanger water heaters on the market, but many are quite bulky. The space restrictions aboard our elderly cruiser-racer quickly narrowed the field to Indel Marine’s Isotemp units, which come in a variety of shapes and sizes. The only space we had, under the starboard settee, measured just 29″ long by 14″ wide. Isotemp has a model in its Slim line that was ideal—a 4-gallon heater measuring just 21″ x 12″. Four gallons is not a lot of water, but it’s enough for a couple of showers if you’re frugal and plenty enough to do the dishes and rinse the salt out of your hair. Isotemp heaters are a bit pricey—the Slim 15 we installed retails for over $500—but they are also among the more efficient.

    When installing a heater it’s important to get it as close to the engine as possible to avoid losing heat through the hoses. You also need to make sure that the heater is below the engine’s heat exchanger to avoid trouble with coolant circulation. If this isn’t possible, the experts recommend that you install a header tank for the coolant above the heater.

    This project was very satisfying and not at all difficult. I spent the best part of a day on it, but if I’d been able to simply bolt the built-in brackets to a bulkhead I think I could have hooked everything up in two or three hours. The most time-consuming jobs were strengthening the area under the settee where the heater was installed and routing the hoses to the engine. I was concerned that I would have to bleed the hoses, but in the event just had to keep topping up the coolant reservoir. The kids really appreciate a hot-water rinse after their swim, and it’s nice not to have to wait for a kettle to boil before you can wash the dishes. With the engine running it takes less than 20 minutes to heat up a tankful of water, and the insulation around the heater should keep it hot for up to 24 hours.

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