Hot Water

By DIY Boat Owner

If you live and cook aboard for longer that the occasional weekend cruise, hot running water is more than a convenience.
The good news is that if your boat has a 110-volt shorepower system and cold water, you're already halfway to a hot-water system.
There are two completely different approaches to the problem: a storage-tank heater or a flow-through "demand" heater. The former is the most common type of system. It has a storage tank very similar to a household unit but with a smaller capacity. Heat is usually provided at dockside by a 1,500-watt 110-volt AC heating element and, while underway, by hot engine coolant recirculating through a heat exchanger in the tank.

Units are available from Atlantic Marine, Raritan, Seaward and others. Never install a household water heater which is not ignition protected and could spark and cause a fire.
Demand heaters that do not store water but heat it immediately before use are usually fired by propane, are less common on boats and are beyond the scope of a do-it-yourself article. Work with explosive propane and the handling of combustible gases should be left to a licensed gas fitter.
In a similar vein, if you are not familiar with the basic 110-volt wiring required to hook up your hot-water heating element, you should seek the assistance of someone who is experienced. At the very least, have your connections checked prior to plugging in the heater.
Before cutting or drilling anything when running piping, check on each side of the bulkhead or partition to ensure you don’t cut into structural stringers or tabbing that hold the bulkhead in place.
PLANNING THE SYSTEM
When planning the layout of the system, especially the tank location, try to arrange a close look at the same model boat already equipped with a factory-installed system   and copy it if that is practical. Or you can contact the builder (if they're still in business) for guidelines. A professional-looking "stock" installation will always raise the resale value of your boat. You do, however, have the freedom to install more deluxe fixtures should your budget allow it.
The tank will need to sit on a level surface so a solid 3/4-inch (19mm) plywood epoxy-painted shelf will need to be prepared and screwed or fiberglass tabbed into place. Once the tank is shoehorned into place (usually a tight fit in a cockpit locker or in the engine room) and fastened securely, run your cold water (in) and hot water (out) lines with hose.
For the "hot" side, good quality clear-braid PVC hose is pressure-rated at 150 degrees Fahrenheit and connects the traditional way with adapters and double clamps. (Make sure the clamps are tight, but not so tight they cut into the hose. If you see hose squishing through the cracks in the clamp, you've destroyed the hose.)
An even better choice is semi-flexible plastic pipe, which is more durable, less prone to kinks and usually has fittings that are less likely to leak and easier to assemble.
RUNNING THE LINES
Threading piping through the boat is time-consuming and fiddly work. Access is often difficult and you will probably have to drill many holes through bulkheads. Be very careful!
Before cutting or drilling anything, check on each side of the bulkhead or partition to ensure you don't cut into a tank, wiring or other piping, or structural stringers or tabbing that hold the bulkhead in place. Fasten the plumbing every few feet with cable clamps or wire ties and try to keep it out of the bilge to prevent contamination.
Avoid tight bends that can kink hose; instead, install 90-degree elbows. The tee connection to the cold-water system can be made anywhere downstream of the pressure pump and before the first fixture. Hot water heaters need a check valve, either at the water inlet to the tank or at the tee connection, so the hot water cannot flow back into the cold water. Make sure you have one. You may have to remove the sink to get access for your connections to the mixer or faucet. Provide for easy access to the tank, fittings and all connections for maintenance.
Any 110-volt wire used should be at least 14-gauge copper multi-strand boat cable and must be sized to the amperage and wattage of the heater; depending on the routing, you may need 12 or 10 gauge. Household single-strand cable is not safe in a boat because engine vibration and jarring from when the boat slams into waves fatigues the wire, leading to breaks and shorts and possibly fires. The hot (black) wire connects to one terminal of the element and the neutral (white) wire connects to the other. Ground the tank via the green or bare copper wire.

Assuming you have a 110-volt system   a breaker panel and hull inlet with circuit protection that was installed to nationally-recognized codes (NFPA)   connect the white to the common neutral buss and install the breaker into an empty slot in the panel connecting the hot (black) lead to the breaker and the breaker's other pole to the hot distribution buss. Never directly connect a water heater to a power cord plugged into an outlet on shore.
Don't do any wiring until you have unplugged and stowed your shorepower cord   a well-meaning soul just might plug it back in again at an awkward moment. Keep all 110-volt A/C wiring physically separate from your DC wiring. Make sure no neutral (white) wires connect to any ground within the boat   this is critical   and make sure your shorepower cord and main AC supply is properly grounded through the dockside power system. Now turn on the pump, fill the tank and turn on the power in that order to avoid heater element meltdown. Check for leaks. There will be a few.
Next, hook up your engine hot-water heat exchanger, but consult a mechanic or your local engine representative first. Some raw-water-cooled engines (early Yanmars for example) won't put out a steady supply of hot water; the temperature alternates between hot and cold as the thermostat opens and closes. Others, such as Atomic 4s, need a separate recirculating pump to help move the engine coolant through the heat exchanger.
When all is done, and you are sure the system works, take a hot shower. You will have earned it!

A version of this story ran previously in Mad Mariner's DIY Boat Owner magazine.