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==== Energy Consumption ====
Water tank heaters cycle continuously and are energy-rated in kiloWatt-hours/year (kWh/y). A typical 40-gal tank will consume 4,773 kWh/y or 545 Watts/hour, which is 4.95 amp-hours (AH) at 110 VAC. One example delivers 25 GPH or about 5 GPH/AGPHA. Peak power demand is 3,800 W, which will take up a good chunk of a 5,000-W inverter.
=== Demand Water Heaters ===
[https://en.wikipedia.org/wiki/Tankless_water_heating Demand water heaters are tankless]. They are about the size of a suitcase. They use AC or gas to flash heat the water. They can deliver 200 to 500 gallons per hour, but require 3,500 to 24,000 Watts to do so. Although the peak current is higher than that of a tanked heater, the energy efficiency is higher, e.g., 6.3 to 4.6 GPH/A GPHA (5.45 median), with the larger units being more efficient. [[Image:TanklessWaterHeater.jpg|200px|left|thumb|Tankless hot water heater with solar and engine pre-heat]]
Demand water heaters offer almost unlimited hot water and produce substantial savings in annual energy costs compared to a hot water tank. They have been around for decades and are common in Europe and Japan. Their capital cost is higher than a tank model but they typically last for 20 years; whereas a tank model will last for three to ten years. They are very simple:
==== Capacity ====
With a demand water heater you will never run out of hot water. Demand water heaters are rated by flow and the rise in water temperature. With a standard input temperature of 50° F 50°F (10°C), and hot water regulated at 120°F (49°C), the temperature rise is 70°F (39°C).
To select a demand heater, pick a model rated for the flow rate and input temperature (or temperature rise) you require.
==== Energy Efficiency ====
Although the unit efficiency of a demand heater is only somewhat better than a tanked heater, the tanked heater must operate continuously while the demand heater operates intermittently; thus consuming less energy overall.On average, a tankless water heater uses 25 to 50 percent less energy than a storage water heater.<ref>https://www.householdwatersystems.com/advantage-tankless-water-heater/</ref>
=== Point of Use ===
Point of use water heaters are electrical (AC). They flash heat the water right at the faucet. You've probably seen them in public washrooms. Most models deliver 2.5 to 6.0 GPM, making them suitable only for a galley sink or a shower on a smaller boat.
== Choosing a Heater Type ==
Point of use water heaters are best suited to a cruising sail boat or a small trawler, either as the main heater or as a supplement to a small hot-water tank heated by engine coolant. In a larger boat, the choices are:
* AC demand hot water tank
* AC continuous operation hot water tank
* Heat exchanger hot water tank with AC backup
Provided the house bank, inverter and charging system can be optimized for the extra load, a demand system is a better choice than a continuous-duty heater. The advantage of the demand heater is its smaller size; it never runs out of hot water; the layout is simple; and it is more energy efficient per gallon of hot water. Its disadvantage is the higher current drawn when running. With peak loads of 5-10 kW, it is unlikely to be a feasible choice except in very large boats with constantly running generators.
* A tankless-heater system designed to minimize the electrical load can be designed.
A large and well insulated water tank is pre-heated by a solar collector and/or the engine. A solar collector could be mounted on the pilothouse roof. Flat panels are easy to fabricate and more cost-effective than evacuated tubes but tubes track the sun better and are more energy efficient.<ref>http://www.apricus.com/html/solar_collector.htm</ref> Temperatures in the tank can run over 180°F, so a tempering valve mixes in cold water on the output side to adjust the temperature to 120°F. A small tankless heater only kicks in when the temperature drops below 120°F. The pump circulates a 50:50 water-glycol solution to prevent freezing in cold climates. It doesn’t operate at night.
With a typical load of 3.8 kW, a continuous operation hot-water tank is a better choice for a smaller boat. Despite its lesser energy efficiency, it has less impact on the electrical system.
* A heat-exchanger hot-water tank with AC backup, using surplus heat from engine coolants when available is the optimum choice for a small boat.
The advantage of the heat exchanger is the free energy source when underway or when a diesel stove is in use; and a smaller inverter required overall.
Both the continuous-operation tank and the heat-exchanger tank with AC backup share the disadvantage of larger size, overall energy inefficiency when using electricity, and the possibility of running out of hot water. In addition, the heat-exchanger model requires a more complex layout. The tankless design with solar collector is even more complex. In both cases, the complexity is mainly in mechanical plumbing.
== References ==
[[Category:FreshwaterSystems]]