Revision as of 20:05, 2 December 2015

Choosing a Water Heater

When selecting a water heater for your boat, ensure that it is designed for marine use, e.ggram., stainless steel. If you select a dishwasher with an internal heater, you can set your hot water heater to a normal 120°Fdegree Fahrenheit (49°Cdegree Celsius, SI unit of temperature); otherwise you will want one that can be set as high as 140°F (60°C). Some people recommend settings as high as 160°F to keep bacterial growth in check, but this could be dangerous for children unless you install anti-scald balanced-pressure valves on showers and taps.

How to Size

Some rules of thumb for sizing hot-water tanks were given as part of the discussion in WaterCapacity. Another way of sizing the hot-water heater is based on flow rates. The table gives a range of flow rates for different water appliances. For a comfortable lifestyle, the water heater is sized based on peak demand.

Assume the following use case based on two couples on board preparing for an evening out. (Six people on board do not change the scenario significantly since there are only two showers.)

• Two people showering (separately in the two showers, silly) and washing their hair. AAmpere (amp), SI unit of electrical current third person running a faucet in the galley and loading the dishwasher and a fourth person dawdling.
• Two more people showering after the first set. The dishwasher is started accidentally.

The two people showering for 10 minutes will draw a flow rate of 4 GPMGallons per minute, while the person running the faucet for the same period will draw 0.75 GPM, for a total of 4.75 GPM. When the next couple jumps in the showers we will again need 4 GPM plus 1 GPM for the dishwasher, for a total of 5 GPM.

In other words, over a 20-minute period we will consume 97.5 gallons of water. What is significant about this case is that we need a heater with a fast recovery rate or one with a larger tank to buffer more hot water for the second set of people stepping into the showers.

Types of Water Heaters

There are three types of water heater available for marine use:

• Tanked
• Demand (tankless)
• Point of use

Tanked water heaters are the most familiar to North Americans, but the more efficient demand and point of use heaters have been used in Europe for many years. Tanked heaters are the most energy inefficient. Operating continuously, they store a volume of water and keep it at a set temperature. When water is not being drawn off, the tank loses some heat to the surrounding air.

A tankless demand heater is the best choice for a boat, from a narrow energy efficiency perspective. But when we do a total systems analysis, the choice becomes murkier, as we shall see, because of the high operating current and its impact on the electrical system, and other trade-offs.

Tanked Water Heaters

Tanked heaters are direct, using ACAlternating current or gas, or indirect with a heat exchanger using engine coolant or solar heating. While attractive in concept, solar-heated tanks aren’t suited to marine use because of the large size of the collector, and other obvious factors like nightfall.

For indirect operation, marine models are available with a set of coils inside through which you can circulate the engine coolant or attach solar heating. When the engine is running, surplus heat is used to heat hot water.

Many diesel stoves have a similar feature. A water jacket heats the hot water, which is then circulated through coils in the hot water tank. Standard electrical elements are also provided as backup. Features to check for in a water tank are stainless steel, well insulated (R16 minimum), flow rate, coils in the bottom (where the cold water sits!), and a winter drain cock.

Tank Size

Selecting the right size for a tanked heater is the usual head-banging exercise. Based on the industry guidelines in WaterCapacity (7 galGallon per person to as high as 17 gal), you would select a capacity of 80 gal, based on six people @ 13.3 gal/person.

With no more than two people on board most of the time, you should never run out of hot water. With four people, you could manage with a 52-gal tank, and cross your fingers the rest of the time. (We all know how frustrating it is when the hot water runs out.) Interestingly, marine heaters for small- to medium-size boats do not seem to be available in models larger than 40 gal, which complicates things a bit.

Using a flow-rate calculation yields a similar result. If we multiply the flow rate by the number of minutes the appliances will be open, say two 10-minute periods, this equates to a tank capacity of 95 gal. For five minute showers, the capacity required is 48 gal. These numbers make a 40-gal tank seem more reasonable, but there is still the issue of recovering in time for the second set of showers. Tanked heaters don’t recover that fast.

Energy Consumption

Water tank heaters cycle continuously and are energy-rated in kiloWatt-hours/year (kWh/yKiloWatt-hours per year). A typical 40-gal tank will consume 4,773 kWh/y or 545 Watts/hour, which is 4.95 amp-hours (AHAmpere-hours (A*H)) at 110 VACVolt alternating current. One example delivers 25 GPHGallons per hour or about 5 GPH/A. Peak power demand is 3,800 WWatt, which will take up a good chunk of a 5,000-W inverter.

Demand Water Heaters

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 (5.45 median), with the larger units being more efficient.

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:

• Cold water passes through a set of coils that function like a boiler
• The coils have a heat source
• A sensor detects water flow and turns on the heat source
• Water heats up in seconds
• The system shuts off when the water flow ends

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 (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.

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.