In North American homes, heating domestic water for bathing, dishwashing, and other related tasks is typically the second largest load (space heating and cooling is usually first), consuming 15% to 25% of the energy a home uses. About half of this is in the bathroom, a third in the laundry, and the rest is in the kitchen. Many grid-tied homes rely on electric resistance water heaters, but this is not a viable option for most off-grid homes.
It’s important to ask first what your motivations and preferences are around domestic hot water, and also some specifics of your situation. If your primary motivation is low cost, you may come up with a different solution than if your primary motivation is using renewable energy. Reliability, serviceability, capacity, convenience, and other factors will play into your decision.
Then, you’ll need to identify what fuels and resources are available to you. Your water “load” and your access to sunshine, space for tanks, as well as your budget and abilities as a plumber may affect your decisions as well.
Conservation & Efficiency First
When it comes to heating water, it’s sensible to start with not only energy efficiency but also water efficiency. Using low-flow fixtures should be the first step—they use less water to do the same job. Low-flow showerheads, for example, provide a flow of less than 2.5 gallons per minute (gpm). Laminar (non-aerating) heads provide more accurate temperature control, while aerating creates more steam and moisture. Low-flow sink faucets have a flow of less than 1.5 gpm. After-market flow restrictors or aerators can usually be added inexpensively.
Low-flow fixtures save water and save the energy needed to heat water. Water-saving appliances like clothes washers and dishwashers are also important in reducing the overall water-heating load. Horizontal-axis washers and high-efficiency dishwashers typically use less water, and choosing appropriate settings on all water-using appliances also helps use less hot water. For example, choosing to wash most of your clothes in cold water, rather than warm or hot, can provide significant energy savings. Buying a horizontal-axis machine that’s sized appropriately for your household will also ensure that you’re using every gallon of heated water efficiently. Make sure you skip any washers that use steam to wash or electric elements to “boost” the temperature, since these will tax your RE system. Dishwashers pose similar issues, so select them carefully.
Inefficiency in heating applications manifests primarily as heat loss. Choose a heating method that is inherently efficient and effective—but keep the heat in once you’ve made it. Super-insulated tanks, thoroughly insulating hot water pipes, and either installing tanks with heat traps integrated into their fittings or installing a gravity heat trap in the tank piping will minimize standby heat loss. These strategies will also allow lower temperature settings by minimizing the difference between the tank and the fixture temperatures. If there’s a recirculation system involved, managing it carefully with wise plumbing and timer use is important, too.
The bottom line is to use only as much water as you need to get the job done. Heat it intelligently and don’t lose any more of that heat than is absolutely necessary.
In cold climates, selecting the water heater type is typically not an isolated decision. Since space-heating loads often represent the greatest share of energy, integrating water heating with space-heating tends to be the most cost-effective and simple solution. For example, if you install a propane boiler for space heating, you will likely also use the boiler to heat domestic water by installing an indirect water heater or by installing an add-on option to a wall-hung boiler that allows the boiler to also provide instantaneous domestic water heating. In warm climates, domestic water heating may be accomplished with a simple stand-alone appliance.