A less expensive approach for compact, energy-efficient homes is to use a water heater both for water heating and for space heating through a fan coil—a hydronic coil installed in the air handler that distributes conditioned air throughout the house. Different types of water heaters can be used for this application, but it will take some careful engineering—or experience—by the heating contractor to be sure that hot enough water is produced and that the hot water for domestic uses is tempered so that it isn’t dangerously hot (see “Renewable Hydronic Heating” in this issue).
Water heating is lot like space heating, cooling, or even electrical energy production—it makes sense to focus first on reducing demand. With water heating, we can do this by installing water-conserving plumbing fixtures and appliances. Next, we should look at how hot water is distributed and try to improve efficiency there (see “Reduce Demand—Improve Efficiency” in this issue). Then we can optimize the system’s efficiency by choosing the most appropriate heating source.
With natural gas prices as low as they are, it’s hard to compete with the economics of simple gas-fired water heaters today. The most environmentally attractive options are solar thermal or electric water heating, powered by a PV system. Beyond that, the options can become quite complex and dependent on other factors, such as climate, the type of space heating system, and usage patterns. If you are in a cold climate and have hydronic heat with a high-efficiency boiler, an indirect water heater may be the best bet. If off-peak electric rates are available, a simple, inexpensive electric-resistance water heater can be an attractive option, especially since the tank can be well-insulated.
In other situations, a high-efficiency tankless water heater with electronic ignition, an integrated space-and-water-heating system with a condensing boiler, a heat-pump water heater, or a gas water heater with electronic ignition and condensing technology may be the best choice. All of these options are more expensive than a basic storage water heater, so the life-cycle costs should be considered.
With whatever type of water heater is selected, make sure it is installed properly. Hire an experienced installer, and ask for references if you are not familiar with the company. Proper installation involves safely venting flue gases with gas-fired water heaters, making sure there’s plenty of insulation on a storage-type water heater (adding more than came with the water heater may make sense), installing “heat traps” on the supply and discharge piping where it leaves the water heater to prevent thermosiphoning, and insulating all hot water pipes.
Alex Wilson is the founder of BuildingGreen, in Brattleboro, Vermont, and executive editor of Environmental Building News. He also recently founded the Resilient Design Institute (resilientdesign.org).