Demand Water Heaters
Instantaneous water heaters, sometimes referred to as “demand” or “point-of-use” water heaters, produce hot water only when it is needed. Their big advantage is that there are no stand-by losses because there is no storage tank. Since stand-by losses can be as much as 15 percent of the total cost of water heater operation, demand units can save money on monthly utility bills.
Demand units have a heating device that is activated by the flow of water when a hot water valve is opened. Once activated, the heater delivers a constant supply of hot water. The output of the heater, however, limits the rate of the heated water flow, which is its greatest handicap: somewhat limited flow-rates. They work best when hot water needs are minimal, and sporadic, like in an occasionally occupied weekend home or remote garage or bathroom.
Demand water heaters can be installed either centrally or at the point of use, depending on the amount of hot water required. For example, you can use a small electric unit as a booster for a remote bathroom or laundry. These are usually installed in a closet or underneath a sink. The largest gas units, which may provide all the hot water needs of a household, are installed centrally.
Gas-fired models are available with higher hot water output than electric models. As with many tank water heaters, even the largest whole-house demand models cannot supply enough hot water for simultaneous, multiple uses of hot water like running the laundry and taking a shower at the same time. Large users of hot water, such as the clothes washer and dishwasher, need to be operated separately. Alternatively, separate demand water heaters can be installed to meet individual hot water loads, or two or more water heaters can be connected in parallel for simultaneous hot water demands. Some manufacturers of demand heaters claim that their product can match the performance of any 40-gallon tank heater.
Units are available that use electricity, however, whole-house electric tankless units are usually not recommended for several reasons. For one, they may not be able to provide multiple uses simultaneously, like filling a tub and running the washing machine at the same time. Also, they may require additional expense upgrading the electrical service, which must be factored into the upfront cost and making them uneconomical. Electric units have the advantage of not needing a pilot light, which either must be on for a gas unit to operate or must be relit every time hot water is needed. They come in a variety of sizes for different applications, such as a whole-house water heater, a hot water source for a remote bathroom or hot tub, or a boiler to provide hot water for a home heating system. They can also be used as a booster for dishwashers, washing machines, and solar or wood-fired domestic hot water systems.