The next class of water heaters are tankless water heaters, or demand type heaters. These heaters provide hot water only when it is needed, therefore having zero standby energy losses since no heat is actually stored.Tankless Water Heaters Guide
As cold water flows into the unit, the heat source is triggered and the cold water is brought up to usable temperature on demand. The water is usually heated by a gas burner or electric elements, which have to heat the water extremely rapidly as it flows through tubes.
As a result these tankless water heaters supply a steady flow of hot water without ever running out. The downside though, is that the hot water flow is limited by the heater design. Most tankless water heaters operate with a flow rate between 2-5 gallons per minute. Usually problems with flow can happen when multiple fixtures call for hot water simultaneously. To overcome this flow problem, two or more tankless heaters can be installed in parallel to meet large simultaneous hot water flow demand.
Gas tankless water heaters generally heat faster and therefore have a higher flow rate, and are the most popular choice because of this. Models that make use of gas pilot waste energy, because normal storage type water heaters use the pilots energy to heat the water, while a on demand water heater has no stored water to heat with the standing pilot. It is generally more efficient to choose a model with a intermittent ignition system.
Electric tankless are generally more limited in function due to heating generally slower than gas-fired units and this having limited flow rates. Another downside is that they require large incoming electrical power in order to rapidly heat the incoming cold water fast enough, so if you are adding an electric tankless water heater to your home, you want to make sure that your existing electrical service can handle the demand. The advantage of electric tankless water heaters, is they are generally cheaper and easier to install than gas tankless, due to the fact that they do not require a combustion gas flue, or gas piping.
Above right is the highly efficient Tempra electric tankless model manufactured by Stiebel-Eltron. A good tankless water heater can cut your water heating costs by 30%, but make sure you factor in the initial costs before buying. Get some estimates from local contractors that specialize in tankless water heaters so you can weigh the initial cost versus the longterm fuel savings.
The next article will discuss the classification of super efficient hybrid water heaters, which are usually even more efficient that traditional storage type water heaters, or tankless demand type water heaters.