Tankless Water Heaters

Tankless water heaters, also known as instantaneous or on-demand water heaters, provide hot water without using a storage tank. Like tank water heaters, tankless water heaters use either gas or electricity to operate. Cold water travels through a pipe into the unit, and either a gas burner (see Figure 1) or an electric element heats the water. Tankless water heaters can be supplementary units placed at the point of use or can replace a centralized tank water heater.

Figure 1: Gas tankless water heater
When a flow sensor detects cold water entering the unit’s heat exchanger, the fan provides combustion air and the controls fire the burner to indirectly heat the water. The controls adjust burner output in proportion to the amount of water flow to produce hot water at the setpoint temperature.

Tankless water heaters are very efficient because unlike conventional water heaters, they do not have standby losses incurred by continuous use of energy to maintain water in a tank at a set temperature. Tankless heaters are space savers, which can be particularly useful for a small business or where a faucet or shower is some distance from the current water heater.

Although they theoretically provide endless hot water, most tankless water heaters, especially electric units, provide it more slowly than conventional tank water heaters and will limit output pressure to maintain the correct water temperature. Because a significant drop in water pressure can develop if many users are trying to draw hot water at once, it’s especially important to properly size a tankless system before installation.

It is also possible to inadvertently increase hot water usage after switching to a tankless system because of the seemingly endless supply of hot water; activities like showering may last longer when no one is worried about getting a sudden flood of cold water. In some cases, the increase in hot water demand can actually outweigh the energy benefits of switching to a tankless system in the first place.

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