Tankless (or “instantaneous” or “on-demand”) water heaters are becoming popular because they are smaller and use less energy than regular tank water heaters.  Tankless water heaters use natural gas, propane or electricity to rapidly heat water to a pre-set temperature as the water flows through the unit.  They come in all sizes, from very small point-of-use units under a sink, to whole-house and commercial scale systems.

A whole-house tankless water heater can be used as a backup to a solar water heater as long as it meets these criteria:

• It must accept variable incoming water temperatures from the solar storage tank, and heat the water only the amount needed to reach the pre-set temperature.
• It must pass pre-heated water that is at or above the pre-set temperature straight through the unit without adding heat.  (Remember that a tempering valve will add cold water to bring the hot water temperature down if necessary before it goes to the hot water faucet.)

Some units meet these criteria and others don’t.  Look for a product that specifically claims to work with solar water heating systems.  Make sure your installer understands your objective of using the tankless unit as a backup for the solar water heater.