The Drain Back Design – Understanding Solar Hot Water Systems

While referring to the placement of the heat exchanger, There are many kinds of solar types hot water systems -pressurized closed loop, pressurized open loop, drain back, thermosyphon, and variations called direct and indirect.

There are variations among the variations.

The two dominant types today are the pressurized glycol system and the drain back system. One the drain version back system was in continuous operation since the late ’70s. I’m sure you heard about this. It has the following six fundamental characteristics. One is the Hitemp sensor on the outlet of the collectors. Now regarding the aforementioned fact… The other is the Lotemp sensor on the coldest part of the tank. The question is. How exactly do drain back operate?

There are two temperature sensors that control a solar system.

While heating the tank continually, This process goes on as long as the collectors are at least 5oF hotter than the tank.


The warmed water spills down the return line into the drain back tank. In the morning when the solar collector temperature rises to about 18oF hotter than the tank temperature, the controller turns the collector pump ON. Let me tell you something. Whenever picking up heat as it goes, Water is pumped from the bottom of the tank through the collectors. Of course, in the evening when the difference falls below 5oF, the controller turns the pump off and all the water drains from the collectors back into the tank. The warmer the water is coming into the water heater, the less heat it must add.

It is important to note that potable water never mixes with the water inside the solar hot water tank.

Cold water goes through a copper coil heat exchanger in the solar tank and into the regular water heater, whenever someone uses hot water in the home.

The solar tank preheats the water and the regular water heater finishes raising it to the final temperature. Properly designed solar hot water systems can supply 3060 of the year round hot water needs of a family. Solar heats water for dishwashing, clothes washing, bathing, spa, hot tub, pool, and all that stuff, that is referred to as domestic hot water, or DHW for short.


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