Solar Thermal Drain Back Systems

Solar Thermal Drain Back Systems
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The solar hot water drain-back system is famous for its simple structure and low maintenance cost compared to other active systems available on the market.The main heat transfer fluid used in this system is water which isn’t expensive when it is changed periodically.

The solar drain back system is a very efficient device to heat water for households and industrial appliances. Because it uses solar energy to heat the water it is extremely cost effective and an environmentally friendly option as well.

Types of Solar Thermal Systems

There are two types of solar drain back systems; a drain-back system and closed loop system.

Drain-Back Solar System

This system is also called an open loop system. In this system, the water is directly circulated in the collector to heat and is then returned to the tank when heating is completed.


  • The main advantage of this system is that it can be simply closed when the system has no place to put the heat. It means the system will shut down if the tank has no water to circulate.
  • The structure of this system is simpler than closed loop system.
  • You don’t need to use any antifreeze with the water that makes its maintenance cost lower.


  • This system needs to circulate water which creates the need for a high power pump to force the water through the system.
  • During the installation if there remains some amount of water in the collector, it may freeze during winter. That makes the system installation very complex, and only experienced solar contractor should perform the installation.

Closed Loop System

In this system the water that needs to be heated doesn’t get sent directly to the collector. Instead, a solution of water and propylene glycol (usually in the ratio of 1:1) is circulated in closed loop piping. This solution takes heat from the collector, and transfers it to the water present in tank.

Propylene glycol is an antifreeze agent; due to this there is no chance of freezing of solution inside the closed loop. Because of this property, it is widely used in areas when winter temperatures dip below the freezing mark.



  • This system is more reliable and offers more flexibility during winter and cold season.
  • It requires circulating only a fixed amount of water and glycol solution. Therefore, it only needs small pump, tank, and small area to install.
  • You won’t face the freezing problems in this system.


  • The structure of this system is little more complicated compared to drain-back system.
  • It requires heat transfer fluid change periodically, and thereby, high maintenance cost.
  • The most important disadvantage is the stagnation of the collector. When the water supply in the tank anyhow stops, the circulating solution starts receiving heat from the collector again and again. That develops two problems; the glycol becomes more acidic at high temperature, and the solution starts boiling due to high temperature. These problems overpressurize the fluid, which is very dangerous.

Components of Solar Water Drain-Back System

The components of both types of solar hot water system are the same.

Solar Collector

This component is used to collect solar energy to heat water . A flat plate collector is commonly used for such systems. It is made up of metal or plastic box, which is insulated and weatherproofed. It contains an absorber plate beneath a tempered translucent cover. Copper piping is added in an S-shape between the absorber plate and translucent cover. This copper piping carries a heat exchanger (fluid).

Metal collectors are somewhat better than plastic collectors because they are sturdier and provide better fire resistance. However, they are heavy and more expensive than plastic collectors.


This is another main component of this system. It is used to collect hot water, and cold water is also passed to solar collectors from it. The size of the tank used in drain back system is significantly larger than that used closed loop system.

Thermostat Element (Controller)

The thermostat element is a differential temperature controller that controls the circulator and maintains the temperature of hot water. When the temperature of the tank is lower than the collector it senses the situation and turns the circulator on. Due to this, the heat transfer fluid starts moving and the procedure of heating water starts again.

At the time of its failure in drain-back system, the circulator doesn’t start at the right time. Therefore, the water is not circulated for further heating, and remains inside the tank.


The circulator uses a pump that sends the heat transfer fluid from tank to collector.

Inlet and Outlet

They are two valves present on the boundary of the tank. The inlet valve is used for sending cold water to the tank, and the outlet valve is for receiving hot water from the tank.

Heat Exchanger

A heat exchanger is also used in this system to transfer heat from working fluid to the water present in the tank. It is a series of copper coils, and submerged in the water tank.


This component has different structures for drain-back solar systems and closed loop solar systems.

In drain-back system, a couple of pipes are used for transferring cold and hot water separately. One end is connected to the collector and the other end is to the tank.

In closed loop system, a closed loop piping is used between the collector and the tank, in which the mixture of water and glycol is circulated.


This is all about solar thermal drain back system. The drain back systems are used specially in household usage. They are not used in cold areas due to the chance of water freezing. The size of closed system is comparatively small; therefore, they are used in large commercial and industrial applications. However, if high temperature water is required then a closed loop system should be used.