Solar heating integration

The Radiant Solar system can be linked to the main heating system so that surplus solar heat (once the hot water tank is satisfied) can be used.

Which way is best?

There are basically three different ways to link the solar system in with the main heating.

Thermal Store

The first way is through a type combination tank, sometimes called a thermal store. This tank replaces the normal hot water tank and has a number of different coils inside to do various functions.

thermal store

This tank is OK if the other heat source is a standard boiler because the main body of water in the combination tank needs to be kept at 70 to 80oC. These tanks are not suitable for use with a heat pump. Also there is a limited amount of hot water available because the lower part of the tank is required for the central heating. Radiant do not recommend this type of tank because of the limited amount of hot water.

Tank – in - Tank

The second way is through a different type of combination tank, sometimes called a tank in tank. This tank has a hot water storage tank inside the main tank which is permanently immersed in the heating water. The solar coil heats the main heating water which in turn heats the hot water.

tan in a tank

This is better than the first in that the main body of water need only be heated to 50 to 70oC. There is more hot water available and the other heat source can be a heat pump.

The main drawback with this tank is that during the summer the solar collector has to heat the total capacity of water in order to heat the hot water. Radiant recommend this type of tank for some system integration schemes.

Two Tanks

The third way is to use two separate tanks; this is more suited to heating systems using heat pumps as the prime energy source. One tank is for the hot water and the other for the central heating. In this way the hot water tank can be sized correctly for the amount of hot water need. For the heating system a buffer tank is used which also has a high performance solar coil fitted. This buffer tank is used to link the heat pump and any other heat source such as a wood stove or cooker in the heating system.

cylinder and buffer tank

An advanced solar controller now controls both tanks and gives priority to the hot water cylinder. Any surplus solar heat can be diverted into the heating buffer tank to assist in the central heating.

This system allows for totally independent control of the hot water cylinder and the central heating system. It is therefore the most appropriate way to integrate different heat sources, especially those that incorporate a heat pump.

Radiant recommend using two tanks for some system integration schemes using heat pumps.


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A special controller with additional features for solar systems using two or more water storage tanks. The heat from the collector is put into the tanks in an order of selected priority. Furthermore any tank that has a lower temperature than the collector will receive any surplus heat. These tanks can be either domestic hot water or heating water such as a buffer tank as used with heat pump systems.
A buffer tank is a small water tank similar to a normal hot water tank in appearance. When using solar to integrate into the heating system the buffer tank has a heat exchanger coil fitted. The buffer tank is mainly used with a heat pump. The buffer tank acts as a heat store between the heat pump and heating circuits and maintains correct operation of the heat pump when radiators or UFH circuits become satisfied. The heat pump will shut down when the buffer tank reaches a normal running temperature (30 - 45°C). Therefore the buffer tank prevents the heat pump from continually turning on and off when running small or partial heat loads.