The simple answer is, a coil or heat exchanger is a device that allows the transfer of heat between two fluids without having them come in contact with each other.
A standard immersion element is by default integrated into the average solar ready cylinder and is considered the primary heat source. When an external secondary heat sources such as solar, heat pumps or even gas boiler are used, most hot water cylinders will not have a heat exchanger to accommodate and potable water is heated directly within the tank.
Systems of this type (without a heat exchanger) are sometimes called “direct systems”, they can be a problem for a number of reasons:
- The mains water may be corrosive
- The mains water may be mineralised
- Mains water will almost certainly have oxygen dissolved in it
- The location where the system is used may suffer from frosts
Corrosive water will attack the collector tubes, this process will be accelerated by any release of oxygen from solution in the water (gasses are less soluble in water at high temperatures). Minerals in the water will form deposits inside the collector tubes at high temperatures (in the same way that deposits are formed inside a jug or kettle). these deposits will eventually block your solar tubes, gas heat exchangers, pumps, valves and other system components.
Heat exchangers in closed loop systems are critical to the performance of the system and should be matched with the heat source (gas boiler or solar for example) being used and the flow rate through the circuit so that the heat being gained by the heat source is efficiently transferred to the potable water by the heat exchanger.
The most common type of heat exchanger is the coil heat exchanger this is integral to the storage tank, although sometimes external plate heat exchangers are used.
Here is one example of a double coil hot water cylinder which can be used for a solar system and gas boiler backup.