Category Archives: Wetback Systems

What is a hot water cylinder coil or heat exchanger?

hot_heater_cylinder_coils

What is a heat exchanger

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 frostscylinder_heat_exchanger

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.

what_is_a_heat_exchanger

 

 

What a beautiful wetback installation

2 guys and 8 hours later this wetback installation was compete. Quick recovery time by having a wetback ready hot water cylinder just above the wetback.

Some Essentials in the design of the flow and return pipes when it comes to wetbacks:

 

  1. Pipes connecting the wetback and the storage vessel must have adequate rise and fall. This will wetback-hwcallow convectional flow and prevent air locks. G12 AS1 states a minimum of 1 in 7 slope to ensure heat will transfer between wetback and storage heater by thermo siphon.
  2. Flow and return pipes should be in copper and generally be a minimum of 25mm diameter. AS/NZS 3500 part 4 section 7 includes a table to determine pipe size according to length of run. The open vent should be a minimum of 20 mm diameter.
  3. If the hot water cylinder is not positioned high enough above the heat source it is possible for reversed circulation to occur. Reversed circulation, also known as back circulation, takes place when heat is transferred from the water cylinder back to the heat source.
  4. The bottom of the water cylinder should be at least 300 mm above the heat source assuming the flow line is connected to a riser pipe or coil within the cylinder. If no riser is fitted a distance of at least 600 mm should be made between the two appliances.

To find out more about wetback and hot water cylinder regulations please visit our regulations page. If in doubt give us a ring and speak to a qualified technician 🙂