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Heat pump hot water systems

Air-sourced heat pumps absorb heat from air and transfer it to heat water. They run on electricity but are roughly three times more efficient than conventional electric water heaters, so when used in the right environment they save energy, save money and reduce greenhouse gas emissions.

Heat pumps work on the same principle as your refrigerator, but instead of pumping heat out of the fridge to keep it cool, they pump heat into water. Electricity is used to pump a refrigerant around the system, which picks up heat from the air and transfers it to the water.

 

How Does It Work?

An air-sourced heat pump works by absorbing heat from the outside air and transferring it to water in a tank. Like a water pump moves water uphill, a heat pump moves heat to where it is needed, in the hot water tank. The secret to making a heat pump work is the use of a refrigerant that evaporates at low temperatures. There are several steps in the process:

  • Liquid refrigerant passes through an evaporator where it picks up heat from the air and becomes a gas.
  • The gas refrigerant is compressed in an electric compressor. Compressing the gas causes its temperature to increase so that it becomes hotter than the water in the tank.
  • The hot gas flows into a condenser, where it passes its heat to the water.
  • The gas then flows into an expansion valve where its temperature drops and it returns to liquid form.

 

As long as the outside temperature is higher than the cold refrigerant the heat pump will absorb heat and be able to move it to the water. As the outside temperature decreases, this becomes more difficult, which is why heat pumps don't work as well in places where temperatures are low.
Fresh air needs to flow across the evaporator to enable heat to be absorbed continuously. A fan is used to assist air flow and remove the cooled air. A ventilated space is necessary for the evaporator to extract heat effectively.
A heat pump uses electricity to drive the compressor and the fan instead of using electricity to heat the water directly. The heat pump is able to transfer heat energy from the surrounding air to the water, which makes it highly efficient. How much heat is transferred depends on the ambient temperature.
For example, when air temperature is 15°C and the desired water temperature is 60°C, the heat transferred into the water is typically around 3 times as much as the electrical energy used. On a 30°C day, the heat energy transferred to the water is over 5 times the energy used by the system.

 



Integrated Systems

In an integrated system, the evaporator and other components are fixed to the top of the hot water tank. The advantages of integrated systems are that they are compact and easy to transport. They are connected in exactly the same way as standard electric hot water tanks and can be installed by a qualified plumber. An integrated system must be placed outside the house, or in an indoor space with good air flow like a well ventilated garage.

 

 

 



Split Systems

In a split system, the evaporator unit is placed away from the hot water tank, with the condenser coils still wrapped around the tank. A split system can offer flexibility that may be needed in some situations. For example, the tank can be placed in a poorly ventilated space (directly replacing an electric hot water tank in a cupboard, for example), and the smaller evaporator unit can be placed in the ventilated space. Or in hot climates, the evaporator can be placed in a ventilated ceiling space which will produce cooling as an added benefit.

The pipes for the refrigerant may need to be connected by a suitably licensed plumber depending on the manufacturer's requirements.

In a less common split configuration, the condenser is not part of the water tank but separated along with the other components of the heat pump to form a separate unit. Additional plumbing allows water from the tank to be pumped across the condenser and back. The tank can be placed in locations that would otherwise not be permitted, such as beneath a wooden floor. Additional energy will however be used for pumping.