The purpose of this user manual is to inform users how their air source heat
pump heating system works, how to run the system at its most efficient and how
to change settings on the main remote controller.
overview of the System
The Mitsubishi Electric Air to Water (ATW) heat pump system consists of the fol-
lowing components; outdoor heat pump unit and indoor cylinder unit or hydrobox
incorporating main remote controller.
How the Heat Pump Works
Space heating and DHW
Heat pumps take electric energy and low grade heat energy from the outdoor air
to heat refrigerant which in turn heats water for domestic use and space heating.
The efficiency of a heat pump is known as the Coefficient of Performance or COP
this is the ratio of heat delivered to power consumed.
The operation of a heat pump is similar to a refrigerator in reverse. This process
is known as the vapour-compression cycle and the following is a more detailed
Low temperature renewable heat
energy taken from the environment
This appliance is not intended for use by persons (including children) with
reduced physical, sensory or mental capabilities, or lack of experience and
knowledge, unless they have been given supervision or instruction con-
cerning the use of the appliance by a person responsible for their safety.
Children should be supervised to ensure they do not play with the appli-
This user manual should be kept with the unit or in an accessible place for
The first phase begins with the refrigerant being cold and low pressure.
1. The refrigerant within the circuit is compressed as it passes through the com-
pressor. It becomes a hot highly pressurised gas. The temperature also rises
typically to 60°C.
2. The hot refrigerant gas is then condensed as it passes across one side of a
Heat energy output
plate heat exchanger. Heat from the refrigerant gas is transferred to the cooler
side (water side) of the heat exchanger. As the temperature of the refrigerant
decreases its state changes from a gas to a liquid.
3. Now as a cold liquid it still has a high pressure. To reduce the pressure the liq-
uid passes through an expansion valve. The pressure drops but the refrigerant
remains a cold liquid.
4. The final stage of the cycle is when the refrigerant passes into the evaporator
and evaporates. It is at this point when some of the free heat energy in the out-
side air is absorbed by the refrigerant.
It is only the refrigerant that passes through this cycle; the water is heated as it
travels through the plate heat exchanger. The heat energy from the refrigerant
passes through the plate heat exchanger to the cooler water which increases in
temperature. This heated water enters the primary circuit and is circulated and
used to serve the space heating system and indirectly heat the contents of the
DHW tank (if present).
Schematic of package cylinder system
(Plate heat exchanger)
3. Expansion valve
(Outdoor unit air heat exchanger)