Ariston Heat pump water heater User Manual page 40

Heat pump water heater
A - a compressor, which allows the cycle to run by increasing the pressure and temperature of the refrigerating
fluid, which in this phase is in the gas state.
B - a first heat exchanger located inside the water heater's water tank: through its surface, heat is exchanged
between the refrigerating fluid and the domestic water to be heated. As in this phase the hot refrigerating gas
changes state and condenses into a liquid upon transferring its heat to the water, the heat exchanger is defined
a condenser.
C - an expansion valve: a device through which the refrigerating liquid passes once its pressure and temperature
decrease sensibly following the liquid's expansion due to an increase in the tube's section upstream of the
valve.
D - a second heat exchanger located on the upper part of the water heater, whose fins are designed to increase
its surface, allows for heat to be exchanged between the refrigerating fluid and the free source, or the forced
flow of the ambient air suitably conveyed by a special fan. Given that the refrigerating fluid evaporates in this
phase and subtracts heat from the ambient air, the heat exchanger is defined an evaporator.
As thermal energy can only move from a higher to a lower level of temperature, the refrigerant in the evaporator (D)
must necessarily have a lower temperature than the ambient air constituting the free source. On the other hand, the
temperature of the refrigerant in the condenser (B) must necessarily be higher than that of the water to be heated in
the tank, in order for it to transfer heat.
The difference in temperature is produced inside the heat pump circuit by the compressor (A) and the expansion valve
(C) located between the evaporator (D) and the condenser (B), thanks to the physical properties of the refrigerating
fluid.
The efficiency of a heat pump cycle is measured by the Coefficient of Performance (COP), i.e. the ratio between the
energy supplied to the appliance (in this case, the heat transferred to the water to be heated) and the electrical energy
used (by the compressor and the appliance's auxiliary devices). The COP varies according to the type of heat pump
and to its relative conditions of operation.
For example, a COP value equal to 3 indicates that for every 1 kWh of electrical energy used, the heat pump supplies
3 kWh of heat to the substance to be heated, of which 2 kWh are extracted from the free source. The rated values for
the COP relative to NUOS water heaters appear in the technical data table in paragraph 2.8.
The typical heat pump cycle temperatures, in relation to the features of the refrigerating fluid and of the free source,
allow for heating the domestic water inside the NUOS water heater tank up to a temperature of 55°C in normal conditions
of use. Together with the different capacities available on the various models, this temperature is capable of satisfying
the entire range of domestic uses.
However, NUOS water heaters are fitted with a supplementary heating element (a standard feature) which allows for
additional options: these include accelerated attainment of full operation by combining heat pump and heating element
operation, or water temperatures of up to 65°C, which are used in running anti-bacteria protection cycles.
To ensure a rational use of energy in operating the water heater, appropriate visual indicators remind the user that
the appliance does not operate in the most energy-efficient way when the heating element is activated.
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