economical best Practice
Air source heat pumps can provide both hot water (providing a suitable DHW tank
is used) and space heating all year. The system is different to a conventional fossil
fuel heating and hot water system. The efficiency of a heat pump is shown by its
coefficient of performance as explained in the introduction. The following points
should be noted to achieve the most efficient and economical operation of your
Important points about heat pump systems
• Domestic hot water and legionella functions are only available on cylinder units
or hydroboxes plumbed to an appropriate storage DHW tank.
• In normal operation simultaneous DHW and space heating is unadvisable.
However during periods of extremely low outdoor ambient temperature, the im-
mersion heater (if present) can be used for DHW whilst the heat pump contin-
ues to provide space heating. Please be aware that the immersion heater, used
alone, is not an efficient method to heat the whole DHW tank. Therefore it
should only be used as a back up in normal operation.
• The hot water produced by the heat pump is typically at a lower temperature
than a fossil fuel boiler.
overview of Controls
Built into the cylinder unit and hydrobox is the Flow Temperature Controller(FTC).
This device controls the function of both the outdoor heat pump unit and the cyl-
inder unit or hydrobox. The advanced technology means that by using an FTC
controlled heat pump you can not only make savings compared to traditional fos-
sil fuel type heating systems but also compared to many other heat pumps on the
As explained in the earlier section, 'How the Heat Pump Works,' heat pumps are
most efficient when providing low flow temperature water. The FTC advanced
technology enables the room temperature to be kept at the desired level whilst
utilising the lowest possible flow temperature from the heat pump.
In room temp (Auto adaptation) mode the controller uses temperature sensors
around the heating system to monitor space and flow temperatures. This data
is regularly updated and compared to previous data by the controller to predict
changes in room temperature and adjust the temperature of water flowing to the
space heating circuit accordingly. By monitoring not only the outdoor ambient, but
the room and heating circuit water temperatures, the heating is more consistent
and sudden spikes in required heat output are reduced. This results in a lower
overall flow temperature being required.
• If the heat pump is being used for DHW the time at which tank heat up occurs
should be scheduled using the SCHEDULE function (see page 12). Ideally
this should be during the night time when little space heating is required and
economy electricity tariffs can be taken advantage of.
• In most situations space heating is best performed using the room tempera-
ture mode. This enables the heat pump to analyse current room temperature
and react to changes in a controlled manner utilising the specialised Mitsubi-
shi Electric controls.
• Using the SCHEDULE and HOLIDAY functions prevent unnecessary Space
or DHW heating when the property is known to be unoccupied for instance
during the working day.
• Due to lower flow temperatures, heat pump heating systems should be used
with large surface area radiators or under-floor heating. This will provide a
steady heat to the room whilst improving efficiency and so lowering running
costs of the system as the heat pump does not have to produce water at very
high flow temperatures.
Flow temp sensor
Return temp sensor
Room temp sensor