Technical data

11
4.4.1 Type of DHW Tank
The larger size the coil within the tank, the better the heat transfer area and hence the better the DHW
performance will be.(Refer to table 3.2)
4.4.2 DHW Tank Size
The tank will need to be carefully sized to meet the DHW demand, based on the number of occupants and
should have an acceptable recovery rate. Due to the lower DHW temperature achieved by the heat pump,
a tank 30% larger than normal is recommended. This is due to the higher demand on the tank, as less cold
water is used at the point of use to mix the lower temperature DHW to an acceptable temperature.
4.4.3 Immersion Heater
Although not required by Building Regulations, it is generally advised that to provide legionella protection
the tank is raised above 60
o
C at least once a week. To provide this we would recommend that a 3 kW
electric immersion heater is fitted to the bottom of the tank, with its own dedicated 7- day timeclock. If
DHW is required higher than 50
o
C then it is advisable that the immersion heater is programmed to
operate for a period following the heat pump operation period to raise the temperature. This avoids the
immersion heater taking all of the load.
4.4.4 Three Port Diverting Valve
If the DHW option is ordered, a 3 port diverting valve (‘W’ plan) is provided by Vent-Axia and is used to
divert the flow when the timeclock calls for DHW production from space heating to the DHW tank. The
valve’s electrical connections are connected to the heat pump’s internal wiring. Please note connection ‘A’
is DHW and ’B’ is space heating.
4.4.5 Tank Thermostat
A tank thermostat is not required but maybe fitted and used as a tank safety stat if wired in series with the
time clock. This should be set at not less than 65
o
C.
programmed to operate for a period immediately following the DHW production. This means that the
majority of the heating load for the DHW is produced at a lower cost using the heat pump, as opposed to
using only the direct immersion heater.
If 50
o
C water is acceptable, then it is recommended that the immersion heater is programmed to raise the
temperature to 65
o
C once a week.