Shoebox Heat Pump Manual Version 9.4
Page 11 of 40
4.4.1 Type of DHW Tank
The larger the size of 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
than a traditional fossil boiler , 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 Three Port Diverting Valve
If the DHW option is ordered, a 3 port diverting valve
(‘
W
’
plan) is provided by Kensa 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. Please note the valve should be installed with the motor at any angle verti-
cal to 30° above the horizontal plane .
4.4.4 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.
4.4.5 DHW timeclock
A 24 hour time clock is required to control the production of DHW and is connected to the heat pump
’
s
internal wiring. (See section 4.7.4). This timeclock is supplied by others.
4.4.6 Secondary Returns
In long DHW pipe runs, to avoid excessive water draw off before the water is up to temperature at the
point of usage, it is common to install cylinders with a secondary return. This is not recommended for
systems using heat pumps as it promotes mixing in the tank and a lower flow temperature off the
cylinder.
For long pipe
runs, to avoid
excessive
cold
water draw offs it
is
recommend
that a flow boiler
is used and the
pipe
is
well
insulated.
The operation of the water pump and flow boiler should be timed to a period/s around the time the most
hot water is used, i.e. early morning and evening.
If the water pipe is well insulated and the system is timed, the amount of energy this system will use is
minimal. It is also possible to use trace heating tape, this removes the additional cost of installation of the
secondary return and water pump and the associated running costs of this equipment.
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rr
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ro
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ra
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n
el
ectr
ica
l
In
sta
lla
tio
n
me
ch
an
ica
l
In
sta
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tio
n
sch
ema
tics
In
sta
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n
G
en
er
al
p
ro
d
u
ct
in
fo
rma
ti
o
n
Sa
fe
ty
in
fo
rma
ti
o
n
In
tr
o
d
u
cti
o
n
He
at
P
u
mp
se
tti
n
gs
s
h
ee
t
DHW
out
Hot
water
cylinder
Flow boiler
Timed
water pump
Insulation
Return pipe
(smaller
diameter)
Timeclock
TimeClock
Point of usage
(taps)
Point of usage
(taps)
Automatic Air vent (installed at
the highest point)
The system uses a return loop, however instead of connecting to the tank it is connected to
the tank outlet. A timed water pump is also used and a flow boiler (Willis heater), which
makes up any losses to atmosphere from the pipework. The return pipe should also be a
smaller diameter than the flow pipe. It is important that an automatic air vent is installed at
the highest point of the system.
Fig 5 Secondary Return
