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5 System Design Criteria
Unlike a typical condensing oil or gas
fired boiler that operates at a flow of
70˚C and a return of 50˚C, a heat pump
operates with a flow of between 30˚C
and 50˚C. The return temperature will
depend on the load of the system at a
given point in time.
The design of any system in the UK is
typically based on 2 parameters.
1. That the outside air temperature
can fall to as low as -3˚C and that
the house comfort temperature will
be 21˚C.
The BTC incorporated in the heat
pump will adjust the output
according to the external ambient
air temperature but the system
must be designed in the first place
to meet this maximum demand.
2. The second factor to consider is
achieving this maximum demand
using much lower water
temperatures than with oil or gas
fired appliances.
Designing a new system for use
with a low-grade heat source is
straight forward, and assuming the
insulation properties of the dwelling
meets or exceeds current building
regulations, there should be no
issue with achieving the heat
demand.
The use of a heat pump in an existing
system can be straightforward if the
following rules are followed.
1. The loft has insulation to a depth
of 270mm
2. Cavity wall insulation has been
installed
3. The radiators have been changed
or upgraded to match the new
water temperature
4. An accurate heat loss calculation
for each room of the house has
been carried out
5. All primary and secondary pipes
have been well insulated to prevent
heat loss
While underfloor heating is the preferred
heat emitter, a combination of
underfloor heating and radiators, or
radiators only, works just as efficiently. It
is necessary, however, to calculate the
size of radiator required accurately
– if this is not done, the house will fail
to reach the target temperature and will
be costly to rectify after the installation
14
is complete.
It must be understood that your final design working
temperature will have an effect on the overall system
efficiency, the COP of the heat pump and the
complete system. Put simply, the lower your design
working temperature, the better the COP. If you are
in any doubt about the suitability of the heating
system, stop and seek the advice of a qualified
heating engineer or experienced system designer.
Refer to Section 6 to determine the size
of radiators required for your
installation.
When tested to BS EN14511, the
Coefficient of Performance and Heat
Output for an Air Source Heat Pump
are declared at the test conditions of
7°C outside air temperature and 35°C
water flow temperature.
At all other values of outside air
temperature and water flow
temperature the actual Heat Pump
output will vary, e.g. the heat output will
a) decrease with lower outside air
temperatures and increase with higher
outside air temperatures at any given
water flow temperature, and
b) decrease with higher water flow
temperatures and increase with lower
water flow temperatures at any given
outside air temperature
The table below gives the approximate
percentage output at different
temperature conditions, compared to
the rated heat output at 7°C ambient
air/35°C water flow.
Air Temp
Approximate Heat Percentage (%) Heat Output
21˚C
212
19˚C
186
17˚C
164
15˚C
145
13˚C
132
11˚C
124
9˚C
118
7˚C
110
5˚C
105
3˚C
100
1˚C
95
-1˚C
91
-3˚C
88
-5˚C
85
-7˚C
80
-9˚C
77
Water Temp
30°C
Example: A Grant Aerona HPAW65
Heat Pump with a rated output of
6.78kW at 7°C air/35°C water would
give approximately 83% of this value
(5.6kW) at -1°C outside air temperature
and approximately 80% (5.4kW) at -
3°C.
However, the same Heat Pump
operating at 11°C outside air
temperature would give approximately
113% of the rated output (7.6kW).
The factory fitted 3kW electric
immersion heater should not be added
to the rated output of the Heat Pump
for sizing/selection purposes. This
electric immersion heater serves as a
back-up and will only be called into
operation when the heat pump is
operating at low air temperatures.
Provided that the Heat Pump is sized
correctly for the system, this back-up
heater will compensate for any short fall
in meeting the design heat load for the
property at the minimum design air
temperatures.
193
181
167
169
159
146
149
140
129
132
124
114
120
113
104
113
106
98
107
101
93
100
94
86
95
89
82
91
86
79
86
81
74
83
78
72
80
75
69
77
72
67
73
69
63
70
66
61
35°C
40°C
45°C
150
129
132
113
116
100
103
88
93
80
88
76
83
82
78
67
74
64
71
61
67
58
65
56
62
54
60
52
57
49
54
47
50°C
55°C
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