5 system design criteria, System design criteria – Grant Products HPAW155 User Manual

Page 18

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System Design


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 60˚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

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


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
is complete.

Refer to Section 6 to determine the size
of radiators required for your

5 System Design Criteria

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

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