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DESIGN
SYSTEM DESIGN
General
• All Torrent HP models covered by this manual, are only suitable for open vented
primary (heat pump and space heating) circuits. Therefore the pressure in the
primary circuit depends upon the height of the feed and expansion cistern and
could be as low as 0.20 bar. Therefore ensure that the heat pump selected will
operate at primary circuit pressures e.g. it is not fitted with a non-adjustable low
system pressure switch.
• The heat pumps normally operate at lower temperature difference (5-10°C)
betweenflowandreturnthanthatforagasboilers(10-20°C)resultinginhigherflow
rates in the heat pump primary circuit. Therefore it is important that the primary
pipeworkconnectingtheheatpumptothethermalstoreandthecirculatingpump
are sized to achieve flow rates recommended by the heat pump manufacturer.
• The efficiency (i.e. coefficient of performance, COP) of a heat pump depends
upontheprimaryflowandreturntemperaturesanditisgenerallyhigheratlower
primarycircuittemperatures.Alsotheheatpumpsnormallyoperateatlowerflow
temperatures;typically45-55°Ccomparedwith75-80°Cforgasboilers.Thereforeif
traditionalradiators areusedthen, theseshouldbe oversizedinaccordance with
the heat pump manufacturers recommendations.
• For optimum operational efficiency and hence lower running costs, a low
temperaturespaceheating(e.g.under-floorheating)systemshouldbeusedwhere
feasible.
Heat Pump Sizing
• The efficiency (i.e. coefficient of performance, COP) of an air source heat pump
and hence its heat output generally decreases as the ambient air temperature
decreases. Therefore, this factor should be taken into account when sizing and
selecting the heat pump.
• The design space heating load of a dwelling should be calculated in accordance
with BS 5449. The allowance for domestic hot water heating (shown in table
below),dependsupontheoperatingmode.Forexample,ifthesystemisdesigned
to operate with priority for domestic hot water, then no additional allowance for
hot water is required for sizing the heat pump.
Allowance for Domestic HotWater Heating
Model Allowance for Domestic HotWater
HotWater Priority Mode Flow Share Mode
THP150
THP180-SOL 0kW 1.5kW
THP180
THP210-SOL 0kW 2.5kW
THP210
THP250-SOL 0kW 3.0kW
THP250
THP300-SOL 0kW 3.5kW
THP300
THP450-SOL 0kW 4.0kW
THP450-SOL 0kW 5.0kW
Heating System Arrangement
The Torrent HP models are designed to be
heated directly by an air or a ground source
heat pump and the space heating can be
supplied directly from the heat pump circuit
asshowninfigures6and8orfromthethermal
storeasshowninfigures7and9.Theselection
betweenthetwooptionswilldependuponthe
operating characteristics of the heat pump,
type of heating system and control strategy.
For general guidance: -
• If the heat pump requires a buffer for
operation; for example to reduce rapid
cycling etc, then the system would benefit
if the heating system is supplied from
the thermal store (figures 7 and 9). In
this configuration, the operation of the
heat pump Is only controlled by the store
thermostat(s).
• Iftheheatpumpismulti-stageormodulating
typeorhasweathercompensatingcontrols,
then the space heating circuit should be
supplied directly from the heat pump as
shown in figures 6 and 8. In this system
configuration,thethermalstoreisonlyused
for producing hot water.
• A hybrid system arrangement can also
be used (not shown). For example; a low
temperatureweathercompensatedheating
circuit e.g. under floor heating can be
supplieddirectlyfromtheheatpumpcircuit
andconstanttemperaturee.g.towelrailscan
be supplied from the thermal store.
Ballvalve/overflow - All Models
Where the whole of the heating system is
supplied from the thermal store,figures 7 and
9,thenaballvalveandoverflowshouldbefitted
in the feed and expansion tank.
Where the heat pump supplies the heating
circuit directly, figures 6 and 8, then fitting a
ballvalveandoverflowisoptionalifitispractical
onoccasionstomanuallyfillthestore.However,
fittingaballvalveandoverflowwilleliminatethe
needforthehouseholdertoundertakethistask
and is therefore preferable.If the siting of the
Torrent HP makes it difficult to fit an overflow
then the‘manual fill’option can be used.In all
cases,if a ballvalve is fitted,then an overflow/
warning pipe must always be fitted.
