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Winter conditions challenge ASHPs
Heat pump customers needn’t be left cold or out-of-pocket by
underperforming systems during the winter months, argues heat pump specialist
Bob Long
A
s recent freezing conditions have
reminded us, winter is not quite
over yet. And I continue to receive
calls from heat pump users who are finding
that their heat pump is falling short on the
quantity and quality of heat output, and/
or is expensive to run during lower winter
temperatures.
When complaints of this nature are
received, I can’t stress the importance of heat
metering too much. I am still amazed that
many systems still don’t have a heat meter
fitted.
Knowledge is all-powerful and to
have the precise knowledge regarding the
economic output of the heat pump is as
essential as the fuel computer in a hybrid car.
Although most manufacturers continue
to show the best aspects of heat pump
performance on advertising brochures, I am
pleased to see a number of the big names are
now making the actual data for low ambient
operation available in their sales literature.
Generally, ASHPs are boldly advertised at
+7° C ambient temperature and +35°C water
temperature, as these are the parameters
where the most boastful COP can be
achieved.
This prominently displayed COP figure
30 | www.renewableenergyinstaller.co.uk
is likely to indicate a very respectable 4:1,
or more. We must therefore forgive the
prospective customer for believing this is
representative of the economic advantage
they are to enjoy. Electrical energy costs of
around 12p per kW/hr would indicate 3p per
kW/hr for the energy supplied by the heat
pump.
In principal, this assumption would be
correct but, in practice, rarely achieved.
The problem arises when you want
to actually use this economically derived
energy for the purpose of space heating.
A maximum economical temperature of
35°C is not very high up the scale when
you consider a conventional heating system
using conventional fuel would produce useful
temperatures somewhere between +55°C
and 85°C, effectively delivering the energy to
the space via relatively cheap, conventional
panel radiators.
To make adequate use of temperatures
around 35°C, the installation must employ
either a close-pitched underfloor heating
matrix, or specially sized fan/coil units.
Whichever method is chosen, the
success is dependant on the heat emitter
having sufficient surface area to transmit the
thermal energy to the space being heated.
Without exception, surface area is never
cheap, and therefore price considerations will
lead the installer to look at higher operating
temperatures, and smaller heat exchangers.
As operating temperature is increased,
so the COP is lowered and very quickly the
anticipation of heating for as little as 3p per
kW/hr becomes a myth.
Outdoor ambient temperature has a
similar effect on the operating economics and
as winter temperatures fall below the average
+7°C, as stated in the sales brochure, the COP
also proportionately falls.
It is no surprise that a prospective
customer who has listened to the sales
patter and installed a heat pump system
now expects to enjoy at least a good level of
comfort and affordable running costs.
All too often it seems, they realise the
cold facts and are either not warm enough,
financially disappointed, or both.
It doesn’t need to be this way, but
installing a heat pump system that provides
economical results is by no means a low cost
exercise.
Once again, honesty is the best policy
and essential in ensuring the value of the
heat pump can be recognised, and not
compromised!