Renewable Energy Installer June 2014 | страница 13

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Heat pumps and flexible living
Following on from last month’s article discussing suitable
choices for heat emitters, heat pump specialist Bob Long looks
at the importance of lifestyles requirements and occupants’
specific heating needs
C
onsideration of lifestyle should be
an integral part of system design,
particularly when the heat source is
to be a heat pump. Designing a heat pump
system to optimise upon the economics of
operation can be much more complex than
designs employing combustible-fuelled
counterpart.
Heating systems powered by
combustible fuels such as natural gas or oil
achieve much greater operational flexibility,
able to match lifestyle needs much easier than
a heat pump powered system.
When referring to lifestyle, I am referring
to the basic use of the property, and by whom.
No matter what you may have read about
it being more economical to maintain a steady
temperature in a home as opposed to allowing
a dwelling to cool down and re-heat, it is
absolutely wrong!
There is no economic merit in heating an
empty home.
A conventional combustible-fuelled
heating system has the advantage of being
able to input large amounts of energy into a
dwelling in a relatively short period of time,
through the ability of the heat source to run
high water temperatures, causing the emitters
(radiators) to deliver large amounts of energy
when needed.
This is not generally possible with a heat
pump, unless the heat emitters are capable of
delivering similar large amounts of energy, on
demand.
In many instances, a high temperature
radiator panel will raise the air temperature
in the room to a habitable level, before the
temperature of the actual building fabric is
satisfied.
Many heat pump system emitters,
such as underfloor (UFH), or even retrofits
employing existing panel radiators, cannot
achieve this and so users find themselves
running the heating system for many hours,
sometimes unoccupied, to achieve the correct
level of heating for the hours of occupation.
Heat pump systems designed for low
occupancy homes should therefore differ from
those designed for high levels of occupancy.
Assuming the dwelling may one day be
sold, a low occupancy design will cater for all
eventualities, but will necessitate the use of
high output emitters wherever rapid heating
results are required.
The type of high output emitter is largely
confined to wall or duct mounted fan/coil
units, as the output from panel radiators is
limited by surface area and will, in almost all
cases, be physically impractical.
UFH systems are also limited by water
temperature and surface area.
When selecting fan/coil units, it is
important to note the output at a specific
working fluid temperature, and this can be
quite difficult as manufacturers’ brochures are
not generally too keen to display the relatively
poor output achievable.
A further limiting factor is the amount
of on demand energy available from the heat
pump.
In an example of a heating system
designed to produce 10kWh of thermal energy,
it would be impossible to supply the total
quantity of energy required to heat a whole
property in one hour, but it is possible to heat
one or two priority areas in this short time
period, and allow a longer time frame to heat
low priority areas.
High and low priority areas should be
defined by occupancy habits, suggesting
that maybe the lounge and kitchen/diner are
high priority, whereas bedrooms and hallways
could be rated as low priority, and where an
extended warming-up period is not going to
impact too severely on living comfort.
Remember, in retrofit installations, your
customer will have lived with the flexibility
of a combustible fuelled boiler, and it is
paramount that they are not disappointed by
their green energy replacement technology.
www.renewableenergyinstaller.co.uk | 13