1.
Can the heat pump heat the whole house?
2. How do I ensure that
the system is energy efficient?
3. There is a big variation
in efficiency claims, who do I believe?
4. What is the environmental
benefit?
5. Can I power a Heat Pump
from a renewable source like a windmill, hydro
or solar panels?
6. What is the best system?
7. Why is the size of the
heat pump that I have been quoted for so much
smaller than my existing boiler?
8. I have an average town
house, with a small garden. Can I install a
heat pump?
9. Why is underfloor heating
coupled with a heat pump so highly rated?
10.
Can I fit a system that blows warm air into
the house?
11. Is a vertical borehole
better than a horizontal pipe trench system?
12. How much ground do
I need?
13. Is an air-source system
a viable option?
14. Can my plumber instal
a system?
15. How long will a heat
pump last?
16. Do I need a buffer
tank?
17. Can a heat pump also
heat the domestic hot water?
18. What is 'Weather Compensation'?
19. What is the 3-phase
electric supply issue?
20. I have heard of heat
pump systems providing heat for under £50/
year. How can this be achieved?
Q.
Can the heat pump heat the whole house?
A. Yes it can. Newly built or well-insulated
buildings can be easily heated with a heat pump.
Old, badly insulated buildings can however be
a problem in cold weather, with heat pumps struggling
to satisfy them. In these buildings, the temperature
of the radiators or underfloor heating may have
to exceed the efficient working temperature
levels for the heat pump. Fires or boilers are
often necessary to ensure adequate heating in
the winter. Even in very well insulated houses,
it is not uncommon to retain a fire or stove
for extra comfort in cold weather. Air source
systems require more back-up heat then ground
source systems.
Q.
How do I ensure that the system is energy efficient?
A. A heat pump can heat water up to around 55°C
(depending on model type). It is vital to understand
that the hotter the water, the lower the energy
efficiency, so running at a lower temperature
will save a lot of energy. The following chart
illustrates this. Figures for a typical ground
source heat pump system.
Water heated to 55° COP = 2.4
Water heated to 45° COP = 3.2
Water heated to 35° COP = 4
(COP is the energy efficiency ratio. COP of
4 means you get 4 kW of heat for 1kW of electricity
consumed. See (Glossary)
for better description.
Q.
There is a big variation in efficiency claims.
Who do I believe?
A. There are a great variety of heat pump applications
and uses, resulting in a wide range of efficiencies.
Just be mindful that there is sometimes a tendency
to quote best-case figures. The average annual
efficiency could be significantly less than
first expected due to many reasons. The IMPORTANT
chart above demonstrates how dramatically the
efficiency varies with heated water temperature.
Attention to design details is the key to achieving
high energy efficiency.
Q.
What is the environmental benefit?
A. We believe that only systems having COP's
(efficiency) of 3.5 and above give a significantly
large enough environmental benefit over gas.
Oil and coal are, however, not as clean, so
heat pumps generally compete well here. If comparing
with electric heating, then even a poor heat
pump would be an improvement since electric
heating is not good for the environment. Wood-burning
is usually the best environmental heating solution..
Q.
Can I power a heat pump
from a renewable energy source like a windmill,
hydro or solar panels?
A. Yes you can. A hydro-powered heat pump is
probably the most viable renewable power-source
since it is fairly constant. Solar or wind inputs
would be difficult, especially on a small scale.
The outputs are variable and dependent on the
weather, so there can be problems matching the
input power requirements. The very large number
of photo-electric cells required to power a
heat pump would probably make this option impractical.
Powering
a heat pump from grid electricity derived from
a renewable source (e.g. wind-farm or hydro)
is a good option for the environment. However,
there is some debate about the actual worthiness
of these schemes.
Q.
What is the best system?
A. In a nut shell, a spring water sourced system
heating a well-insulated building through underfloor
heating. Since springs are uncommon then ground
source would be the next best option. If using
a back-up to support the heat pump then wood
is the best option, but it is labour intensive.
An oil or gas boiler would be a good back-up,
but this adds considerable cost. A direct electric
back-up is cheap to install, but not ideal environmentally.
Q.
Why is the size of the
heat pump that I have been quoted for so much
smaller than my existing boiler?
A. Boilers are usually larger than they need
to be, and are often used with time switches
to quickly heat a building. It makes sense for
a plumber to play safe to avoid any complaints
that the house is not warm enough. However,
it is too expensive to install an oversize heat
pump system, therefore, the heat output is more
accurately matched to heat demand. Furthermore,
heat pumps are happy running continuously without
a rest. Don't forget you are saving energy with
a smaller system. This is also true for boilers,
but to a lesser extent
Q.
I have an average town
house, with a small garden. Can I install a
heat pump?
A. Installing ground-source is often very difficult
in this situation. You can probably do better
things with your money to save energy and fuel
costs. Don't neglect the obvious draught-stripping
etc. Consider investment in some serious insulation
in the form of either internal or external walls
cladding. This is not as interesting as a heat
pump but it will save you energy for the lifetime
of the building. If you have dealt with heat
loss then you could fit an air-source system.
Unfortunately it may not be cheaper to run than
a very well-controlled condensing natural-gas
system at present. However, if your only fuel
option is oil, then a well optimised air-source
system could save you a significant amount of
money and benefit the environment. Be mindful
that these units can be a little noisy, will
not last as long as a ground source system,
and are not very efficient in mid-Winter.
Q.
Why is underfloor heating coupled with a heat
pump so highly rated?
A. The lower the temperature of the heated water,
the better the heat pump's efficiency. Since
standard radiators can reach as high as 80 deg.C
(175 deg.F), far hotter than a heat pump can
achieve, you will have to significantly increase
the area of the radiators to be able to utilise
the working temperature of 50 deg.C (122 deg.F),
from a heat pump. However, if the water temperature
can be further reduced to around 35 deg.C or
less, then there is a considerable energy benefit.
Underfloor heating pipes can give sufficient
heat at these low temperatures and are therefore
a good match with a heat pump. This system works
best in insulated homes where the heat required
is less than 50watts/sq.m. of floor area. For
the average older house, the floor would have
to be too warm to provide full heating. It is
important to realise that heat pumps like lower
temperatures and higher water flow within the
uderfloor pipes. This means more pipe and a
lower pressure drop than average. MAKE SURE
THAT THE UNDERFLOOR COMPANY ARE HAPPY TO ACCOMMODATE
THE SPECIAL REQUIREMENTS OF A HEAT PUMP. Also,
make sure that there is enough insulation below
the floor to minimise heat-loss into the ground,
which can be significant. Concrete screed systems
are by far the best, far better than wood. Be
mindful of floor coverings. Carpets can reduce
the system efficiency significantly when a heat
pump is used.
Q.
Can I fit a system that blows warm air into
the house?
A. Air-movement is very good for cooling applications
but not so good for heating, we feel more comfortable
in minimal draughts and with the most heat-radiation.
Radiation is emitted from an underfloor system
and from conventional radiators. Blown air systems
may be acceptable in very well insulated houses,
but it is hard to match the comfort of good
underfloor heating.
Q
Is a vertical borehole better than a horizontal
pipe trench system?
A. The heat from either if these systems is
mostly stored solar heat in the mass of earth
near the surface. Either system will produce
similar results. It is usually a matter of cost
and practicality. e.g. if land is available
the horizontal trench system will usually be
cheaper to instal than a borehole.
If cooling is required, then the borehole may
prove to be better
The
use of the term "geothermal" is commonly
used in conjunction with heat-pumps, but it
is not strictly correct. You have to drill a
long way down before you experience any worthwhile
rise above mean ground temperature due to heat
from the earth's core.
Q
How much ground do I need?
A . The more the better, but this strategy is
expensive. The average garden is often too small
to get sufficient heat output. However, as houses
become better insulated, then ground collectors
don't need to be so big. Do not underestimate
the upheaval of digging trenches, but when the
grass and plants have grown back it will all
seem worthwhile. This component of the system
should last well over 50 years, extra pipe will
eventually pay for itself since the efficiency
of the system will be better. Ground conditions
will also have an effect on performance for
example, wet conditions assist the heat transfer
process. Dry, sandy ground is inferior, so would
require more pipework and a larger area. As
a rough guide, you may need 150m of narrow trench,
or 400 sq. m. of land for a 10kW (output) heat-pump.
Bigger if dry sandy soil, smaller if very wet.
There is some debate about the depth, but 2m
deep is ideal. Excessive cost and health &
safety regulations usually mean that shallower
trences are used. It would be during long cold-spells
in mid winter where deeper trenches would be
beneficial. In recent years, such conditions
seem infrequent. The pipeowrk length, diameter
and manifolding is calculated carefully so as
to minimise required pumping power to circulate
the fluid.
Q.
Is an air-source system a viable option?
A. Air-source is much easier and cheaper to
install. But it does not promise very high energy
efficiencies for year-round heating in the UK,
particularly in the North. The main reason being
that the efficiency drops when you need most
heat. A back-up from a boiler at such times
is usually required. Ground source, by contrast,
maintains a constant heat output unaffected
by the daily temperature changes above. Smaller
air systems may give reasonable overall results
if used alongside a boiler, but only if used
when air temperatures are not too cold. Air
source units usually have a shorter life since
they have to work out in the elements.
Q
Can my plumber instal a system?
A . Yes, but there are some potential pitfalls
to avoid. Pipe connections are usually larger
in diameter than rule-of-thumb sizing. A heat
pump cannot simply be fitted in place of a boiler
as there are some fundamental differences in
the operation. But the actual handy work is
exactly the same.
Q.
How long will a heat pump last?
A. Most good water-to-water type heat pumps
will far outlast even the best quality boiler.
They should operate for over 20 years, and with
minimal maintenance.
Air source systems are usually exposed to the
elements and have a slightly harder life, so
may have a similar life to a boiler.
Q.
Do I need a buffer tank?
A. The manufactureres do not all agree on this
point, but it is suggested that you go with
thier specific recommendations.
A buffer tank is simply a quantity of water
that can help to reduce the number of times
the heat-pump has to 'cycle' (times it has to
stop and start). It is particularly necessary
in a larger property where many heating zones
are involved. In well-insulated and open-plan
houses a buffer tank may not be needed, in these
cases, the floor itself can act as the buffer.
However, the floor must have plenty of pipe
in it with good thermal contact within a thick
screed. High water-content radiators can act
as a buffer.
In summary; having a buffer tank is playing-safe,
and recommended if the radiator or underfloor
system is unknown, or un-matched. With well
designed house and emitter circuits, you might
be better off without one.
Q.
Can a heat pump also heat the domestic hot water?
A. It certainly can, but whilst heating to the
high temperatures required, the efficiency reduces.
However, even low efficiencies are far better
than an electric immersion heater. Most of the
latest heat pump units have the hot water function
built in, so it is usual to use this facility.
As the insulation levels in buildings increases,
the room-heating demand drops, but the hot water
demand is, if anything, increasing. It is therfore
becoming more important to optimise the hot
water facility. i.e. the size and design of
the hot water cylinder is very important.
Q.
What is 'Weather Compensation'?
A. As you now know, it is important to keep
the heated water as low as possible if high
efficiencies are to be attained. It is sensible,
if not vital, to have the water temperature
vary dependent on outside conditions. i.e. if
you require water for the floor to be at 40°C
when at -5°C outside, you may require only
32°C when it is +5°C outside. Weather
Compensation does this automatically, and is
an integral part of most heat pumps.
Q.
What is the 3-phase electric supply issue?
A. Heat pumps are driven by fairly large electric
motors. Such motors work best with a 3-phase
electrical supply. Unfortunately this supply
is not common for houses in the UK. Having said
that, small heat pumps (say up to 8kW output)
work just as well on single phase. Multiple
compressor systems are available. A 3-phase
unit will debatably last longer, or may be a
little more reliable and efficient than a single
phase unit so we favour that type. It may be
worthwhile asking your electricity supplier
the cost of installing a 3-phase supply. Most
people however have no option otherthan single
phase.
Q
I have heard of heat
pump systems providing heat for under £50/
year. How can this be achieved?
A. The reason that such a house is
so cheap to heat is that it is super-insulated.
Sorry, but there is no magic box that can reduce
fuel costs so dramatically.
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