1.
How do heat pumps work?
Heat
pumps extract heat energy from the ground and convert this into
usable heat. The ground temperature is always at a relatively
stable 10 - 12 deg C. Plastic pipes are buried at a depth of
about 1 metre, and a water/antifreeze solution is circulated
through them. This brine solution then passes through an evaporator/heat
exchanger, where the low temperature heat energy is transferred
to a refrigerant. The refrigerant then passes through a compressor,
which increases the pressure and temperature of the solution.
The now hot refrigerant passes through a condenser/heat exchanger
where the heat energy is transferred to the heating system water.
The now cold refrigerant passes through an expansion valve,
which regulates the system pressure, and the cycle starts again.
Put simply, it works like a fridge in reverse.
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2.
What fuel do heat pumps run on?
Heat
pumps run on electricity. Generally speaking for most domestic
applications, 240 Volt single phase is the only option available.
For single phase, heat pumps providing up to 12 kW heat output
are available (depending on manufacturer). For higher heat outputs,
it would be necessary to have 400 Volt three phase electricity.
As an example, for the NIBE heat pumps we supply and install,
a 12 kW single phase heat pump will have an electrical consumption
of 2.5 kW and a heat output of up to 11.5 kW at a water output
temperature of 35 deg C, which gives a CoP
of 4.6. Back to top
3.
Can I run a heat pump if my house is off the grid system?
Heat
pumps need electricity to run them. This is usually supplied
from the grid system, but if you are able to install a wind
turbine or a small hydro system from a fast running stream or
river, you could generate enough electricity to run the heat
pump. We know of one person who has a heat pump supplied by
a wind turbine, which actually generates more electricity than
he uses. This is great for him because instead of getting a
bill from his electricity provider, he gets a cheque!
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4.
Do they really save money?
Heat
pumps are generally at their most efficient when producing water
at an output temperature of around 25 - 35 deg C. This makes
them ideal for use with underfloor heating in a very well insulated
building where the heat losses can be kept as low as possible.
At this level of output it can be possible to get out up to
5 times more usable heat energy than the electricity used to
run the heat pump. However, as the output temperature rises,
this value drops away quite quickly. (See CoP
below). Heat pumps can certainly save you money when compared
against an oil boiler or bottled gas, but are not so competitive
when compared to natural gas. However, as fossil fuels are rising
sharply, heat pumps are becoming the preferred option, even
against natural gas. Back to top
5.
Are
they easy to install?
Heat
pumps are no more difficult to install than a conventional gas
or oil boiler. They have the advantage that they are not covered
by installation restrictions as is the case for gas, where these
must be installed by a CORGI registered installer. There is
a bit more work involved with a heat pump, because of the external
ground loops, which must be laid into excavated trenches, but
the connections into and out of the heat pump are generally
the same as for a normal boiler, requiring the same amount of
safety equipment to be fitted.
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6.
What if I don't have a big garden, or I don't want to dig it
up?
Some
form of external heat source is required for a heat pump. Usually
this is in the form of horizontal pipes laid out over a large
area of ground. However, if space is limited, you can have vertical
pipes inserted into a bore hole. A bore hole is a lot more expensive
and must be drilled to a depth of around 100 metres. This must
be done by a specialist drilling company. If you have ground
water available in the form of a lake, large pond or a stream,
then the external pipes can be submerged in the water to provide
the external heat. Even in winter, water can provide a usable
temperature. If none of these options are available, you can
have an air to water heat pump. This is a stand-alone unit which
extracts heat from the outside air and transfers this to the
heating in the same way as a normal heat pump. An air source
heat pump is not quite so efficient as a ground source heat
pump.
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7.
What
about servicing?
Heat
pumps generally do not need servicing, unlike a conventional
fossil fuelled boiler, which should be serviced annually. There
are very few moving parts to a heat pump, so not so much to
go wrong.
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8.
Where
do I find out about grants?
You
can get information from www.energysavingtrust.org.uk For households
or communities in Scotland, go to www.energysavingtrust.org.uk/scotland
where you'll find links whether you're a householder or a housing
professional. If you're a householder, look for the link to
the Scottish Community Housing Renewable Initiative. If you're
in England go to www.energysavingtrust.org.uk
for general information, or www.lowcarbonbuildings.org.uk
for information on what grants are available on which devices.
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9.
Will
I need a boiler as well?
Not
normally, but it depends on whether or not the heat pump is
big enough to cope with the total heating and hot water demand
from the building. This in turn is a factor of the size of the
building. In a well insulated building with very low levels
of heat loss, a heat pump will generally be able to keep up
with replacing the heat being lost to the outside, assuming
that the heat pump has been correctly sized. However, if the
heat pump is undersized for the size of the building, and/or
the building is not well insulated, for example, in an older
house, then the heat pump may not be able to keep up with replacing
the heat being lost. In this case, a secondary heat source,
such as an oil or gas boiler may be required.
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10.
Can
I couple it with another heat source?
Yes.
As in the case above, if the building is too big for the heat
pump to be able to keep up with replacing the heat being lost
to the outside, then a secondary boiler may be required. Coupling
a heat pump and boiler together is not difficult, but it does
require a bit of space to fit in all the plumbing required to
dock the two together. When a heat pump and boiler are docked
together, the boiler will only kick in when the heat pump is
unable to keep up with the demand. Back
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11.
I've
heard people talk about pay-back time; what is it?
Pay-back
time is the amount of time it takes for the heat pump to have
saved you the amount of money by reducing your fuel bills that
it cost you to install the heat pump in the first place. Be
careful with this. When people talk about pay-back time, they
often consider the full cost price and use this figure to illustrate
the point, but they fail to account for the cost you would have
incurred if you had fitted a conventional boiler. In the case
of an oil boiler, there is also the cost of an oil tank. By
the time you calculate all the costs and include the cost of
buying and fitting the conventional boiler and tank, plus the
fuel costs and service costs over the life of the boiler, there's
really not much in it. We never ever talk about pay-back times
to make a point in favour of heat pumps or underfloor heating.
These items are designed into the fabric of the building and
should last the lifetime of the building. Yet people still insist
on asking how long the payback time is. In reply we say that
you wouldn't consider the payback time of a fitted kitchen,
a flat screen TV or a new car, all of which will depreciate
immediately you buy them, and will be replaced some years down
the line with no consideration of the cost. An efficient heating
system has the added benefit of giving your property the edge
if you should ever come to sell it.
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12.
Can
I fit a heat pump to my existing house?
Yes,
but be careful. Heat pumps are at their most efficient when
running at an output temperature of between 25 - 35 deg C. As
with any boiler, they work by simply replacing the amount of
heat being lost to the outside in order to maintain the house
at a given temperature. In an existing house, especially an
older house, it is unlikely that the levels of insulation will
be sufficient to prevent the heat being lost from exceeding
the heat output capabilities of the heat pump on the coldest
of days. This, of course, is when you need an output temperature
in the range of 45 - 55 deg C (for underfloor heating) so the
heat pump would probably not be sufficient on its own. Back
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13.
If
it works like a fridge, can it have a cooling effect in summer?
Yes.
By including an additional piece of equipment to the heat pump,
surplus heat can be removed from the house and recycled into
the ground, where it will be stored until winter when it is
needed again to heat the house.
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14.
What does CoP mean and why is it important?
CoP
stands for Coefficient of Performance. This is stated as a number
between 1 and 5. In the simplest form it is the factor used
to express the heat output from the heat pump against the electrical
input used to run the heat pump. So if a heat pump has a CoP
of 4, this means that for every 1 kW of electrical energy used
to run it, there will be an equivalent output of 4 kW of heat
energy. The CoP is important because it is a quick way of comparing
one heat pump against another. Some heat pump manufacturers
are now claiming that they can achieve a CoP of over 5. However,
beware of extravagant claims. Watch out for the dreaded 'up
to' words. Remember that a heat pump is most efficient when
producing lower water temperatures in the range of 25 - 35 deg
C. This is OK if you only use the heat pump to run your underfloor
heating and the building is very well insulated, but as soon
as you ask the heat pump to heat the domestic hot water to a
temperature of 55 deg C then the CoP will fall away dramatically.
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