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Heat pumps Heat pumps do exactly what the name suggests, they pump heat from a relatively low grade source and turn it into higher grade, more usable heat for your heating or hot water system.  They just happen to be fantastically efficient at doing it, this makes them cheap to run and exceptionally good for the environment in reduced carbon emissions.  When we talk about low grade sources of heat, we generally mean air, the ground or water.  The reason we call these sources of energy low grade, is that the temperature of them is usually too low to be of any use for heating homes, offices, schools etc or for providing hot water, without the use of a heat pump.  Heat pumps utilise the refrigeration cycle, which is exactly what a standard fridge, freezer or air conditioning unit relies upon.  The two main types of heat pump we fit are air source and ground source.  Air source heat pumps are cheaper to fit, ground source heat pumps are cheaper to run.  The efficiency performance of a heat pump is quoted as its Coefficient Of Performance (COP) and as a general rule, air source heat pumps have a Cop of between 2.5 and 3 and ground source heat pumps have a COP of between 4 and 5.  As a rule of thumb, a modern condensing gas boiler has a COP of 0.9.  If a heat pump has a COP of 4, then for every kilowatt of electricity it uses to operate, you get 4 kilowatts of heat out into the heating system.  Heat pumps work best with low temperature heating equipment, such as under-floor heating or fan coil heaters. Heat sources for heat pumps: There are three main types of heat pump available for central heating and hot water supply.  Air to water, brine to water and water to water.  It therefore follows that there are three sources of heat for the heat pumps to extract.  These are air, water and the ground.  Air source heat pumps extract their heat from the external air.  Water source heat pumps extract their heat from a body of water, a large pond, lake or river.  And ground source heat pumps extract their heat from the soil, using an antifreeze mixture of water and ethylene glycol in sealed pipe work (this is often known in the industry as brine).   Air source heat pumps:  These are air to water heat pumps and they extract heat from the outside air and put it into the heating system water.  As their heat source is the outside air, then their efficiency is affected by how cold the air happens to be.  Now the sharp ones among you will have realised, that the time we want the most heat from our heat pump (winter), is when the outside air is generally at its most cool.  This is why air source heat pumps are less efficient than ground source heat pumps, as their COP is measured over the whole year, and the cooler the weather, the less efficient they are.  Many people think that an air source heat pump cannot operate when the air temperature is below freezing.  This is not the case, they can operate perfectly well with air temperatures down to –20° Celsius, they just get less and less efficient as the air gets cooler.  They are also affected by the humidity of the air, as if the air is very humid, and the heat pump is working very hard, then ice can form on the evaporative heat exchanger.  This has to be periodically defrosted which takes energy and also affects the COP of the heat pump.  Whenever we quote a COP for an air source heat pump, we always take into account these factors.  All heat pump manufacturers should quote the Cop of their unit at a given air temperature and a given flow temperature of the hot water going to the heating system Ground source heat pumps: These are known as brine to water heat pumps and they extract heat from the ground via pipes buried in it, either horizontally or vertically.  The liquid in these pipes is actually water mixed with anti freeze, it is often called brine, and the wetter the ground in which the pipes are buried, the better the heat pump works.  If you have room, then laying the ground source pipes in a horizontal grid is cheaper, but if the land available is limited, then the pipes can be fitted in vertical bore holes.  The holes are up to 100 metres deep and we use specialised contractors to bore the holes and install the pipes, known as probes.  The liquid in the ground loop is a mixture of water and ethylene glycol, which is an antifreeze solution.  The loop is normally protected against freezing down to about –15° Celsius.  It is quite normal in winter for the ground loop to run at a constant temperature close to or below 0° Celsius.  This is particularly true of horizontal collector ground loops, as they rely on heat in the ground which comes from the sun.  This is built up over the year by rainfall soaking into the ground, carrying solar energy with it.  In a horizontal collector the pipes are normally buried approximately 1.2 meters deep, as the temperature at this depth hardly varies throughout the year.  It is very important that the ground loop is correctly sized, as if it is too small, then the heat pump, which is really just a large refrigeration unit, can freeze the ground, and ultimately freeze the liquid in the ground loop.  Before it gets to that stage, though, the efficiency of the unit will drop off dramatically, because in the same way that the higher the source temperature the more efficiently the heat pump can run, then it follows, that the lower the source temperature, the less efficient the heat pump will become.  Also, permafrost in the garden is a bit of a problem, as it takes a long time to thaw out!   Vertically fitted ground loops are a better option, but obviously cost more.  The benefits are that because they normally go at least 50 metres deep, they usually pass through a water table.  This means that because of the movement of the water in the earth, there is more heat available for extraction by the heat pump.  This usually means that the ground loop is physically smaller if you fit vertical bore holes as opposed to a horizontal collector.  The other benefit of the vertical bore hole installation is that if the correct heating equipment is installed in the property to be heated, then the ground source can be used for passive cooling in the summer, if required.  The most important rule is; If you are spending the money to fit a ground loop for a heat pump, it might as well be done right first time.  We can calculate the required size of the ground collector, once we know the size of the heat pump installation.  As a rough ‘rule of thumb’ a 5 kW heat pump requires one 100 metre deep bore hole. Water source heat pumps: These extract heat directly from the source water, possibly a lake or river, and as a result are very efficient.  They require a suitable clean water supply, which could be from a bore hole.  The problem is, if the water supply has nay contamination at all it can muck up the operation of the heat pump.  Also if the water is being ‘lifted’, i.e. pumped from a source below the level of the heat pump, then the cost of running the lifting pump has to be taken into account when looking at the overall running cost, because the COP of a water to water heat pump is very impressive.  Generally, they work best if there is a large supply, such as a lake, above the level of the heat pump.  This would allow the water to be fed by gravity into the circuit through a suitable filtration system.  However, if large amounts of water are required for a big heat pump installation, then it may be necessary to obtain an extraction licence from the environment agency.  Usually, water to water heat pumps are only a suitable solution in a very small number of very specific situations. Coefficient Of Performance: This term relates to the efficiency of a heat pump unit, and is normally abbreviated to COP.  If a COP is quoted as 2.8, this means for every kilowatt of electricity the heat pump uses when it is operating, 2.8 kilowatts of heat is generated.  It seems too good to be true, doesn’t it?  It happens because of the refrigeration cycle within the heat pump, which uses the electrical energy to create heat, which is given up to the heating or hot water system.  The key thing when a COP is quoted is that the source and output temperature must be quoted, so if you want to compare the efficiency of different heat pumps, the same source and output temperatures must be used.  We are approved installers for Stiebel Eltron which is a German manufacturer that has been making heat pumps for over 25 years, as an example, they make an air source heat pump, romantically called the WPL13E.  This has a nominal output of 8.1 kilowatts.  This is at an air temperature of +2° Celsius, and an output temperature to the heating system of +35° Celsius. The COP at these temperatures is 3.4.  So to get 8.1 kW of heat from the unit, it would use 2.4 kW of electricity.   The operating range of the heat pump is –20° Celsius to +40° Celsius.  This temperature is only suitable for heating via fan coils or under-floor heating. It is obviously not suitable for hot water provision, however, the figures tell us that at the same source temperature, if the output temperature from the heat pump is +50° Celsius, then the COP drops to 2.7.  This then has implications for how the system is installed and controlled.  If a heat pump is fitted to an existing  heating system with traditional radiators, then the heat pump will probably need to run at an output temperature of at least +50° Celsius.  This would mean that the COP would be lower than if an energy efficient, low temperature form of heat emitter were used.  If the heat pump is fitted to say under-floor heating throughout the property, then the output temperature for heating can be far lower, improving the COP.  The controls would then operate the heat pump at a higher output temperature only for heating the domestic hot water supply.  It is possible to install a mixed system, part radiators, part under-floor heating, for example, as the control unit with the heat pump is capable of adapting to the system to which the heat pump is fitted.  The thing to remember is that a modern (high efficiency!) condensing boiler operates at a COP of around 0.9, i.e. for every kilowatt of gas you buy, about 90% gets into the heating system.  Until now, because gas is nearly one third the price of electricity, air source heat pumps have not been much cheaper to run than a condensing gas boiler.  However, from 2010, the government are introducing new lower electricity tariffs for electricity supplied to heat pumps, this is likely to halve the running cost of a heat pump compared to a gas boiler. Refrigeration Cycle: A heat pump operates using the refrigeration cycle.  This relies on the principal that when a gas is compressed, it heat up, this is the heat we get from a heat pump.  Inside the heat pump is a sealed circuit with refrigerant in it (often called Freon).  This is a liquid only below about –35° Celsius.  So the liquid is boiled by the heat source, whether that be air, water or the ground, via the evaporative heat exchanger.  The refrigerant gas is then compressed, and it is the compressor that uses most of the electricity in the heat pump.  As the gas is compressed, it heats up.  The gas is then cooled on the condensing heat exchanger, where the heat is transferred to the heating or hot water system and the refrigerant is condensed back into a liquid.  The pressure of this liquid is then reduced via a pressure reducing valve, the liquid is drawn back into the compressor and the cycle starts again. Under-floor heating & Fan coil heaters: These types of heating are particularly well suited to use with heat pumps because they are designed to operate at lower temperatures than traditional convector radiators.  If it is properly designed, under-floor heating is happy to operate at temperatures as low as 35° Celsius.  Under-floor heating can be ‘retro-fitted’, though this is often quite expensive, however in new properties, it is becoming a standard form of heating, especially on the ground floors.  If a ground source heat pump, with vertical bore holes for the ground source, is fitted to under-floor heating, it is possible in the summer to utilise the ground source to ‘passively’ cool the under-floor heating circuit. By passive cooling, we mean using a heat exchanger between the ground source circuit and the under-floor heating to lower the temperature of the water in the under-floor pipes.  This does not require active refrigeration, like in an air conditioning unit, which can be expensive to run.  In this country this form of cooling is generally sufficient, as we do not experience prolonged periods of very high temperatures most summers.  Because it relies on the temperature of the ground source, which is usually around 12° Celsius, the only running cost of operating it, is that of the two circulating pumps.  The heat pump controller is able to handle the operation, as it is important that the floors are not cooled too much, otherwise condensation may form, which is not ideal. Fan coils are a type of fan convector radiator that is designed to operate at temperatures of around 35° to 40° Celsius.  They can also be used for either active or passive cooling.  For passive cooling the heat pump needs to be a ground source with vertical bore holes. Active cooling is what air conditioning units do, they actively refrigerate the air in the room.  Some air source heat pumps are designed to operate as both heating and cooling units, which means in the summer, they can act as a central air conditioning unit.  This means that if they are connected to fan coils, or ceiling cassettes (like normal air conditioning units), they can actively refrigerate the air in the building, as long as the fans can cope with any condensation which may form.  The benefit of this type of system, is that a central air conditioning unit has a better COP (it is more efficient) than the more normal, individual split systems.  Also fan coils can often be more easily ‘retro-fitted’ than under-floor heating Condensing Boiler: This is the most efficient type of traditional boiler currently on the market.  They are more efficient than older boilers, because they recover more heat from the flue gases, this improves their efficiency to around 90% (equivalent to a COP of 0.9) We do not believe that we will be fitting many boilers in 5 or 10 years time as heat pumps and solar heating look set to replace them for efficiency and performance.  However, if you are interested in a replacement boiler either as an option or instead of a heat pump, we would be delighted to provide a quotation. Fridge: This is an appliance used for keeping important and delicate food stuffs, such as chocolate, wine or beer cool.  In just the same way that your fridge cools important produce that you put in it, by extracting heat from the goods inside and then putting that heat out of the back of the appliance (just feel the back of your fridge or freezer), so a heat pump takes heat from the source, cooling it down in the process, and puts it into your properties heating system, or hot water tank.
Ground source heat pumps - solar power (thermal) - Air source heat pumps - combined heat and power (CHP) - Solar power (PV) - under floor heating - Condensing boilers - Un-vented hot water
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