Where the heat comes from.

There are various types of heat sources which broadly fall into the following categories:-

Ground source
Water source
Air source

(How we utilise the heat from the above is covered further down the page)

Ground source is attracting greatest interest at present since it promises to deliver the best year-round energy-efficiency. The ground source heat pump (gshp) extracts heat stored in the ground. This is sometimes referred to as “geothermal”, however, the heat is mostly from the surface (solar gain), and very little is from the earth’s core. (True geothermal is found in Iceland, Japan and even Southampton).
At depths of 2m and more, the ground temperature does not deviate very much from the average summer/winter surface temperatures (around 9 -12C in the UK depending on location). At this depth, there is an enormous store of heat that can be usefully tapped for heating in the winter.
The most practical way of extracting this energy is to bury a large amount of pipe in the ground. This is usually laid in horizontal trenches at 1.5 – 2m depth, but vertical bore-holes are an alternative method giving similar results.
Click here for further information on ground source heat pumps.

Water source
A river or small stream can be utilised, and, in the past systems using copper coils in the water have been used. However, rules and regulations and the lack of available manufactured equipment have made this type of system less attractive.

Pumping river water through a heat pump is another option, and can give very good results, but heat pump units require water at temperatures above 5 to 8°C (varying depending on type).
Whilst delivering very high-efficiencies for much of the time, this system will fail to operate in the middle of winter during lower temperatures- just when you need the most heat, so a back-up heat source will be needed.
Oxygen and contaminants in the river water may also be a concern in some circumstances, causing pump failure and possibly a system refrigerant leak. But don't let me put you off, this system can give excellent results if installed correctly.

For those lucky enough to have a spring, this is a much more stable and better heat source. Its an opportunity not to be overlooked, offering excellent efficiencies. Again, acidity and impurities in the water can sometimes make its use prohibitive (see glossary re water purity). We have successfully used plastic heat-exchangers in such circumstances.

The water source should ideally be fairly close to the property, and should not require pumping up any significant height or the power for pumping it may detract from the energy savings. Having said that, water could be taken over considerable distances if the pipe diameter is big enough, especially in a downward direction. It is a relatively simple exercise to calculate the pump power required (if any) to get the water to and through the heat pump unit.

Permission should be sought from the relevant authorities as an abstraction licence may be needed.

Air source is probably the most common type of heat pump. Many of the roof-top air-conditioners that you see on offices and homes are reversible, and can operate in heating mode as well as cooling, but these are usually optimised for cooling mode. Non-reversible air source types for heating-only are usually much more energy-efficient, but the build quality and efficiency will vary greatly.. you only get what you pay for.

The air source system however will be less effective for heating in winter since the air temperature fluctuates and can become very cold. Furthermore, at low air temperatures, ice will form on the heat-exchanger requiring a mechanism of reverse-heating to melt the ice. This process is not as wasteful as maybe first thought, but it still contributes to a reduction in efficiency.
Air source units are being improved all the time, and are a simpler and cheaper option to install. But there is no way getting around the fact that the air is coldest when you need most heat for the house.
A back-up heater in the form of a conventional electric heater is usually included within the heat pump package. This is far more necessary for air source systems, and usually controlled automatically.

Exhaust air heat-recovery heat pumps take their heat from within a building. Such systems can help in buildings with damp problems. However, the electricity consumption of such heat pumps can be considerable. Such systems are common in countries like Sweden where electrical heating is more common, in-part due to their cleaner electricity generating network which has a large hydro-power input.
In new-build houses, such systems can work well if designed well, but there is always a danger that badly installed systems increase the demand on the houses heating system. i.e. ventilation is in excess of requirements, and the boiler load goes up.

Passive heat recovery has improved greatly in recent years, and promises much better solution for ventilation and damp problems. They now can recover 90% of the heat from exhaust air simply using 2 small low-power DC fans. These fans can consume as little as 50watts which is only the power of a light bulb. It is very hard to beat the simplicity and energy efficiency of such passive (non-heat pump) systems.

Very low energy 'Passive house' designs use a combination of passive heat recovery and a heat pump. This is appropriate where heat demand is very low.

If any ventilation recovery system is used, then the house needs to be air-tight, otherwise too much air is lost through natural ventilation to make these systems very energy efficient.

HYDRO-POWERED HEAT PUMP SYSTEMS
The mechanical power from a water turbine or wheel is usually used to generate electricity. A heat pump can extract energy from the same water to produce a heat-energy output of three or more times that of a conventional hydro-electric system. Since space-heating is usually the biggest single energy load, it is sometimes better to put all the hydro energy into a heat pump, whilst remaining grid-connected for general electrical needs. The heat pump can be driven directly by a mechanical belt-drive etc. However, this system requires a lot of maintenance. An electric drive heat pump driven from a hydro-electric source is probably the most practical solution, since electric compressors are now so well developed and significantly more efficient than any belt-drive design available.

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How is the heat delivered to the building?
Heat pumps usually deliver heat in the form of hot water, as do most conventional central-heating systems. However, to maintain a high energy-efficiency, the emitter system should be designed so that the water temperature is as low as possible. Ideally, a well-designed underfloor heating system should be used. Such systems are energy efficient and very comfortable. Radiators may be the only alternative, but should be significantly larger in area than normal. Several radiators in one room is advantageous.
Ducted hot air is an alternative method of distributing heat into a building, however, this is often not as comfortable as radiant (underfloor) heating and should be installed with caution. It is particularly undesirable in badly insulated buildings.

COOLING.

In summer, buildings can overheat. The main cause is often sunlight (solar gain). One square metre of sunlight through a window can contribute almost 1kW of heat to the room. This is the same as the heat from a small electric room heater. The solar heat often falls on the floor, and therefore heats the room. Air-to-air systems (air-conditioners), are used throughout the world for air cooling. Their energy consumption is significant, so they are to be avoided if possible. It is far more energy-responsible to reduce the solar gain in the first place by shading the sun. There are many ways to minimise heat build-up in houses including limiting the heat sources and good ventilation. Older buildings that have a high thermal mass tend to have less problems. It is perfectly possible to design modern buildings that keep sufficiently cool without the need for air-conditioning.

If air-conditioning is deemed to be necessary, then a water or ground-coupled heat pump system will be the most energy efficient. This type rejects the heat to the ground coil or borehole.

If the liquid in the ground source is pumped directly around the underfloor heating pipes, then a certain amount of ‘free’ cooling can result. This is known as 'Passive' cooling. It will have a limited effect, and will only work with a borehole, or with a very good ground collector with moving ground water . So consider this option with caution. But if coupled with good housekeeping as outlined above, it can help to curb excessive internal temperatures with minimal use of energy.