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Heat Pumps

Heat Pumps

Heat Pumps and heat recovery systems explained

>>> Go Ground Source Heat Pumps <<<        >>> Go Air Source Heat Pumps <<<

What is a heat pump?

The most familiar form of heat pump is the domestic refrigerator. The refrigeration system extracts heat from the refrigerator cabinet and expels it at the back or underneath the unit as warm air into the room. For a refrigerator heat is a waste product.

In a heat pump system the process is reversed, collecting warmth and expelling cold air - which is, for obvious reason regarded as the waste product and dumped to atmosphere outside. By collecting the warmth we harvest it can be utilised as a heat source and put to work warming things that require a heat source.

For example, if we were to place the radiator grill located on the back of your fridge in a vessel of water it would impart heat to the vessel and eventually you would have warm water.

Diagram illustrating how a heat pump works
A Fridge - the common example of a Heat Pump

Heat recovery systems are designed to use less energy during the process of recovery than they actually recover, so for intents and purposes you are gaining FREE energy.

Heat may be recovered from various sources, but broadly speaking the most popular are Air source, Ground Source and Water source. Recovery systems (referred to as heat pumps) are units in which the heat gathering process is conducted.

Air Source heat pumps (View products)

Air Source heat recovery pump

You may have deduced from the description above that the heat transfer process is reversible. So, a heat pump can be used to refrigerate or to warm depending on which way the designated waste product is expelled. Indeed many air conditioning units can be utilised to provide cool air when it's hot and a degree of heating when it's cold.

In the main, air conditioning units cool air to provide a pleasant climate during hot weather and heat pumps recover energy to be utilised as a heat source.

Airborne energy is of course most abundant when the climate is hot and inversely so during cold weather. For this reason air source recovery is not ideal for heating when demand may be at it greatest and the air is at its coldest.

Ground source heat systems (see below) are a little less susceptible to climate and on average are believed to be around 20% more efficient. However, it should be said there are many factors affecting the efficiency of a heat pump system.

At lower air temperatures there is a propensity for ice to build on the heat exchangers of air source pumps. This of course adversely affects performance so anti-icing facilities are necessary. While this reduces overall efficiency the reduction is generally less on more expensive units that employ more sophisticated and energy efficient methods to deal with the issue.

Ground Source heat pumps (View products)

Ground source heat pumps recover heat stored in the ground. Sometimes referred to a 'geothermal energy' this rather grand term is a little misleading as heat collected at a fairly shallow depth is more likely to be the product of heat from the surface (i.e. solar) rather than from the earth's core. True geothermal energy can be found and is exploited commercially is several parts of the world (generally where volcanic action is evident near the surface). In the UK the process of harvesting enough geothermal energy to make it commercially viable requires boring deep into the ground. Such a commercial enterprise is in fact operated in a district in the City of Southampton where the discovery of warm water springs was made almost by accident while test boring for oil several years ago.

Domestic Ground Source heat recovery systems operate at much more shallow depths. At depths of over 2 metres the ambient average temperature deviates little all year round. This of course will vary slightly in the UK depending on location. At this depth there is an abundant amount of energy that can be collected and utilised for heating during winter months.

Ground Source heat recovery pump
Coils of pipe laid in a trench absorb ground source heat.

Subterranean pipe run

The simplest way to extract ground source energy is via a large length of pipe buried horizontally in the ground in trenches between 1 and 2 metres deep.

Fluid passing through the pipe absorbs heat which is processed by the Ground Source heat pump station.

Vertical bore holes may also be used where land is less abundant but this is generally a more expensive method and requires good access for the machinery necessary to bore into the ground.

Click here to view our range of Ground Source Heat Pumps

Water Source heat pumps

Where there is water there is often thermal energy. Rivers and streams can be utilised but are not overly popular due to regulatory constraints and lack of suitable equipment. Lower winter water temperature can lead to system failure just when you need it most.

Water springs can also be used and provide a more stable heat source and high efficiency. However, impurities and acidities can sometime negate this as a viable option. Whilst there are ways around these problems additional equipment may reduce operational efficiency and affect perfromance. A licence from relevant Water Authorities may be necessary.

Output from a heat pump

A heat pump will typically deliver heat as hot water. Conveniently, this is how many conventional household heating systems work. (For convenience we will confine our considerations to 'wet' heating systems throughout the following.)

Within a typical dwelling in the UK one can usually define two distinct functions of a 'wet' household heating system.

1. Space heating using heat pumps.

When heat for the entire property emanates from a single or central location and is distributed via a network of pipes we tend to call it 'central heating'. The pipework is a means of transporting warm water which in turn heats areas of dwelling by means of convection, conduction and/or radiation. Methods may include radiators, warm air blowers (e.g. Myson hydronic plinth heaters) or Underfloor Heating systems. For further discussion please refer to a later section on this page.

Heat pump system schematic diagram

2. Domestic hot water (DHW) using a heat pump

The water you see appearing from your hot water taps is referred to as Domestic Hot Water and abbreviated to DHW. It differs from say the hot water circulating around your radiator system or through your boiler's heat exchanger which is often contaminated with inhibitor and waterborne debris and sometime referred to as 'primary hot water'. DHW can be produced by various methods.

  1. Combination (Combi) Boiler. It is necessary to acknowledge this method of heating although Combi systems have little or no interaction with heat pumps. In a Combi system hot water is produced instantaneously by heating on demand and delivered directly to whatever outlet might be calling. Whilst this method has the advantage of space saving (because there is little or no storage required) the hot water delivery rate is often frustratingly slow.

  2. Vented or unvented 'Indirect' hot water cylinder. By this method a volume of water contained within a cylinder is heated by a remote heat source using a heat exchanger (coil) through which hot water from the heat source is passed. (An 'Indirect' cylinder may be fitted with immersion heater/s, but a cylinder heated exclusively by electrical immersion heater/s is deemed to be 'Direct'). Indirect and Direct hot water cylinders may be vented or unvented by design. A vented cylinder is, as the description suggests, vented to atmosphere and only ever subjected to the pressure gravity from the water feeding into it from a header tank. Vented hot water cylinders typically only supply a low pressure 'gravity' feed of hot water. An unvented cylinder is a pressure vessel fed with water directly from a mains supply. Generally regarded as the high performance option because of higher hot water output pressure, they are only as good as the supply pressure to them and bring with them a raft of conditions and regulations as they are potentially dangerous if incorrectly installed or not serviced. A cylinder constructed for use with a gravity supply must never be connected directly to a mains pressure supply as the risk of explosion exists. (Gledhill Unvented Heat Pump Cylinders) (Newark Vented & Unvented Heat Pump Cylinders)

  3. Thermal Store cylinder. Using a simple bit of reverse logic a cylinder with multiple capabilities was born. Instead of heating a volume of clean water contained within a vented cylinder by a remote heat source via a heat exchanger, a Thermal Store uses the water contained within a cylinder as a medium in which thermal energy may be stored (hence the title). (See Heat Pump Thermal Stores)

Thermal stores can produce mains pressure hot water and a reservoir from which space heating (UFH or Rads) may draw their hot water.

By immersing a heat exchanger within the cylinder through which mains water is passed a rapid heat transfer takes place whereby the incoming cold mains water becomes hot water at mains pressure which is then delivered to taps and outlets.

A circuit of water for space heating can be drawn directly from the cylinder - in effect stealing the preheated water.

One of the beauties of a Thermal Store is the fact that the large volume of hot water in the store (that features as a concern with an unvented cylinder) remains at atmospheric pressure, only the significantly small amount contained within the mains water coil is at pressure at any given moment. For this reason a Thermal Store is regarded as the 'safe' way to achieve mains pressure hot water as it utilises an inherently safe vented store of water.


Additional considerations for the type of cylinder to use.

Heat Sources

There are certain heat sources that are incompatible with certain types of cylinder. The cylinder type that gives the cause for most concern is the 'unvented' type.

More about unvented hot water cylinders

An unvented hot water cylinder is essentially a pressure vessel in which water is heated, then by virtue of pressure maintained at the inlet, contents are forced out at mains pressure whenever an outlet calls for hot water. The problem with unvented cylinders is the risk of explosion if an 'over temperature or over pressure' scenario were to go unchecked. For this reason pressure relief equipment must be fitted to an unvented cylinder. Other notable issues are G3 Building Regulations for these devices and the fact that ONLY controllable heat sources may be connected. This precludes the use of such things as wood burning stoves, AGA's , Rayburns, etc as they cannot generally regulate their output.

Solar systems, heat pumps, gas and oil boilers are normally controllable and therefore safe to use with unvented hot water cylinders and may be used in combination. (see Multi coil unvented cylinders)

Using 'uncontrollable' heat sources

If however you wish to use a heat source deemed 'uncontrollable' such as a wood burner you may only do so with a vented cylinder (being vented there is zero risk of explosion). As referred to earlier in this section, there are essentially two families of vented cylinder -

1. An open vented gravity hot water cylinder (for gravity hot water), and

2. A thermal store (for mains pressure hot water from vented cylinder).

Thermal stores are considered attractive because of their ability to deliver mains pressure hot water while remaining open vented, being able to accept heat from multiple heat sources simultaneously (including those deemed uncontrollable) and being able to supply stored energy directly to a wet heating system (rads or UFH). In short, any and all heat sources contribute to any and all outputs. Quite unique! (See Heat Pump Thermal Stores and Thermal Stores / Solar Thermal Stores)

A vented gravity cylinder by contrast can be configured to accept different heat sources by the inclusion of several heat exchanger coils (multi coil cylinders) but can only ever deliver gravity pressure hot water. Extraction for heating purposes is not an option. (See multi coil cylinders , Stainless Lite Vented and Heat Pump Cylinders)



For more information on the various cylinder types compatible with heat pumps you might like to follow these links:

Gravity open vented Heat Pump Cylinders




Mains pressure unvented Heat Pump Cylinders






Mains pressure Thermal Store Heat Pump models





Buffer Stores for use with Heat Pumps




Further discussion on the output of Heat Pumps

To appreciate the operational interaction of Heat Pumps as a heat source for space heating or hot water you must consider the temperature to which the heat pump can deliver water.

Remember, the achievable output temperature from a heat pump is somewhat cooler than one might expect from a gas boiler for example. While a gas boiler might be able to heat water to 80oC a heat pump might only be able to achieve say 40oC. This of course impacts on how well the output may be utilised.

If you are heating a cylinder of water For example, it is essential that the heat exchange mechanism is highly efficient so that as much energy recovered by the heat pump is deposited in the cylinder. For this reason good quality Heat Pump Cylinders my employ a duplex heat exchanger and/or a radial finned tubing thereby greatly increasing surface area and maximising heat transfer. There are certain regulations and guidelines about cylinders heated by heat pumps requiring the stored water temperature to be raised periodically in order to prevent the growth of Legionella bacteria. In case a heat pump is unable to lift water temperature to the required level (circa 60oC) it is necessary to do so by the use of an electrical immersion heater. Use of electric heaters is of course expensive and flies in the face of attempts to save energy and money by using heat pumps. However, when all the figures are examined an overall saving in cost will be enjoyed.

If you wish to use a heat pump to warm a property you must consider that the output water temperature will be way below that of a conventional boiler. If it is your intention to use wall hung radiators it would be wise to up-size or increase their number within each area you wish to heat - replacing existing ones if necessary. The lower temperature of the circulating water will thereby have a better chance of dissipating into the room. It is also advisable to use a buffer store and/or a thermal store with a radiator system to reduce pump cycling and ensure the most efficient operation of the source heat pump.

Underfloor heating is considered to be an excellent system type for use with heat pumps. As underfloor heating operates at a lower temperature and over a much greater area (the entire floor of a room) a heat pump can function within the temperature range at which it operates most efficiently.

Ensure you do the calculations. You will still need to match the capabilities of your heat pump to your expectations of service.


You are welcome to call if you would like to discuss these matters further.

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