Heat Pumps; Making Use of Low Grade Heat

warm home with heat pumps

What are Heat Pumps?

Heat pumps are any device which transfers heat from one place to another. Regarding commercial and residential use, they move residual warmth in the air or ground from outside a building to inside. Heat pumps can extract low-grade heat needed to create useful heat transfer into the desired area. This can be from any source above -30°C, although they work better the higher the temperature of the source.

Traditionally ground and flowing water can be used. This is because they keep a mostly constant temperature all year round, no matter what the conditions. These methods of obtaining heat, however, require an increased initial investment. Faults are rare but can happen. The heat pipe system can be buried up to 100m underground so difficult to access if there is a fault. Also, if refrigerant were to leak into the waterways it could cause environmental damage.

Sourcing from the air

Air source heat pumps (ASHP) are becoming more widely used in the UK, as they are elsewhere in the world. They are essentially reversed air-conditioners. They are the easiest to install and the most cost-effective forms of heat pumps (circa £7000 as opposed to GSHP, circa £18-£23,000). This is because they do not require bore holes or underground systems with nearby waterways, lakes or ponds. They do function better in higher temperatures though and so one downfall of the ASHP is that air is the most seasonally diverse source. In winter, when heat is required the most, there is less available heat to be transferred. In Summer it is vice versa, (although they can be reversible and used to cool in the Summer). As a result, the seasonal performance factor (SPF) is 10-30% worse than ground source heat pumps (GSHP), especially on windy and cold days. The UK, however, has a pretty stable and agreeable range of temperature throughout the year, so they can operate at a high efficiently.


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Working in the UK

The UK’s temperature range normally sits between 0°C and 15°C, which increases the SPF compared to other countries. This is because constant temperatures can raise the efficiency of a heat pump, especially if it can stay above 0°C. Due to the intermittent nature of the air source, in the past, ASHPs have required a fossil fuel powered backup system. This was as an insurance against loss of a heating device for installed buildings. Modern ASHPs, however, do not require this and can operate efficiently to provide hot water and space heating for most reasonably well-insulated homes.

For residential properties, our single-phase supply has had very little investment from manufacturers in developing heat pump design, especially regarding large capacity heating. That was, however, until Kensa Heat pumps, based in the UK started to manufacture heat pumps. These pumps have a capacity up to 24kW for single phase electricity supplied homes. This is a lot larger than the previous common 12kW capacities. The larger output models, however, require a larger initial start-up input. Particularly in older properties, this can have side effects, such as lights flickering when they come on. Also, if a power supply is shared with others, this could become an installation obstacle, as the demand for, and on, these units is understandably very high.

“We need to stop living in poorly insulated, high heat-loss homes using radiators containing water at 70°C and start constructing super insulated buildings that are heated using water at 35°C, through under floor and wall heating systems”

Andy McCrea, Renewable Energy, 2013

Fitting and efficiency

Due to the fact that any heat pump circulates heated output at a temperature of around 35°C, the traditional radiators are inefficient at distributing the heat to the room. This is because it has a relatively small surface area. At these lower temperatures, larger radiators or a network of under floor heating is required to distribute the heat evenly and effectively. This fact means installation and retrofitting of heat pumps can be labour intensive and not suited to certain buildings (particularly listed buildings). They also require a better base level of insulation as they cannot allow for as large a heat loss as the conventional 70°C wet central heating systems.

A new construction can most efficiently use the pumped heat. This is done by designing heat outlet pipes in the core of any building (between walls of two rooms or floors of two levels). This also means there is less pipe network required as the rooms can use the same heat source. It is best paired with high thermal retaining materials and buildings. This allows for short spells of insulation reductions with minimal heat loss (windows opening etc.). This also again increases the surface area the heat must emit from. In existing buildings this may be prohibitive and more complicated to achieve. This may be due to any existing infrastructure, leading to expenses mounting and financial inviolability becoming a problem. Commonly, retrofits overcome this issue by using existing piping but fitting special air radiators which better circulate the lower level warmth produced by the heat pumps.

Benefits to current gas consumers of switching

If we assume, for example, that 1 kWh of electricity is 15p, and 1kWh of gas is 5p, heat pumps as an example, can be said to convert 1kWh into 3.5 (worked out using the COP, see below) units of useful heat. The traditional heating system works at a 1:1 ratio of 1kWh to 1 unit of heat. Therefore, the one unit of electricity which the heat pump would use, could actually become a more efficient method of heating the desired area, so long as insulation is enough.

To work out the coefficient of performance (COP), which is the formula used to assess the efficiency of the system in question, there is a simple formula:

Direct ground heat source pumps, (when the refrigerant is directly circulated into the ground), can work at up to 500%, or 5.0 efficiency levels at certain times of the year. On average, though, heat pumps will work at a 3.5 or 350% efficiency. This means that for every unit of energy put into the pump as electrical energy, 3.5 units of heat energy are put into the home. This is in comparison to 0.96, or 96% average rating of a gas boiler. This shows the potential benefit a gas consumer can have by converting to a heat pump (~250%~). The difference in actual heat output, however, may mean that a property wanting to swap between the two supplies, may have to insulate before installation to gain the full possible improvements.

A Consumer Guide to Heat Pumps in 2019

air source heat pump

There are different types of heat pumps, the main types being geothermal and air source heat pumps. To work, they transfer heat energy from a source such as the ground or air. Although heat pumps do require some power, they are still considered as clean because they don’t rely on fossil fuel being burnt, therefore reducing your carbon footprint and your energy bills.

Heat pumps work well in the UK as they can work during even the coldest of winters to provide heating to homes and businesses. They can be used to heat radiators, underfloor heating and warm the air directly using convectors. They can also heat water and reverse this process to produce cooling in summer months.

There are many benefits to heat pumps including:

  • high efficiency ratings
  • providing space and water heating and space cooling
  • substantially lowering energy bills
  • lowering carbon footprint

Are you really interested in getting a heat pump? We or one of the nations installers will offer free advice on the best options for you, either contact us directly or search for a local, trusted supplier HERE.

Types of heat pumps

Air source heat pumps

Air source heat pumps use heat from the air which they turn into a liquid which then passes through a compressor and the temperature is then raised. It then circulates this heat throughout the heating and hot water circuits in your home. Air source heat pumps are cheaper than others on the market. There are two types of heat pumps within the air source category:

Air to water heat pumps

This works better for things such as underfloor heating and large radiators as it works at a lower temperature, distributing heat throughout your wet central heating system. By working well at a lower temperature, this type of heat pump is more efficient.

Air to air heat pumps

This type of heat pump produces warm air which is circulated around your home using fans. Air to air pumps are usually only used for one function at a time and this tends to be electricity. They could also provide hot water but not at the same time.

Ground source heat pumps

Ground source heat pumps or geothermal heat pumps are used for hot water and warm air heating systems. They harness the heat from the ground using buried pipes. The heat is harvested and transported to a heat exchanger where it is then utilized in heating sources within the home. The different types include:

Open or Closed loop system

Predominantly a ground source or geothermal heat pump system is closed loop. A closed loop system uses a single tube connected to either end of the heat pump that loops out through the ground or a water source extracting residual warmth from the ground or the water source to feed the heat pump. An open loop system draws the water directly from a water source like a well, lake, river etc and pumps it directly through the heat pump system extracting the energy directly from the source.

Pond and standing well systems

In water that is deep enough, an open loop system can be used. The pipes will lie beneath the water at the bottom of the pond utilizing the constant temperatures down there. Standing well systems require a well to be dug and a pump system inserted. Water can then be pumped through the system and extract heat from the water before returning it to the well. These systems are cheaper than closed loop well systems.

Vertical ground source systems

These systems are generally used when less outside space is available, as a result, holes need to be drilled down vertically for the heat exchangers. Water can then be pumped through the inserted pipes and into the ground. The temperatures below ground are very constant and below a certain level so the water can be warmed and then extracted through a separate hole, the warm water being used to heat up a refrigerant liquid in the system. This can then be used for various heating systems within a house, such as radiators, underfloor heating etc.

Horizontal ground source

If your property has enough space around it, this space can be utilised to lay pipes in. The pipes will be laid in trenches 1 to 3m deep in a uniform and evenly spread pattern over the largest area available. This is the cheapest of the two heat pump options, but problems can arise in colder weather.

Alternatives to trenching

If having trenches dug in your garden doesn’t sound appealing or if you simply have less space, there is the option of radial or directional drilling. This means piping can be laid under the ground without the land being disturbed and which also doesn’t require much ground space. The cost of this is somewhere in the middle of the trenching and vertical drilling techniques.

Direct exchange geothermal heat pump

This Is a closed loop system which uses copper pipes placed in the ground to circulate refrigerant, the copper tubes exchanging heat with the earth. A water source is not necessary for this system. This kind of system is similar to, but more efficient than an air source heat pump.

What are the maintenance requirements of a ground source heat pump?

If installed correctly a quality system should last at least 20-30 years. They do require some minor maintenance, and this is essential for efficiency reasons, as a heat pump not working correctly can lose 25% of efficiency. Different systems will have different requirements but generally speaking, if you feel your system is working to specification, we recommend a full service every 2/3 years. However, if you suspect any issues or your system is heavily used, then annual check-ups are recommended. The warranty should last up to 3 years but there are other ways to insure your system and this should be researched to find the best protection.

RHI and Government financial incentives

The government does currently offer some incentives for heat pumps, the first being for domestic use heat pumps. Domestic Renewable Heat Incentive means you will receive money for every kWh of heat/energy you generate, please see below for current rates;

There is a similar incentive for business and industry purposes, but they do differ and are based on the installation.

Planning permission is not usually required for heat pumps, however there are some requirements that have to be met.

Limits to be met:

  • Development is permitted only if the air source heat pump installation complies with the Microgeneration Certification Scheme Planning Standards (MCS 020) or equivalent standards. Read more about the scheme.
  • The volume of the air source heat pump’s outdoor compressor unit (including housing) must not exceed 0.6 cubic meters.
  • Only the first installation of an air source heat pump would be permitted development, and only if there is no existing wind turbine on a building or within the curtilage of that property. Additional wind turbines or air source heat pumps at the same property requires an application for planning permission.
  • All parts of the air source heat pump must be at least one meter from the property boundary.
  • Installations on pitched roofs are not permitted development. If installed on a flat roof all parts of the air source heat pump must be at least one meter from the external edge of that roof.
  • Permitted development rights do not apply for installations within the curtilage of a Listed Building or within a site designated as a Scheduled Monument.
  • On land within a Conservation Area or World Heritage Site the air source heat pump must not be installed on a wall or roof which fronts a highway or be nearer to any highway which bounds the property than any part of the building.
  • On land that is not within a Conservation Area or World Heritage Site, the air source heat pump must not be installed on a wall if that wall fronts a highway and any part of that wall is above the level of the ground story.

In addition, the following conditions must be met. The air source heat pump must be:

  • used solely for heating purposes.
  • removed as soon as reasonably practicable when it is no longer needed for microgeneration.
  • sited, so far as is practicable, to minimise its effect on the external appearance of the building and its effect on the amenity of the area.

Rules for Scotland and wales again vary slightly, but a local installer will be well informed as to the process for getting subsidies and will often help you with the process. Search here for installers in your area.



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