Homeowner's Guide to Heat Pumps



So what is a Heat Pump?



Air Source Heat Pump

Air Source Heat Pumps (ASHP) use the ambient temperature of air, drawn through a fan unit to extract the heat from the atmosphere to expand the compressed refrigerant gas. They can be a ‘monobloc’ in that the heat pump and fan evaporator are housed in single unit, or a split system where the compressor and heat exchange is in an inside unit and the fan unit outside.


Ground Source Heat Pump

Ground Source heat pumps (GSHP) use the more stable temperature of the earth at around 1m depth to extract the heat from. Pipes, usually containing the glycol are sunk horizontally underground and extract the heat from the soil. As the earth 1m below ground rarely falls below 8°C the performance is more stable and predictable all year round. They require a lot of land to be effective.


GeoThermal Heat Pump (GTHP)

Often confused with GSHP because they too use the earth as the heat source, but in this case a series of bore holes are sunk vertically to much greater depths; sometimes as much as 100m. Much higher temperatures can be found the deeper you go, but the installation costs can be much greater then either ASHP or GSHP.


Air to Air Hot Water

Similar in principle to an ASHP but extract heat from the air and use it to heat the air (or cool if reverse cycled). Usually found in smaller properties and often in hotels and apartments for example. They are effective but it does look like you have an A3 printer on the wall of each room...

Is it right for me?


The Government has indicated that gas boilers will be phased out in the next 10~15 years and all properties will be using heat pumps, with 600,000 being fitted every year, so in theory, you won’t have a choice.

However, in reality, a Heat Pump cannot replace a gas boiler in every instance, and the gas energy suppliers and boiler manufacturers are starting to push back highlighting some of the practical difficulties of the Governments position. Hydrogen and bio-gas are just some of the alternative technologies being proposed to supplement or replace methane or natural gas in the network. The result is likely to be a mix of technologies (known in Political speak as a ‘Fudge’) and so you should consider what’s best for you and your property. Here are some examples for you to consider:


New build detached property in a rural/off-gas area

In essence, this is the holy grail of heat pump applications and where the technology first found a foothold. If the options are between oil, LPG , biomass or direct electric heating, the convenience as well as the grants and RHI (Renewable Heat Incentive) available to a new build home owner for heat pumps make this a ‘no-brainer’.


New Build property in an Urban Environment

A more nuanced decision given the availability and cost of gas. The higher insulation levels of a new build will make a heat pump £ for £ on a par with a gas boiler, although servicing and maintenance might be higher. If you have any grants or RHI available to you, this will tip the balance in an ASHP or GSHP’s favour


Renovation or Refurbishment

This will depend on a) the level of insulation that can practically be achieved and b) the availability of mains gas. Older properties do not have the inherent insulation levels to make a heat pump a viable and economical proposition without remediation. Long abandoned building techniques such as solid outer walls, uninsulated solid asphalt or suspended timber ground floors, single pane glazing all need attention if the target heat losses of under 50w/m2 are to be achieved. Other considerations such as existing pipework means that a careful costing should be undertaken first – especially if a gas boiler is a viable option.


High rise apartments and terraces

Even ASHP require some outside space and good air circulation to facilitate the fan unit, and an internal cylinder to store the hot water. Not all small properties have been designed with this is mind, and other considerations such as access for maintenance, noise pollution and routing pipework into and through the property may be problematic.

Which Should I Have?


Assuming the previous considerations have confirmed the suitability of your property for a heat pump, then you will need to consider the environment around and about your property.

Air Source Heat Pump

most suited to semi-detached and detached properties as they will require some external space to locate the fan unit and be ground or lowly wall mounted for ease of access and servicing. They require at least a metre of clearance and should not be within 1 metre of a boundary or under trees or plants as this can affect their performance

Typical installed cost: £10~£14K


Ground Source Heat Pump

Requires at least 250m2 of garden area for the coils of pipes to absorb enough heat from the ground, and often 500m2 for larger properties. It is important that there is sufficient earth to provide enough heat for the property. If too much heat is extracted from too small an area, then the ground can actually develop a permafrost which will severely deteriorate the performance until the ground thaws out.

Typical installed cost £12~£18K

GeoThermal Heat Pump (GTHP)

Given the depths of the bore holes, very often permissions and assessments may be required to avoid drilling into under ground water courses, mine workings and other subterranean hazards. With the cost of a bore hole typically £30~£50 per metre, and generally 3 x 100m bore holes required, costs of £10~15K for the bore holes is not uncommon.

Typical installed Cost £20~£25K


Air to Air Hot Water

Generally for a single room or small dwelling they extract the air from within the property and induct air from outside. These can often provide a heating and cooling function (like an air-con unit) and can substitute for a mechanical ventilation and heat recovery unit (MVHR). Often combined on top of water cylinder, they can remove warm stale air from bathrooms and kitchens, extract the heat through the heat pump cycle and then reintroduce fresh air into the property.

Typical installed cost; £4~£6k each.

What Else is Required?


As well as being well insulated and have space for a hot water cylinder of at least 150L, there are other considerations when retrofitting a heat pump into a property where previously a high grade heat source such as a gas or oil boiler or direct electric heating has been used



Many properties built in the last 40 years have used pipework of 15mm Ø and even 10mm to radiators (a few even have 8mm microbore). In order to heat the property using low grade heat from a heat pump the flow rate must increase to provide the same level of heating.

Typical installed cost; £4~£6k each.


The formula for calculating the volume of hot water required to heat a room is Q = M x C x (dT)
where Q is the heat load of the room (in Kw), M is the mass/volume of the water, C is the specific heat capacity of water (4.2) and dT is the difference between the room temperature and the average water temperature of the heating system (and thus the radiators).

If a room has a heat load of 1kw (1,000w), a desired room temperature of 20°C, and the water temperature is 70°C for a boiler and 45°C for a heat pump you can soon see that the dT is halved (70°-20° = 50°, and 45° – 25° = 25°). Therefore, it is easy to see that the Mass/Volume of the water must double to achieve the same output of 1kw.


This means that whilst a boiler heated property can comfortably deliver enough heat using 22mm main feed pipes and 15mm or 10mm tails to each radiator, when using a heat pump, the pipe sizes need to increase to 28mm (or even 35mm) for the main feed and return and 22mm for as far as practically possible to the radiators or heat emitters (the last ½ metre could be reduced to 15mm for ease of final connection).

The alternative is to double the flow rate through the existing pipes by using additional pumps or a low loss header (don’t ask!) which can introduce other issues such as noise and vibration as well as additional cost.

In summary its not always necessary to change the pipework in your home if retrofitting a heat pump, but it must be considered and calculated.

Do I Need to Replace my Rads?


As discussed, if the flow temperature is reduced and thus the flow rate needs to be increased, then likewise the ability of the emitter to radiate that heat also needs to be sized accordingly.



This may mean the radiators increasing in size by as much as double, although many radiators are already oversized by up to 25% (the typical plumbers’ factor of safety) so it’s worth doing the maths first. It is of course possible to save some money on replacement radiators by moving larger radiators from, say, the living room into a bedroom and then only replacing the living room with a larger one etc. As radiators are really convectors and thus rely on hot air rising, it might be beneficial to fit 2 normal sized radiators rather than one large one to compensate for the reduced heat distribution caused by operating at low surface temperatures.


Fan Assisted Radiators

These include an internal fan unit to extract more heat from the hot water by blowing air across a heat exchanger. They will require a new and separate electrical connection to provide the power (they should not really be plugged into a socket outlet as they are a fixed heating appliance for Building Control purposes) and a suitable sturdy wall to fix to.


Skirting Heating

This replaces the traditional skirting boards with a radiant panel disguised to look like one, and can provide the larger surface area associated with installing an oversized radiator as well the wider heat distribution typical of underfloor heating.


Underfloor Heating

This is often installed with heat pumps as it naturally operates at lower flow temperature and has a large surface area. Retrofitting to existing properties can be difficult and often new floors are required to provide the necessary insulation underneath and the choice of floor coverings can be limited to tiles and hard surfaces if performance is not to be degraded by carpets and wood. Overlaid systems may require doors to be planed down and skirting boards to be removed in any event and so larger radiators or skirting heating may prove to be more cost effective.


How Do I Maximise my System Efficiency?


If you are going to the expense of installing a heat pump, it would be a false economy to penny pinch on the controls.

As heat pumps often have to operate 24/7 during the colder months to keep a house warm, it is important that heat is preserved wherever possible to avoid unnecessary expense. Ideally, smart heating controls with time and temperature and heat pump interface should be employed to ensure that only the rooms that require heat are heated and at the appropriate time (for example bedrooms OFF in the daytime).


Ensuring that your thermal store of hot water is not wastefully discharged when not needed by closely controlling each room can enable the heat pump to operate when it is most efficient to do so; typically during the day rather than at night. This is especially important for an ASHP that extracts heat from the ambient air and therefore its efficiency deteriorates sharply when the night time air is very cold.

Skirting Heating has a lower water volume than oversized radiators and UFH and so can form part of a balanced approach to maintaining cylinder temperature.


Is My System Operating Properly?




Your system needs to be properly pressurised to work efficiently. This is typically 1.5~ 2.2 bar on the boiler pressure gauge (usually indicated by the green section on the gauge).
If your system continuously needs topping up then you may have a leak somewhere on the system, although it will usually go into fault mode if the pressure drops below 0.5bar.
If you have a leak on a radiator or skirting heater, these are very easy to find and remedy- usually it will be where the pipework connects onto the radiator panel and simple to get at.

Leaks in pipework or underfloor heating buried in the floors or walls can be very destructive over time and so need to be addressed quickly to avoid more significant repair costs later. Specialist companies are available to trace leaks in the floors or walls


Skirting Heating is completely above ground meaning it is completely serviceable. Any unlikely leaks can be spotted immediately....



It is common in areas with high levels of calcium carbonate dissolved into the water, to use a water filter or softener to reduce limescale and improve the taste. Under no circumstances should chemically softened water be plumbed into the central heating system. Whilst limescale can build up in appliances that are constantly being refilled (such as kettles, baths etc) the water in the central heating is a ‘closed’ system and apart from the occasional top up (see above) the calcium carbonate levels are static. As such limescale build up is minimal in closed loop systems especially if plastic pipe has been used. Introducing chemically softened water can damage boilers, heat exchangers, aluminium radiators etc. and should be avoided at all costs. Mechanical or centrifugal filters do not present such a hazard, but double check the suitability of your heating system to accept softened water. If in doubt, treat only the domestic potable water.

You're Ready!


  • Heat Pumps are an excellent alternative to oil & LPG but more nuanced against mains Gas on new build projects.
  • Insulation levels to achieve heat losses of less than 50w/m2 should be the goal to make heat pumps viable in retrofit applications.
  • Pipe work usually needs to be upsized as well as the radiators to achieve suitable flow rates and emitter outputs
  • UFH under hard surfaces provide the necessary output at lower temperatures but may struggle under wood or heavy carpets
  • Oversized or fan assisted radiators as well as skirting heating can provide a simpler solution on refurbishment and retrofit projects
  • If replacing a combination boiler, space must be found for a storage cylinder or thermal store.

DiscreteHeat can assist with your understanding of your heating requirements, and provide system design to suit your heat pump system. We will provide impartial advise on what is right for you.

ThermaSkirt- The Skirting Board That Heats your home is the perfect partner for heat pumps in both new build and refurbishment projects...



DiscreteHeat Company Limited
1 Victoria Works Industrial Estate
Coal Pit Lane, Atherton
Manchester, M46 0FY

 01942 88 00 60

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