Start With the Simplest Explanation

 

Think of a heat pump as having two sides.

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The first side is where the heat comes from.

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That might be:

• Outside air

• The ground

• A lake, river or other body of water

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The second side is where the heat goes.

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That might be:

• Water flowing through radiators or underfloor heating

• Warm air supplied directly into rooms

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Once you understand those two things, the terminology becomes much easier.

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What Is an Air Source Heat Pump?

An air source heat pump takes heat from the outside air.

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Even when the weather feels cold, the outside air still contains thermal energy that a heat pump can extract.

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The outdoor unit absorbs that heat and uses a refrigeration cycle to increase its temperature.

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That heat is then transferred into the home.

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Air source heat pumps are generally the most common type considered for ordinary houses because they do not require large areas of land or underground pipework.

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What Is an Air to Water Heat Pump?

 

This is where the terminology often causes confusion.

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An air to water heat pump is a type of air source heat pump.

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It takes heat from the outside air and transfers that heat into water.

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That heated water can then supply:

• Radiators

• Underfloor heating

• A hot water cylinder

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So when somebody talks about installing an air source heat pump in a typical UK home, they will very often be talking about an air to water system.

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The source is air.

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The heat is delivered through water.

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Air Source Does Not Mean It Blows Warm Air

 

This is a very common misunderstanding.

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An air source heat pump does not necessarily mean that warm air is blown into the rooms.

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Most domestic air source systems proposed as replacements for gas boilers are air to water systems.

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They heat water which then circulates around the central heating system.

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The outdoor unit may look a little like an air conditioning unit, but inside the house the heating can still come from radiators or underfloor heating.

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What Is an Air to Air Heat Pump?

 

An air to air heat pump also takes heat from the outside air.

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The difference is that instead of heating water, it delivers warm air directly into the rooms.

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These are closely related to modern air conditioning systems.

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Many air conditioning units can operate in both directions.

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In summer they remove heat from the home and provide cooling.

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In winter they reverse the process and provide heating.

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An air to air system can therefore provide both heating and cooling.

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However, it normally does not provide domestic hot water.

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A separate system would still be needed for baths, showers and hot taps.

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Air to Water or Air to Air?

 

The right choice depends upon what you want the system to do.

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An air to water system may make sense if you want to replace a traditional boiler and continue using a wet heating system.

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An air to air system may be more attractive if:

• Cooling is important

• You do not need to retain traditional radiators

• You want rapid room heating

• The property already has suitable air conditioning units

• Domestic hot water can be provided separately

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They solve slightly different problems.

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What Is a Ground Source Heat Pump?

 

A ground source heat pump takes heat from the ground rather than from the air.

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Pipes are installed underground and filled with fluid.

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The ground remains at a relatively stable temperature compared with the outside air.

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The fluid circulating through the underground pipes absorbs heat from the ground.

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The heat pump then increases that temperature and transfers the energy into the home's heating system.

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How Are Ground Source Pipes Installed?

 

There are generally two main approaches.

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The first is horizontal ground loops.

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Long lengths of pipe are buried across a reasonably large area of land.

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This requires substantial garden or land space.

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The second is boreholes.

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Deep holes are drilled vertically into the ground and pipes are installed within them.

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Boreholes require much less surface area but drilling can make the installation considerably more expensive.

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This is one reason ground source systems are more common on larger properties, new developments or homes with sufficient land.

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Why Consider Ground Source?

 

The ground temperature is generally more stable than the outside air temperature.

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This can allow a ground source heat pump to operate very efficiently.

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There is also no large air source fan unit operating outside the property.

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However, the installation can involve significant groundworks.

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For a typical London terrace with a modest garden, digging large ground loops is unlikely to be practical.

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For a detached house sitting on substantial land, the possibilities may be completely different.

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What Is a Water Source Heat Pump?

 

A water source heat pump takes heat from a body of water.

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Possible sources could include:

• A lake

• River

• Large pond

• Canal

• Suitable groundwater source

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Pipework or another heat collection system allows energy to be extracted from the water and transferred to the heat pump.

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Water can provide a relatively stable source of heat and can make water source systems extremely effective where the right conditions exist.

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Is a Water Source Heat Pump Suitable for an Ordinary House?

 

Usually not.

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The obvious problem is that most homes do not have a suitable body of water next to them.

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There can also be issues involving access, permissions, environmental requirements and installation.

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For certain rural properties, estates or developments beside suitable water, a water source heat pump may be worth considering.

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For the average suburban house, air source will normally be much more realistic.

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So What Is the Difference?

 

The easiest way to remember it is:

Air source

Takes heat from the outside air.

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Ground source

Takes heat from the ground.

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Water source

Takes heat from a body of water.

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Then there is the second part of the description.

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Air to water

Takes heat from air and uses it to heat water for the home.

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Air to air

Takes heat from air and delivers warm air directly into the rooms.

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That is really the key to understanding the terminology.

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Which Type Is Most Common for Homes?

 

For most existing UK homes considering replacing a boiler, an air to water air source heat pump is likely to be the system they encounter most frequently.

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It generally requires much less disruption than installing a ground source system.

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The outdoor unit needs a suitable location and the home's heating system needs to be assessed carefully.

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The existing radiators may need to be enlarged and the property may require a hot water cylinder.

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Insulation and heat loss should also be considered.

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Which Heat Pump Works With Underfloor Heating?

 

Both air source and ground source heat pumps can work particularly well with water based underfloor heating.

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Heat pumps generally operate most efficiently when supplying water at lower temperatures than a traditional gas boiler.

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Underfloor heating uses a large floor area to distribute heat, which means it can operate effectively at relatively low water temperatures.

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This can make it an excellent partner for a heat pump.

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Large correctly sized radiators can also work well.

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What About Existing Radiators?

 

Installing a heat pump does not automatically mean every radiator has to be replaced.

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But they need to be checked.

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A traditional boiler may send very hot water through relatively small radiators.

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A heat pump normally works more efficiently with lower water temperatures.

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Some existing radiators may therefore need to be replaced with larger units capable of delivering enough heat at the lower temperature.

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This is one reason a heat loss assessment is important before choosing the system.

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Do You Need a Hot Water Cylinder?

 

With most air to water heat pump systems, yes.

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A traditional combination boiler can produce hot water instantly when a tap is opened.

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A heat pump usually works differently.

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Domestic hot water is normally heated and stored in a cylinder.

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That means space needs to be found for the cylinder and this should form part of the design from the beginning.

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For some properties this is straightforward.

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For smaller homes it can be one of the practical challenges.

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Which Type Can Provide Air Conditioning?

 

Air to air heat pumps are particularly interesting because many systems can both heat and cool rooms.

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In effect, this is reversible air conditioning.

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During winter the system transfers heat into the building.

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During summer the process is reversed and heat is removed from the building.

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That can make air to air technology particularly attractive where summer overheating is already becoming a problem.

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An air to water heat pump may also have cooling capabilities depending upon the system, but the heating distribution system needs to be designed appropriately.

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Which System Is Cheapest to Install?

 

In general, air source heat pumps tend to involve a lower initial installation cost than ground source systems because there is no need to install extensive underground pipework.

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Ground source systems can require excavation or boreholes.

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Water source systems can involve specialist design and installation.

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But installation cost should not be the only consideration.

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The correct choice depends upon the property, available space, heating demand, existing heating system and future plans.

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Which Heat Pump Is Best?

 

There is no single answer.

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For a typical suburban home, air source may be the most practical.

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For a large rural property with plenty of land, ground source may deserve serious consideration.

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For a property with access to a suitable lake or water source, water source could potentially offer another option.

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For somebody primarily interested in heating and cooling individual rooms, air to air may be particularly attractive.

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The important question is not:

Which heat pump is best?

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It is:

Which heat pump makes the most sense for this particular home?

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Look at the Whole Home Before Choosing

 

Heat pumps should not be selected in isolation.

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Before making the decision, consider:

• Insulation

• Heat loss

• Existing radiators

• Underfloor heating

• Hot water requirements

• Space for a cylinder

• Outdoor unit location

• Solar panels

• Battery storage

• Electricity tariffs

• Cooling requirements

• Future electric vehicle charging

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A heat pump that looks attractive on paper may not be the best first investment if the house is losing large amounts of heat through poorly insulated walls, roof or windows.

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Likewise, a well insulated home with solar panels and underfloor heating could be extremely well suited to heat pump technology.

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Start With the Home, Not the Heat Pump

 

The technology should follow the property.

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APN Green Homes looks at the entire home before deciding which improvements make sense.

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That means considering heating alongside insulation, glazing, ventilation, solar energy, battery storage and cooling.

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The aim is to help you understand which technology suits the property and how the different improvements can work together.

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Every home is different. Your green home plan should be too.

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Before spending thousands changing your heating system, find out which solution is right for your property.

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Green Home Assessment £495 + VAT

 

Tip: Before you spend thousands improving your home, find out where your money will make the biggest difference.