How do Heat Pump Systems Work?

Ground source heat pumps work by absorbing energy from the ground and compressing the low-grade energy into high-grade heat. They then deliver the heat through a heating distribution system, such as radiators or underfloor heating, for reliable heating and domestic hot water through the seasons.

Ground source heat pumps can provide temperatures up to 65°C. As well as heating buildings of all sizes and ages, they can also deliver active or passive cooling.

Can I use a ground source heat pump with underfloor heating?

Yes. Underfloor heating with a ground source heat pump is a very effective way to heat your property. Together, they can achieve efficiencies of around 400%.

Underfloor distribution systems work especially well with ground source heat pumps because they operate at lower flow temperatures. The large surface area of underfloor heating means the heat pump can deliver temperatures of 35°C. This lends itself to efficiency, and can leave you with cheaper heating bills and running costs.

Underfloor heating vs. radiators

When underfloor heating, you can achieve higher efficiencies because of the low temperature required from the heat pump. Radiators either need to be correctly sized or replaced with bigger ones to cater for a lower flow temperature, or the heat pump needs to produce higher temperatures to emit sufficient heat from a smaller surface area.

How do I get the best efficiencies from underfloor heating?

Low flow temperatures

To get the most efficient operation from a heat pump, it’s important that the outlet temperature of the heating distribution system is kept as low as possible. That way, the heat pump has to perform less work to upgrade the energy from the ground to a usable temperature inside the property.

With underfloor systems, the ideal installation is to mount the underfloor in screed. Due to its larger surface area, underfloor in screed can run at lower temperatures of around 35°C. The screed can even be used as a thermal mass to allow the heat pump to run on off-peak electricity tariffs. This further reduces the running costs of a ground source heat pump.

Ideal materials for different floor types

For first floor applications:

The ideal building construction is beam and block with the underfloor and screed laid on top.

For suspended floors:

A dry or sand screed can be used either between or over joists. However, structure and height need to be taken into account with these systems. It might be that the heat pump will need to run at a higher temperature to drive the heat through the overlying chipboard and final floor finish. Bear in mind that this will reduce its efficiency.

Ground Source Review: Rimmer's Farm Heat Pump - Underfloor Heating

What should I consider when using underfloor heating?

Additional costs for suspended timber floors

If you have a suspended timber floor, any underfloor heating pipework – which is typically installed within the joist voids – needs to be supported by a steel heat transfer plate. This not only increases cost, but could also slow down the build programme.

Efficiency of underfloor vs. radiators for suspended floors

Since the flow temperature for suspended floors would need to be increased to around 45ºC (to drive the heat through the overlying chipboard and final floor finish), there is no greater efficiency for underfloor than if the heat pump was serving radiators.

No off-peak tariffs for heat emission plates

If heat emission plates are used in the installation, off-peak tariffs cannot be effectively used as there is no thermal storage and, again, higher temperatures may be required.

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We now have a beautiful home which is comfortably warm throughout the cooler or cold months, but is able to retain its cool interior at the height of the summer. Hot water is constant and plentiful. It almost feels magical to be able to flick a shower on each morning without having to rely on gas or electric central heating boilers. The system is intuitive to use and easy to adjust seasonally if required.


Do ground source heat pumps work with radiators?

Yes. When radiators are used with ground source heat pumps, the radiators are typically oversized to deliver the appropriate flow temperatures. To provide sufficient movement of air and hence heat flow, the radiators need a certain flow temperature or size, as heat flow is proportional to temperature and surface area.

As the flow temperature is decreased, the surface area of the radiator needs to be increased to keep the same heat output. This is why underfloor heating – with its larger heat emitting area – tends to be more popular with ground source heat pumps.

How do heat pump systems work?

Use fan assisted radiators to enhance performance

Fan assisted radiators, such as Jager DBE units, can be used with a heat pump to enhance performance. These units combine a copper aluminium finned heat exchanger with a low water content and a number of small fans. As the fans increase airflow around the heat exchanger, the output of the radiator rises and can give up to 3 times more heat output than a conventional radiator with the same dimensions.

As these units contain a low water content, they are quick to react to ambient changes and night set-back temperature. Fan assisted radiators operate with electrical fans, so they need to be connected to the electrical supply and have a small electrical consumption of around 2-3 watts. They also generally come with a boost button which provides the maximum heat emission for approximately 15 minutes to rapidly heat a cold room.

What should I consider before using radiators with a heat pump?

Radiator size must cater for flow temperatures up to 55ºC

Radiators should be oversized to deliver flow temperatures compatible with the heat pump. Higher flow temperatures reduce the Coefficient of Performance (CoP) and therefore the efficiency of the heating system.

Use bypass radiators to avoid short cycling

To avoid short cycling of the heat pump in space heating mode, approximately 25% of the radiators should be used as bypass radiators, i.e. with no thermostatic controls on them. These bypass radiators should be in areas that do not require close temperature control, such as hallways. If close temperature control is required in all zones, you should use a buffer vessel.

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Can I use my existing radiators with a ground source heat pump?

You can, but if the radiators are not already oversized, the heating system will not be as efficient as it could be.

In order to obtain adequate heat from a radiator, the outlet temperature of a heat pump needs to be increased to approximately 45°C – 50°C. To produce outlet temperatures of 50°C, the COP of a heat pump is approximately three. So for every one unit of electricity used to power the heat pump, it produces three units of heat.

As radiators in retrofit properties are typically sized for a flow temperature of 71°C – 82°C, they may be undersized. In this case it would be advisable, if possible, to replace these with oversized radiators which operate at lower flow temperatures compatible with the heat pump (45ºC-50ºC). Any microbore pipe to the radiators would also need to be replaced.

Remember that as the outlet temperature of the heat pump increases, its efficiency decreases. If the radiator outlet temperature is more than 50°C, this will reduce the efficiency and COP of the heat pump system, reducing the running cost advantages.

How do I find out if my radiators are suitable for a heat pump?

To find out if your existing radiators are compatible or large enough for a ground source heat pump, find an installer through Kensa’s trusted network.

You can also test this out by trialling your radiators over the heating season. See more here.

Can I use a combination of underfloor heating & radiators with a heat pump?

Yes. Heat pumps can be used to heat buildings either with underfloor systems, with radiators or a mix of both. While underfloor systems are often used on lower floors, it might be that your preferred choice for heating upstairs is radiators.

If you use a mixture of both types of heating systems, there are a number of things you must take into account:

The heat pump will need to run at a higher temperature

Due to their smaller surface areas, radiators need a higher flow temperature to provide heat into a room. This temperature  means that the heat pump runs less efficiently than if the heating distribution was solely underfloor.

There could be a delay in heat output

The underfloor mounted in screed acts as a large heat sink and absorbs most of the heat produced by the heat pump. This keeps the return temperature to the heat pump low.

However, at low flow temperatures the radiators in the system will not provide heat and feel lukewarm until the underfloor is up to running temperature. This could cause a delay between turning on the heating system and the radiators actually providing heat. This delay is more pronounced at initial start up, but can also occur during normal running conditions.

Underfloor pipe density

Due to the lower flow temperatures, Kensa recommends that pipe density is increased for all underfloor systems used with heat pumps. This maintains the heat.

In systems with radiators, the 45°C flow temperature can be mixed down to 35°C by the use of mixing valves. However, it is important to remember that floor coverings acting as an insulating layer may need a higher flow temperature than 35°C for the underfloor system.

Can I use a ground source heat pump for hot water?

Yes. Kensa ground source heat pumps are capable of producing stored hot water up to 60°C.

The Shoebox heat pump can generate hotter water than any other Kensa system (65°C). Meanwhile, the Twin Compact high temperature, Evo*, and hybrid ground source heat pump ranges can deliver 60°C hot water temperatures.

Kensa has pioneered an approach to domestic hot water production in ground source heat pump models, which avoids the need for hot water thermostats.

*excluding the 17kW Evo – space heating only.

Domestic hot water

How do ground source heat pumps produce hot water?

When the domestic hot water timeclock calls for production of hot water, a three-port valve diverts flow from the heating distribution circuit into an indirect coil within the hot water cylinder. The temperature of the water from the heat pump is increased.

When the hot water production time period ends, the three-port valve switches back to the underfloor distribution and the temperature drops back to its space heating design temperature. The heat pump then reverts to space heating mode or switches off if no zones are calling for heat.

Due to the low flow temperatures generated by the heat pump, the hot water tank must have an oversized coil to provide the correct heat transfer. The larger the size of the coil within the tank, the better the heat transfer area and the better the domestic hot water performance will be.

How hot can a ground source heat pump heat water?

The maximum hot water temperature that the heat pump can produce is approximately 65°C.

Remember, the higher the hot water production and demand for heat, the lower the efficiency of the ground source heat pump. The maximum output water temperature of a heat pump depends on many factors, including:

  • The refrigerant circuit’s maximum pressure
  • The hot water flow rate through the domestic hot water cylinder coil
  • The ground temperature
  • The flow rate through the pipework

Kensa has pioneered an approach to domestic hot water production across an entire ground source heat pump range, which ensures optimum and efficient hot water temperatures whilst removing the requirement for thermostats. Controlling the maximum output water temperature with a thermostat or fixed temperature switch can result in a lower water temperature than would otherwise be possible.

This is why Kensa uses a refrigerant pressure switch which automatically terminates the domestic hot water cycle at the point of the highest pressure and thus highest temperature. This ensures the heat pump delivers the hottest water possible at the best efficiency.

Once the Kensa heat pump has terminated the domestic hot water cycle, an internal timer prevents another cycle from beginning for two hours. This timer is adjustable during the heat pump commissioning process.

Do I need an immersion heater with a heat pump?

As standard, immersion heaters are not used in any Kensa heat pump models due to their potential costly implications for end users.

However if 65°C is required all year round, it is recommended that an immersion heater is linked to the automatic temperature boost function on Kensa Evo models. Our Shoebox models deliver 65°C without immersion.

If 60°C water is acceptable, it is recommended that the immersion heater is programmed to raise the temperature to 65°C once a week using the domestic hot water timeclock or the Evo controller (Genesis System Manager).

If a ground source heat pump produces hot enough hot water temperatures, there is no need for an electrical top-up from an immersion heater. However, immersions can be fitted into hot water cylinders for back-up measures.

Can a heat pump be used for cooling?

Yes. A ground source heat pump in cooling mode offers a lower-carbon and lower-cost alternative to air-conditioning or chiller systems.

Uniquely, Kensa ground source heat pumps deliver passive cooling for ultra-low-cost comfort in the summer, whilst re-charging the ground for a more energy-efficient ground source heating system.

Our ground source heat pumps can also be manufactured to provide active cooling for a building by operating in reverse-cycle mode. This operates in the same way as a chiller.


Can I use a ground source heat pump with a boiler?

Yes, this is called bivalent heating. A bivalent heating system combines a ground source heat pump with a secondary boiler. This system is designed to provide heat into the distribution system when the heat pump isn’t sized for 100% of the peak load. Bivalent systems are generally found in retrofit applications where insulation levels of the building are insufficient and a heat pump cannot meet all of the heating load effectively.

Bivalent systems have to be carefully designed to avoid the return temperature of the heating circuit being too high. If this return temperature is above the in-built temperature set point at which the heat pump turns off, the heat pump will never actually turn on and the whole of the load will be taken by the secondary boiler – resulting in higher than expected energy bills and carbon emissions.

For the majority of heat pump projects, Kensa aims to size the system to meet 100% of the heating demands, so bivalent systems aren’t often necessary. Contact Kensa to discuss the type of set-up that could suit your project.

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How do I get optimum performance from a bivalent system?

The simplest and most effective way of providing the maximum efficiency of a bivalent heating system, while retaining the occupant’s comfort, is to use an ‘either/or’ control logic. Simply put either the heat pump or the secondary boiler is operating, but not both together.

The system operates by use of an external temperature sensor. This is set at an external temperature, above which the heating load is satisfied by the heat pump alone. If the external ambient temperature drops below this set point then the heat pump is switched off and the secondary boiler switched on to supply heat into the heating distribution system. Due to the higher output temperature of the secondary boiler, it is important that the boiler’s flow is mixed via a mixing valve with the return flow to lower the temperature to a suitable level for the heating distribution system.

See example schematics for bivalent systems

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