One of the issues which is often raised when talking about heat pumps is the low flow temperature of water it produces in comparison with a boiler.

Conventional heat pumps typically operate with a water flow temperature of 35-50℃. The lower the flow temperature, the more efficient the heat pump becomes. As efficiency is one of the big selling points for a heat pump, that means there is an incentive to keep the temperature as low as possible.

This is in comparison to a boiler, which will generally produce water with a flow temperature of 60-80℃.

Retrofitting Ireland’s housing stock with heat pumps is a major part of the government’s plan to reduce emissions. However, that can often involve a lot of work on making a home more energy efficient before it is even suitable for a heat pump.

High temperature heat pumps have emerged as an option for homes that can more easily replace a boiler, as well as for commercial premises with greater heating demands.

High Temperature Heat Pumps

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What is a High Temperature Heat Pump

This is a general term given to a number of different systems and technology types which are meant to produce water temperatures far greater than the majority of heat pumps used in residential properties.

A high temperature heat pump should have a hot water temperature output which matches or exceeds what can be produced by an oil or gas boiler.

How do high temperature heat pumps work

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Benefits of High Temperature Heat Pump

Producing hot water up to 80℃ means that a high temperature heat pump can more easily replace a gas or oil boiler, without requiring any upgrades to the fabric of a building or its heating systems.

A conventional heat pump, working at 35-50℃ requires a well insulated house and properly sized radiators or underfloor heating systems. 

Without this too much heat is lost through the fabric of the building, and the heating system doesn’t have the required surface area to deliver enough heat.

There are multiple reasons why it might be impossible or impractical to make home improvements which would allow a regular heat pump to be installed.

If a building is a protected structure, or located in an architectural conservation area, it could be impossible or cost-prohibitive to install new insulation, replace the existing radiators, or install underfloor heating.

Even if there are no legal hurdles, it might simply be cheaper to replace an old boiler with a modern high-temperature heat pump.

Meeting Hot Water Demands

For larger homes with a high demand for hot water a specialised high-temperature heat pump might be better suited to your needs than a larger conventional heat pump.

Benefits and Drawbacks of a High Temperature Heat Pump
Pros Cons
Can easily replace an existing boiler Often more expensive than regular heat pumps
Doesn’t require upgrading of radiators Can be more expensive than a boiler to run
Produces hotter water for use in your home. More complex systems may require greater maintenance
Can provide both hot water and space heating even without a hot water cylinder. Bulkier than regular heat pumps

Types of High Temperature Heat Pumps

There are multiple ways in which a heat pump system can achieve high temperature outputs by making use of different refrigerants and system setups.

The older method of creating high water temperatures is with a cascade system, where to low and high heat refrigeration cycles are used to push the heat level higher than a regular heat pump can.

The other most prominent depends on making use of refrigerants which can be compressed to a level which will bring them to extremely high temperatures. Natural refrigerants such as propane (R-290) and carbon dioxide (R-744) .

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Cascade Heat Pump System

  • Daikin Altherma 3 H HT

A cascade heat pump system essentially operates as two refrigerant cycles working in tandem to produce water up to 80℃ in order to cater for larger heating and hot water demands from a building.

Cascade systems operate with two refrigeration cycles, one at a low temperature and the other at a much higher temperature.

In the first heat pump, low temperature air is used to heat the refrigerant to levels typical of a normal heat pump.

However, instead of using that to heat water or air, that refrigerant is then used to heat a second heat pump.

The heat coming from that refrigerant is far greater than from an ambient air source. This then allows the second heat pump to create hot water with a flow temperature of up to 80℃.

The disadvantage of this system is that it needs a second compressor, which will raise the operating cost when compared to non-cascaded systems.

  • Produces much hotter water than a normal heat pump.

  • Different refrigerants can be used for different heating cycles to optimise performance.

  • Good for low temperature climates.

  • Two compressors in the system will increase operating costs.

Cascade Heat Pump System

Transcritical CO2 Heat Pumps

Transcritical heat pumps make use of R-744 (Carbon Dioxide) as a refrigerant, which allows them to achieve water temperatures in excess of 80℃ by making use of CO2’s unique properties compared with synthetic refrigerants.

Using CO2 as a refrigerant allows it to deliver heat to the system past its critical temperature. This is a point where synthetic refrigerants would break down and become useless. This allows R-744 to deliver heat across a wider range of temperatures in a subcritical and supercritical state.

One technical issue with CO2 heat pumps is that they must have a return temperature for the refrigerant of 30℃ or less, which can require specialised designs.

  • R-744 (Carbon Dioxide) is extremely cheap and abundant.

  • The refrigerant is also comparatively non-toxic and completely non-flammable.

  • New to the market systems can be more expensive up front.

  • Requires specialised design to ensure low return temperature.

CO2 heat pumps are still uncommon. Vattenfall is introducing a new model in the Netherlands and UK, with no announcement as to when it will reach other markets. Mitsubishi also has a CO2 heat pump.

Transcritical CO2 Heat Pumps

Propane Heat Pumps

  • Vaillant Aerotherm Plus

Propane (R-290) is another natural refrigerant which is capable of working efficiently at flow temperatures in the region of 60℃. This is, however, still more common in commercial sized heat pumps.

Heat Pump manufacturers are looking at R-290 as a refrigerant of the future. It can be easier to work with than CO2 and has a lower environmental impact than HFCs. 

The Vaillant Aerotherm Plus already boasts an SCOP of 3.5 at 55℃, and we can expect to see more mid to high-heat systems coming with propane in the future.

FAQs

Depending on the system design and refrigerant used, high temperature heat pumps can produce water temperatures anywhere from 60-80℃.

Whether a high temperature heat pump is best suited for you depends on your property and domestic hot water needs.

A home which is very difficult to insulate, or which requires a large amount of hot water, may benefit from a high-temperature system, as opposed to an oversized normal heat pump.

However, to really improve the energy efficiency of your home upgrading the fabric of the building should still be the first choice.

While normal heat pumps are at their most efficient with a flow temperature of around 35℃, advancements in technology have meant that high temperature systems no longer have to sacrifice efficiency. However, such systems are likely to be more expensive.

heat pump FAQs

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