High-Temperature R290 Heat Pumps: What 35°C, 55°C, 65°C and 75°C Really Mean

When comparing air-to-water heat pumps, one specification attracts a lot of attention:

maximum water temperature.

Modern R290 heat pumps can reach considerably higher water temperatures than many earlier generations of air-source heat pumps. Tongyi’s R290 platform, for example, is designed to provide outlet water temperatures of up to 75°C, depending on operating conditions and system configuration.

That capability can be particularly useful in retrofit projects and buildings with radiator heating.

But there is an important distinction:

The maximum temperature a heat pump can produce is not necessarily the temperature at which it should operate all the time.

To understand why, we need to look at heat pump flow temperature, heating-system design and efficiency together.

What Is Heat Pump Flow Temperature?

Flow temperature is the temperature of the water leaving the heat pump and entering the building’s heating system.

The water then passes through radiators, underfloor heating or fan coils, transfers heat into the rooms, and returns to the heat pump at a lower temperature.

For example, a heating system might operate with:

These are not universal design values. The correct temperature depends on the building heat loss, emitter size, outdoor temperature and heating-system design.

This is why asking simply, “Can the heat pump produce 75°C?” does not provide enough information.

A better question is:

What is the lowest flow temperature that can comfortably heat this building under the required design conditions?

Why Does Flow Temperature Affect Heat Pump Efficiency?

A heat pump moves heat from a lower temperature source — such as outdoor air — to a higher-temperature heating system.

The greater the difference between the outdoor source temperature and the required water temperature, the harder the refrigeration cycle has to work.

This difference is often described as the temperature lift.

For example, producing 35°C water when it is 10°C outside requires a much smaller temperature lift than producing 65°C water when it is -5°C outside.

As the required lift increases, compressor work generally increases and COP decreases.

This is why heat pumps normally achieve their highest efficiency with lower-temperature heating systems.

The engineering principle is therefore straightforward:

Use the lowest flow temperature that still delivers the required indoor comfort.

What Does 35°C Heat Pump Operation Mean?

Around 35°C is typically associated with low-temperature heating systems such as underfloor heating.

Large emitter areas allow useful amounts of heat to be transferred even when the circulating water is relatively cool.

Because the heat pump does not need to raise the refrigerant to such a high condensing temperature, this operating condition is usually favourable for COP.

This is also why European heat-pump performance data commonly distinguish between low-temperature operation around 35°C and medium-temperature operation around 55°C.

Current UK product-policy work similarly uses 35°C, 55°C and 65°C operating categories when assessing low-, medium- and high-temperature heat pumps.

What About 45°C to 55°C?

The 45–55°C range is particularly important for residential retrofit projects.

Many houses do not require traditional boiler temperatures throughout the heating season.

Radiators may already be larger than necessary, the building may have received insulation improvements, or the original heating system may have been designed conservatively.

In these situations, a properly sized heat pump may be able to operate existing or upgraded radiators at considerably lower temperatures than the previous boiler.

The key is to perform a room-by-room heat-loss and emitter assessment rather than assuming that the old boiler setting is the temperature the new heat pump must reproduce.

Operating around 45–55°C can therefore offer a useful compromise between radiator output and heat-pump efficiency.

When Are 60°C to 65°C Flow Temperatures Useful?

Higher flow temperatures can become valuable in buildings where:

• radiator surface area is limited;
• design heat loss is relatively high;
• substantial building renovation is impractical;
• domestic hot water requires higher temperatures;
• or the heating system must maintain output during particularly cold weather.

High-temperature capability can therefore increase the number of buildings in which an air-to-water heat pump is technically feasible.

Government retrofit guidance has noted that while heat pumps generally perform most efficiently at lower temperatures, systems capable of approximately 60–65°C can provide additional options for more difficult existing buildings.

However, higher-temperature capability should be treated as design flexibility, not as an instruction to operate permanently at the maximum temperature.

What Does a 75°C R290 Heat Pump Actually Offer?

Modern R290 systems have expanded the practical operating envelope of residential air-to-water heat pumps.

The ability to produce water temperatures approaching 75°C can be useful for several reasons.

First, it creates greater flexibility when replacing boilers in buildings with existing radiators.

Second, it can support high domestic-hot-water temperatures and faster cylinder recovery where required.

Third, it provides additional temperature headroom during demanding operating conditions.

Tongyi’s R290 monobloc heat pump platform is designed for outlet temperatures of up to 75°C, while also supporting heating operation in low outdoor temperatures.

But 75°C should not automatically become the normal space-heating setpoint.

If the building can remain comfortable at 50°C, continuously heating water to 70°C simply creates unnecessary temperature lift.

Maximum capability and optimum operating temperature are two different specifications.

Why Is R290 Well Suited to High-Temperature Heat Pumps?

R290, or propane, is increasingly used in modern air-to-water heat pumps.

Its properties allow manufacturers to design refrigeration circuits capable of combining low-temperature heating efficiency with relatively high outlet-water temperatures.

However, the refrigerant alone does not determine performance.

A high-performance heat pump also depends on factors such as:

compressor design, inverter control, heat-exchanger sizing, expansion control, refrigerant-circuit optimisation, defrost logic and system controls.

Tongyi combines R290 with technologies including twin rotary compressor technology, inverter control and EVI architecture to support operation across changing load and outdoor-temperature conditions.

This is an important distinction.

An R290 label does not automatically guarantee identical performance between different heat pumps.

The complete refrigeration and hydraulic system must be evaluated.

Can an R290 Heat Pump Work With Existing Radiators?

Often, yes — but the answer depends on the building.

Radiator output decreases when water temperature decreases.

A radiator that produced sufficient heat with very hot boiler water may deliver substantially less heat when supplied at 45°C or 50°C.

That does not automatically mean every radiator must be replaced.

The installer should first determine:

1. the design heat loss of each room;
2. the heat output available from the existing radiator at the proposed water temperature;
3. the required outdoor design condition;
4. whether selected emitters need to be enlarged;
5. whether building improvements could reduce heat demand.

Some rooms may already contain sufficiently large radiators.

Other rooms may require larger radiators or alternative emitters.

A high-temperature R290 heat pump provides additional flexibility because the system can increase flow temperature when necessary instead of requiring a single fixed low-temperature operating point.

Is a 75°C Heat Pump More Efficient Than a 55°C Heat Pump?

Not because it reaches 75°C.

Maximum water temperature describes capability, not seasonal efficiency.

A heat pump capable of reaching 75°C can still be operated at 35°C, 45°C or 55°C when the building allows it.

In fact, that is normally the preferred approach.

The useful advantage is that the system has a wider operating envelope.

This allows the designer to use lower temperatures during mild weather while retaining access to higher temperatures when required.

Weather Compensation Makes Flow Temperature Dynamic

A well-designed heat pump system does not necessarily use the same water temperature every day.

When outdoor temperatures are mild, the building loses less heat.

The heat pump can therefore reduce its flow temperature.

As outdoor temperatures fall, the control system gradually increases the target temperature.

This strategy is known as weather compensation.

Instead of repeatedly switching the heat pump between maximum output and off, weather-compensated control allows an inverter system to modulate output according to the building’s changing heat demand.

The result is more stable operation and potentially better seasonal efficiency.

Flow Temperature Must Be Considered With the Whole System

A heat pump cannot be selected correctly from one temperature specification.

System designers should consider:

building heat loss + heat pump capacity + emitter output + flow temperature + outdoor design temperature + hydraulic flow + controls

Hydraulic design is also important.

Stable water flow, correct system volume and proper zoning all influence how effectively the heat pump operates.

In some installations a buffer tank can help provide additional water volume or hydraulic separation, while other systems can operate correctly without one.

The correct arrangement depends on the complete heating system rather than on the heat pump alone.

35°C, 55°C, 65°C or 75°C: Which Temperature Is Best?

There is no single correct temperature for every building.

A modern R290 heat pump may be capable of operating across a broad temperature range.

The system designer’s objective should therefore not be to operate at the highest possible temperature.

It should be to identify the lowest practical temperature that can meet the building’s heat demand under the required conditions.

For a new low-energy building with underfloor heating, that might be around 30–35°C.

For a radiator retrofit, it might be 45–55°C.

Some existing buildings may require 60–65°C during colder periods.

And a heat pump capable of reaching 75°C provides additional headroom for demanding heating or domestic-hot-water conditions.

That is the real value of high-temperature R290 technology:

not that the heat pump must always run hot, but that the heating system has more design flexibility when higher temperatures are genuinely required.

FAQ

What flow temperature should an R290 heat pump use?

The lowest temperature that can meet the building’s heating demand comfortably. The correct value depends on heat loss, emitter size and outdoor conditions.

Can an R290 heat pump run existing radiators?

Yes, in many installations. The existing radiators should be checked against the building’s room-by-room heat demand at the intended flow temperature.

Can an R290 heat pump reach 75°C?

Some modern R290 systems can. Tongyi’s R290 air-to-water platform can provide outlet-water temperatures of up to 75°C depending on operating conditions and configuration.

Is 75°C operation more efficient?

No. Higher maximum temperature is primarily a capability advantage. Heat pumps generally operate more efficiently when the required water temperature is lower.

Is 55°C enough for existing radiators?

It can be, but this must be verified through heat-loss and radiator-output calculations. Some buildings will require lower temperatures, while others may need higher temperatures during design winter conditions.