A high temperature R290 heat pump can make radiator-based home renovation more practical by providing higher leaving-water temperatures than conventional low-temperature heat-pump applications. However, maximum water temperature alone does not determine whether the original radiators can be retained.

Installers must still check the building heat loss, radiator output at the proposed flow and return temperatures, heat-pump capacity at the local outdoor design temperature, hydraulic flow and domestic hot-water requirements.

EXINDA supplies R290 monoblock air-to-water heat pumps for European residential and selected light-commercial heating projects. The EXINDA Storm platform supports high-temperature water production up to 75°C on applicable models and operating conditions, while actual capacity, efficiency and available leaving-water temperature must be confirmed from the published data for the exact model and design point.

Can a High Temperature R290 Heat Pump Work With Existing Radiators?

Yes. Existing radiators can often be retained when they deliver enough heat at the new design water temperature.

This is particularly relevant for European homes previously heated by:

  • Gas boilers

  • Oil boilers

  • LPG boilers

  • Pellet or biomass boilers

  • District-heating interfaces

  • Older electric boilers

The presence of radiators does not automatically require a 70–75°C heat-pump design. Many existing systems were oversized, and some buildings can maintain comfort at substantially lower temperatures after hydraulic balancing or envelope improvements.

The installer should determine the lowest water temperature that still allows every room to meet its design heat loss. This improves the opportunity for efficient heat-pump operation while limiting unnecessary radiator replacement.

Why Radiator Output Changes With Water Temperature

Radiator output is usually rated against a temperature difference between the radiator’s mean water temperature and the room temperature.

For example, a radiator tested at 75°C flow, 65°C return and 20°C room temperature has:

  • Mean water temperature: 70°C

  • Room temperature: 20°C

  • Mean temperature difference: ΔT50 K

If the same radiator operates at 55°C flow and 45°C return in a 20°C room, its mean temperature difference falls to ΔT30 K. Its heat output will therefore be substantially lower.

The following table gives an indicative illustration using a typical panel-radiator exponent of approximately 1.3. The exact output must be checked against the radiator manufacturer’s correction data.

Flow / return / room Mean temperature difference Approximate output compared with ΔT50
75 / 65 / 20°C ΔT50 100%
70 / 60 / 20°C ΔT45 87%
65 / 55 / 20°C ΔT40 75%
60 / 50 / 20°C ΔT35 63%
55 / 45 / 20°C ΔT30 51%
50 / 40 / 20°C ΔT25 41%
45 / 35 / 20°C ΔT20 30%

These percentages are not product guarantees. Radiator construction, enclosure, airflow, installation position and manufacturer data can change the actual output.

The table explains why a room that previously required the radiator’s full ΔT50 output may not remain warm at 55/45°C without a larger emitter or reduced building heat loss.

Does a Radiator Retrofit Always Need 75°C Water?

No. A high temperature R290 heat pump may be capable of producing water up to 75°C under applicable conditions, but the heating system should not automatically be designed to operate continuously at the maximum temperature.

Higher leaving-water temperature generally means:

  • Greater compressor lift

  • Lower COP than at low-temperature conditions

  • Higher electrical input for the same delivered heat

  • Possible changes in available heating capacity

  • Greater importance of model-specific operating-envelope data

The maximum leaving-water temperature is valuable because it increases retrofit flexibility. It can support colder design days, difficult rooms, domestic hot-water production or systems that cannot immediately be converted to low-temperature emitters.

For normal space heating, the preferred approach is still to use the lowest water temperature that meets the building load.

What Is the Difference Between Maximum and Design Water Temperature?

These terms should not be confused.

Term Meaning
Maximum leaving-water temperature The upper operating limit available under stated model and ambient conditions
Design flow temperature The temperature required to meet the building load at the outdoor design condition
Heating-curve temperature The variable target selected according to outdoor temperature
Domestic-hot-water target The cylinder temperature required by the DHW strategy
Test-point temperature The water condition used for published capacity or efficiency data

A model capable of 75°C water may spend most of the heating season operating at 35–55°C under weather-compensated control.

Installers should request capacity and COP data at conditions close to the actual design point rather than assuming that a maximum-temperature capability guarantees the required output at every outdoor temperature.

How Should Installers Assess Existing Radiators?

1. Calculate the Room Heat Loss

Use a room-by-room heat-loss calculation at the local outdoor design temperature. Property area, boiler size or previous fuel consumption may provide context, but they are not a substitute for the design calculation.

2. Identify Each Radiator

Record:

  • Radiator type

  • Height and length

  • Number of panels and convectors

  • Manufacturer and model where available

  • Installation position

  • Enclosures, shelves or obstructions

  • Thermostatic radiator valve condition

3. Determine Output at the Proposed Temperature

Use the radiator manufacturer’s published correction factors where possible. If the exact product cannot be identified, apply a conservative engineering estimate and state the uncertainty.

4. Compare Output With Room Load

Each room should have sufficient emitter output at the proposed design flow and return temperatures.

5. Upgrade Only Where Necessary

Possible measures include:

  • Replacing selected single-panel radiators with double- or triple-panel models

  • Increasing radiator length or height

  • Adding an additional radiator

  • Using low-temperature radiators with enhanced convection

  • Installing fan-assisted radiators or fan coils

  • Improving insulation and airtightness

  • Correcting hydraulic imbalance

The objective is not necessarily to replace every radiator. A room-by-room review identifies which emitters can remain and which ones limit the whole system temperature.

A Practical Radiator-Retrofit Decision Table

Existing-system finding Likely action
Radiators meet the room load at 45–55°C flow Retain radiators and design for lower-temperature operation
Most radiators are adequate but one or two rooms are short Enlarge only the limiting radiators
Radiators require 60–65°C on design days Evaluate high-temperature R290 performance at the exact design condition
The building requires 70°C or more throughout winter Review insulation, emitters and operating cost before selection
Boiler is oversized but radiators are generous Do not size the heat pump from boiler output; calculate actual heat loss
Several zones close independently Verify minimum flow and consider hydraulic separation or buffer volume
Pipework is small or highly restrictive Calculate pressure drop and required pump head
Existing cylinder has a small coil Evaluate a heat-pump-compatible DHW cylinder

Why Weather Compensation Matters

Weather-compensated control adjusts the target water temperature according to outdoor conditions.

Instead of sending a fixed 60°C to the radiators throughout the heating season, the system may operate at a lower temperature during mild weather and increase the target only as outdoor temperature falls.

This can:

  • Improve seasonal efficiency

  • Reduce unnecessary compressor lift

  • Support longer, steadier operating cycles

  • Improve comfort consistency

  • Reduce the risk of frequent on/off cycling

The heating curve should be commissioned for the actual building. Setting the curve unnecessarily high can eliminate much of the efficiency benefit of the retrofit.

How Does High-Temperature Operation Affect Heat-Pump Selection?

A heat pump described as 9 kW, 12 kW or 15 kW does not necessarily provide its nominal output at low ambient temperature and high leaving-water temperature.

For radiator retrofit projects, installers should review:

  • Heating capacity at the outdoor design temperature

  • Capacity at the required leaving-water temperature

  • COP at the relevant test condition

  • Maximum leaving-water temperature at low ambient conditions

  • Compressor operating envelope

  • Minimum modulation and cycling behaviour

  • Defrost performance

  • Required water flow

  • Electrical input and backup-heater demand

  • Sound performance at the proposed location

Performance at A7/W35 should not be used alone to select a heat pump for a radiator system designed around A−7/W60 or another high-temperature condition.

What Project Information Does EXINDA Need?

Required information Why it matters
Country, city or postcode Establishes climate and outdoor design conditions
Building type and floor area Provides project context
Room-by-room or total heat loss Determines required heating output
Current boiler fuel and output Identifies the existing heat source
Current flow and return temperatures Shows how the radiator system presently operates
Radiator schedule or photos Helps assess existing emitter capacity
Required indoor temperatures Supports room-load evaluation
Domestic-hot-water demand Determines cylinder and control requirements
Available electrical supply Confirms voltage, phase and capacity
Hydraulic schematic Helps review integration and flow strategy
Noise and siting restrictions Affects outdoor-unit placement

Final model selection should be based on reliable project data and the published performance of the exact product.

Can Existing Pipes and Radiator Valves Be Retained?

Often yes, but they must support the required water flow and pressure drop.

A heat-pump system may circulate more water than a boiler system delivering the same heat at a larger temperature difference. The installer should check:

  • Pipe diameter and length

  • Available pump head

  • Pressure drop through valves and fittings

  • Thermostatic radiator valve operation

  • Minimum open water circuit

  • Differential bypass arrangements

  • Dirt and magnetic separation

  • Air removal

  • Hydraulic balancing

  • System cleanliness and water quality

Existing pipework should not be retained merely because it is physically intact. Insufficient flow can reduce delivered capacity, trigger alarms and interfere with defrost operation.

Is a Buffer Tank Necessary for a Radiator Retrofit?

Not always.

A buffer tank or hydraulic separator may be useful when the project has:

  • Multiple small radiator zones

  • Frequent thermostatic-valve closure

  • Low active system water volume

  • A primary and secondary pump arrangement

  • A hybrid boiler and heat-pump system

  • Insufficient minimum flow under part-load conditions

  • Short cycling risk

  • Limited available system energy during defrost

However, unnecessary buffer volume can add standing heat loss and system complexity. The decision should reflect minimum flow, minimum water volume, zoning, heat-pump modulation and the complete hydraulic design.

Can the Existing Boiler Remain as Backup?

Yes. A hybrid or bivalent design can retain the existing boiler for peak-load, emergency or economically selected operation.

This approach may be considered when:

  • The property has very high design heat loss

  • Existing radiators require high temperatures during short cold periods

  • Electrical capacity is limited

  • The renovation will be completed in stages

  • Redundancy is required

  • Local energy pricing supports dual-fuel operation

The system needs a defined control sequence, including heat-source priority, changeover temperature, water-temperature targets, pump operation and safety interlocks.

R290 Monoblock Installation Considerations

R290 is propane and is classified as a flammable refrigerant. In a monoblock air-to-water heat pump, the factory-sealed refrigerant circuit remains in the outdoor unit, while water pipes connect the heat pump to the building.

Installers should follow the current product manual and applicable local requirements regarding:

  • Outdoor-unit location

  • Product-specific safety clearances

  • Doors, windows, drains and ignition sources where applicable

  • Air intake and discharge space

  • Service access

  • Sound impact

  • Defrost-water drainage

  • Snow, ice and flood protection

  • External water-pipe insulation

  • Freeze protection

  • Electrical isolation and protective devices

  • Transport, handling and service procedures

Requirements should be confirmed for the exact model and project country.

EXINDA High Temperature R290 Heat Pump Support

EXINDA works with European distributors, regional partners, heating contractors and qualified installers.

Depending on the model and cooperation arrangement, support can include:

  • Model-specific performance data

  • Technical datasheets and submittals

  • Operating-envelope information

  • Hydraulic and electrical connection information

  • Controller and heating-curve guidance

  • Installation, operation and maintenance manuals

  • Fault-code information

  • Commissioning support

  • Training for distributor and installer teams

  • Spare-parts identification and planning

  • Applicable CE, ErP and Heat Pump KEYMARK documentation

Certification must be checked against the exact product. A certificate covering selected models should not be presented as covering every capacity or configuration.

Review the EXINDA high-temperature R290 heat pump product page for the Storm platform and published high-water-temperature capability.

Installers can also review the wider EXINDA R290 heat pump range for Europe and the European R290 heat pump supplier page.

European Radiator and Boiler-Retrofit References

Germany Radiator Heating Renovation

This German villa project retained radiator-based hydronic heating while replacing the former fossil-fuel heat source with an EXINDA R290 monoblock system.

View the EXINDA R290 radiator heating project in Germany

United Kingdom Boiler Replacement

This UK residential retrofit integrated an EXINDA R290 heat pump with existing radiators, domestic hot water and a PV-supported control arrangement.

View the UK boiler-to-R290 heat pump project

UK MCS and Retrofit Guidance

UK installers and distributors can review product selection, model-specific MCS status and project-support considerations.

Read the MCS-certified R290 heat pump guide

Boiler-Replacement Selection Guide

For a broader assessment of heat loss, DHW, hydraulics and full versus hybrid replacement, read the dedicated boiler-replacement guide.

Read the R290 heat pump boiler-replacement guide

More application and technical articles are available in the EXINDA HVAC Blog.

Frequently Asked Questions

What is a high temperature R290 heat pump?

A high temperature R290 heat pump is an air-to-water heat pump designed to provide higher leaving-water temperatures for hydronic heating and domestic-hot-water applications. Maximum temperature, capacity and efficiency depend on the exact model and operating condition.

Can an EXINDA R290 heat pump supply 75°C water?

The EXINDA Storm R290 platform publishes leaving-water capability up to 75°C for applicable models and operating conditions. The required capacity and available temperature at the project’s outdoor design point must be confirmed from model-specific data.

Can old radiators be used with a heat pump?

Yes, if their output at the proposed flow and return temperatures meets each room’s design heat loss. Some projects can retain all radiators, while others require selected emitters to be enlarged.

Does every radiator retrofit need high-temperature water?

No. The installer should determine the lowest water temperature that meets the design load. Maximum-temperature capability provides flexibility but should not automatically become the continuous operating setpoint.

Why does radiator output fall at lower water temperature?

Radiator output depends on the difference between mean water temperature and room temperature. As that difference decreases, the radiator transfers less heat to the room.

Should the heat pump be sized from the existing boiler output?

No. Boilers are frequently oversized. Heat-pump selection should be based on calculated building heat loss and model performance at the required outdoor and water temperatures.

Can only the smallest radiators be replaced?

Often yes. A room-by-room calculation can identify the limiting emitters. Enlarging only those radiators may allow the complete system to operate at a lower temperature.

Is a buffer tank always required?

No. The need depends on minimum water volume, minimum flow, zoning, heat-pump modulation, defrost requirements and hydraulic design.

Can the existing boiler stay in a hybrid system?

Yes. A hybrid system can retain the boiler for peak or defined operating conditions, provided the hydraulic arrangement and control sequence are properly engineered.

Are all EXINDA R290 models Heat Pump KEYMARK certified?

Certification is model-specific. The exact capacity and configuration must be checked against the applicable certificate scope.

Request a Radiator-Retrofit Assessment

If you are an installer, heating contractor, distributor or project company evaluating a high temperature R290 heat pump, please provide:

  • Company name and country

  • Project city or postcode

  • Building type and heated floor area

  • Calculated room-by-room or total heat loss

  • Outdoor design temperature

  • Existing boiler fuel and output

  • Current flow and return temperatures

  • Radiator list, dimensions or photos

  • Required indoor temperatures

  • Domestic-hot-water demand

  • Available electrical supply

  • Hydraulic schematic, if available

  • Required quantity and project schedule

EXINDA will use this information to help identify the appropriate model data and the next technical steps.