When replacing a gas or oil boiler with an air-to-water heat pump, one question appears almost immediately:
Do all the radiators need to be replaced?
Not necessarily.
Some existing radiators may already be large enough. Others may need to be upgraded, particularly in rooms with higher heat loss or limited radiator capacity.
The answer does not depend simply on the radiator’s age or physical dimensions. Correct heat pump radiator sizing requires comparing two figures:
- the design heat loss of each room;
- the heat output the radiator can deliver at the proposed water temperature.
Radiators release less heat when their average water temperature is reduced. A radiator that comfortably heated a room with high-temperature boiler water may provide much less heat when connected to a heat pump operating at a lower temperature.
The correct question is therefore not simply:
“Can a heat pump work with radiators?”
It is:
“Can these radiators deliver enough heat at the intended operating temperature?”
Why Heat Pump Radiator Sizing Matters
A heat pump and its radiators form one heating system.
The heat pump generates and transfers heat into the water circuit, while the radiators must release enough of that heat into each room.
This relationship can be summarised as:
room heat loss + radiator output + operating temperature = emitter design
If the radiators can satisfy the room heat demand using lower-temperature water, the heat pump can operate with a smaller temperature lift.
If a radiator is too small, the system may need a higher water temperature to maintain comfort during cold weather. Alternatively, that radiator may need to be replaced or supplemented.
This is why radiator sizing should be based on a proper heat loss calculation for heat pumps, not only on property area or the nominal capacity of the heat pump.
Why Radiator Output Changes with Water Temperature
A radiator transfers heat because its surface is warmer than the surrounding room.
The greater the difference between the radiator’s average water temperature and the room temperature, the more heat the radiator can release.
Radiator catalogues commonly state output at ΔT50. In this context, ΔT means:
mean radiator water temperature − room temperature
For example:
- flow temperature: 75°C;
- return temperature: 65°C;
- mean water temperature: 70°C;
- room temperature: 20°C;
- emitter temperature difference: 50 K.
This is therefore a ΔT50 rating condition.
It is important not to confuse this with the other ΔT used in heat-pump commissioning: the difference between the system’s flow and return water temperatures.
Approximate radiator-output factors
The following table illustrates how the output of a typical panel radiator may change as the emitter temperature difference decreases.
| Emitter temperature difference | Approximate output relative to ΔT50 |
|---|---|
| ΔT50 | 100% |
| ΔT40 | 75% |
| ΔT35 | 63% |
| ΔT30 | 51% |
| ΔT25 | 41% |
| ΔT20 | 30% |
These are approximate values based on a commonly used radiator exponent of around 1.3. The exact correction factor depends on the radiator design, so manufacturer data should be used for final selection.
The calculation can be expressed as:
Corrected output = rated output × (design ΔT ÷ rated ΔT)ⁿ
Where n is the radiator’s manufacturer-specified exponent.
The MCS Heat Emitter Guide similarly uses the relationship between room heat loss, emitter output and operating temperature to assess radiator systems.
A Simple Heat Pump Radiator-Sizing Example
Imagine a bedroom with a calculated design heat loss of:
1,000 W
The existing radiator has a catalogue output of:
2,500 W at ΔT50
Suppose the proposed design conditions produce an emitter temperature difference of approximately ΔT25.
Using an indicative correction factor of 41%:
2,500 W × 0.41 ≈ 1,025 W
At first glance, the radiator appears capable of matching the room’s 1,000 W design heat loss.
However, the margin is very small. The final decision should use the radiator manufacturer’s data and account for factors such as radiator covers, restricted airflow, pipe arrangement and the project’s design assumptions.
Now consider a radiator rated at:
1,500 W at ΔT50
Under the same indicative conditions:
1,500 W × 0.41 ≈ 615 W
That radiator would not provide enough output for a room with a 1,000 W design heat loss.
Possible solutions could include:
- installing a higher-output radiator;
- changing from a single-panel to a double- or triple-panel radiator;
- using a fan-assisted low-temperature emitter;
- adding another emitter;
- reducing the room’s heat loss;
- or designing the system around a higher water temperature.
This assessment should be performed for every heated room.
Start With Room-by-Room Heat Loss
Radiator selection should begin with the building rather than with the heat pump.
Each room loses heat through a different combination of:
- external walls;
- windows and doors;
- roofs or ceilings;
- floors;
- ventilation;
- air leakage;
- and adjoining unheated spaces.
Heat loss also depends on the intended indoor temperature and the local outdoor design temperature.
A living room with extensive glazing may therefore require considerably more heat than an internal hallway of the same floor area.
This is why a whole-house figure is not sufficient for radiator selection. A system designer needs the design heat loss of each room before checking the available emitter output.
The current MCS Heat Load Calculator likewise calculates both whole-property and individual-room heating requirements for emitter sizing.
How to Check an Existing Radiator
A practical radiator assessment follows five steps.
1. Establish the room’s design heat loss
Calculate how much heat the room loses under the selected winter design condition.
2. Identify the radiator
Record its:
- radiator type;
- height and length;
- number of panels;
- number of convector fins;
- manufacturer and model, if available;
- and nominal output.
3. Confirm the rating condition
Determine whether the stated radiator output is based on ΔT50 or another condition.
4. Select the intended operating temperatures
Calculate the mean water temperature and its difference from the target room temperature.
5. Correct the radiator output
Use the manufacturer’s correction table, software or exponent to estimate the output at the proposed conditions.
The corrected output should then be compared with the room’s design heat loss.
Do Existing Radiators Always Need Replacing?
No.
Some existing radiators are already larger than the rooms strictly require. This may be because:
- the original system was designed conservatively;
- the building has since received better insulation;
- windows have been upgraded;
- previous renovations reduced heat loss;
- or larger radiators were installed for faster warm-up.
That spare capacity can be useful in a heat-pump retrofit. An oversized radiator may still provide adequate output when operated with cooler water.
The correct approach is therefore not:
“Replace every radiator.”
It is:
“Check every room.”
After assessment, the result may be:
- the existing radiator can remain;
- a larger radiator is required;
- another emitter should be added;
- the room’s heat loss should be reduced;
- or a different design water temperature is needed.
Frequently, only the most demanding rooms require changes.
Can Better Insulation Make Existing Radiators Suitable?
Yes, potentially.
Improving the building fabric reduces the rate at which a room loses heat. The radiator then needs to deliver less heat to maintain the same indoor temperature.
Relevant improvements may include:
- roof or loft insulation;
- wall insulation;
- improved glazing;
- floor insulation;
- draught reduction;
- and better airtightness with appropriate ventilation.
For example, if improvements reduce a room’s design heat loss from 1,200 W to 850 W, a radiator delivering approximately 900 W at the proposed water temperature may become suitable without being replaced.
The MCS emitter guidance illustrates how reducing fabric and ventilation losses can enable a lower radiator flow temperature or reduce the required emitter size.
What If a Radiator Is Too Small?
Replacing a radiator does not always mean using something dramatically longer or taller.
A single-panel radiator may sometimes be replaced with:
- a double-panel radiator;
- a double-panel, double-convector model;
- a triple-panel radiator;
- a fan-assisted low-temperature radiator;
- or another high-output emitter.
These options increase the heat-transfer surface or enhance air movement.
The correct choice depends on:
- the room’s design heat loss;
- available wall space;
- required water temperature;
- acceptable noise level;
- pipe sizing;
- hydraulic flow;
- and the intended control strategy.
Underfloor heating can also work well with heat pumps because its large surface area can distribute useful heat at relatively low water temperatures. However, its suitability and available output still depend on floor construction, covering and room heat loss.
How Does Flow Temperature Affect Radiator Sizing?
Radiator size and water temperature are directly connected.
When the target water temperature decreases, radiator output also decreases. The system must compensate through:
- greater radiator surface area;
- lower building heat loss;
- another type of emitter;
- or a combination of these measures.
Conversely, increasing the water temperature allows the same radiator to release more heat.
This does not mean the system should automatically use the highest available temperature. The designer should identify the lowest practical operating temperature that can satisfy the room heat loads under the required conditions.
For a detailed explanation of the different operating-temperature categories, see Tongyi’s guide to the high-temperature R290 heat pump.
That article explains temperature capability. This radiator-sizing article explains how to determine whether individual emitters can use a chosen temperature successfully.
What Does R290 Change in a Radiator Retrofit?
High-temperature R290 technology can provide additional flexibility in buildings with existing radiators.
The Tongyi R290 monobloc supports heating, cooling and domestic hot water, with product information stating hot-water production up to 75°C.
This high-temperature capability should be understood as operating headroom, not as a recommendation to run the space-heating circuit continuously at the maximum temperature.
A retrofit could operate at moderate water temperatures for much of the heating season while using higher temperatures when building demand increases, subject to the selected model’s performance envelope and system design.
The preferred solution still combines:
suitable radiator capacity + appropriate operating temperature + accurate heat-pump sizing + good hydraulic design and control
Maximum temperature alone does not establish system efficiency or confirm that the emitters are correctly sized.
Radiator Sizing and Heat-Pump Sizing Are Different
These calculations are related, but they answer different questions.
Heat-pump sizing determines the heating capacity required for the building.
Radiator sizing determines whether each room can receive the heat it requires.
A correctly selected 10 kW heat pump cannot compensate for a bedroom radiator delivering only 600 W when that room needs 1,000 W.
Similarly, installing very large radiators will not correct a heat pump that is incorrectly selected for the building’s design load.
Both parts of the system must be assessed.
Do Water Flow and Hydraulic Balancing Matter?
Yes.
Even a correctly sized radiator must receive sufficient circulating water to transfer its intended heat output.
Poor balancing can cause some radiators to receive excessive flow while others receive too little. This can produce uneven room temperatures and prevent the system from operating as designed.
Commissioning should therefore include:
- confirming the required system flow rate;
- balancing the radiator circuits;
- checking pump settings;
- removing air;
- verifying flow and return temperatures;
- and testing operation under representative conditions.
Our guide to heat pump water flow explains these hydraulic requirements in more detail.
Radiator sizing is one part of the system. It does not replace correct hydraulic design.
Common Radiator-Sizing Mistakes
Using floor area alone
Two rooms with identical floor areas can have very different heat losses.
Using the ΔT50 output without correction
The catalogue rating may substantially overstate the output available at heat-pump operating temperatures.
Checking only the total radiator capacity
The total capacity might look sufficient while one individual room remains underheated.
Assuming every existing radiator must be replaced
Some radiators may already have adequate spare capacity.
Selecting maximum water temperature first
The intended operating temperature should emerge from the relationship between building demand, emitters and heat-pump performance.
Ignoring pipework and water flow
Larger radiators still require suitable pipework, circulation and balancing.
Practical Heat Pump Radiator-Sizing Process
A well-designed retrofit normally follows this sequence:
- Calculate the design heat loss of every heated room.
- Identify the existing radiator type and rated output.
- Confirm the radiator’s nominal rating condition.
- Choose the proposed flow and return temperatures.
- Calculate the emitter temperature difference.
- Apply the manufacturer’s output correction.
- Compare corrected radiator output with room heat loss.
- Upgrade only the emitters that cannot satisfy the design load.
- Check pipework, water flow and hydraulic balancing.
- Select the heat pump using the building’s calculated design requirement.
- Configure weather compensation so water temperature can reduce when outdoor conditions allow.
This process can avoid unnecessary radiator replacement while providing a stronger basis for system design.
Do You Need Bigger Radiators for a Heat Pump?
Sometimes—but not automatically.
The important question is whether each radiator can deliver the heat required by its room at the intended water temperature.
Some existing radiators may already be sufficiently large. Others may require upgrading, and reducing building heat loss can sometimes make an existing emitter suitable.
High-temperature R290 technology can provide additional retrofit flexibility, but correct design still begins with the same process:
Calculate the room heat loss, correct the radiator output and use the lowest practical water temperature that maintains comfort.
That is how radiator sizing becomes part of a complete heat-pump design rather than simply a radiator-replacement exercise.
FAQ
Do all radiators need replacing when installing a heat pump?
No. Each radiator should be checked against the room’s design heat loss at the proposed operating temperature. Some may remain suitable while others require upgrading.
How much larger should a radiator be for a heat pump?
There is no universal percentage. The required size depends on room heat loss, radiator type, mean water temperature and target indoor temperature.
Can a heat pump use existing radiators?
Often, yes. Their corrected output must be sufficient at the intended operating temperature, and the hydraulic system must provide adequate water flow.
What does ΔT50 mean on a radiator?
It means the radiator’s mean water temperature is 50 K above the room temperature. It does not mean there is a 50°C difference between flow and return water.
Can insulation reduce the need for larger radiators?
Potentially. Lower room heat loss means less radiator output is required to maintain the same indoor temperature.
