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Do I Need Bigger Radiators for a Heat Pump? A Commercial Guide to Emitter Sizing & Heat Pump Efficiency

 

Heat pumps are transforming how UK buildings stay warm, efficient, sustainable, and ready for the net-zero future. But one question comes up in almost every installation conversation:

“Will my existing radiators and heat emitters work with a heat pump?”

The short answer is sometimes. But in commercial settings, achieving the correct emitter capacity is essential for meeting heating demand at low flow temperatures and unlocking high system efficiency.

In this guide, we’ll explain how heat pumps and radiators work together, how to know if your current setup is compatible, and how to figure out exactly what size radiators your building needs.

Why Radiator Size Matters for Heat Pumps

Traditional gas or oil boilers heat water to high temperatures, typically around 70–80°C, before pumping it through your radiators. Modern condensing boilers operate at lower flow temperatures as required under various rules, typically about 55°C. This means there are a lot of different variations of emitter size out there in the UK’s building stock. 

Heat pumps, on the other hand, work differently. They use electricity to extract heat from the air or ground and deliver it into your building at a lower flow temperature, usually between 35°C and 55°C. Some heat pumps, like our Elm and Yew, can match traditional boiler temperatures but are still more efficient at low flow temperatures. 

However, it’s not only the flow temperature that matters. The temperature differential (ΔT) between flow and return sets the mean water temperature (MWT) through the radiator, and this is what determines emitter output and required radiator size.

Mean Water Temperature, ΔT, and Radiator Output

Radiator output depends on the temperature difference between the mean water temperature and the room temperature.

A smaller ΔT at a given flow temperature results in a higher mean water temperature, which increases radiator output and allows smaller emitters. A larger ΔT lowers the mean water temperature, which reduces radiator output and requires larger emitters.

For example:

  • A CO₂ heat pump operating at 70°C flow and 30°C return has a mean water temperature of 50°C.
  • An R290 heat pump operating at 55°C flow and 45°C return also has a mean water temperature of 50°C.

In this case, both systems would require the same size radiator, because the mean water temperature, and therefore the radiator-to-room temperature difference, is the same.

By contrast, if the flow temperature is held constant:

  • 55°C flow with a 5°C ΔT gives an MWT of 52.5°C
  • 55°C flow with a 10°C ΔT gives an MWT of 50°C

The larger ΔT reduces the mean water temperature, which would require larger emitters to achieve the same heat output.

This is why ΔT should never be considered in isolation. Emitter sizing must always be based on mean water temperature, not flow temperature alone.

It’s also worth noting that a larger ΔT mainly benefits system hydraulics, which enables reduced flow rates, smaller pipework, and lower pumping energy, rather than reducing emitter size.

How to Work Out Radiator Size in Commercial Buildings

The good news is that many modern radiators are already suitable for heat pump systems, especially if a building is well insulated. Here’s how to get a sense of whether yours will be up to the job:

1. Calculate the heat loss of each zone

This determines the wattage the radiator must deliver.
Heat loss depends on a number of factors, which all engineers should be familiar with. The more detailed the calculation, the more accurate the outcome; equally, the more estimates and assumptions that are used, the less reliable the number.

2. Look at the required flow temperature

Often:

  • R290 systems operate around 45–65°C, or high-temperature options at 70–80°C, with a 5–20°C ΔT
  • CO₂ systems can deliver up to 80°C with a high ΔT

Emitter output at 80°C is vastly different from output at 45°C, so a radiator heat output chart should always be used to match the actual operating conditions.

Radiators are rated at a ΔT, which is the difference between the mean water temperature and the room temperature. Common reference points include ΔT50 and ΔT20, with manufacturers able to provide correction factors where required.

In many commercial buildings, with the right heat pump and the correct combination of flow temperature and ΔT, no emitter changes are needed. This can result in significant installation cost savings, although it may lead to a lower SCOP. The customer and designer need to find the right balance.

The Best Radiators for Heat Pumps (Commercial)

Here’s how the main emitter types compare in real commercial environments.

Steel Panel Emitters

These are the most common in commercial buildings.

  • Higher surface area due to multiple panels and convector fins
  • Strong performance at ΔT30–ΔT20
  • Good output-to-size ratio, ideal for offices, corridors, classrooms, and mixed-use spaces
  • Easy to source and install, with predictable performance data across manufacturers

In many retrofit projects, replacing these is relatively straightforward.

Fan-Assisted Radiators / Low-Temperature Convectors

A smart choice where wall space is limited, or heat loss is higher than standard radiators can cover.

  • Integrated fans help maintain high heat output even at very low water temperatures
  • Ideal for legacy buildings, public sector estates, schools, and large open-plan areas
  • Fans allow rapid response to changing loads, helping heat pumps avoid cycling
  • Often used to avoid oversizing radiators when aesthetics or space are a constraint

These are a favourite in commercial retrofits where ΔT20–ΔT25 systems need extra output without upsizing pipework dramatically.

Fan Coil Units (FCUs)

Many commercial buildings already use FCUs, making heat pump integration simpler than expected.

  • Designed for variable flow temperatures
  • Provide forced convection, meaning strong output at ΔT30 or lower
  • Often only need minor adjustments (e.g., coil cleaning, valve updates, or flow balancing)
  • Work well with R290 and CO₂ heat pump systems, depending on building demand

For offices, retail units, universities, and mixed-use buildings, FCUs can often remain unchanged during a heat pump retrofit.

Talk To Us About System and Emitter Design

Getting radiator design right is one of the most important steps in a successful heat pump installation. 

From heat loss calculations and emitter selection to system balancing and commissioning, every detail affects performance, comfort, and long-term efficiency.

That’s where we can help.

At Clade, our engineers design, manufacture, and support complete natural-refrigerant heat pump systems to ensure that radiators, controls, and flow temperatures all work together seamlessly.

Just get in touch to discuss your project or learn how we can help you design the right radiator setup for your heat pump system.