Home Heat Pump Guide

Heat Pump Radiators: What You Need to Know

Radiators are the link between your heat pump and the warmth you feel in each room. Get them right, and your heat pump runs efficiently, your home stays comfortable, and your electricity bills stay low. Get them wrong, and the heat pump has to work harder, run at higher temperatures, and consume more energy — potentially undermining the whole point of switching from a gas boiler.

This guide explains how radiators work with heat pumps, why low flow temperatures matter, which radiator types perform best, and how to make sure your heating distribution is properly matched to your system.

How Heat Pump Radiators Differ from Boiler Radiators

Technically, there is no such thing as a "heat pump radiator." Any radiator will work with a heat pump — the question is whether it is large enough to deliver sufficient heat at the lower flow temperatures a heat pump operates at.

With a gas boiler, water flows through your radiators at 60–80°C. That high temperature creates a large temperature difference between the radiator surface and the room air (known as Delta T), which drives heat output. Even a modest-sized radiator can produce plenty of heat at these temperatures.

A heat pump works most efficiently at flow temperatures of 35–50°C. The lower temperature means a smaller Delta T, which means less heat output per square metre of radiator surface. To compensate, radiators for heat pump systems need to be larger than those designed for boilers — or of a type that enhances heat transfer (like fan convectors).

Understanding Delta T and Radiator Output

Delta T (ΔT) is the temperature difference between the average water temperature in the radiator and the room temperature. It is the single most important factor in determining radiator output.

Example

  • Boiler system: Flow 75°C, return 65°C, average 70°C. Room at 21°C. Delta T = 70 - 21 = 49°C (rounded to ΔT50 in manufacturer data).
  • Heat pump system at 45°C: Flow 45°C, return 40°C, average 42.5°C. Room at 21°C. Delta T = 42.5 - 21 = 21.5°C (approximately ΔT25).

At ΔT25, a radiator produces roughly 44–50% of its output at ΔT50. This is why the rated output on the manufacturer's label (always given at ΔT50) is misleading for heat pump use — you need to apply correction factors to find the actual output at your design flow temperature.

Correction Factors

Radiator manufacturers publish correction factors for different Delta T values. Here are approximate outputs as a percentage of the ΔT50 rating:

  • ΔT50 (boiler at 75°C flow): 100% of rated output
  • ΔT40 (55°C flow): approximately 75%
  • ΔT30 (45°C flow): approximately 56%
  • ΔT25 (40°C flow): approximately 44%
  • ΔT20 (35°C flow): approximately 34%

These figures show why flow temperature matters so much. Running a heat pump at 55°C instead of 45°C means your existing radiators produce roughly 35% more heat — but the heat pump operates significantly less efficiently.

Types of Radiators for Heat Pumps

Double-Panel Convectors (Type 22)

These are the workhorses of heat pump installations. A Type 22 radiator has two panels and two sets of convector fins, delivering roughly twice the output of a single-panel (Type 11) radiator of the same physical size.

If your home already has Type 22 radiators of a reasonable size, they may well be adequate for heat pump operation — especially if your home is well insulated. If you need to upgrade from Type 11 to Type 22, the pipework connections are usually in the same position, making the swap relatively straightforward.

Type 33 (Triple-Panel) Radiators

Triple-panel radiators (Type 33) have three panels and three sets of convector fins. They deliver even more heat than Type 22 but are deeper (roughly 160mm vs 100mm for Type 22). They are useful in rooms with high heat loss where wall space is limited, but they are heavier and more expensive.

Fan Convector Radiators

Fan convectors have an internal fan that forces air over the heat exchanger, dramatically boosting output at low flow temperatures. A fan convector can produce 2–3 times the heat of a standard radiator of the same size at 45°C flow temperature.

They are particularly useful in:

  • Rooms where wall space prevents fitting a larger standard radiator
  • Open-plan living areas with high heat loss
  • Kitchens and utility rooms

The trade-off is that they are more expensive (£400–£800 per unit) and produce some fan noise — though modern units are very quiet (25–35 dB).

Aluminium Radiators

Aluminium radiators heat up and cool down faster than steel, making them responsive to temperature changes. They also tend to have a slightly higher output per kilogram, meaning they can be lighter for the same heat output. They are well-suited to heat pump systems and are increasingly popular in the UK.

Aluminium radiators are available in modern column designs that look quite different from traditional steel panels — which some homeowners prefer aesthetically.

Cast Iron Radiators

Traditional cast iron column radiators have a loyal following for their appearance, particularly in period properties. They retain heat well, which can be beneficial for maintaining even temperatures. However, they have lower output per square metre compared to modern convector designs, and they heat up slowly.

Cast iron radiators can work with heat pumps, but they often need to be very large — which is fine if you have the wall space and prefer the aesthetic. They are not the most efficient choice purely from a heating engineering perspective.

How to Size Radiators for a Heat Pump

Radiator sizing for a heat pump follows a logical process:

Step 1: Room-by-Room Heat Loss Calculation

Your installer calculates the heat loss for each room based on dimensions, insulation, windows, and the design outdoor temperature. This gives the required heat output in watts for each space.

Step 2: Determine the Design Flow Temperature

The installer selects the flow temperature — ideally 45°C or lower for optimal heat pump efficiency. This determines the correction factor to apply to radiator ratings.

Step 3: Calculate Required Radiator Output

Using the correction factor, the installer works out what size radiator is needed to deliver the required heat at the design flow temperature. For example:

  • Room heat loss: 1,200W
  • Design flow temperature: 45°C (ΔT25)
  • Correction factor: 0.44
  • Required radiator rating at ΔT50: 1,200 ÷ 0.44 = 2,727W

So you need a radiator rated at approximately 2,700W at ΔT50 to deliver 1,200W at a 45°C flow temperature. This is roughly 1.5–2 times larger than what a boiler system would need.

Step 4: Select the Radiator

The installer selects a radiator (or combination of radiators) that delivers the required output while fitting the available wall space. This might be a large Type 22, a Type 33, a fan convector, or a combination of standard radiator plus underfloor heating.

Do "Low-Temperature Radiators" Exist?

You will see some products marketed as "low-temperature radiators" or "heat pump radiators." In most cases, these are simply oversized standard radiators or fan convectors — there is nothing fundamentally different about the technology. The marketing term recognises that they are designed with heat pump flow temperatures in mind.

Some manufacturers do produce radiators specifically optimised for low-temperature operation, with enhanced convector fin designs or integrated fan systems. These can be genuinely better performers, but the core principle is the same: more heat transfer surface area at a lower temperature difference.

Radiator Positioning for Heat Pumps

Traditional advice says "put radiators under windows" because that is where the most heat loss occurs (from cold air dropping down the glass). This remains good practice with heat pumps, but there are additional considerations:

  • Do not block radiators with furniture. At lower flow temperatures, restricted airflow over the radiator surface has a more noticeable impact on output.
  • Use radiator reflector panels. These reflect heat from the back of the radiator into the room instead of heating the wall. They are cheap (£10–£20 per radiator) and effective.
  • Ensure adequate clearance. Leave at least 50mm between the floor and the bottom of the radiator, and 100mm between the top of the radiator and any windowsill, to allow natural convection.

System Balancing

System balancing is the process of adjusting the flow rate to each radiator so that heat is distributed evenly across the house. It is essential for heat pump systems because:

  • Heat pumps produce lower flow temperatures, so every degree counts
  • An unbalanced system means some rooms overheat while others stay cold
  • Proper balancing ensures the return temperature is consistent, which helps the heat pump operate efficiently

Balancing involves adjusting the lockshield valve on each radiator to control the flow rate. Your installer should do this during commissioning — it is not optional.

Radiators and Weather Compensation

Weather compensation adjusts the flow temperature automatically based on the outdoor temperature. On a mild 10°C day, the flow might drop to 35°C. On a freezing -3°C day, it rises to 50°C. This keeps the heat pump running as efficiently as possible at all times.

For weather compensation to work properly, radiators need to be sized for the lowest expected flow temperature on the coldest design day. If they are borderline at the design temperature, the system may struggle on the coldest days even though it performs beautifully for 95% of the year.

A well-designed system accounts for this by sizing radiators with a small margin, ensuring comfort even on the very coldest days of a typical UK winter.

Mixing Radiators and Underfloor Heating

Many modern installations combine radiators upstairs with underfloor heating downstairs — or use underfloor heating in specific rooms (kitchen, bathroom) alongside radiators in others. This is a perfectly good approach, but the system needs a mixing valve to deliver different flow temperatures to each circuit:

  • Underfloor heating circuit: Typically 30–35°C
  • Radiator circuit: Typically 40–50°C

Your installer will design the system to manage these different circuits, ensuring each receives the right flow temperature for efficient operation.

Frequently Asked Questions

What type of radiator is best for a heat pump?

Double-panel convector radiators (Type 22) are the most common and cost-effective choice. They deliver good output at low flow temperatures and are widely available. Fan convectors are excellent where space is limited. Aluminium radiators are a premium option with good responsiveness.

Will my radiators feel warm with a heat pump?

Yes, they will feel warm — but not burning hot as they might with a boiler. At 45°C flow temperature, a radiator surface will be around 35–40°C to the touch, which is noticeably warm but comfortable to lean against. This is normal and the rooms will still reach the set temperature.

Can I use my existing radiator pipework with a heat pump?

In most cases, yes. Standard 15mm and 22mm copper or plastic pipework used in UK central heating systems is typically adequate. If you have microbore pipework (8mm or 10mm), some sections may need upgrading. Your installer will assess this during the survey.

Do I need thermostatic radiator valves (TRVs) with a heat pump?

TRVs are useful for controlling individual room temperatures, but they work slightly differently with a heat pump than with a boiler. Your installer should ensure the system has adequate flow even when multiple TRVs are closed, typically by including a bypass valve or buffer. Some modern heat pump systems use electronic TRVs for more precise control.

How much do new radiators for a heat pump cost?

Standard double-panel convector radiators cost £100–£300 each to buy, plus £100–£200 for fitting. Fan convectors cost £300–£600 each, plus fitting. Most homes need 0–5 radiator upgrades, so budget £0–£2,500 as part of the overall installation cost.

Is underfloor heating always better than radiators for a heat pump?

Underfloor heating is more efficient because it operates at lower flow temperatures. But it is expensive to retrofit (£1,000–£3,000 per room) and impractical in many existing homes. Well-sized radiators at 45°C flow temperature are a very good second-best and far more practical for most retrofits.

Summary

Heat pump radiators are not a mystery. The principles are straightforward: lower flow temperatures require more radiator surface area. Whether you achieve this with larger standard radiators, fan convectors, or a mix of solutions depends on your home's specific characteristics.

The single most important step is a proper room-by-room assessment by your MCS-certified installer. With accurate data, your installer can design a system that keeps every room comfortable while allowing the heat pump to run as efficiently as possible — and that is the whole point.