The main reason heat pumps need larger radiators than gas boilers
If you are switching from a gas boiler to a heat pump, the radiators in your home will almost certainly need to change. Heat pumps operate at lower flow temperatures than gas boilers, which means each radiator transfers less heat into the room.
The direct answer is that heat pumps typically need radiators with 2 to 3 times the surface area of those used with a gas boiler, and the average cost to upgrade a single radiator is between £150 and £300 (2026 prices).
A gas boiler sends water through your radiators at 60–75°C. A heat pump, to remain efficient, sends water at 35–55°C (Energy Saving Trust, 2026). Because the temperature difference between the radiator surface and the room air is smaller, the radiator must be physically larger to release the same amount of heat. A standard double-panel radiator that works well with a gas boiler may need to be replaced with a triple-panel or double-convector model for a heat pump. The surface area increase is achieved through extra panels, deeper fins, or a longer and taller unit.
How to calculate the correct radiator size for your heat pump
Radiator sizing for a heat pump is not guesswork. The key metric is the radiator’s delta-T rating. Delta-T (ΔT) is the average temperature difference between the water inside the radiator and the room air, measured at a standard test condition. Gas boiler radiators are typically rated at ΔT50. Heat pump radiators should be rated at ΔT30 or ΔT40. This reflects the lower flow temperatures.
To calculate the required radiator size, you first need the room heat loss in kilowatts (kW). This is calculated by a heat pump installer using the MCS Heat Pump Installer Standard (MIS 3005) and CIBSE Guide B1. The formula is: required radiator output at test delta-T = room heat loss (kW) × (test delta-T / actual delta-T). For example, a room that loses 1.5 kW of heat and will be served by a heat pump at ΔT30 needs a radiator rated at about 2.5 kW at ΔT50. That is a 67% increase in rated output.
Online calculator tools from the Energy Saving Trust and the Heat Pump Association (HPA) can help homeowners input room dimensions and insulation levels to get a rough idea (Heat Pump Association, 2026). However, the final sizing must be done by a certified installer as part of a room-by-room heat loss calculation.
Quick numbers radiator size increases and upgrade costs
The table below shows typical output ratings for common radiator types at the two key delta-T values, along with the size increase factor and average installed cost in 2026.
| Radiator type | Output at ΔT30 (kW) | Output at ΔT50 (kW) | Size increase factor | Average installed cost (2026) |
|---|---|---|---|---|
| Single panel, single convector (Type 11) | 0.6 | 1.0 | 1.7x | £100–£200 |
| Double panel, single convector (Type 21) | 1.0 | 1.7 | 1.7x | £150–£300 |
| Double panel, double convector (Type 22) | 1.4 | 2.4 | 1.7x | £200–£400 |
| Triple panel, double convector (Type 33) | 2.0 | 3.4 | 1.7x | £300–£500 |
Costs are based on the DESNZ Consumer Cost Database and EST installation cost data (DESNZ, 2025–2026). A 1.2m double-panel radiator (Type 22) used with a gas boiler may cost £150–£300 to replace with a triple-panel model (Type 33) for a heat pump. Installation costs include pipework adjustments and removal of the old radiator.
The direct answer do you need new radiators for a heat pump?
Often yes. Most existing UK radiators were designed for the high flow temperatures of gas boilers and are too small for efficient heat pump operation (Energy Saving Trust, 2026). Exceptions exist. Homes with oversized radiators, such as those originally installed for a large gas boiler, or homes with convector-style radiators that have high surface area, may not need replacements. Underfloor heating is also an excellent match for heat pumps because of its large surface area and low temperature requirement.
A heat pump installer must calculate room-by-room heat loss and radiator sizing as part of an MCS-compliant design. This is not optional. The MCS Heat Pump Installer Standard (MIS 3005) requires that the system be built to meet the full heat load at the lowest expected outdoor temperature. If your installer does not offer a room-by-room heat loss calculation, that is a red flag.
Eligibility for heat pump grants and the radiator upgrade link
The Boiler Upgrade Scheme (BUS) in England and Wales provides £7,500 towards the cost of an air-source heat pump in 2026 (GOV.UK, 2026). The grant does not explicitly cover radiator upgrades. However, many MCS-certified installers include radiator replacement in their fixed-price quote because the system cannot function properly without correctly sized radiators. If the installer charges separately for radiators, the grant amount remains fixed at £7,500. You pay the difference.
Homeowners in England and Wales apply through their MCS-certified installer. Scotland has separate grants through Home Energy Scotland, which may offer additional funding for radiator upgrades. The key point is that radiator costs are not reimbursed separately under the BUS, so check your installer’s quote carefully for what is included.
How to apply for the Boiler Upgrade Scheme in 2026
How to verify your heat pump installer is properly certified
Only MCS-certified installers can access the BUS grant. Check the MCS Installer Database at mcscertified.com to confirm certification (MCS, 2026). Installers should also be registered with TrustMark for consumer protection and, if they work on existing gas pipework, with the Gas Safe Register.
Before agreeing to any work, ask for a room-by-room heat loss calculation and a radiator sizing report. This report should list each room, its heat loss in kW, the proposed radiator type and size, and the expected flow temperature. If the installer cannot provide this in writing, do not proceed. The MCS standard requires it, and without it your heat pump will likely underperform.
What happens if you don’t upgrade radiators for a heat pump
Installing a heat pump with undersized radiators leads to several problems. The heat pump will run for longer periods to try to reach the target temperature, increasing electricity bills and reducing efficiency. It may also struggle to maintain comfort during cold weather, leaving rooms below the set temperature (Heat Pump Association, 2026). The DESNZ Heat Pump Field Trials found that systems with undersized radiators had significantly lower seasonal performance factors, meaning higher running costs.
There is also a warranty risk. Some heat pump manufacturers require that the system be built to their specifications, including correct radiator sizing. If a warranty claim is made and the installer cannot demonstrate that the radiators were sized correctly, the claim may be rejected (MCS, 2026). Always check the manufacturer’s installation requirements before proceeding.
Frequently Asked Questions
Yes, because heat pumps run at lower flow temperatures (35-55°C) than gas boilers (60-75°C). The Energy Saving Trust states that the smaller temperature difference means each radiator transfers less heat, so you need 2-3 times the surface area to achieve the same output.
Between £150 and £300 per radiator at 2026 prices, according to industry averages. A typical 3-bed semi with 8 radiators could cost £1,200-£2,400 in total. Check with MCS-certified installers for exact quotes.
Use a delta-T30 or delta-T40 rating, not the delta-T50 used for gas boilers. Calculate by multiplying room heat loss (kW) by the ratio of test delta-T to actual delta-T, as per CIBSE Guide B1. An installer will do this per MCS standard MIS 3005.
Usually no, unless they are already oversized. Existing radiators designed for gas boilers (delta-T50) will not output enough heat at the lower temperatures. The Energy Saving Trust advises replacing them to avoid cold rooms and poor efficiency.
Triple-panel or double-convector radiators with large surface areas and deep fins work best. Ofgem recommends models rated at delta-T30 to match heat pump flow temperatures. Vertical or horizontal designs can be used as long as the surface area is sufficient.
