Switching from a gas boiler to a heat pump often requires radiators that are 2.5 to 3 times larger in heat output to work efficiently at lower flow temperatures.
If you are planning to replace a gas boiler with an air source heat pump, you may be wondering whether your existing radiators will be adequate. The short answer is that they almost certainly will not be, unless they are already oversized for the gas boiler.
The average UK gas boiler runs at a flow temperature of 65-75°C, whereas a typical air source heat pump operates most efficiently at 35-55°C (Energy Saving Trust, 2026). A radiator’s heat output, measured in watts, is directly linked to the temperature difference (ΔT) between the water inside it and the room air. Lower flow temperatures mean a lower ΔT, which drastically reduces output. To maintain the same room temperature with a heat pump, the radiator surface area must be increased to compensate for the lower water temperature. This size increase is the single most common retrofit requirement when replacing a gas boiler, and it is often overlooked in initial quotes (Energy Systems Catapult, Electrification of Heat Demonstration Project reports; MCS installation standards).
How radiator output changes with flow temperature (the ΔT rule)
Radiators are traditionally rated at ΔT50 (75°C flow, 20°C room) or ΔT60 (90°C flow, 20°C room) under the standard BS EN 442. A heat pump typically runs at a ΔT of 25-35. The consequence is significant: a radiator rated at 1,000W at ΔT50 will only output approximately 500W at ΔT30, meaning you need roughly double the radiator size for the same heat delivery (BS EN 442; manufacturer technical data tables). The exact correction factor can be calculated using the formula: Output at new ΔT = Rated output x (new ΔT / rated ΔT)^1.3 (CIBSE Guide B; Stelrad or Myson radiator technical manuals). For a real-world example, a radiator designed for a gas boiler (ΔT50) may lose 40-50% of its stated output when run at a heat pump’s typical 45°C flow temperature (EST field trial data; DESNZ heat pump performance reports).
Quick numbers radiator size comparison for a typical UK home
The table below shows the radiator length required for a typical 4x4m room in a home built to 2026 Building Regulations insulation levels. The figures are based on a standard Stelrad type 22 double-panel double-convector radiator and the MCS Heat Loss Calculator methodology (MCS Heat Loss Calculator; Stelrad technical data).
| Room type | Required heat loss (watts) | Gas boiler radiator width (mm) at ΔT50 (75°C flow) | Heat pump radiator width (mm) at ΔT30 (45°C flow) | Percentage increase in length |
|---|---|---|---|---|
| Living room | 1,500 | 600 | 1,400 | 133% |
| Bedroom | 1,000 | 500 | 1,100 | 120% |
| Kitchen | 1,200 | 550 | 1,250 | 127% |
| Hallway | 800 | 450 | 900 | 100% |
These figures assume a standard 600mm height radiator. If your existing radiators are single-panel (type 11), the increase required will be even greater, often exceeding 200% in length.
The direct answer radiator sizing for a heat pump means using the correct flow temperature and ΔT
Radiator sizing for a heat pump is not about the radiator’s physical dimensions alone. It is calculated by matching the radiator’s output at the heat pump’s design flow temperature (typically 35-55°C) to the room’s calculated heat loss. You must first know your heat pump’s design flow temperature, which is set by the installer and is usually 45°C or 50°C for retrofits. Then, use the radiator manufacturer’s correction factor table for that specific ΔT (flow temp minus room temp) to find the actual output. A properly sized heat pump radiator will be significantly larger and often longer or deeper (double-panel, double-convector, e.g., type 22) than a gas boiler radiator for the same room (MCS 022; EST guide “Heat pumps in existing homes”).
guide to heat pump flow temperatures and efficiency
How to verify your existing radiators are large enough for a heat pump
The installer should perform a room-by-room heat loss calculation using MCS 022 methodology to determine the wattage required for each room at the heat pump’s design flow temperature. Compare this required wattage to the actual output of your existing radiators at that same flow temperature (using the ΔT correction). If the existing radiator output is lower, it must be upsized. A simple rule of thumb: if your existing radiators are single-panel (type 11), they are almost certainly too small. Most heat pump retrofits require double-panel (type 22) or triple-panel radiators (Energy Saving Trust, 2026; EST guide “Heat pumps: a guide for householders”). The MCS installer is contractually required to ensure the radiators are correctly sized for the heat pump’s design conditions (MCS installation standards; DESNZ heat pump consumer guidance).
Eligibility and installer certification who can size radiators for a heat pump correctly?
For a government grant, such as the Boiler Upgrade Scheme in England and Wales, or equivalent schemes in Scotland and Northern Ireland, the heat pump and radiator sizing must be carried out by an MCS-certified installer (GOV.UK, Boiler Upgrade Scheme guidance; Ofgem administration rules). The MCS certification (Microgeneration Certification Scheme) requires the installer to follow MCS 022 (Heat Emitter Guide) for radiator sizing and to produce a system design document. TrustMark is a separate but required registration for all work funded by the Boiler Upgrade Scheme. Always check the installer is on the MCS Installer Database and has TrustMark (GOV.UK; TrustMark website). For safety, any pipework or radiator replacement work must be done by a Gas Safe registered engineer if gas is involved in the existing system, or a qualified plumber or heating engineer who can evidence competence (Gas Safe Register).
guide to the Boiler Upgrade Scheme eligibility and application process
