Choosing the right radiator is partly an engineering decision — if the output is too low the room will never reach temperature — and partly aesthetic. This guide explains the main radiator types available in the UK, how to calculate the BTU output you need, and what supply and installation typically cost in 2026.
What BTU Means and Why It Matters
BTU (British Thermal Unit) is the traditional unit used to quote radiator heat output in the UK; it measures the amount of heat energy transferred per hour. The equivalent SI unit is Watts (1 W = 3.41 BTU/h). Both units appear on product data sheets — some manufacturers use Watts, some BTU, and some list both.
The BTU figure quoted by manufacturers is measured at delta T (ΔT) 50 °C — the difference between mean water temperature and room temperature in the test. In a real installation with a modern condensing boiler running at lower flow temperatures (typically 60–70 °C flow, 50–60 °C return), the actual ΔT in a 21 °C room is closer to 34–44 °C. This means real-world output is roughly 25–35% lower than the quoted figure. Always apply a correction factor, or use the manufacturer’s de-rated output tables.
Sizing a Radiator: The Method
Step 1 — Calculate Room Heat Loss
The accurate method uses a heat loss calculation (BS EN 12831), but a practical rule of thumb for most UK properties:
| Property type | BTU per m³ of room volume |
|---|---|
| New build / well insulated (post-2000) | 100–130 BTU/m³ |
| Standard semi-detached (1960s–1990s) | 140–180 BTU/m³ |
| Older Victorian / Edwardian / poor insulation | 190–250 BTU/m³ |
Example: A 4 m × 3.5 m × 2.4 m living room in a 1970s semi = 33.6 m³. At 160 BTU/m³ = 5,376 BTU required. In Watts: 5,376 ÷ 3.41 = 1,576 W.
Step 2 — Add Adjustments
- Large window area (more than 20% of wall area): add 15%.
- North-facing room: add 10%.
- Ground floor over uninsulated void: add 10%.
- Open-plan room: size for the entire open area.
Step 3 — Account for ΔT Correction
If your boiler runs at lower temperatures (or you plan to in future with a heat pump), apply a ΔT correction. At ΔT40 instead of ΔT50, output drops to approximately 75% of the quoted figure. Choose a radiator rated at 25–35% more than your calculated heat requirement, or use the manufacturer’s ΔT40 figure if available.
Radiator Types
Panel Radiators (Convector Radiators)
The standard choice for most UK homes. A flat steel panel with welded internal water channels, usually backed by corrugated convector fins that increase airflow and heat output. Available in single-panel (P1), double-panel single convector (P+), and double-panel double convector (P2/22) configurations.
- Type 11 (single panel, single convector): lowest output; suitable for small or well-insulated rooms.
- Type 21 (double panel, single convector): mid-range; the most common choice.
- Type 22 (double panel, double convector): highest output for a given footprint; preferred for larger rooms.
Panel radiators are the cheapest option and are available in standard heights (300, 400, 500, 600 mm) and widths from 400 mm to 3,000 mm+, allowing precise output matching.
Column Radiators
Traditional multi-column design in cast iron or pressed steel. Cast iron retains heat longer (thermal mass) and continues radiating after the boiler shuts off. Pressed steel columns are lighter and less expensive.
Column radiators suit period properties aesthetically and are often used to maintain the look of an original Victorian or Edwardian interior. They tend to be physically larger for the same output as a panel radiator, and significantly heavier (cast iron requires proper wall fixings and may need two people to install).
Designer / Low-Surface-Temperature Radiators
Flat-panel or geometric designs in steel or aluminium. Often used in contemporary bathrooms, home offices, and hallways. Aluminium radiators heat up and cool down more quickly than steel — advantageous with modulating boilers and smart controls.
Low-surface-temperature (LST) radiators are specified for schools, care homes, and any space where vulnerable people may contact hot surfaces. Not usually required in standard domestic installations.
Underfloor Heating (UFH)
Not a radiator in the traditional sense, but a competing product for domestic heat delivery. Wet UFH distributes heat via pipes in or under the floor screed at low flow temperatures (35–45 °C), ideal for heat pump systems. The floor acts as a very large low-temperature radiator.
Cost Comparison by Type
| Type | Supply cost range | Typical output range | Notes |
|---|---|---|---|
| Panel type 11 (single) | £20–£80 | 500–3,000 BTU | Budget; limited output |
| Panel type 22 (double) | £30–£200 | 1,500–20,000+ BTU | Most common choice |
| Steel column (white) | £80–£400 | 2,000–15,000 BTU | Stylish; mid-weight |
| Cast iron column | £150–£800+ | 2,000–12,000 BTU | Heavy; period look |
| Aluminium designer | £100–£500 | 1,500–10,000 BTU | Fast response; modern |
| Low-profile (skirting) | £150–£600/m | 500–2,500 BTU/m | Discreet; below windows |
Installation Costs
| Job | Typical cost (2026, England) |
|---|---|
| Replace existing radiator (same size, same pipework) | £150–£250 including supply |
| Replace existing radiator (different size, pipe adjustments) | £200–£350 |
| Add new radiator (extend pipework from nearby circuit) | £300–£500 |
| Full system drain and refill (for multiple swaps) | £100–£200 additionally |
| Thermostatic radiator valve (TRV) upgrade | £30–£60 per valve (supply + fit) |
London and South East pricing typically runs 20–30% above these figures. A plumber or gas engineer charges £200–£300/day in most areas.
Valves and Controls
Every radiator needs two valves — a flow valve (on the inlet) and a lockshield valve (on the return, used for balancing the system). Options for the flow valve:
- Manual valve — turn by hand to adjust flow. Simple, low-cost, no automatic temperature regulation.
- Thermostatic radiator valve (TRV) — senses room temperature and adjusts flow automatically. Part L of the Building Regulations requires TRVs on all radiators in new and replacement installations, with the exception of the one radiator used for the room thermostat (typically the hallway or living room).
- Smart TRV — connects to a smart home system (Hive, Tado, Honeywell) and allows room-by-room scheduling via an app. Adds £40–£100 per valve over a standard TRV.
Balancing the System
When adding or replacing radiators, the system may need balancing — adjusting the lockshield valves so heat is distributed evenly across all radiators. An unbalanced system results in radiators near the boiler getting very hot while those at the far end of the circuit remain cool. Balancing can be done DIY using a clip-on thermometer on the flow and return pipes; the aim is a 10–12 °C temperature difference across each radiator.
Summary: Choosing the Right Radiator
- Living room or bedroom: type 22 panel radiator — best output for the price.
- Period property: steel or cast iron column — suits the aesthetic and is compatible with standard pipework.
- Bathroom: towel radiator (dual fuel if year-round use is needed) — see separate guide.
- Contemporary space: aluminium or flat-panel designer radiator — fast response, slim profile.
- Heat pump system: large low-temperature radiator or UFH — sized to run at ΔT20–30 rather than ΔT50.
