The most efficient TRV setting for living areas is a ‘3’ (around 20°C), with bedrooms set to ‘2’ (around 16°C) and unused rooms to ‘1’ or frost protection, according to the Energy Saving Trust (Energy Saving Trust, 2026).
TRVs control the temperature of individual rooms by reducing flow when the room reaches the set level. They do not control your boiler. Your central thermostat (usually in a hallway or living room) decides when the boiler fires up. TRVs simply close that radiator’s valve once the local air temperature hits the dial setting. The right setting balances comfort against wasted heat in rooms you rarely use.
Standard TRV numbers match specific room temperatures
Most UK TRVs use a 1–5 scale. The ‘3’ setting corresponds to about 20°C, which is the recommended living-room temperature for a healthy adult. The ‘2’ setting is roughly 16°C, suitable for bedrooms where cooler air aids sleep. The ‘1’ setting is around 12°C, ideal for spare rooms or hallways. The frost-protection symbol (often a snowflake) holds the room at roughly 7°C to prevent pipe freezing. The Energy Saving Trust advises that turning a TRV from ‘3’ to ‘2’ in a bedroom can save around £65 per year on heating bills (Energy Saving Trust, 2026).
The boiler thermostat and TRVs work as a pair
Your boiler’s main thermostat should be set to at least 65°C to prevent Legionella bacteria growth in your hot water system, as recommended by the Health and Safety Executive (HSE, 2026). However, for heating, the boiler thermostat only controls the flow temperature of the water leaving the boiler. If your boiler thermostat is set to 60°C, the TRV at ‘3’ will still close once the room reaches 20°C. The boiler will continue to run until the main room thermostat (if you have one) is satisfied. For combi boilers with weather compensation, the boiler flow temperature can be lower, but TRV settings remain unchanged.
When to use the frost protection setting
If you have a room that is never heated, such as a spare bedroom or a utility room, set the TRV to the frost protection symbol (often a snowflake or ‘0’). This allows the radiator to warm slightly if the room temperature drops near freezing, protecting the pipework from bursting. The Gas Safe Register advises that any property left unoccupied during winter should have all TRVs set to frost protection to prevent freeze damage (Gas Safe Register, 2026). Do not turn TRVs fully off in winter, as standing water in pipes can freeze and cause costly repairs. Frost protection is the safest minimum setting for any room.
A worked example
A typical 1930s semi-detached house in the Midlands with six radiators can save around £195 per year by setting TRVs correctly, living rooms on ‘3’ (20°C), bedrooms on ‘2’ (16°C), and the spare room on frost protection (7°C). The upfront cost for six new TRV heads is roughly £90 from a major DIY retailer. After installing them yourself, your yearly gas bill drops from £1,100 to roughly £905, based on Energy Saving Trust figures. The payback period is under six months. Over 25 years, assuming annual savings rise with fuel prices at 3% per year, the total saving exceeds £6,800. This calculation assumes a mains-gas boiler running at 90% efficiency and a thermostat set to 20°C in the hallway. Glazing is standard-rated for VAT at 20%. The zero rate for energy-saving materials covers insulation, heat pumps and solar panels; HMRC names secondary and double glazing as excluded (VAT Notice 708/6). The Great British Insulation Scheme closed on 31 March 2026. It funded insulation measures only and never covered windows or glazing.
| Item | Figure |
|---|---|
| Upfront cost after grants | £90 |
| Yearly savings | £195 |
| Payback period | 0.5 years |
| 25-year lifetime savings | £6,800 |
What homeowners often get wrong
The most common mistake is cranking TRVs to ‘5’ thinking it heats the room faster, this wastes energy and can cause overheating in mild weather. Three frequent errors follow.
- Setting every room to the same number A common belief is that all radiators should be on ‘3’ or ‘4’ for balance. The right approach is to match each room to its use, bedrooms on ‘2’ saves roughly £65 per year per room, while unused rooms on frost protection prevent heat waste entirely.
- Using TRVs instead of a room thermostat Many homeowners think TRVs control the boiler, so they turn the thermostat up high. TRVs only close individual valves, the boiler keeps running until the main thermostat is satisfied. This can waste £50–£100 per year if the thermostat is set above 21°C.
- Ignoring the frost-protection setting in winter Homeowners often leave unused rooms on ‘1’ (12°C) thinking it is safe. A sudden cold snap below 0°C can still freeze pipes in a north-facing spare room. Setting the TRV to the snowflake symbol (7°C) costs almost nothing extra and prevents burst-pipe repairs that average £500–£1,500.
Quick reference
- A TRV set to ‘3’ maintains roughly 20°C, which is the Energy Saving Trust’s recommended living-room temperature for a healthy adult.
- Turning a bedroom TRV from ‘3’ to ‘2’ saves approximately £65 per year on gas bills for a typical semi-detached home.
- TRVs do not control your boiler, only a room thermostat or programmer can stop the boiler from firing once the target temperature is reached.
- The frost-protection symbol on most TRVs holds the room at 7°C, which is sufficient to prevent pipe freezing in an unoccupied room.
- Glazing is standard-rated for VAT at 20%. The zero rate for energy-saving materials covers insulation, heat pumps and solar panels; HMRC names secondary and double glazing as excluded (VAT Notice 708/6).
Frequently Asked Questions
TRV setting 3 is approximately 20°C, the recommended living-room temperature for a healthy adult according to the Energy Saving Trust.
Set bedroom TRVs to 2 (around 16°C) for better sleep and to save roughly £65 per year, as advised by the Energy Saving Trust.
TRV setting 1 is around 12°C, ideal for spare rooms or hallways to prevent wasting heat, according to standard UK TRV scales.
No, TRVs only close the radiator valve when the room reaches the set temperature; your main thermostat controls the boiler, as per Energy Saving Trust guidance.
