Thermostatic Radiator Valves: Selection, Setting Temperature Ranges & Lockshield Balancing

Summary

TRVs are the most common form of individual room temperature control in UK wet central heating systems and have been standard practice since building regulations began requiring room-by-room control in 1994. Despite being commonplace, they are frequently misunderstood and often left on incorrect settings — factory-set to maximum and left there, defeating their purpose.

A TRV works by containing a wax or liquid-filled capsule (the thermostatic head) that expands as room temperature rises, pushing down a pin that closes the valve and restricts hot water flow to that radiator. When the room cools, the capsule contracts, the pin rises, and flow resumes. This is entirely mechanical — no electricity required. The thermostat on the boiler or the programmer controls when the system runs; the TRV controls how much heat each radiator emits when the system is running.

Understanding TRV selection, correct settings, bypass requirements, and system balancing is essential for any heating engineer. A well-balanced system with correctly set TRVs is more efficient, more comfortable, and less likely to cause boiler short-cycling.

Key Facts

• Building Regulations Part L1B — TRVs (or other zone control) required in all rooms except those containing a room thermostat. Required when replacing a boiler or carrying out significant heating upgrades
• TRV position scale — typically 1–5 or 1–6, with a frost symbol (❄) below 1. Position 3 ≈ 20°C, position 2 ≈ 18°C, position 4 ≈ 22°C. Exact temperatures vary by manufacturer
• One radiator without TRV — required as a bypass for pump protection. The hallway radiator is conventional (no occupant temperature preference). Alternatively, fit a bypass TRV set to maximum, or a manual valve set fully open
• Lockshield valve — the non-adjustable (usually plastic cap) valve on the return side of the radiator. Used by the heating engineer to balance the system — not adjusted by the occupant
• Balancing requirement — without balancing, radiators nearest the boiler will be hottest. The lockshield valve is opened partially to create an artificial flow restriction on near radiators, ensuring even distribution to distant radiators
• Frost setting — the ❄ symbol maintains approximately 5–7°C to prevent freezing. Use in unoccupied rooms or when the property is vacant in winter
• Flow direction — most TRVs have an arrow on the body indicating flow direction (from boiler, not from radiator). Installing backwards prevents correct operation
• Angled vs straight vs corner — TRVs come in straight, angled (90°), and corner (15°) configurations to accommodate different pipework entry positions (bottom, side, corner)
• Remote sensor heads — available for TRVs installed in confined positions (behind curtains, in cupboards, under windowsills) where the standard head would sense an unrepresentative temperature
• Smart TRVs — motorised TRVs with wireless control (e.g., Tado, Drayton Wiser, Honeywell Evohome). Can be programmed remotely, integrate with smart home systems, and provide better comfort control. Require a boiler communication protocol (OpenTherm) or a smart relay to work correctly

Quick Reference Table

Detailed Guidance

TRV Selection

Standard TRV heads: Most UK TRVs use a standard M30×1.5 thread for the valve head connection, meaning heads from different manufacturers are often interchangeable. However, always confirm the thread specification before mixing brands. Common brands: Drayton, Honeywell, Danfoss, Pegler, Giacomini.

Valve body material: TRV bodies are typically chrome-plated brass. Nickel-plated options available for aesthetic purposes. Confirm the connection size (typically 15mm compression or BSP thread for the pipe connection).

Angle of installation: The thermostatic head must be mounted horizontally, or within 30° of horizontal, for accurate temperature sensing. A vertically mounted head will sense the warm air rising from the radiator pipe and not the room air — it will close prematurely, causing the radiator to underperform. Where the pipe configuration forces a vertical mounting, use a remote sensor head connected by capillary tube and mounted on the wall in the room.

Draught risk: The TRV head must not be in a draught (e.g., positioned under a permanently open vent or beside a door in a cold hallway). A draught will cause the valve to open fully and the radiator to overheat, because the head senses cold air rather than room temperature.

Installing a TRV

Isolation: Drain the system down (or use a freeze kit) before removing the old valve body. Open the lockshield valve first — if the system is still charged, this slightly reduces pressure before breaking the compression.

Flow direction: Check the arrow on the TRV body — it must point in the direction of water flow from the boiler. Incorrect installation causes the differential pressure across the valve to hold it closed, meaning the radiator may fail to heat up. If you cannot determine flow direction from the pipework, a non-directional TRV body is available.

Thread sealing: Use PTFE tape or an appropriate jointing compound on BSP connections. Compression fittings do not require sealant. Pressure test after installation before committing to refilling.

Head fitting: Screw the thermostatic head onto the valve body — M30×1.5 thread typically requires only hand-tightening, though most heads have a locking ring that tightens with a small spanner. Set to position 3 initially.

Setting TRV Positions

Initial commissioning: After installation or system re-commissioning:

1. Set all TRVs to position 3 (living areas) or position 2 (bedrooms)
2. Leave the bypass radiator without a TRV (or set to maximum)
3. Run the system and allow to reach equilibrium (30–60 minutes)
4. Walk the building — check that all radiators are warming. Adjust positions based on occupant preference

Occupant guidance: Many occupants increase TRVs to maximum when rooms are cold, then wonder why the room overheats and they have to open windows. Explain the correct settings and the approximate temperatures clearly.

Rooms that need no TRV:

• Room with the room thermostat (the thermostat controls the boiler for that zone)
• Bypass radiator (must remain fully open to protect the pump)
• Airing cupboard — if heated towel rail or cylinder coil is in here, usually manual valve

Bypass Requirements

Without a bypass, when all TRVs close (because all rooms are at their setpoint temperatures), the pump has no flow path and pressure builds rapidly. This causes:

• Pump noise (cavitation)
• Boiler lockout on overheat
• Premature pump failure

Solutions:

1. One radiator with no TRV — leave the hallway radiator on a manual valve set fully open. Simple and reliable.
2. Automatic bypass valve (ABV) — a pressure-differential valve fitted in the pipework that opens when system pressure exceeds the bypass setpoint. Required on sealed systems with all TRVs and no manual bypass. The ABV is set using the adjustment screw — typically 0.2–0.3 bar differential.
3. Bypass TRV set to maximum — technically works but less reliable than an ABV.

Modern condensing boilers with built-in pumps often include an ABV internally — confirm with the boiler manufacturer's documentation before deciding whether an external ABV is needed.

Lockshield Valve and System Balancing

Why balancing is needed: Water, like any fluid, takes the path of least resistance. Radiators closest to the boiler (shortest pipework run, lowest resistance) receive more flow and therefore more heat than distant radiators. Without balancing, the first radiator may be 65°C flow temperature while the last is only 45°C — a significant imbalance.

Target temperature differential: A correctly balanced radiator should have a temperature difference of approximately 10–12°C between the flow and return (the temperature drop across the radiator). At higher flow temperatures (older gas boilers at 70–80°C), the differential can be 10–15°C. At heat pump flow temperatures (35–45°C), the differential should be 5°C.

Balancing procedure:

1. Preparation: Open all lockshield valves fully. Set all TRVs to maximum. Turn the system on.

2. Identify the radiator nearest the boiler — this will heat first and fastest.

3. Temperature measurement: Using a clip-on pipe thermometer or a contactless infrared thermometer, measure the flow pipe temperature and return pipe temperature at each radiator.

4. Restrict the near radiators: Start with the radiator nearest the boiler. Gradually close the lockshield valve (in small increments — quarter-turn at a time) until the temperature differential across that radiator is 11°C (flow minus return). Allow 5 minutes to stabilise after each adjustment.

5. Work outward: Move to the next nearest radiator and repeat. The furthest radiator from the boiler should have its lockshield valve fully open (or nearly so) — the restriction must be on the nearer ones.

6. Re-check: After adjusting all radiators, walk the system again — the adjustments interact, so first-set radiators may need minor re-adjustment.

7. Count lockshield turns: Note the number of turns required to balance each radiator. Record this data — it is useful if the system needs re-commissioning.

Tools needed:

• Pipe thermometer (clip-on type) or infrared thermometer
• Small adjustable spanner (for lockshield cap removal)
• Thermometer probe or meter

Smart TRVs

Smart TRVs (Tado, Drayton Wiser, Honeywell Evohome, Netatmo, Hive) replace the standard wax-capsule head with a motorised valve actuator controlled wirelessly. Each radiator has its own schedule and setpoint, controlled via a smartphone app.

Benefits:

• Individual room scheduling (bedroom cooler until waking time, living room warming before occupant returns)
• Geofencing — heating adjusts based on occupants' phone locations
• Integration with weather forecast compensation
• Energy monitoring and savings reporting

Installation requirements:

• The smart TRV hub must have a communication path to the boiler. Most systems use a wired relay connected to the boiler's room thermostat terminals.
• For true modulation (not just on/off), the boiler must support OpenTherm and the smart system must be OpenTherm-compatible.
• A bypass valve is essential — smart TRVs may all close simultaneously; an automatic bypass valve is more reliable than relying on a manual bypass radiator.
• Battery replacement — smart TRV heads are battery-powered. Typical battery life 1–2 years. Budget for this ongoing cost.

Frequently Asked Questions

Should I turn TRVs down when leaving the house?

It depends on how long you're away. For a working day (8–10 hours), leaving TRVs at their normal settings is often more efficient than turning down and reheating — a well-insulated house loses heat slowly. For a few days or longer, turn down to frost setting (❄) or position 1. Never turn TRVs fully off in winter — this risks freezing in a cold snap and causes radiators to accumulate cold water that takes much longer to reheat.

Why is one of my radiators always cold when the others are hot?

This is typically a balancing problem or an air lock. Bleed the radiator first (key vent at the top corner) — if air comes out, bleeding it will resolve the issue. If no air comes out and water flows freely, the lockshield valve on that radiator may be closed — check it. If the radiator is the furthest from the boiler, the lockshield valves on nearer radiators may need to be closed slightly to divert more flow to that radiator.

My TRV head clicks and the radiator keeps cycling on and off. What's wrong?

This is normal TRV operation — the clicking sound is the valve opening and closing as the room temperature oscillates around the setpoint. However, if cycling is very rapid (clicking every few minutes), the TRV head may be in a draught or near a heat source (e.g., a television), causing it to sense misleading temperatures. Relocate the TRV head or fit a remote sensor head.

Can I fit a TRV on a bathroom radiator or heated towel rail?

Yes, but with considerations. For a heated towel rail, a TRV is useful if you want to limit the bathroom temperature, but many customers prefer to leave the bathroom warmer. If the bathroom radiator is the designated bypass radiator, do not fit a TRV — leave it on a manual valve. A thermostatic element with a high maximum setting (position 4–5) is usually appropriate for bathrooms.

Do TRVs work with a combi boiler?

Yes — TRVs work on any wet central heating system, combi or heat-only. On a combi boiler system, the bypass is particularly important because there is no buffer cylinder — when all TRVs close, the pump has nowhere to circulate water, which can cause boiler lockout. Fit an automatic bypass valve.

Regulations & Standards

• Building Regulations Approved Document L1B — Zone control (TRVs) required when replacing a boiler in an existing dwelling, and in all heated rooms (except the room thermostat room)

• BS EN 215:2004 — Thermostatic radiator valves: requirements and testing (European standard for TRV performance)

• CIBSE Guide C — Pipework and system balancing guidance for wet heating systems

• MCS 021 — Heat pump system commissioning (balancing requirements for low-temperature systems)

• Drayton TRV Installation and Setting Guide — Product-specific setting guidance and bypass valve specification

• Danfoss Radiator Thermostat Guide — European market leader; detailed technical documentation on valve selection and balancing

• Honeywell Home (Resideo) TRV Guidance — UK-specific installation and compatibility guidance

• HHIC (Heating and Hotwater Industry Council) — Industry guidance on system balancing and Part L compliance

• heat pumps — Low-temperature systems require careful balancing and appropriate TRV settings

• system flush — Inhibitor treatment and powerflush before TRV installation

• cold radiators — Cold radiator fault-finding including TRV, lockshield, and air lock diagnosis

• radiator replacement — Radiator replacement including valve selection