Radiator Valve Types: TRV, Lockshield and Manual Explained

The Three Valve Types

Every radiator in a wet central heating system has two valves: a control valve on the flow side (the pipe bringing hot water into the radiator) and a lockshield valve on the return side (the pipe taking cooled water back to the boiler). The control valve is what you interact with; the lockshield is set during installation or system commissioning and then largely left alone.

Thermostatic Radiator Valve (TRV)

A TRV is a self-actuating valve that senses the air temperature around the valve head and automatically adjusts the flow of hot water into the radiator to maintain the desired room temperature. The mechanism is entirely mechanical — no electricity or batteries are required.

Inside the TRV head is a liquid- or wax-filled capsule that expands as the room temperature rises. As the capsule expands, it pushes against a spring-loaded pin that closes the valve, restricting water flow to the radiator. As the room cools, the capsule contracts, the spring returns the pin to the open position, and water flows freely into the radiator. The TRV head number (1, 2, 3, 4, 5 on most brands) corresponds to the temperature setpoint:

• Setting 1: approximately 10–12°C (frost protection)
• Setting 2: approximately 15–16°C (cool room)
• Setting 3: approximately 18–20°C (comfortable — standard for most rooms)
• Setting 4: approximately 21–22°C (warm)
• Setting 5: approximately 24–25°C (maximum — effectively fully open)
• Snowflake (or * symbol): frost protection setting, approximately 7°C — keeps the radiator barely warm to prevent pipe freezing in unoccupied rooms

Recommended settings for typical London homes: living rooms and kitchens: 3–4; bedrooms: 2–3; hallways and landings: 1–2.

Lockshield Valve

The lockshield valve is the valve on the return side of the radiator — the side where cooled water exits the radiator and returns to the boiler. It is identified by a plastic cap covering the valve spindle; the cap must be removed to access the adjustment. Under the cap is a valve stem that can be turned with a lockshield key or adjustable spanner.

The lockshield's function is system balancing. In a central heating system, hot water naturally takes the path of least resistance — radiators closest to the boiler (with short, low-resistance pipe runs) receive more flow and heat up quickly, while radiators further away may receive insufficient flow. Balancing involves partially closing the lockshield on radiators that are receiving too much flow, increasing their hydraulic resistance and redirecting more flow to distant radiators. The objective is for all radiators to heat up to roughly the same temperature in the same time.

The lockshield is set during commissioning or when the system is balanced by a heating engineer. It should not normally be adjusted by the homeowner. When counting turns for the lockshield setting, make a note — if the valve is ever fully closed to drain a radiator for maintenance, you need to know how many turns to reopen it to the correct position.

Manual Radiator Valve

A manual valve is simply a quarter-turn or multi-turn ball or gate valve — it is either fully open or fully closed, with intermediate positions possible but without any automatic modulation. It provides no temperature regulation — if the radiator has manual valves on both sides, it either delivers full heat output (valves open) or no heat (valves closed).

Manual valves are now less common as the primary control valve in modern systems, where TRVs are the standard. They may be found in older properties that have never been updated, or in locations where a TRV is specifically not recommended (see below).

What Happens When a TRV Sticks

TRVs can fail in two ways: stuck closed or stuck open.

Stuck closed: The pin inside the TRV body becomes seized in the closed position — it no longer moves regardless of the temperature around the valve head. A stuck-closed TRV results in a radiator that remains cold even when the heating is running and all other radiators are hot. This is the most common TRV failure. The fix is to remove the TRV head and manually push the pin down — if it is seized, try pressing it repeatedly or applying a small amount of penetrating fluid around the pin stem. If it cannot be freed, the valve body needs replacement by a plumber.

Stuck open: Less common. The pin stays in the retracted (open) position regardless of room temperature — the radiator runs at full output and cannot be controlled. The room overheats and the TRV provides no temperature regulation. If the radiator is in the room where the wall thermostat is located, a stuck-open TRV causes the thermostat to never be satisfied, running the boiler unnecessarily. Replacement of the TRV head (usually possible without draining the system — the head removes by unscrewing the body from the valve with the heating off) typically resolves the issue.

TRV pins also seize after extended periods of being set at maximum — the pin is never used and corrodes in the open position. Setting TRVs appropriately throughout the year (not leaving all on maximum over summer when the heating is off) reduces this risk, as does exercising the TRVs periodically.

Where TRVs Should NOT Be Fitted

There is one location in any central heating system where a TRV must not be installed: the radiator in the room containing the main room thermostat.

The room thermostat controls the boiler — when the room temperature meets the thermostat's setpoint, the thermostat signals the boiler to stop. A TRV on the radiator in the same room would also respond to the room temperature and close before the thermostat is satisfied, reducing the room's heat input and preventing the thermostat from ever reaching its setpoint. The result is a feedback conflict — the thermostat keeps the boiler running to satisfy a demand that the TRV is preventing from being met.

The radiator in the thermostat room should either have a manual valve left fully open, or a lockshield/blank head in place of a TRV. The thermostat controls the room temperature in that room; all other rooms are controlled by their individual TRVs.

Lockshield Adjustment for System Balancing

If your central heating system has significant imbalance — some radiators very hot, others barely warm — balancing the lockshield valves can improve this. The procedure:

1. Start with all lockshields fully open (turn fully anti-clockwise).
2. Set all TRVs to maximum and turn the heating on.
3. Using a clip-on thermometer (or a pipe thermometer on the flow pipe), identify the radiator that heats up first — this is the one receiving the most flow, closest to the boiler.
4. Partially close the lockshield on that radiator (try one turn clockwise) and wait for the system to re-equalise.
5. Continue partially closing lockshields on fast-heating radiators until all radiators reach comparable temperatures in a similar time.

Professional balancing uses flow measurements and temperature differentials (aim for approximately 10–12°C difference between flow and return pipe temperature on each radiator). For a homeowner without professional equipment, approximating by feel produces a significantly better result than an unbalanced system. Count the turns on each lockshield and note them — you will need to return to these settings if you ever need to close and reopen a lockshield during maintenance.