Radiator Valves Explained
Manual, thermostatic, and lockshield valves each do a different job. Understanding them is the key to a balanced heating system.
By Pansa Editorial Team · Published Nov 4, 2025
Every radiator has two valves, and each one does a different job. Confusing them — or leaving them poorly set — is one of the most common reasons heating systems feel uneven from room to room.
What radiator valves do
One valve controls how much hot water enters the radiator. The other is used to balance flow so that every radiator on the system gets a fair share. Together they decide how warm each room becomes and how evenly heat is distributed.
Manual radiator valves
A manual valve is the simplest type: a wheel that you turn to open or close flow into the radiator, similar to a tap. It does not respond to room temperature, so the radiator runs at whatever setting you choose until you change it.
Thermostatic radiator valves
A thermostatic radiator valve (TRV) has a head with a temperature-sensitive capsule. As the air around the head warms up, the capsule expands and gradually closes the valve, throttling flow into the radiator. As the room cools, the valve opens again. This gives each room its own basic thermostat.
What TRV numbers actually mean
The numbers on a TRV are not exact degrees. They are relative comfort settings that depend on the valve model, the position of the radiator, and how freely air can move around the head. A setting that feels right in a sheltered bedroom may be too low in a draughty hallway or too high behind heavy curtains.
Lockshield valves
The valve at the opposite end of the radiator is usually a lockshield, identified by a plastic cap that hides the adjustment. It is set once during balancing by a heating engineer and then left alone. It restricts return flow on radiators that would otherwise grab more than their fair share of water.
| Valve type | Purpose | Adjusted by |
|---|---|---|
| Manual | Open/close inlet flow | User, as needed |
| Thermostatic (TRV) | Maintain room temperature | User, set and forget |
| Lockshield | Balance system flow | Heating engineer |
Valve placement
Inlet valves are usually at the bottom of the radiator. Lockshield valves sit at the opposite end. Some valves are angled, some are straight, and some are corner valves — the choice depends on pipework routing.
Balancing a heating system
Balancing means adjusting lockshield valves so that hot water reaches every radiator at roughly the same temperature drop. A well-balanced system warms upstairs and downstairs evenly, eliminates 'always-cold' radiators, and helps the boiler or heat pump run efficiently.
Common valve problems
- Stuck TRV pin — the valve appears open but flow is restricted.
- Leaking spindle — small drips around the valve stem.
- Painted-over heads that can no longer turn.
- Reversed flow direction in older valves.
A simple valve check before calling for help
Room-by-room valve review
- Confirm the TRV or manual valve is open enough for the room to call for heat.
- Check that furniture, curtains, or radiator covers are not trapping warm air around the valve head.
- Compare the affected radiator with one that heats normally on the same floor.
- Do not disturb lockshield settings unless you are intentionally balancing the system.
- If several rooms are cold, treat it as a system issue rather than a single faulty valve.
When to replace a valve
Persistent leaks, seized pins that cannot be freed, and old valves that lack TRVs are common reasons to replace. Valve replacement usually involves draining at least part of the system and should be done by a qualified heating engineer unless you have the right experience and tools.
Frequently asked questions
Q.Should every radiator have a TRV?
Most rooms benefit from a TRV. The room containing the main thermostat is often left on a manual valve to avoid two thermostats fighting each other.
Q.Why won't my TRV head turn?
Pins can seize after a long warm season. Removing the head and gently freeing the pin (or having a heating engineer do it) usually solves the problem.
Q.What number setting should a TRV be on?
Numbers do not map to specific temperatures. Set the head to the lowest number that keeps the room comfortable and adjust from there.
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