Radiator Valves: Purpose, Types, & TRVs

Every radiator in your home has two valves. 

One controls how much hot water flows in. The other controls how much flows back out. 

Together, they determine how well your radiator heats your room and how efficiently your heating system runs.

This guide explains the different types of radiator valve, how to choose the right one, and what to do if a valve starts to leak.

What are the two valves on a radiator?

At the bottom of most radiators, you will find a valve at each end. Both valves work together to control temperature and balance (evenly distribute heat) the system.

The valve on the flow side controls how much hot water enters the radiator. This is where you will find either a manual valve or a thermostatic radiator valve (TRV). 

The valve on the return side controls how much water flows back out into the heating system.

This is always a lockshield valve. It’s a plain-capped valve adjusted with a spanner rather than by hand.

Types of radiator valve

Manual valves

A manual valve works like a tap. You twist the head to open or close the flow of water into the radiator. Fully open means maximum heat; fully closed means the radiator is off.

Manual valves are simple and reliable. But they need adjusting by hand whenever you want to change the temperature. 

So, they are best suited to rooms where you want a constant level of heat rather than spaces where you want finer control (e.g., a hallway or utility room).

Thermostatic radiator valves (TRVs)

A TRV contains a temperature-sensitive element inside its head, usually made of wax or liquid. 

TRVs don’t turn the boiler on or off. They simply regulate water flow. As the room warms up, this element expands and restricts the flow of water into the radiator to prevent the room overheating.

When the room cools down, it contracts and allows more water through, stopping the room from cooling below the heating system’s current setting.

This automatic regulation saves energy because the radiator only uses as much hot water as the room actually needs.

Types of TRV head

There are two types of TRV head. Wax heads are the most common in UK homes.

Liquid heads respond slightly faster to temperature changes but the practical difference is minimal in most domestic settings.

Older TRV heads often have a numbered dial (1–5 or 1–6). These numbers are not degrees Celsius, they are relative settings. 

As a rough guide, position 3 is approximately 20°C, with each step representing around 3–4°C difference. 

Some heads also have a snowflake symbol for frost protection mode and a star symbol for off.

Which side should the TRV go on?

The TRV should be fitted on the flow pipe (the pipe that brings hot water into the radiator). 

This typically (though not always) is on the left side when facing the radiator. If you’re unsure, a heating engineer can check for you.

A note on TRVs and room thermostats

Do not fit a TRV in the same room as a wall-mounted room thermostat. The two will conflict. causing the boiler to short-cycle and reducing efficiency

The TRV will restrict flow when the room reaches its set temperature, whilst the room thermostat simultaneously tries to regulate the same space. 

Leave the radiator in the room with the main thermostat on a manual valve instead.

Smart TRVs (eTRVs)

Smart TRVs replace the wax or liquid head with an electronic motor and wireless connectivity. 

Instead of a scale from 1 to 5, they give you more precise temperature targets (e.g. 19°C) and can be programmed and controlled via an app.

Smart TRVs typically cost £40–£80 per valve and can be retrofitted to most existing radiator valve bodies. 

The Energy Saving Trust estimates that using TRVs to reduce heating in unused rooms can save between £70 and £150 per year (though savings vary depending on your home and usage).

Lockshield valves

A lockshield valve is fitted on the return side of every radiator. It’s covered by a plastic cap and adjusted using an adjustable spanner. 

Unlike other valves, you do not operate it day-to-day. It’s set each time the radiator is balanced (usually every 5 – 10 years).

Without a correctly set lockshield valve, radiators nearest boilers tend to get hotter than those furthest away from them.

Straight or angled valves?

Radiator valves come in straight and angled versions. The one you need depends on where your pipework runs.

If your pipes come up from the floor and connect to the bottom of the radiator, you need straight valves. 

If your pipes come out from the wall and connect to the side of the radiator, you need angled ones.

A third option is the H-block valve (also called a centre connection valve). It’s used where both pipes emerge from the floor at a central point below the radiator. These are common with towel rails and some designer radiators.

	Through Wall (Pipework)	Through Floor (Pipework)
Bottom inlets	Angled radiator valve	Straight radiator valve
Side inlets	Angled radiator valve	Angled radiator valve
Middle inlets	H-Block valve	H-Block valve

Are radiator valves universal?

Not entirely. Most standard UK radiators use a ½ inch BSP thread, which means the majority of valves are interchangeable. 

However, some older radiators use different thread sizes. And some designer or towel rail radiators have non-standard connections. 

If you’re replacing a valve, check the thread size before buying its replacement.

Radiator valve sizes

Standard radiator valves in the UK are most commonly 15mm (to match standard 15mm copper pipework).

Some larger systems or commercial installations use 22mm. The valve size refers to the pipe connection, not the radiator inlet.

If your valve is leaking, the fix depends on where the leak is coming from. See our step-by-step guide to fixing a leaking radiator valve.

When to replace your radiator valves

Most radiator valves last 15–20 years with no maintenance required. Signs that a valve needs replacing include:

• Persistent leaking that tightening does not fix
• A TRV head that no longer responds to temperature changes (the wax element can fail over time)
• A valve that is stuck fully open or fully closed and cannot be turned
• Visible corrosion or pitting on the valve body

Replacement manual or TRV valves typically cost £10–£50 per valve depending on style. 

Smart TRV heads that fit existing valve bodies cost £40–£80. A heating engineer will typically charge £50–£100 to replace a valve including labour.

Protecting your heating system

If you are replacing or fitting new valves, it is also worth checking that your system has a magnetic filter and the correct inhibitor fluid. 

These protect your pipework and valves from the corrosive sludge that builds up over time.

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