Thermostatic Radiator Valves (TRVs): What the Numbers Mean and How to Set Them

What is a thermostatic radiator valve and how does it work?

A thermostatic radiator valve is the valve fitted to one end of a radiator — the end with the numbered dial on top. It controls the temperature of that single radiator and nothing else in your home.

Inside the TRV head is a small capsule of wax or liquid. When the room warms up, the capsule expands and pushes down on a pin, which closes the valve and reduces the flow of hot water into the radiator. When the room cools, the capsule contracts, the pin releases, and hot water flows in again. The whole process is mechanical and automatic — no electricity required on a standard TRV.

This is quite different from a manual (non-thermostatic) valve, which simply stays at whatever position you leave it, regardless of how warm the room gets. A TRV actively responds to the air temperature around it.

One important point that trips a lot of people up: a TRV is not the same as your room thermostat. Your wall-mounted room thermostat tells the boiler when to fire and when to stop. A TRV only throttles the flow of water into the one radiator it sits on. It cannot switch your boiler on or off. The two controls work together, but they do different jobs.

For this reason, you should never fit a TRV to the radiator in the same room as your main room thermostat. If the TRV closes the valve because that room is warm, but the room thermostat is still calling for heat, the system becomes confused and inefficient. Leave that radiator on a manual valve and let the thermostat do its job.

What do the numbers on a thermostatic radiator valve mean?

The numbers on a TRV dial — typically 0 to 5 or 0 to 6, often with a snowflake symbol at the low end — do not represent degrees Celsius directly. Instead, they represent a target room temperature range. The higher the number, the higher the room temperature must reach before the valve closes and cuts the flow of hot water.

Here is the approximate guide that most standard UK TRVs follow:

• ❄️ Snowflake / Frost setting: approximately 5–7°C — frost protection for unused rooms
• 1: approximately 10°C — a cool setting, suitable for a hallway or utility space
• 2: approximately 15°C — a comfortable cool, often used for bedrooms
• 3: approximately 19–20°C — standard room temperature, right for living rooms, kitchens and dining areas
• 4: approximately 21–22°C — a warm setting, well suited to bathrooms
• 5: approximately 25°C — very warm; rarely needed in a well-insulated UK home

These figures are approximate because the actual temperature at which your TRV opens and closes depends on several factors: the temperature of the water flowing through your system, how much air circulates freely around the valve head, the size of the room, and how well insulated it is. Different manufacturers also use slightly different scales — Danfoss TRVs, for example, often start frost protection at 5°C rather than 7°C — so always check your own product's documentation if you need precision.

The key takeaway is this: a higher number means the valve stays open for longer, allowing more heat into the room. It does not make your radiator heat up faster.

What number should my radiator valve be on? A room-by-room guide

Getting your TRV settings right room by room is the most effective way to balance comfort with efficiency. The Energy Saving Trust recommends keeping living spaces between 18°C and 21°C, which typically corresponds to a setting of 3 on most TRV scales.

Here is a practical starting point for each room type:

• Living room and lounge: setting 3–4 (approximately 19–21°C). This is where you spend most of your waking time, so comfort matters. Start at 3 and nudge up if needed.
• Kitchen: setting 2–3 (approximately 15–20°C). Cooking generates its own heat, so a lower setting is usually sufficient.
• Bedrooms: setting 2–3 (approximately 15–19°C). Most people sleep better in a cooler room. Setting 2 is a sensible default; adjust to personal preference.
• Bathroom: setting 3–4 (approximately 19–22°C). Bathrooms benefit from a slightly warmer setting, especially in colder months.
• Hallways and landings: setting 1–2 (approximately 10–15°C). These transitional spaces rarely need to be fully heated.
• Unused or rarely used rooms: snowflake / frost setting. This keeps the room just warm enough to prevent frozen pipes without wasting energy.
• Room with the main thermostat: use a manual valve, not a TRV (see above).

If you are going away in winter — even for a few days — turn all your TRVs down to the snowflake setting. This protects your pipes from freezing while keeping your energy use to a minimum.

These are starting points, not rules. Every home is different. Give the settings a week or two to settle, then adjust if a room consistently feels too warm or too cold.

The biggest mistakes people make with TRV settings

Understanding what not to do with a thermostatic radiator valve is just as important as knowing the right settings.

Turning the dial to maximum when you feel cold is the most common mistake. Setting your TRV to 5 does not make the radiator heat up faster or push more hot water through — it simply tells the valve to stay open until the room reaches around 25°C. In most UK rooms this means the radiator runs continuously, which wastes energy and overshoots your comfort level considerably.

Blocking the airflow around the TRV head is another frequent problem. The valve senses the air temperature immediately around it. If it is tucked behind a sofa, boxed inside a radiator cover, or surrounded by curtains, it will read a higher temperature than the rest of the room and close the valve prematurely, leaving you with a cold room. Always keep the area around the TRV head clear.

Fitting a TRV inside a radiator cover is particularly problematic. The heat trapped inside the cover gives the TRV a false reading. If you have radiator covers, either keep the TRV head protruding through the side, or replace the standard head with a remote-sensor TRV that can be mounted on the wall away from the heat source.

Setting TRVs higher than your main boiler thermostat achieves nothing. If your room thermostat is set to 20°C, cranking a TRV to its highest number cannot push the room above that ceiling — the boiler will have shut off already. Set your TRVs at or below the level of your main thermostat for the settings to have any meaningful effect.

Common TRV problems and how to troubleshoot them

Even well-maintained TRVs can develop problems, and most of the common ones have straightforward solutions.

A stuck or seized valve pin is probably the most frequently reported TRV issue in UK homes. It happens because the pin — the small metal rod the wax capsule pushes against — can become corroded or stuck in one position after sitting unused all summer. When the heating comes back on in autumn, the radiator either stays permanently cold (pin stuck closed) or permanently hot (pin stuck open).

To check: remove the TRV head by unscrewing it. You should see the pin sticking up from the valve body. Press it down gently with a finger or flat-head screwdriver — it should spring back up. If it does not move at all, apply a small amount of penetrating oil around the base of the pin, leave it for five minutes, then gently work it up and down. Do not yank it upward sharply, as this can damage the valve body. If the pin still will not move or the valve starts to weep water, stop and call a Gas Safe registered engineer or qualified heating engineer.

A radiator that is cold even when the TRV is fully open may have a blocked or partially closed lockshield valve — the plain-capped valve on the opposite end of the radiator. This controls the system's flow balance. If it has been accidentally turned down too far, hot water cannot enter regardless of what your TRV is set to. Try opening the lockshield valve a little (turn anti-clockwise) and see if the radiator warms up. System-wide rebalancing is best done by a professional.

A radiator that is cold at the bottom but warm at the top probably has trapped air and needs bleeding. This is straightforward: turn off the heating, use a radiator bleed key on the bleed valve (usually at the top end of the radiator), and open it slowly until water trickles out, then close it. Check your boiler pressure afterwards — it may have dropped below the normal range of 1–1.5 bar and need topping up via the filling loop.

Whistling, knocking, or gurgling noises from a radiator with a TRV fitted are usually a sign of too-high water flow speed or trapped air. Bleeding often helps with gurgling; persistent knocking or whistling is worth mentioning to an engineer during your next service.

Sludge and mineral buildup is a longer-term issue, particularly in hard-water areas of England. If multiple radiators are underperforming and cold at the bottom, a power flush or magnetic filter check is likely needed — this is a job for a heating engineer.

Maintaining your TRVs and when to call a professional

A little regular attention keeps TRVs working well for years. At the end of each heating season, before you switch off the central heating for summer, turn each TRV to its fully open position. This keeps the pin from seizing in the closed position over the warm months. When you fire the heating back up in autumn, check each valve responds — if a radiator fails to heat up on a setting of 3 or above, a stuck pin is the first thing to investigate.

Keep the valve head free of dust and grime. A quick wipe with a damp cloth or a careful pass with a vacuum cleaner nozzle every few months is enough. Avoid getting water or cleaning sprays into the mechanism.

Visually inspect valves each autumn for signs of corrosion, lime-scale crust around the base, or small drips. Catching a slow weep early can prevent a bigger leak developing.

There are some situations where you should always call a Gas Safe registered engineer or a qualified heating engineer rather than attempting work yourself:

• The valve body is visibly cracked or leaking steadily
• You cannot stop a drip after tightening the gland nut
• The pin is completely seized and penetrating oil has made no difference
• You want to replace the full valve body (this involves draining part of the system)
• Multiple radiators are cold and the problem does not resolve after bleeding
• You are fitting TRVs for the first time as part of a boiler replacement — this work may need to be notified under Building Regulations

Building Regulations in England (from June 2022) and Wales (from November 2022) require thermostatic room controls to be fitted when a boiler is replaced in an existing home. Adding TRVs to all radiators except the one in the room with the room thermostat is the standard way to meet this requirement. Your installer should handle this as part of the boiler replacement.

Step by step

1. Check the room and locate the TRV

   Find the thermostatic radiator valve on your radiator — it is the end with a numbered dial, usually 0 to 5 or 0 to 6 with a snowflake symbol. Make a note of its current setting. Also confirm that the radiator on the other end has a plain-capped lockshield valve (no numbers) — make sure this is not fully closed.

2. Make sure the heating system is running

   Turn your boiler and heating on as normal. Set your main room thermostat to the temperature you want the house to reach — for example, 20°C. The TRV settings you choose need to be at or below this level to have any effect. If the thermostat is set to 20°C and the TRV is at a position that corresponds to 22°C, the thermostat will have already cut the boiler before the TRV can do anything useful.

3. Turn the TRV to its maximum setting briefly to check the radiator heats up

   Turn the dial fully clockwise to the highest number, then wait 15–20 minutes. The radiator should warm up across most of its surface. If it stays cold, you may have a stuck pin, trapped air, or a closed lockshield valve — see the troubleshooting section. Do not leave the dial at maximum permanently.

4. Bleed the radiator if needed

   If the radiator is warm at the bottom but cool at the top, trapped air is the likely cause. Switch the heating off and allow the water to cool slightly. Insert a radiator bleed key into the bleed valve (the small square fitting, usually at the top of one end). Turn slowly anti-clockwise until you hear hissing air. Once a steady trickle of water appears, close the valve. Check your boiler pressure gauge — it should read between 1 and 1.5 bar. If it has dropped, top it up using the filling loop (consult your boiler manual for the correct procedure).

5. Check the TRV pin is not stuck

   If the radiator is cold even with the TRV on its highest setting, remove the TRV head by unscrewing the collar ring (usually hand-tight). Look for the small metal pin protruding from the valve body. Press it down gently with a fingertip — it should spring back up freely. If it does not move, apply a small amount of penetrating oil around the base, wait five minutes, then gently work it up and down. If the pin remains stuck or any water escapes, replace the TRV head and call a qualified heating engineer.

6. Set the TRV to the right number for the room

   Once the radiator is confirmed to be working, set the dial to the appropriate position for that room: snowflake for unused rooms, 1–2 for hallways and bedrooms, 3 for living rooms and kitchens, and 3–4 for bathrooms. These are starting points — give each room a few days to settle at the new setting before deciding whether to adjust.

7. Leave the room for 30 minutes and check the temperature

   If you have a room thermometer, place it at seated height in the middle of the room, away from windows and heat sources. After 30 minutes, compare the reading to the comfort level you want. If the room is too warm, turn the TRV down by one mark. If it is too cool, turn it up by one mark. Repeat over a few days until you find the right balance. If you do not have a thermometer, use your own comfort as the guide.

8. Confirm the TRV head has free airflow

   Check that the valve head is not obstructed by curtains, furniture, or a radiator cover. If the TRV is inside a cover, the trapped heat will give it a false reading. Move furniture back at least 10 cm from the valve if possible. If you cannot avoid a radiator cover, consider fitting a remote-sensor TRV head, which mounts separately on the wall where the air temperature is representative of the whole room.

9. Repeat the process for each radiator in the house

   Work through every radiator in your home, setting each TRV to the appropriate level for that room. Remember to leave the radiator in the same room as your main thermostat on its manual valve without a TRV. Once all valves are set, leave the system to run for a full day, then walk around and check each room feels right. Small adjustments are normal — the goal is a comfortable home that is not wasting heat in rooms you rarely use.

Typical costs

Typical UK ranges as a guide only — prices vary by region (expect the top end, or 20–30% more, in London and the South East) and by how accessible your system is. Always get a written quote.

Frequently asked questions