Guide · Radiators
Thermostatic radiator valves: how they work and what the numbers mean
The thermostatic valve opens and closes the flow of hot water to the radiator by itself, depending on the air temperature in the room. The number on the knob shows the temperature you want. On many heads, 3 is about 20 °C, but the scale differs from brand to brand. Setting them is your job, while fitting them where they are missing is for an engineer.
10 minutes Difficulty: EasyUpdated 25 October 2026
Removing and refitting the thermostatic head does not let water out. The valve body, the part screwed to the pipe and the radiator, must never be taken apart with the system full because it is under pressure.
What you need
- A room thermometer
Step by step
Tap a step to mark it as done.
How it works and what the numbers mean
Inside the head there is a sensor filled with liquid or wax. When the room warms up, the liquid expands and pushes the pin, which gradually closes the valve. When the room cools, the valve opens again. It works without electricity or batteries, radiator by radiator.
The number shows the temperature at which the valve closes. Turning the knob to 5 only raises the temperature at which the valve closes, and the room does not heat up any faster. Used well, thermostatic valves save on the bill: according to the manufacturers' estimate reported by ENEA, up to 20% in some cases.
Are they mandatory?
In apartment blocks with central heating, yes. D.Lgs. 102/2014 requires temperature control and heat metering to be installed in each apartment, that is thermostatic valves and heat cost allocators, unless a technical report shows it is not worthwhile. Anyone who does not comply risks a penalty of 500 to 2,500 €.
With an independent boiler there is no general obligation for existing systems. The rules on minimum building requirements, DM 26 June 2015 updated by a new decree at the end of 2025, do however require room-by-room or whole-home regulation when a new system is installed or the boiler is replaced. The engineer who replaces it tells you whether valves are needed in your case.
Thermostatic valves on old radiators
They can be fitted to cast iron radiators too. If your manual valve is compatible with thermostatic heads, you just remove the knob and screw the head on in its place. If not, the valve body has to be changed, often together with the lockshield, and the circuit has to be shut off or drained: this is done by the engineer.
The cost depends on how many radiators there are, the type of valve and whether the system has to be drained: we give you the price before we start. In central systems the work has to be coordinated with the building manager.
Smart thermostatic valves
Smart heads have batteries, a small motor and an app: you set schedules and temperatures room by room, even when away from home. They are fitted in place of the normal head, on valves that accept thermostatic heads. The most common connection is M30x1.5, and in the boxes of brands like tado there are adapters for Danfoss, Caleffi and Giacomini valves.
Usually a bridge is also needed, which connects the heads to your home wifi. You can install them yourself and, as tado and Netatmo explain, no water comes out when you remove the old head. If no adapter fits your valve, the valve body has to be replaced.
When to stop and call a technician
- The pin stays stuck down even after moving it, or the valve drips under the head
- You have manual valves that do not accept thermostatic heads and you want to fit them: the valve body has to be changed
- Water leaks from the joint between valve and pipe or between valve and radiator
- With all valves open some radiators stay cold: bleeding or balancing the system may be needed
Frequently asked questions
What does number 3 on the thermostatic valve mean?
It is the room temperature at which the valve closes. On many heads it corresponds to about 20 °C, but the exact scale is in your model's data sheet.
Are thermostatic valves mandatory?
Yes in apartment blocks with central heating, together with heat cost allocators, unless a technical report rules it out. With an independent boiler, the rules require room-by-room or whole-home regulation when a new system is installed or the boiler is replaced.
Can they go on old cast iron radiators?
Yes. If the valve accepts thermostatic heads, the head alone is enough, otherwise the engineer replaces the valve body.
Do smart thermostatic valves fit all radiators?
They fit valves that accept thermostatic heads, with or without an adapter. Before buying, check compatibility on the manufacturer's website with the brand and model of your valve.
Is it better to fully close the radiators in empty rooms?
The frost setting is better: it keeps the room at around 7 °C and protects it from freezing. Keep the door closed, so cold air does not reach the sensors in the other rooms.
How much does it cost to fit thermostatic valves?
It depends on the number of radiators, the type of valve and whether the system has to be drained. In Campania we fit them ourselves and give you the price before we start.
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