Leave radiator valves on (open) in rooms you use and in the room that holds the room thermostat. Turn them off, or well down, in rooms you rarely heat, such as a spare bedroom or a box room. Closing a radiator does not switch the boiler off, because the room thermostat does that, and the rooms next door can end up slightly cooler.
Radiator valves on (open) or off (closed): how the two settings compare
Radiator valves on or off changes one thing: how much hot water flows through that one radiator. The table sets the open setting against the closed setting on the points that actually differ in a house.
| Criterion | Valve on (open) | Valve off (closed) |
|---|---|---|
| Rooms it suits | Rooms used daily, plus the room holding the room thermostat | Spare bedrooms, box rooms and stores you rarely heat |
| Effect on the boiler | A thermostatic head throttles the flow once the room reaches its set temperature | Closing one radiator does not stop the boiler, because the room thermostat still calls for heat |
| Effect on the rooms next door | A warm wall passes some heat into adjoining rooms | Adjoining rooms can end up slightly cooler, as the shared wall stays cold |
| Effect on running costs | You pay to heat every room the open radiators serve | Evidence reviews for government found savings only where controls are genuinely set and used |
| After a long period unused | Regular movement keeps the valve pin free | Valve pins tend to stick once a radiator has stood unused, so it can stay cold next autumn |
| Bleeding trapped air | Thermostatic heads should be fully open before a radiator is bled | A closed valve can hide trapped air until the room is wanted again |
Every radiator has two valves, one at each bottom corner. One is the control valve: either a manual wheel or a thermostatic radiator valve (TRV), the head marked with numbers. The other is the lockshield valve under a plastic cap, which limits the water leaving the radiator and is used for balancing. Anticlockwise opens a valve, clockwise closes it.
When to leave a radiator valve on (open)
Leave a radiator valve on (open) in any room you want warm at some point in the heating season. That covers living rooms, kitchens in daily use, bedrooms you sleep in and the hallway. The radiator in the room with the room thermostat is a special case. It is normally left open, and often carries no TRV at all, so the thermostat reads a fair temperature for the house. Our guide on TRVs on radiators explains which radiator should not have one.
Open does not mean flat out. A TRV set to a middle number still cuts the flow back once the room reaches that temperature. Work on heating controls commissioned for government describes a typical pattern of medium to high downstairs during the day and lower upstairs.
Leaving valves open through the summer has one practical benefit. TRV pins tend to stick when a radiator stands unused for months. A stuck pin usually shows up as a cold radiator in October, when the room is wanted again.
When to turn a radiator valve off (closed)
Turn a radiator valve off, or well down, in a room you rarely heat. A spare bedroom used twice a year, a box room, a hallway cupboard or a store are the usual cases. Ideal Heating lists turning down radiators in unused rooms among its energy-saving steps for central heating. The same point applies to a small room that already stays warm from the space below it.
Three side effects are worth knowing before you shut a radiator down for the season:
- Rooms next to the closed one can feel a little cooler, because heat no longer crosses the shared wall.
- The saving only appears if the setting is really used. Evidence gathered for government found that controls left unadjusted change neither demand nor the bill.
- Valve pins stick after long periods shut. Opening the valve every few weeks keeps it moving.
- Closing one radiator does not switch the boiler off. The room thermostat decides when the boiler fires.
What do the numbers on a TRV mean, and does 0 mean frost protection?
The numbers on a thermostatic radiator valve set a target room temperature, not a flow rate. Inside the head sits a material that expands as the air around it warms. It pushes a pin down and throttles the hot water entering the radiator. As the room cools, the material contracts, the pin lifts and more hot water flows in.
A TRV does not control the boiler. It only limits flow through its own radiator once the set temperature is passed. The room thermostat is what switches the boiler on and off.
The 0 or off mark simply closes the valve. It is not a frost protection setting. Boiler and control manuals describe frost protection as a function of the appliance or the system control, which brings the heating on when the temperature drops below a set limit.
Why is a radiator cold when the valve is already fully open?
A radiator that stays cold with the valve open has a different fault, and the pattern of cold tells you which. Manufacturer advice points to five common causes:
- Cold at the top, warm lower down: air trapped in the radiator, cleared by bleeding.
- Hot at the top, cold at the bottom: sludge lying in the base, not air. Our guide on a radiator hot at the top and cold at the bottom explains what that means for the system.
- Stone cold all over while others heat: a stuck TRV pin, often after months shut. Freeing or replacing the head is work for an engineer, not a householder step.
- Some radiators hot, others lukewarm across the house: the system needs balancing at the lockshield valves.
- Warm radiator, cool room: a radiator cover, a shelf or a long curtain holding the heat in.
Which radiator jobs you can do, and which need a Gas Safe registered engineer
Radiator valves, bleeding and balancing are water-side work, not gas work. Anything behind the boiler casing is gas work: the burner, the flue, the seals and the condensate inside the appliance. In England, regulation 3 of the Gas Safety (Installation and Use) Regulations 1998 as amended allows that work only by a competent person belonging to an HSE-approved class of persons. In practice that means a Gas Safe registered engineer, and the HSE says you should ask to see the ID card.
Four checks are safe for a householder: the boiler pressure gauge, the thermostat and programmer settings, the reset button, and bleeding a radiator. For bleeding, manufacturers say to switch the heating off and let the radiators cool first, and to open thermostatic heads fully. A quarter to half a turn anticlockwise on the bleed valve is enough, and it is closed again as soon as water appears. The bleed screw is never wound right out, and the pressure gauge is checked afterwards.
Setting lockshield valves, freeing a seized TRV, flushing and fitting radiators all sit outside that list. They are jobs to book, not to attempt.
Radiator valves on or off: the setting to pick room by room
Radiator valves on (open), if the room is used in the heating season, holds the room thermostat, or has stood shut long enough for the pin to seize. Radiator valves off (closed), if the room is genuinely unused and you accept that the rooms around it may feel slightly cooler. A middle TRV number suits most occupied rooms, because the valve then holds the room rather than running the radiator flat out.
Neither setting fixes a system fault. If some radiators never warm up while others are scalding, or a valve head no longer moves the pin, the problem is in the valves or the water, not in the number you chose. A boiler service or repair visit is the point to raise it, since the engineer is already at the system.
Sources
- Viessmann – Radiator valves explained
- Ideal Heating – 8 top tips for saving energy with your central heating
- Loughborough University – Heating Controls Scoping Review Project (final report)
- How heating controls affect domestic energy demand: a Rapid Evidence Assessment
- Vaillant – How to bleed a radiator
- Baxi – Why are my radiators cold
- The Gas Safety (Installation and Use) Regulations 1998, regulation 3
- HSE – Check an engineer: are they Gas Safe registered?