Opening a radiator means turning its valve counterclockwise. That’s the universal direction for every type of radiator valve, whether you’re switching the heating on, bleeding trapped air, or adjusting flow between rooms. The specific steps depend on which valve you’re working with and what you’re trying to accomplish.
Opening the Main Valve
Every hot water radiator has at least one control valve, usually located at the bottom corner where the pipe connects. On a manual radiator, this is a simple twist knob. Turn it counterclockwise to open and allow hot water to flow in, or clockwise to shut it off. Turning it all the way counterclockwise gives you maximum heat.
If your radiator has a thermostatic radiator valve (TRV), you’ll see a numbered dial instead of a plain knob. These work the same way: counterclockwise increases heat, clockwise reduces it. The numbers typically range from 0 (off) to 5 (fully open). Setting it to 3 keeps most rooms comfortable, while 4 or 5 is better for larger or poorly insulated spaces. The TRV automatically adjusts flow based on room temperature, so you don’t need to keep turning it manually once it’s set.
Some older radiators have no knob at all, just a bare metal valve stem. If the handle has broken off or been removed, grip the valve with a pair of pliers and turn it counterclockwise. Go slowly so you don’t strip the metal.
Bleeding Air From a Radiator
If your radiator is on but has cold spots near the top, trapped air is blocking hot water from filling it completely. Bleeding releases that air through a small valve at the top of the radiator, usually on one side.
Before you start, turn off your heating system and let the radiators cool. Working with hot water under pressure increases the risk of scalding and makes the process messier. Place a towel or small container under the bleed valve to catch any drips.
The bleed valve has a small square socket in its center. Insert a radiator bleed key (a simple tool available at any hardware store for a couple of pounds or dollars) and turn it counterclockwise about a quarter to half turn. You’ll hear a hissing sound as air escapes. Keep the key in place and wait. Once the hissing stops and a steady trickle of water appears, close the valve by turning it clockwise. That’s it.
Standard UK bleed valves have a 5mm square socket, and virtually any universal radiator key will fit. Some valves accept a flat-head screwdriver instead, and others take an Allen key. Check what yours looks like before heading to the shop. A brass bleed key tends to give the most precise fit and is less likely to slip and damage the valve.
After bleeding, check your boiler’s pressure gauge. Releasing air often drops system pressure slightly, so you may need to top it up using the filling loop (the braided hose near your boiler) until the gauge reads between 1 and 1.5 bar.
Opening the Lockshield Valve
On the opposite end of the radiator from the main control valve, you’ll find a lockshield valve. This one is covered by a plastic cap and isn’t meant to be adjusted regularly. It controls how much water flows through the radiator and is used to balance heating across your home so that every radiator heats evenly.
To open it, pop off the plastic cap by hand or with a flat-head screwdriver. Underneath is a valve spindle that you turn with an adjustable spanner. Like every other radiator valve, counterclockwise opens it and clockwise closes it. When balancing radiators, the standard approach is to close all lockshield valves fully, turn the heating on, then open each one roughly a quarter turn counterclockwise, starting with the radiator closest to the boiler. Radiators further from the boiler typically need to be opened a bit more to compensate for the longer pipe run.
Steam Radiators Work Differently
If you live in an older building with steam heating (common in apartment buildings in the northeastern United States), the setup is different. Steam radiators typically have a single supply valve at the base and a small air vent on the side, about a third of the way down from the top.
The supply valve should generally be either fully open or fully closed. Leaving it halfway creates banging and uneven heating because steam and water condensate fight each other in the pipe. Turn it counterclockwise until it stops to fully open it.
The air vent is an automatic device. As steam fills the radiator, it pushes air out through the vent, which then closes on its own once steam reaches it. You don’t normally open or close this vent manually. If your steam radiator isn’t heating properly, the vent may be clogged or stuck. These vents unscrew from the radiator body and can be replaced without any special tools.
Freeing a Stuck Valve
Radiator valves that sit in one position for months can seize up, especially thermostatic valves. If turning the knob or dial does nothing, the internal pin is likely stuck.
Start by removing the thermostatic head. Most TRV heads detach by loosening the threaded ring just below the head, either by hand or with an adjustable spanner. Once the head is off, you’ll see a small metal pin sticking up from the valve body. This pin should move up and down freely. Push it with your finger. If it doesn’t budge, grip it gently with pliers and work it up and down until it loosens. Don’t yank it or use excessive force, as the pin connects to the internal mechanism.
Once the pin moves freely again, spray a small amount of lubricant like WD-40 around it to prevent future seizing. Reattach the thermostatic head and test it. If the pin won’t move at all despite gentle effort, the valve body itself may need replacing, which involves draining the radiator and is better left to a plumber.