A TRV not working properly can stem from several distinct faults. This guide explains how to diagnose and address a stuck valve pin, a worn seat causing drips, incorrect placement, and trapped air.
Stuck Pin: Radiator Always On or Always Off
One of the most frequent TRV faults is a stuck pin. This is the small metal pin in the valve body that the TRV head pushes down to restrict water flow. If it's seized, your radiator may stay permanently on, making the room too hot, or permanently off, leaving it cold.
Diagnosis is straightforward. Remove the TRV head by unscrewing it or releasing the locking mechanism. You will see the pin in the centre of the valve. Using a pair of pliers or a small spanner, gently try to press the pin down. It should move smoothly and spring back up when released. If it's stuck fast or only moves with extreme force, it's seized.
The fix involves carefully freeing it. Apply a small amount of penetrating oil around the pin's base and let it soak. Then, gently work the pin up and down with your tool until it moves freely. Never use excessive force, as you could break the pin. Once free, wipe away excess oil, refit the TRV head, and test the system.
Worn Valve Seat Causing Drips
If you notice a persistent drip from the valve body or the connection point when the TRV head is removed, the issue is likely a worn valve seat. This is the internal seal that the pin closes against. Over years of operation, debris in the system or simple wear can damage this seat, preventing a proper seal.
This fault is distinct from general plumbing leaks in pipework. The drip will be isolated to the TRV valve itself, often worsening when the pin is in the closed position. You may also see minor corrosion or pitting around the valve assembly.
Fixing a worn seat usually requires replacing the entire TRV valve body, a job for a qualified heating engineer. It involves draining the relevant section of your heating system, removing the old valve, and installing a new one. This is a good opportunity to install a magnetic filter if you don't have one, as system debris often causes this wear.
TRV Set Correctly But Room Temperature Is Wrong
Sometimes the TRV mechanism is perfectly functional, but the room temperature doesn't match the setting. This is typically an issue of placement or environment, not a valve fault. The TRV head contains a sensor that reacts to the ambient air temperature around it.
If the TRV is blocked by long curtains, furniture, or tucked behind a sofa, it cannot sense the room's true temperature. Similarly, if it's placed in a draughty hallway or directly above a radiator, it will get false readings, causing premature shutdown or constant operation.
The fix is about improving placement. Ensure there is at least a foot of clear space around the TRV head for air to circulate. Relocating furniture or curtains often solves the issue. For problem locations, consider a remote sensor TRV, where the sensor can be placed on a wall away from draughts and heat sources, communicating wirelessly with the valve head.
Airlock Behind the Valve
An airlock trapped directly behind the TRV can prevent hot water from entering the radiator, making it seem like the valve has failed in the 'off' position. This can happen after system drainage or if air has gradually accumulated at this high point in the circuit.
The symptom is a cold radiator despite the TRV being set to a high number and the lockshield valve on the opposite end being open. You might also hear gurgling or knocking sounds from the valve area when the heating runs.
To fix this, you need to bleed the specific airlock. Start by turning off the heating system. Place a towel and a small container under the valve. Carefully loosen the union nut connecting the valve body to the radiator tail. As you loosen it slightly, air and a small amount of water will escape. Once a steady trickle of water appears, tighten the nut again. Refill the system pressure if necessary, restart the boiler, and check if the radiator now heats. This is a precise task; if unsure, consult a heating professional for system balancing and air removal.
Frequently asked questions
Can I replace just a TRV head, or do I need the whole valve?
You can usually replace just the TRV head if the valve body is in good condition. Ensure the new head is compatible with your existing valve's pin type and brand. If the valve body is leaking or the seat is worn, the entire valve must be replaced by a heating engineer.
Why does my TRV make a knocking or clicking noise?
Knocking is often caused by water flow turbulence, while clicking is usually the sound of the wax capsule inside the TRV head expanding and contracting. Ensure the TRV is not oversized for your radiator and that your system water pressure is correct. Persistent loud knocking may indicate a need for system balancing.
Should all radiators have TRVs installed?
No. It is a building regulation requirement that at least one room, typically with the main room thermostat, should have a radiator without a TRV or with a permanently open TRV. This provides a circuit for hot water to circulate and allows the boiler to be controlled correctly.
How often should TRVs be maintained or replaced?
TRVs require minimal maintenance. It's good practice to exercise the valve by turning the head from low to high and back a few times each season to prevent the pin from sticking. The heads themselves can last 10-15 years, but valve bodies may last the lifetime of the system unless wear or corrosion occurs.