Why Won't My Radiator Turn Off?

In the chilling grip of winter, a properly functioning heating system can make your home a sanctuary.

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However, it's an entirely different story when your radiator seems to have a mind of its own, relentlessly pumping out heat and refusing to shut off.

There are several components that, if not functioning properly, can prevent your radiator from turning down or turning off completely. The most common culprits include:

Faulty Thermostatic Radiator Valve

A thermostatic radiator valve works by sensing the air temperature in the room and adjusting the flow of hot water into that individual radiator. It’s typically located on the right side of the radiator and will have a large, numbered dial (the “head”) ranging from 0 to 5 or 6. This is used to manually adjust the radiator’s heat output. The higher the number, the hotter the radiator will get.

If a TRV is faulty, it might not correctly sense the room's temperature and cause the valve to remain open. This allows hot water to continuously flow into the radiator and remain hot even when it should be off.

To determine if the TRV is likely to blame, turn the dial to “0” and see if it cools down. If the radiator doesn’t turn off, your TRV might need to be repaired or replaced.

Stuck Lockshield Valve

The primary function of a lockshield valve is to balance your radiators. Balancing involves adjusting the flow of water in your central heating system to ensure each radiator heats up at the same rate. This adjustment creates an even distribution of heat throughout your home, which can help the system run more efficiently and potentially reduce energy costs.

The lockshield valve is typically located on the left side of the radiator, opposite the thermostatic radiator valve. Unlike the TRV, which you can easily adjust by hand, the lockshield valve is usually adjusted with a plastic cap or a flathead screwdriver.

A lockshield valve isn’t often responsible for a radiator not turning off. However, if the lockshield valve is stuck open due to corrosion or debris in the valve, it could allow continuous hot water to flow into the radiator, keeping it hot.

To determine if the lockshield valve is the culprit, you can try to turn the valve clockwise to restrict the flow of hot water. Before doing so, keep in mind that the adjustments you make may affect the rest of the radiators in your home. Either set the valve back to its original position after you’re done testing, or have a professional HVAC technician perform the test for you.

Malfunctioning Thermostat

The thermostat in a central heating system controls the temperature of your home by communicating with the boiler. It tells the boiler when to produce heat and when to stop. If the thermostat isn’t working correctly, the boiler can stay on and constantly send hot water to the radiators throughout your home. If all of the radiators in your home aren’t turning off despite the thermostat being set to a reasonable temperature, the thermostat may be acting up.

Try turning the thermostat to its lowest setting and observe whether the boiler turns off and the radiators cool down. If not, try resetting the thermostat according to the manufacturer’s instructions. You can also turn the thermostat off entirely to see if that resolves the issue. If your radiators are still hot, you may need to replace the thermostat.

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Broken or Dirty Diverter Valve

In a combi boiler system, the diverter valve directs the flow of heated water to either the domestic hot water system or the central heating radiators, depending on where it is needed most. When you turn on a hot water tap, the diverter valve should prioritize sending hot water to your taps over the radiators. When the tap is closed, the valve redirects hot water to your radiators.

If a diverter valve is dirty, it may get stuck in one position. If it gets stuck in the position that directs hot water to the radiators, this could cause your radiators to stay on continuously, even when they shouldn't be. This is because the heated water continues to flow to the radiators regardless of whether the thermostat or TRVs are calling for heat.

Another sign the diverter valve is to blame is if the hot water coming out of your faucets, or shower heads is lukewarm or the temperature fluctuates substantially. That’s because the diverter valve is directing some of the water to the radiators instead of the taps.

If you suspect your diverter valve is faulty or dirty, it's best to call in a professional HVAC technician who has the specialized tools and knowledge.

Can I Resolve These Issues on my Own?

With the exception of the diverter valve, the primary reasons a radiator won’t shut off can be fixed with a minimal amount of tools, time or expertise. That being said, if you're unsure or uncomfortable performing any of the steps outlined below, it's best to hire a professional who can ensure it's done correctly and prevent any possible damage to your heating system.