There are few things more frustrating than a radiator that refuses to do its job, especially when the temperature drops and the rest of your home is warm. Whether it’s stone cold, only lukewarm, or just heating unevenly, a radiator that isn’t working properly is always worth investigating, and more often than not, the fix is simpler than you’d expect.
At NJ Harkus, we get calls about this all the time. So here’s a straightforward rundown of the most common reasons your radiator isn’t heating up, and what to do about each one.
Cold At The Top, Warm At The Bottom? You Probably Need To Bleed It
This is the classic sign of trapped air in your radiator. When air builds up inside, it prevents hot water from filling the whole unit, so the top stays cold while the bottom heats up just fine. Luckily, bleeding a radiator is something most homeowners can do themselves.
All you’ll need is a radiator bleed key (usually available for a few pounds at any hardware shop) and a cloth. Simply open the bleed valve slowly until you hear a hiss of air, then close it once water starts to appear. Job done. We’ve actually put together a detailed guide on how to bleed a radiator if you want to walk through it step by step.
Cold At The Bottom, Warm At The Top? That Points To Sludge
If your radiator is hot at the top but cold or barely warm at the bottom, the issue is usually a build-up of limescale and rust debris, commonly known as sludge. Over time, this settles at the base of the radiator and blocks the flow of water through it.
This one is harder to fix yourself, and in most cases, it’s a sign your whole system could do with a power flush. This process clears sludge and debris from your pipework and radiators, restoring proper circulation throughout the system. It’s one of the best things you can do for the long-term efficiency of your heating, and it often makes a noticeable difference to how quickly your home warms up.
The Whole Radiator Is Cold
If the radiator is completely cold, there are a few likely culprits:
The thermostatic radiator valve (TRV) is stuck
The TRV is the valve on the side of your radiator, usually with a numbered dial. If it’s been set to zero or has seized up (which happens on radiators that haven’t been used for a while), no hot water will flow through. You can always try turning it up and tapping it gently to free it off.
The lockshield valve is closed
This is the capped valve on the opposite side of the radiator. It’s adjusted during balancing and should be left partially open. If it’s been fully closed, the radiator won’t heat.
Your boiler pressure is too low
A combi boiler needs to be running at the right pressure to push water around the system effectively. If the pressure gauge is reading below 1 bar, you’ll need to repressurise it.
Only Some Radiators Are Affected
If certain radiators in the house are fine but one or two aren’t heating properly, the issue is usually localised rather than system-wide. This often comes down to:
- An imbalanced system, where some radiators receive too much flow and others too little
- A stuck or faulty TRV on the affected radiator
- Localised sludge build-up in that particular radiator or the pipework feeding it
Balancing radiators is something we (a heating engineer) can sort out relatively quickly, and it can make a meaningful difference to how evenly your home heats.
It Could Be Your Boiler
Sometimes the radiator itself is absolutely fine, and the issue is with the boiler. If your central heating isn’t working as it should across multiple radiators, or your boiler is making unusual noises, displaying a fault code, or struggling to maintain temperature, that’s a different conversation.
For example, if the boiler needs repairs that are costly or are becoming more frequent, or it’s more than 10 years old, it may also be worth considering a boiler replacement.
You might find our post on the most common boiler problems useful, as well as our no-heating troubleshooting guide.
When To Call A Professional
Some radiator issues are genuinely straightforward DIY fixes, with bleeding a radiator being the obvious example. But if you’ve worked through the basics and still can’t get things right, or if you’re dealing with leaks, persistent cold spots, or a system that just isn’t performing the way it should, it’s time to get someone in.
At NJ Harkus, we’ve been keeping Cardiff homes warm for over 15 years. We’re Gas Safe registered, carry over 1,000 five-star reviews, and we approach every job, whether it’s a single radiator or a full system flush, with the same level of care and attention.
If your radiator isn’t heating up and you’d rather not guess your way through it, give us a call on 07859 896019 or get in touch here, and we’ll get it sorted.
