Why Is My Radiator Cold And How To Fix It

Is your Radiator cold? Find out with our in-depth guide and how-to fix it guide.

We rely on our radiators to spark into action and keep us warm year after year, but what if they don’t?!

The humble radiator is a relatively simple device, but they’re still prone to several issues that can affect your central heating performance. 

First and foremost, don’t panic - heating issues may seem ominous, but many are reasonably easy to solve. 

Heating systems are full of moving components that suffer from wear, tear and degradation. The older your heating system is, the more likely it is that issues will arise at some point. 

Inefficient or faulty radiators will cost you heat, but they’ll probably cost you money too. As energy prices soar, maintaining a smooth, efficient central heating system is more important than ever. 

This is a guide to cold radiators, answering the question “how do radiators work” and much more!

How do radiators work?

First and foremost, it’s helpful to know what sort of central heating system your home has. 

Gas central heating systems account for 77% of all heating systems in the UK. Not all radiators are hooked up to gas central heating systems, but the vast majority of flats and houses on the mains gas grid use traditional plumbed radiators (as do homes on LPG gas). 

Other less-common central heating systems include electric or oil heating. The information in this guide is relevant to the majority of central heating systems in the UK. 

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Central heating systems explained

Most homes in the UK are heated by a gas central heating system powered by a boiler. The boiler ignites gas to heat water and pump it through your radiators. The radiators then warm your home via heat radiation and convection. 

While all gas central heating systems are similar in principle, their components do differ. First and foremost, there are three different types of boilers: 

  • Combi boilers: Combi boilers are compact, small and provide direct hot water. They’re common in flats and small houses. However, they may not meet hot water demands in larger homes. If you have a combi boiler, you likely have a sealed system. 
  • System boilers: System boilers also take water straight from the system, but unlike a combi boiler, they have a separate hot water tank. This allows them to meet higher hot water demands. If you have a system boiler, you likely have a sealed system. 
  • Conventional, traditional or regular boilers: in addition to a hot water tank, conventional boilers also have a cold water feed (typically in the loft). These are still the most common form of boiler in the UK. If you have a conventional boiler, you likely have an open-vented system. 

If you have a modern combi or system boiler, then you won’t have a feed tank in the loft. Your radiators may also be built slightly differently to accommodate the higher pressure of the sealed system. This isn’t guaranteed, though; modern boilers also work with most older radiators. 

Radiator valves

All radiators have valves that intersect the piping heading in and out of the radiator. There are two common types of valves:

  • Lockshield valves: These are fitted on the side where the water flows into the radiator. They’re designed to regulate the radiator's heat output to balance the heating system. Once the lockshield valve is ‘set’, it’s usually left until changes are made to the heating system. 
  • Thermostatic Radiator Valves (TRVs): On the other end of the radiator, you’ll likely find the TRV valve, which is used to adjust the radiator’s heat output. Older radiators may have a manual valve instead of a TRV valve, but they perform the same fundamental function. 
Horizontally mounted white TRV valve
Above: Horizontally mounted TRV valve

Understanding what sort of central heating system you’re working with will help you troubleshoot heating issues. For example, open-vented traditional systems are more prone to a build-up of sludge, rust and other contaminants, whereas sealed systems are prone to pressure issues. 

Moreover, conventional boilers have separate pumps, whereas system boilers and combi boilers typically build their pump into the boiler unit itself. 

In summary, radiator problems might emanate from within your radiator, the valves, the pipework between radiators, the boiler itself, or the tanks that feed the boiler. 

Without further ado, let’s take a look at some of the most common radiator issues: 

Common radiator issues

radiator against concrete wall

What causes cold spots on radiators?

Radiators often suffer from cold spots due to a build-up of sludge, limescale, rust and other grime. If this is the culprit, your radiator will likely feel cold across the bottom. 

Flushing your radiator could rid it of some of the blockages. Sludge build-up is more common in older central heating systems with older radiators that aren’t corrosion-resistant. 

If you have a relatively new system installed in the last thirty years or so, then your system may also need balancing, or there may be an airlock, blockage or obstruction in the pipes. Bleeding your radiator should be the first port of call. 

Why is half of my radiator cold?

If the bottom half is cold, then sludge is the likely culprit. If the top half is cold, then your radiator might have an airlock, which is essentially trapped air that inhibits the flow of water through the radiator. Bleeding the radiator should alleviate the airlock. 

Should radiators be hot all over?

In a word, yes. A proper-functioning radiator should be warm all over. 

Why are there cold radiators in some parts of the house?

There are a few possible causes. First, if you have a zonal heating system, then check to see if the cold radiators are confined to one of your heating zones and check the controls. 

Otherwise, cold radiators upstairs could signal an issue in your heating feed, such as a blocked ballcock valve. Cold radiators downstairs are more likely to do with the heating pump (which is usually located by the boiler or hot water tank). This could also be a balancing issue, or the cold radiators might need bleeding. 

Cold radiators upstairs?

Cold radiators upstairs but normal downstairs could result from individual faults in your upstairs radiators (e.g. valve blockage, airlock or sludge). However, if all your upstairs radiators are cold and not just one or two, it’s worth checking the feed and expansion tank in the loft. 

Cold radiators downstairs?

This is symptomatic of pump issues and is especially likely when you first turn your heating on at the start of autumn or winter. Your central heating pump should be located either by the boiler or hot water tank. It’s sometimes possible to start it by gently tapping it, and you should be able to remove the cover to see if the pump spindle is turning. 

Why is my brand new radiator cold at the bottom?

If your brand new radiator isn't heating up well or at all, then it’s possible that the system wasn’t rebalanced after installation. If you turn all of your radiators off and the new radiator still fails to heat up, then re-balance the system. If it still doesn’t heat up, there might be an issue with the pipework or valves. Ask the heating engineer to check the installation. 

One radiator not working?

If just one radiator isn’t working at all, then check the lockshield and thermostatic valves. The TRV pin under the valve cap might have become stuck - you can try and lubricate it and loosen it manually. You can also bleed the radiator.

Bare TRV pin
Above: TRV valve pin

How to fix a cold radiator?

Bleed your radiators

Consider bleeding your radiators if: 

Identify which radiators are problematic, turn your heating off, and leave your radiators to cool completely. The bleed valve is usually located in the top corner of the radiator. 

To bleed a radiator, you’ll need four things:

  • A radiator bleed key
  • A bucket, cup or a mug
  • A cloth to collect any water or leakage
  • Gloves to protect your hands from any hot water 
  1. Place the cloth under the bleed valve and gradually turn the key. The best-case scenario is that air and some water will come hissing out and drip onto the cloth. Wait until the hissing stops and close the valve. If nothing comes out, the valve might be blocked with paint. If so, you’ll need to close the valves at each end of the radiator and partially disassemble the valve by loosening the centre screw until you break any surrounding paint. 
  2. Make sure to close the valve once all air has leaked out. This might take a few minutes of gently opening and closing the valve, so be patient. 
  3. Close the valve and repeat for all other radiators. Afterwards, you may need to check your boiler pressure (normal pressure is typically between 1 and 2 bar).

Tip: Close the valve when the hissing stops. Bleeding the radiators is about releasing air, not water. This is why it’s essential to release the valve gradually, especially if you have to loosen the central screw slightly. If no water comes through at all, you might have a blockage on the way to your radiator or in the lockshield or bleeder valve. 

How does air get trapped in the radiator? 

Central heating systems are full of moving parts and pipes that expand and contract with heating and cooling. Over time, small radiator leaks can introduce air into the system. For older central heating systems, air might enter the system via the pump or cold water feed in the loft. 

Do you let all the water out when bleeding a radiator?

A small amount of water should accompany a hissing sound (the sound of air escaping). Once the hissing stops, close the valve to prevent excess water from escaping. 

What happens if you bleed a radiator with the heating on?

Never bleed your radiators with the heating turned on. If your radiators are hot, any escaping water and air will also be hot. The solution is simple: turn the heating off before bleeding the radiator! 

Perform a radiator powerflush

Powerflushing involves connecting a pressurised device to the heating system and pumping water and cleaning chemicals through it to clean the pipes and remove sludge from radiators. 

This is a complex process that requires the assistance of a heating engineer. If done incorrectly, the risk of damage to your central heating system is high. In addition, powerflushing can cause more problems than it solves with some older heating systems - get in touch if you think you need a powerflush.

What is a radiator balancing issue?

Balancing issues occur when the heating system is not distributing hot water to the radiators equally. 

If your radiators heat up at different times, only partially heat up, or won't heat up at all, it’s probably worth rebalancing the system if bleeding has no effect. 

Balance your radiators

Balancing radiators can help fix problematic radiators that don’t heat up fully. However, bleeding should be the first port of call, so bleed your radiators before balancing them. 

  1. To balance your radiators, make a list or rough diagram of where your radiators are in your home. 
  2. Turn your central heating off and allow all radiators to cool. 
  3. Open every valve on every radiator (both the manual valve/TRV and lockshield valves). 
  4. Turn the heating back on and locate the radiators, which heat up first/get the hottest. Make a note of these in order. 
  5. Turn the heating off, and allow it to cool. 
  6. Turn the heating back on, and turn the lockshield valve on your fastest heating radiator until it’s closed, then open it around ¼ of a turn. You’ll need to follow your radiator/valve manufacturer’s guidance as the procedure might differ between models. 
  7. You’ll need a thermometer to check the temperature difference between the pipework leading into the valve and the pipework leading out. Next, adjust the lockshield valve until the difference between these two pipes is around 12C. The exact figure might vary between manufacturers.
  8. Repeat the process with other radiators.

Tip: This is only a rough guide, and the exact process will differ between systems. Book one of our engineers today for a piece of mind.

How to get a flow diverter installed

Flow diverters are fitted internally to the radiator to ensure water is pumped to all areas of the radiator. Most modern radiators are pre-fitted with flow diverters. If you’ve fitted new radiators but still can’t get them to heat up properly after trying other solutions, then ask a heating e engineer to check the flow diverter. 

Summary: Why Is My Radiator Cold?

Radiators are humble, hardworking devices that are relatively simple once you get to know them. However, correctly diagnosing central heating issues can be tricky, especially if you’re unsure of how your central heating system works. 

When it comes to more complex central heating tasks such as rebalancing, flushing and checking the pump, cold water feed, valves and other heating components, it’s always wise to get in touch with us. One thing is for sure - the cost of advice is less than the cost of replacing damaged components! 

Read our other guides on radiators

Fix your leaking radiator
Fix your hissing Radiator
Fix your noisy boiler
Most common towel radiator problems
Fix your rattling radiator

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