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Radiators Cold At The Bottom? – Easy Fix When You Know How!

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Radiators Cold At The Bottom? – Easy Fix When You Know How!


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Radiators Cold At The Bottom? – Easy Fix When You Know How!

Now that winter is upon us, everyone is switching on their Central Heating and finding after a long sleep, their heating seems to be not heating the house properly.

This blog is all about :

  • Common Central Heating Problems
  • Grants For Free Central Heating
  • Grants For Free Boilers
  • Radiator Valves


Your Radiator Is Cold At The Bottom

A radiator cold at the bottom and hot at the top is usually down to the build-up of sludge in your central heating system. Sludge  is a black, mud-like substance which is a by-product of corrosion inside the heating system usually due to lack of inhibitor ( corrosion preventer)

This sludge would have been built up over many years and will be circulating around all your radiators with deposits sticking in some or all of your radiators.

Sludge does impact a radiator’s ability to heat your room so you do need to take action to fix it.

How Does Sludge Affect Radiators

Sludge being denser than water will sink to the bottom of the radiator.

This stops hot water moving through the radiator evenly and it will block the radiator eventually if not attended to.

Best Way To Remove Sludge


DIY  –  Disconnect the radiator and remove it from the wall, and then flush it out in the garden using a garden hose to the radiator.

If this does not work it may be better to replace the blocked radiator with a new one especially

It may also be wise to do this if the old radiator is showing signs of rust or corrosion on the outside. Replacing it before it develops a leak could save a lot of future headaches. After replacing the radiator, it’s important to add a chemical inhibitor to the system.

This will prevent the formation of sludge in the future.


Power Flush

Powerflush – a heating engineer will use a machine to powerfush water and anti-corrosion chemicals through your central heating system.

All sludge will be forced out  from all of your radiator.the engineer will connect a pump to the heating system which then pushes a chemical through the pipework and radiators at a high speed, thus breaking down and removing the sludge. After this process is complete the engineer will then use a descaler to remove limescale, and also a corrosion inhibitor which will prevent rust from forming again in the future.

Sometimes the most effective solution if the blockage is particularly stubborn is to bite the bullet and  replace the radiator.


Bad Blockages

If the blockage is very bad and the Powerflush is not enough, the engineer might also have to dislodge pieces of sludge using tools on the outside of your radiators.

This happens more frequently in hard water areas as there will be extra limescale which makes the blockage harder to remove.


Other reasons why radiators is cold at the bottom but hot at the top?

Assuming the radiator is free from sludge, It could be is that not enough hot water is flowing through it.

Hot water will naturally rise and could be that the little amount which is getting through is only enough to heat the top – and it probably won’t be very hot either, in comparison to the other radiators.

  • Check that both the  thermostatic radiator valve and  lockshield are open
  • The radiators may need balancing.


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Why Are My Radiators Cold?

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There are several probable causes as to  why your radiators are not not getting hot.

These are the most common reasons for this and you should investigate all of them.

  • Radiators Are Blocked  – debris in the pipework or filter
  • Radiators Need Balancing    
  • Thermostatic Radiator Valve Stuck
  • Pump not working / has air in it
  • Air in the system
  • Lockshield  fully closed
  • Boiler or heat pump not working properly
  • Central heating has turned off

Radiator is Blocked

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If your central heating system has not been serviced regularly, then there could be a build up of debris in the pipework or the filter.

This build up of limescale, sludge, rust or debris flowing through the system will eventually settle in your radiators creating a blockage.

Blockages that start off fairly small can quite quickly become large blockages and interrupt your central heating flow.

Blocked radiators are usually cold at the bottom and warm at the top due to insufficient flow.


Blocked Radiator Fix

 Remove the radiator and flush it out  thoroughly with a garden hose, before replacing it.

If you have several radiators not heating up then it may be necessary to have them power flushed by a professional.

A power flush uses pressure to shoot water and chemicals through the system, removing dirt and flushing it away down an outside drain.


Radiators Need Balancing.

How To Balance Radiators

  1. Turn off your heating
  2. Open all radiator valves
  3. Note the speed each radiator heats up
  4. Allow your heating to cool down
  5. Turn your heating back on
  6. Adjust the fastest radiator
  7. Repeat for other radiators.


A Thermostatic Radiator Valve is Stuck

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If it is just one radiator not heating up, then a stuck thermostatic radiator valve could be the culprit.

A thermostatic radiator valve (TRV) controls the flow of hot water to a radiator and occasionally these do get stuck so not opening to allow the hot water in.

The TRV works by pushing a pin in and out and sometimes this pin will corrode and not move.

Try to free the pin using pliers and grease but don’t use excessive force as you might damage the pin and it can pop out or leak. Replacing the TRV is usually the best practice and will cost you around £12 to DIY.


 Pump is Not Working


If a few, or all, of your radiators are not heating up, the circulation pump could be to blame.

Circulating pumps move hot water around a heating system, taking it from the boiler through the pipes to your radiators and hot water cylinder in some cases — before taking it back to the boiler.

Sometimes, blockages or airlocks in the system will occur.

Good indicators that this may have happened, other than radiators not heating up, include a water leak from the pump, unusual noises coming from the pump, casing that is hot to touch and no hot water.

You can try bleeding the pump or exchanging it.


 Air in Heating System

A very common cause of radiators not heating up is an airlock in the central heating system.

“There could be air in the system which would mean that the radiator may only heat a little bit at the bottom, or not at all if there is a lot of air,

“You will need to bleed the radiator to get the air out.

“As you let air out of the system you need to allow more water in.

A gravity fed central heating circuit should fill automatically but a high pressure system will have a valve (or two) to let the water in.

Don’t leave this valve open. Pressurise the system a bit and then bleed the radiator. It is usually best if two people do this but if in doubt call in a professional.


 Lockshield Valve Closed Or Open Too Much

On the opposite side of the radiator to the TRV is a lockshield valve with a cap on it.

If this is closed no water will flow to the radiator.

If you open it too much (This is often only a quarter turn open ) the boiler water will now circulate more through the closed radiators and hardly ever get heat to furthest away radiators.

A good signal  will be if the radiators upstairs or those nearest to the boiler are getting hot but the downstairs radiators are cold.

The system will need to be rebalanced.


The Boiler is Not Working


An obvious reason for radiators not working is a faulty boiler.

Check your boiler  is on the right setting and appears to be working ok

A fault code will usually alert you to a problem.


 The Water Pressure Has Dropped

If you have a combi boiler, or a high-pressure central heating system, the problem could lie with a drop in pressure, causing the boiler to cut out.

Take a look at the pressure dial — it should be in the range specified in the instruction manual.

If not, you will need to top-up the water (check your manual for instructions) to repressurise the boiler.


The Central Heating is Off


Check that your central heating hasn’t been switched off or set to summer mode, or to heat the water only, by accident.

 How To bleed a radiators – A step by step guide

Bleeding a radiator is just one way to make your central heating more efficient.

It is quite easy and you can do it yourself.

Step 1

If you have a combi , make sure you know how to re-pressurise the boiler . It should be around 1 bar.  Check your manual for details.

Make sure that your central heating is off and that the radiators are cold.

This is to ensure toy avoid  the risk of spraying yourself with hot water when bleeding the radiator.

Check the pressure gauge on your boiler after bleeding a radiator to check that its still in the green band

2. Protect the surrounding floors and walls

You will need to protect the wall and floors around the bleed valve ( usually at the top of the radiator)  as you will get some water escape when the valve is opene.

Place a towel between the radiator and the wall and  hold another towel just underneath the bleed valve.

 A bowl on the floor underneath the bleed valve will protect  from any further drips.


3. Find the bleed valve and release the air

The bleed valve is typically located on the top right hand side of the radiator.

You will need a radiator key or in some cases a flat head screwdriver to move the valve.

You’ll need to turn the key anti-clockwise to open the bleed valve.

You will hear air hissing from the valve meaning the air is being released. But make sure you are ready for a squirt of water. As soon as this happens turn the key clockwise to close the bleed valve.

A multi-plumbing tool with a bleed key or a standard bleed key can be used to bleed a radiator

4. Keep an eye on the boiler pressure

After bleeding a radiator — keep your bleed key (opens in new tab) in a save place, they are easy to lose — check you boilers pressure gauge. Often the pressure will drop and fall outside the optimum green band and into the red.

If the gauge level falls into the red zone, you will need to re-pressurise your boiler to bring it back into the green zone. This is relatively simple — especially on new boilers — but it will differ from brand to brand.

In the image below the two blue handles are used to re-pressurise the boiler. The one on the left opens the water supply and the one on the right re-pressurises the boiler. If unsure on how your filling loop works, check the manual or call in a professional.

You’ll need to locate the filling loop mechanism to re-pressurise your boiler while bleeding radiators

5. Continue to the next radiator

Repeat this step on your other radiators that need attention. You’ll only really need to bleed the radiators that you determine have air trapped inside, working from the furthest radiator away to the nearest to the boiler, starting with the ground floor.

If you do have a radiator repositioned you will need to follow the previous steps once installed. Make sure valves are closed and the radiator is bled to ensure it pumps out plenty of heat.

6. Turn the heating back on

Once you’ve bled all the radiators you need to, turn the heating back on and check that they’re heating up correctly.

If not, there may be another reason for your radiators not working that may require it to be flushed. Also, double check all the valves are properly closed and that none of them are leaking.


What Is Radiator Balancing?

Radiator balancing is the adjustment of your radiator valves to ensure all your radiators heat up at the same speed.

If you find that your individual radiators have cold spots, this could mean trapped air in the system and you’ll need to bleed them.

If your radiators are cold at the bottom, this could be caused by trapped sludge and you’ll need to flush this out.

You Will Need:

  • Radiator bleed key
  • Lockshield valve key
  • Screwdriver
  • Digital thermometer/multimeter with thermometer
  • Adjustable spanner

How To Balance Radiators

  1. Turn off your heating
  2. Open all radiator valves
  3. Note the speed each radiator heats up
  4. Allow your heating to cool down
  5. Turn your heating back on
  6. Adjust the fastest radiator
  7. Repeat for other radiators.

1. Switch Off Your Heating

To begin balancing radiators, all the radiators and towel rails in your home must be completely cold. So, time to turn off your home heating.

It does also help if you have recently bled them, as your upcoming temperature readings will be more accurate.


2. Open Up All Your Radiator Valves

First you’ll need to open all the radiator valves in your home by turning them anti-clockwise

Thermostatic valves and wheelhead valves can be opened easily by turning them by hand.

Lockshield valves will have a plastic cover which will need to be removed. You can then use your adjustable spanner or lockshield valve key to turn the valve anti-clockwise and open it.


3. Note The Speed Each Radiator Heats Up

With all your radiator valves open, it’s time to turn on your heating and make a note of the speed each of your radiators heat up.

It helps to enlist a few friends or the kids at this point so that you can keep an eye on how fast all the radiators warm up. You’ll usually find those radiators nearest the boiler will be faster.


4. Allow Your Heating To Cool Down

Once again you’ll need to turn everything off and allow your heating system to cool down. Letting your system completely cool down may take a while and some people will prefer to leave it overnight.


5. Turn Your Heating Back On

Once you’ve let your radiators cool down completely, you’ll need to turn your heating back on and head to the radiator which warmed up the fastest.


6. Adjust The Fastest Radiator

You’ll need to adjust the fastest radiator in your home first. Close the lockshield valve on this radiator completely, then open it by one quarter turn.

Once it’s started to heat up you’ll need to take some measurements with your digital thermometer or multimeter with thermometer.

First measure the temperature of the pipework next to the valve. Then, take the temperature of the pipework on the opposite side of the radiator, usually where the TRV is. Make sure you take a note of both of these readings.

You’ll now need to turn the lockshield valve very gradually until the difference between the readings of the two pipes is 12°c precisely.

It’s important to remember balancing radiators takes time. You’ll need to wait for the temperature of the radiator to change each time you adjust the lockshield before you can get an accurate measurement.



7. Repeat For Other Radiators

Now that you’ve balanced your first radiator to an exact difference of 12°c, it’s time to tend to your slightly slower radiators and balance them just the same. Do this in the order they took to heat up.

What you’ll find when balancing radiators is that the amount the lockshield valve needs to be opened is correlative to the distance of the radiator from your home’s boiler. When it comes to the slowest of your radiators, you may find the lockshield valve even needs to be opened completely.

If you followed the above steps correctly, you’ll find your radiators are now balanced. The dispersal of hot water from your boiler is reaching the furthest radiator from your boiler as evenly as it is reaching the closest.


Central Heating Guide – Radiators & Valves

When it comes to radiators and valves, they may look quite complicated but really, all they are a taps than open and close to allow water to flow.

In this guide, we’ll take you through the different types of radiators available, the components they use and provide an overview on radiator valves.

We’ll also provide a basic overview on how radiators work.


How do Radiators Work?

Radiators work through a heat transfer process called convection.

When water in the radiator is heated, the surrounding air is also heated up via convection and this hot air is then moved around the room as the air circulates.

Radiators are normally connected to a central heating system via pipes and hot water flows through these pipes and into the radiators themselves transferring hot water into the radiator. As mentioned, this hot water heats the room via convection. The hot water doesn’t remain in the radiator, instead the hot water flows through the radiator and out again to the next radiator in the chain. As the hot water flows through the system it starts to cool down. When it flows back to the boiler the water is in a cooler state, and the boiler then reheats the water. The process repeats so long as the system is turned on.

As the radiators in the central heating system are connected in a chain, the radiator closest to the boiler is exposed to the water when it is at its hottest, whilst the radiator at the end of the chain, located just before the hot water returns to the boiler will be exposed to the coolest water. Because it is desirable to have all the radiators operating at the same heat output a process called balancing is used to ensure that the radiators all work to a similar temperature. Balancing works by restricting the flow of the water into a radiator. The balancing process is carried out by adjusting a valve called the ‘lockshield valve’. Either by restricting or opening the flow of water into the radiator.




What is a Lockshield Valve?


A lockshield valve is a type of valve that is fitted onto every radiator at the side where the water flows into the radiator from the boiler. The purpose of the lockshield valve is to help regulate the heat output of the radiator itself, and to allow for a heating engineer to balance the radiators so that all the radiators in the heating system output heat at around the same rate. Once the lockshield valve has been set, via the balancing process, a plastic cap is placed on top of the valve to stop and ‘shield’ from any further adjustment.

This ‘locks’ the valve in place. Once set, the lockshield valve rarely needs to be adjusted again unless there is a problem with the system or you need to adjust the flow of water. In short, the lockshield valve’s job is to control the maximum and minimum amount of water that can travel through the radiator.


What is a Lockshield Valve?

We’ve already discussed the lockshield valve, but on the other side of the radiator you’ll find another type of valve. This valve is adjustable by the user to control the radiator’s heat output. This is different to the lockshield valve because it can be adjusted by the user as and when heat adjustment in the room is required.

Generally, there are two different style radiator valves (that are adjustable by the user) available. These are manual valves and Thermostatic Radiator Valves (also known as TRVs).


Manual Valves

Manual valves are quite simple devices that you can think of like a tap in your home. You can adjust the heat of the radiator by simply turning manual radiator valves to the desired setting. However, achieving a desirable heat output can only be achieved through trial and error.

These simple valves are low cost and look great.

Take a look at the manual valves that Mr Central Heating has in stock.



Thermostatic Radiator Valves

Thermostatic Radiator Valves are a different type of user-adjustable radiator valve that monitors the temperature of the radiator and automatically adjusts the room temperature to maintain a regular level. Once the TRV is set to the desired level, the valve automatically controls the heat output from the radiator.

These thermostatic valves aren’t that expensive to buy, but they are more expensive than manual valves.

However, the cost outlay for the valves is offset by their energy efficiency. Because these valves can alter the heat output and control how hot the radiator gets in each individual room where a TRV is fitted, they are more efficient and therefore can help reduce your heating bills.

TRV’s in the UK are generally mounted vertically whereas the European style is to leave the TRV in a horizontal position.

Learn more about the thermostatic radiator valves (TRVs) that Mr Central Heating can supply.


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Where and how the valves attach to the radiator may be different depending on the type of radiator you are connecting it to. Modern radiators tend to attach to the bottom of the radiator at opposite ends. This is known in the trade as a BBOE radiator (Both Bottom Opposite Ends).

Other connection types are available too, although these tend to appear in older vintage radiators. TBOE stands for Top and Bottom Opposite Ends, and this as the name suggest means the valves attach at the top and bottom of the radiator at opposite ends. Whereas, TBSE means Top and Bottom Same ends, meaning that both pipes enter on one side of the radiator.

The only other considerations you might need to think about when it comes to valves is how the pipe attaches to the valve itself and in what direction the pipe will enter the valve. This tends to be more related to the way the pipes are set up in your own home.

For instance, you can get straight radiator valves for when the pipework goes straight into the radiator in line horizontally. Straight valves are not the only type of valve available.



Straight Valves



Corner Valves


Angled Valves

Corner Valves are used when the pipework turns at a 90% angle into the radiator. Corner valves are used when the pipework comes from the wall and not the floor.

Angled valves are used for when pipework comes from vertically from the floor and needs to attach at an angle into the radiator. When buying a radiator valve you’ll need to consider how your pipework is installed in the home to ensure you pick the right type of valve.

In the UK, the most common pipe diameter used in a central heating system is 15mm. So, you’ll find that most valves accommodate this pipe size. This is not always the case, however especially in older homes so you may need to use different sized valves or adapters to convert the pipe size to a more traditional size. It’s worth checking this prior to purchase to ensure you buy the right size connector on the valves.

It’s worth noting that if you buy a new radiator they will not normally come with valves.

This is because you need to pick the radiator valve to suit your own individual pipework. Often, people forget that both a lockshield valve, and a manual or TRV valve is required, for each new radiator. The good news is that Mr Central Heating provides valve sets that are perfect for your new radiator that includes both valves.

Central Heating Guide –  Understanding Different Radiator Types

Column Radiators

As the name suggests are constructed using metal columns and these radiators provide a traditional look but with modern efficiency. These radiators come in a range of different shapes, colours and sizes and provide excellent heat output. Individuals looking for more design-orientated look to their radiators tend to prefer this style of radiator. These radiators are available in both double and triple column designs.

Learn about the different types of column radiators that we stock at Mr Central Heating.


Compact Radiators

These are very common radiators and are fitted into a lot of homes in the UK. These radiators use fins to help transfer the heat to the room. These are smaller in size than column radiators but are also available in a lot of different shapes and sizes, along with falling into two distinct types, single panel and double panel radiators. As you might imagine a double panel radiator will provide more heat, at the expense of its size. Double panel radiators take up more room than a single panel radiator. These radiators are typically available in white only.

If you’re interested in buying a compact radiator we have a large selection for you to pick from.


Flat Panel Radiators

These radiators have the lowest depth profile of all the radiators so are great at fitting in to a small or strange space. These radiators are available in a range of different sizes, such as vertical radiators and the more traditional horizontal design. We supply a range of ‘wet’ flat panel radiators, which means that they can be connected to a traditional central heating system just like a column radiator or compact radiator. A ‘wet’ radiator is one that uses hot water for heat. Often, when flat panel radiators are mentioned these can refer to electric powered radiators that are used in situations where rooms or entire buildings do not have a traditional wet central heating system installed. This can be quite common in flats, where space is at a premium.

Explore our wide range of flat panel radiators.


Towel Radiators

Just like a flat panel radiator, a towel radiator can be both ‘wet’ or powered by electricity. The reason these radiators are called towel radiators is because they are often found in bathrooms, and allow for the drying of towels. Most homes will likely have a towel radiator in the bathroom, if a radiator is installed there. Often radiators that fall into the towel rails category are made in stainless steel and are of chrome design.

You can find a wide range of towel radiators for your bathroom at Mr Central Heating.



Radiator Heat Output

One aspect that might not be immediately clear when choosing a radiator to buy (style aesthetics aside) is what heat output you will require for any given situation. Radiator heat output is measured in a metric called British Thermal Units (commonly written as BTU).

A BTU is defined as the amount of energy it takes to raise one pound of water by one degree Fahrenheit. This is all very well, but not that useful for the average consumer. The good news is that to calculate what radiator BTU output you need you can use an online BTU Calculator such as the one featured on Mr Central Heating.

All you need do is enter in some basic information and the tool will give you some guidelines as to the range of BTU you should use in your room. Too low and the room will never get hot enough, and too high the radiator will not operate efficiently, or it will be too hot. This tool will help you pick the right sized radiator for your needs.


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