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Radiator Cold At Bottom

Your radiator will be cold at the bottom because of a build-up of sludge and magnetite waste.

This waste deposit can make things more difficult because it stops the radiators from working at their full capacity. If unattended, a radiator cold at bottom can cause problems like the following:

  • You can potentially ruin your boiler or heat exchanger
  • It will cost you more money
  • Your home will become an uncomfortable place to live

What many homeowners don’t realise is that if you have an excess buildup of sludge, you need to make sure your boiler is still in operation, especially during the summer months. If you don’t, the sludge can dry up and block your pipework (especially if it’s the smaller size) which causes all kinds of trouble.

How does it affect your central heating system?

magnetic sludge blocking pipe

Simply put, it’s affecting your radiators, your pipework and your boiler. At Boilerhut we’ve seen several instances in which a homeowner’s pipework (often an 8mm-10mm microbore system) hasn’t been used for a while. As a result, the sludge has literally dried up in the heating system, which means you’ll have to invest in completely new pipes. If you are doing this however, you would generally need to replace your boiler and radiators at the same time.

Having a Radiator Cold At Bottom catches homeowners off guard if not dealt with, especially during the colder months. But if you plan ahead, you can be prepared for any drops in temperature, and your boiler would never have to break down.

But why does the problem occur? What does it mean? And how can you prevent it from happening again? Getting answers is important to making sure you have decent heating, which you’ll find in this very article!

Here you’ll find information on the right responses and how prevent it from happening again. Beyond this, you can read the following articles for further help and advice:

Radiator Cold At Bottom – What is Sludge?

As we’ve discussed, sludge is most likely the reason behind your radiator cold at bottom. It’s usually a mix of limescale, magnetite sludge and dirty heating water. If you don’t have any filters installed, these can build up quickly.

Limescale is a hard, white substance that comes from natural minerals in water. It’s mainly made up of calcium carbonate and can be every plumber’s worst nightmare. To establish the level of limescale in your area, ‘hard water’ and ‘soft water’ labels are assigned to all areas in the UK, making it easier to determine what you’re dealing with.

water hardness can help solve your radiator problem

Dirty water in your system occurs as a result of rust breaking away from inside radiators, pipework and towel rails. If this goes unnoticed over several years, it can cause a much bigger problem in the long run.

When the dirty water/limescale builds up, the flow of water around the radiator is restricted, making heating very difficult and a lot more costly. This is similar to how the human body works. If a diet is full of fats and sugars, arteries can get clogged up which restricts flow to the brain and the rest of the body. A central heating system is no different. Once the pipework and radiators are clogged with magnetite sludge, it’ll affect your entire heating system.

As water gets pumped through your system, it passes different metals like the brass valves, copper pipes and steel radiators. When an electrolysis breakdown occurs, your steel radiators will be attacked first, causing internal corrosion. This is similar to the hull of a boat travelling through the water – while being an incredibly well designed piece of engineering, components will still need to be protected and preserved.

Radiator Cold At Bottom – How to Respond

Find out which radiators have the problem

Magnetite sludge is heavier than water. So when you have an internal buildup in your system, the lowest radiator in your property will be affected first. Even if it’s only one radiator, find out if any others are affected by a build up of sludge.

You can do this by turning the heating on, then running your hand gently along the radiator’s surface. By going over each one, you can safely determine which areas of the radiator are cold.


Touch your radiator gently to find the cold patches

Moving your hands gently across the radiator surface allows to safely assess where the cold patches are

Perform a standard chemical/power/advanced/mains flush

Because you now know which radiator contains a sludge build-up, you have other options available for you. You can perform one of several types of flushes:

  • Standard Chemical Flush
  • Power/Advanced Flush
  • Mains Flush

We believe a mains flush is the most cost-effective way to get your system clean. For other types you’re much better off getting a professional to carry it out.

Apply a chemical inhibitor to your heating system

Adding a central heating inhibitor is very important because it helps to reduce the chances of an electrolysis breakdown in the system.

a typical chemical inhibitor used when a radiator cold at bottom

This is a typical chemical inhibitor used to clear out your system in a chemical flush


If you’ve noticed that the radiator is cold at the top instead, then you’ll probably have to bleed your radiator. This is because air has become trapped at the top, and you’ll need to let it escape the radiator in order for hot water to fill up.


Radiator Cold At Bottom – Types of Flushes

Chemical Flush

When arranging a flush of your system, you’ll need to consider the age of your property. For instance, a power flush could cause damage if the system is too old, so another option like a chemical flush is often a better choice.

The benefit of chemical flushes is that they’re perfect for breaking down the sludge in older properties with weaker systems. The cost for chemical flushes is usually lower, and can ultimately save you more money by protecting you from further damage. While other installers may try and charge you extra by constantly performing power flushes, Boilerhut will make sure that only the most appropriate choice is made to preserve your heating system and save you money.

For this type of flush, the chemical is sent through the central heating system. It breaks down and disperses the sludge that accumulates over time and with a final flush and chemical inhibitor, gives your system that much-needed clean.

  • Add a rapid heat cleaner chemical to the system – Leave this in for a minimum of 1 hour running at maximum temperature
  • Once the boiler has reached maximum temperature, isolate all the radiators except the one you’re using
  • Open the drain off on flow from boiler
  • Open the filling loop – this will release your cold mains water supply through the radiator
  • Check hosepipe for colouration of water
  • Isolate the radiator once the water coming out the hosepipe starts to run clear
  • Open the next radiator – repeat this process until you’ve flushed out every one.

Power Flush

power flush used to clear out your radiator

To avoid any drops in efficiency for your boiler, a power flush can ensure all the ‘nasty bits’ have been taken out of the system and correct your radiator cold at bottom. They’re often a cleaner and easier option for homeowners to deal with as it’s all done by a professional.

To carry out a power flush, a professional will need to visit the property. They’ll begin by attaching a pump to your central heating system. From here, chemicals will be pushed through the radiators, pipework and boiler at high speed. Any deposits from the system are then collected and removed.

The length of time for a power flush can vary between each property type. The age of the property, the size of the sludge deposit and the number of radiators need to be taken into account as well. Ultimately, your best chance will be to speak directly to a professional installer like Boilerhut to discuss in detail your options.

As successful as power flushes can sometimes be, they’re not always the best choice for cleaning out your system. In some cases where the radiator is damaged and a blockage has caused a breakage, it can actually be more cost effective to replace your radiator entirely rather than fixing or cleaning it.

Mains Flush 

If you’re unable to pay for a professional power flush, a main chemical flush may be the best option for sorting out your radiator cold at bottom.

To carry out a mains flush, you need to:

  • Have towels ready – Lay these around where the radiator will be moved to avoid any splash damage or stains.
  • Turn the radiator valves off – Start by turning the temperature control valve by hand, then use a wrench to turn the lockshield valve (the one on the other side of the radiator). Remember how many times you need to turn them, you’ll be doing the reverse later on.
  • Loosen the nuts connecting the radiator valve
  • Open the bleed valve with a radiator key – Use a bucket or any container to catch everything that comes out of the radiator. Once you’ve extracted what you can, close the bleed valve back up.
  • Remove the radiator – Once you’ve disconnected it from the wall, take the radiator to somewhere safe like the garden so you can hose it through.
  • Hose through the radiator – With the hose over one end, turn your taps on so that water is flushed through the radiator and coming out the other side.
  • Reconnect it to the system – After you’ve flushed the system with water, you can reattach the radiator using the same number of turns on the valves as before.
  • Check the boiler pressure – If the boiler pressure is too low, you can repressurise if required.

While power-flushing is effective at clearing your system, mains flushes can sometimes be a better response if your heating system is old enough. For more information on how this is carried out using MagnaCleanse, please watch the following video:



Radiator Cold At Bottom – How to prevent it from happening again

How it can be stopped

Thankfully, there are things that you can do to help your central heating system. You can significantly lower your energy bills and save yourself the hassle of getting a new boiler. This can be done by:

  1. Having a filter installed – If you’re having a new boiler installed, these work wonders for helping your system. Using filters like the Spirotech UE022WJ Magnetic Filter, they’ll work hard to catch any sludge that is deposited into the system. While filters are unable to remove the problem entirely, they can help prevent the build up of contaminants.
  2. Use a chemical inhibitor – Adding a chemical inhibitor to your system further prevents corrosion and the buildup of sludge
  3. Use a deaerator – You’ll need one of these to help with the issue. When you’re using your central heating, air gets passed through the system as well as dissolved oxygen. The deaerator removes this by sucking it out. A good example is the Spirovent RV2 22mm Deaerator, which is also shown in the image below:

Spirovent RV2 22mm Deaerator

If you’d like to know more about how this component can help you clear your heating system, give us a call on 02920 099898, and one of our heating experts will be happy to chat with you!

How to get a free online quote

If you’re still looking for answers for your radiator cold at bottom, you can call Boilerhut on 02920 099898 to speak to one of our heating experts for some friendly, reliable advice.

You can also get a free, no-obligation quote online by filling in the box below, taking you one step closer to a comfortably heated home.


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