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Why is Your Radiator Cold at Bottom?


Many confused homeowners have called in a plumber stating that their radiators are ‘half hot and half cold.’ They are often sure that it’s a malfunction with the radiators. And while this might be true, it’s not always the case. In some instances, having your radiator cooler at the bottom is normal. But if it’s too cold, then you probably have a sludge problem on your hands.

Before we get to the fixes, let’s ensure you can tell the difference between when it’s normal and when it’s not.

When is it normal for a radiator to be cold at the bottom?

Naturally, radiators are warmer at the top than at the bottom. The temperature differences vary from one radiator to the next. For instance, Convection radiators heat a room through convection. This means that the top is always warmer than the bottom. However, the radiator shouldn’t be ‘cold’ at the bottom.

When is it not normal for a radiator to be cold at the bottom?

If you notice a radiator is unusually cold at the bottom and hot at the top, then it is generating less heat than normal. As such, your house is cold, or you are spending more on energy bills to compensate for the temperature difference. In this case, you need to call in a boiler engineer.

The most common cause of this problem is central heating sludge. On average, 20% of boilers break down yearly. A good number of these boilers are affected by central heating sludge. And while many homeowners think it’s normal for a boiler to break down after five years, the problem can be avoided.

What is Central Heating Sludge?

Central heating sludge is made up of two things:


Limescale is a solid buildup of minerals naturally found in water. Homeowners that use hard water are affected more than those who use soft water. This is because soft water contains fewer natural minerals. As such, limescale build-up takes longer where soft water is involved.

Dirty water

Over the years, the insides of the radiator, pipework and towel rails will develop rust. The rust will break off and get into the central heating system’s components. The more water circulates in the system, the more the system breaks down from the inside. Eventually, a thick, dark, and dirty sludge will build up. And since it’s heavy, it collects at the bottom of the radiator causing it to be cold at the bottom.

Without a filter, the central heating sludge will build up fast. However, the buildup will take several years before it starts causing problems.

How central heating sludge causes radiator problems

The sludge circulating in the heating system will find its way to the pipework, radiator valves and the bottom of the radiator. With time, it will cause a small blockage. This blockage will cut off parts of the radiator from the main system, thus resulting in parts that are hotter than others.

Since sludge is heavy, the bottom parts of the radiator are blocked from receiving hot water. The sludge also causes a spike in pressure in the radiator. The pressure build-up can cause the pump seal to blow. If ignored, sludge build-up could cause major internal damage to your boiler and warrant a replacement.

Other symptoms that manifest alongside cold bottom radiators include:

  • The boiler makes strange noises.
  • The radiator doesn’t get hot enough even when the valve is set to the maximum.
  • The radiator needs bleeding a little too often.
  • Patchy warm spots on the radiator.
  • The pipework feeding the radiator is hot but is itself cold.
  • Valves keep malfunctioning.

How to fix if your radiator is cold at the bottom?

Determine the radiators causing the problem

Before attempting to fix the problem, first determine which radiators are faulty. Are all radiators cold at the bottom? Or are the radiators upstairs working and those downstairs not working?

In some instances, it’s only one radiator that is cold at the bottom. And other times, the one radiator will cause a malfunction in other radiators installed after it.

Use a central heating inhibitor

Central heating systems should be filled with a quality central heating inhibitor. Top boiler manufacturers recommend this. If this is not done, the warranty doesn’t hold. A central heating inhibitor helps to break off the loose limescale and rust pieces in the boiler’s system. For best results, the inhibitor should sit in the system for an hour before it’s drained. For severely blocked systems, the inhibitor should be left inside for a week for excellent results.

There are many central heating inhibitor brands to choose from, including Sentinel, Fernox, Magnaclean, Victas, and Clearwater.

Manually flush the system

Here is a quick guide on how to manually flush your radiator.

  • Turn your boiler off and allow it time to cool. The hot water within the system can cause severe burns.
  • Protect the area surrounding the radiator with old towels. Also, have a bucket on standby to catch the central heating sludge.
  • Turn off the valves located on both ends of your radiator. Count the number of turns you make since you will need the number when resetting your radiator.
  • Loosen the nuts on the thermostatic valve.
  • Open the bleed valve using the radiator key. Be careful since a lot of radiator water will flow out.
  • Carry the radiator outside. Handle it with care since it will still be full of sludge and dirty water.
  • Connect a hosepipe to the valve and flush the radiator to clean out the sludge.
  • Reconnect the radiator to your boiler system.
  • Re-pressurize the boiler.

This fix works if only one radiator is affected. If many are affected, the process is tedious.

Power flush the system

For boilers that have a serious blockage, a Power Flush is recommended. A Power flush is done by forcing a solution of water and cleaning agents into circulation. The solution will force out the central heating sludge. The process will take about 5 hours. And since force is a key ingredient to the process, a flushing pump is needed. Because of this, only a gas safe engineer should do it.

Install a scale reduce and a boiler filter

Now that the blockage is cleared, you need to take measures to prevent it from reoccurring. Generally, preventing the build-up is more cost-effective than cleaning and unblocking the system. Luckily, with regular maintenance and yearly chemical water treatment, you’ll extend your boiler’s life and protect it from frequent breakdowns. Below are some methods you can use to prevent central heating sludge build-up.

Installing a boiler filter – Installing a boiler filter lie the TF1 Omega or the Fernox TF! Will help prevent sludge build-up by filtering it away from the system’s components. The filter will act as the first line of defence to the system.

Use an inhibitor – Using a corrosion protector or inhibitor like the Fernox F1 will help to prevent scale build-up and corrosion of the metallic components of the boiler system.

Replace the radiator

Unfortunately, some radiators cannot be saved. If you live in an area serviced with hard water and your radiator is old, there are high chances that central heating sludge build-up will be stubborn. Not even a power flush will work.

In this case, remove the problematic radiator and install a replacement. Luckily, radiators are affordable. New age radiators are up to 50% more efficient than those built decades ago. So though you’ll spend cash on a new radiator, you’ll save more in energy bills over the years.

What now?

If you still experience problems with your radiator being too cold at the bottom, get in touch with us. We will get 3 companies to send you FREE quotes to help with your job.

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