Radiator cold at the bottom, hot at the top: 2 reasons why

A radiator that’s cold at the bottom and hot at the top is usually because of the build-up of sludge, a black, mud-like substance which is a byproduct of corrosion inside the system. It can seriously impact a radiator’s effectiveness, resulting in a chilly, uncomfortable room. This article will explain why this happens and what can be done to fix it.

Radiator cold at the bottom explained

How exactly does sludge affect the radiators?

The answer is simple: it blocks them. Sludge is denser than water, so it tends to sink to the bottom of the radiator. As it builds up, it prevents hot water from moving through the radiator evenly, before stopping it completely.

How do you remove the sludge?

There are a couple of different ways to do this. The first way is to disconnect the radiator, remove it from the wall, and flush it out in the garden. This can be done by attaching a hose to the radiator. However, if there is sludge in one radiator, then it may easily be in the others. As a result, a powerflush may be worth thinking about. This is where a heating engineer will flush water and anti-corrosion chemicals through the central heating system, removing any sludge from all of your radiators and the boiler.

Of course, the most effective solution is to replace the blocked radiator with a new one. This could be the only option if the blockage is particularly stubborn. 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 is important to add a chemical inhibitor to the system. This will prevent the formation of sludge in the future.

Are there any other reasons why my radiator is cold at the bottom but hot at the top?

Assuming the radiator is free from sludge, one possible explanation is that not enough hot water is flowing through it. Remember that hot water will naturally rise. This potentially means 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 the thermostatic radiator valve is open. If the TRV and the lockshield are open, the radiators may need balancing.

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