Central Heating Faults – why is my radiator hot at the top and cold at the bottom?

At DIY Doctor we use the questions and answers on our DIY Forum to understand what you, and the rest of our users, need help and advice with.

Tracking the questions that arrive in our forum, and on our feedback pages, every day gives us a guide to what are the biggest problems in your homes.

We notice every autumn that questions about making your central heating more effective in order to save money, become more popular. We have a project about that on our main section. However,in the meantime here’s a bit of advice if your radiators are only partially heating up, when you turn on your central heating after the long hot summer. If your radiators are not fully operational, they are not heating your room effectively.

We have put together a little diagnostic blog to help you understand what the problem is when you are not getting the most from your central heating system.

Radiator Hot at the Top but Cold at the Bottom?

• If the bottom of your radiator is cold, but the top is hot, it is usually the result of rust and sludge building up in the bottom of the radiator while the water has settled over the summer. In this situation the system must be drained down, and the radiators flushed through. Flushing is done with flushing agents available from all plumbers’ merchants and, while inconvenient, allows you to thoroughly clean you heating system and keep it in good working order.

Why is my Radiator Cold at the Top, Hot at the Bottom?

• If the bottom of your radiator is hot, and the top of the radiator is cold or cool, this is generally a result of air getting into an unused system which stops incoming water reaching the top of the radiator, the water cannot compress the air and the pressure of the water is not great enough to force it down and out through the pies to the next one.  This problem is easily solved by using a radiator ‘bleed key’ to open the bleed valve, also called a bleed nipple, at the top of the radiator. Turn the key in the valve a little and listen until the hiss of trapped air stops. The air will be replaced by a trickle of water so have a cloth handy. If this problem occurs regularly then you need to call in a heating engineer, because it indicates that too much air is getting into your system, making it very inefficient.

These are the two most common faults with radiators after a good summer so now, might be a good time to test your heating to be sure of it working efficiently when you really need it in the winter.