Guide
Why are my radiators cold at the bottom but hot at the top?
This is the single most common heating question we get, and it is almost always misdiagnosed as air in the system. It is usually not air.
Short answer
Cold at the bottom and hot at the top is sludge, not air. Bleeding will not fix it. A power flush of the full system starts at £500 + VAT and clears it properly.
Cold at the bottom versus cold at the top
This distinction is the whole diagnosis, and it is worth learning because it tells you which problem you have before anyone comes out.
Cold at the TOP, warm at the bottom, is air. Air rises, so it collects at the top of the radiator. Bleeding it releases the air and fixes it, and you can do that yourself with a bleed key.
Cold at the BOTTOM, hot at the top, is sludge. Corrosion and debris are heavier than water, so they settle in the bottom of the radiator and block the flow. Bleeding does nothing at all, because there is no air to let out. People bleed these radiators for years.
What the sludge is actually doing to your bills
A radiator with two inches of sludge in the bottom is heating a fraction of its surface area, but your boiler is still working as hard as if the whole thing were hot. You are paying full price for part of a radiator.
It also wears the boiler. The pump is pushing water through a restricted system, and the debris ends up circulating through the heat exchanger. Fitting a brand new boiler onto a sludged system is one of the quicker ways to shorten its life, which is why a flush is included in our two higher boiler packages.
What a power flush involves and what it costs
We connect a pumping unit to the system and push water and cleaning chemicals through at high velocity, radiator by radiator, until what comes out runs clear. On a badly neglected system that takes a while.
A power flush of the full system starts at £500 + VAT. We fit a magnetic filter afterwards, which catches the debris that forms from here on so you are not paying to do this again in five years.
It is not always the answer. If one radiator is cold and the rest are fine, that is often a single radiator that needs removing and flushing, or a stuck valve, and that is a much smaller job at our hourly rate of £70 + VAT.
Other signs it is sludge
The system is noisy, especially banging or kettling sounds from the boiler. It takes a long time to warm up. Some radiators never get properly hot no matter what you do with the valves. The water that comes out when you bleed a radiator is black rather than clear.
Any of those with cold spots at the bottom, and you are looking at the same problem.
Prices shown are guide prices and exclude VAT. Last updated 4 August 2026. The final figure for any job is confirmed after a free survey.