The short answer
Nine times out of ten, a radiator that's hot at the top and cold at the bottom has sludge in it. Sludge is heavy, so it settles in the bottom of the radiator where the water can't push it out. The hot water flows across the top and the bottom stays cold.
Cold at the top is a different problem
If it's the other way round, cold at the top and warm at the bottom, that's air. Bleeding the radiator with a radiator key usually sorts it. If you find yourself bleeding the same radiators every few weeks, something else is going on and it's worth having the system looked at.
What you can check yourself
Bleed a radiator and look at the water. Hold a cloth under the valve. If what comes out is black or dark brown rather than clear, there's sludge in the system.
Is it one radiator or several? One cold radiator can be a stuck valve. Several cold at the bottom usually means the whole system needs cleaning.
Listen to the boiler. Kettling, banging or gurgling noises often go hand in hand with a dirty system.
What fixes it
Bleeding won't shift sludge, and neither will turning the heating up. The proper fix is a power flush, which pushes water through every radiator and pipe until it runs clean. Fitting a filter afterwards catches new debris before it builds up again.
At DDS, a power flush with a filter fitted is from €450. You can see all our prices on the pricing page.
Not sure?
Text or call Dave on 083 400 0043 and describe what's happening. He'll tell you whether it sounds like sludge, a valve or something else before you spend anything.
Last updated October 2026. Grant amounts and rules change, so check seai.ie for the latest.