1. Radiators are cold at the bottom but hot at the top
Cold bottoms are a classic sign of sludge, because dirt settles low. The dirt settles at the bottom of the radiator and blocks the flow. Bleeding will not fix this.
If the top is cold instead, you may only have air. Check which pattern you have, because the fixes differ.
2. Some radiators are significantly cooler than others
Uneven heat can come from sludge in parts of the system. It can also come from a need to balance the radiators. A heating engineer can tell which.
If balancing does not help, sludge is likely. Because dirt builds up slowly, the change may be gradual.
3. The boiler makes banging or kettling noises
Kettling sounds like a boiling kettle, and scale or sludge in the boiler can cause it. Scale or sludge in the boiler can cause it. The water then boils in small pockets.
This can harm the boiler over time, so get it checked. Ask a Gas Safe registered engineer to look at it.
4. Pump failure or repeated pump replacement
Sludge can wear out a pump, since the pump pushes thick, gritty water. If you have replaced a pump more than once, dirt may be the cause. The pump has to push thick, gritty water.
Ask the engineer to check the water quality. A clean system helps new parts last.
5. The water drained from a radiator is black or dark grey
Dark water shows that rust and dirt are in the system. Draining a little into a jug shows the colour. Clear or pale water is a better sign.
Black water is a clue, not a final answer, because some water darkens quickly. If a leak appears, see our emergency plumber page. For dirty water alone, ask a heating engineer.
6. The boiler is being replaced or is new
A new boiler may suffer if old sludge remains. Makers often ask for the system to be cleaned before a new boiler goes in. Check your boiler’s manual and warranty.
Ask the installer what they will do to clean the system. Because a clean start protects the boiler, it matters.
7. Heating takes a long time to warm up from cold
A slow warm-up can mean the water is not moving well, since sludge may be the reason. Sludge may be the reason. Check for cold patches too.
If the system warms slowly, ask an engineer to test it. Simple steps may help first.
What a power flush costs
The price depends on the size of the system and how dirty it is. We cannot give a fixed figure, since every home differs. Read our powerflush cost guide to see what changes the price.
You can ask for a free quote on our plumbers page or online. We pass your details to a local engineer.
Related guides and local help
If you want to know more, read Underfloor Heating Problems: A Diagnosis and Repair Guide. You can also read Radiator Not Heating: A Diagnosis Guide for Every Fault Type and What Is a Powerflush and Does Your Boiler Need One Now?, because they cover closely related problems.
These guides help too: Powerflush Cost Guide: What Affects the Price You Pay, Annual Plumbing and Drainage Maintenance Checklist, New Boiler Guide: How to Choose the Right Boiler for You, Commercial Drain Maintenance: A Practical Guide for Owners and How to Drain and Flush a Central Heating System at Home. Each one explains a related topic in plain English.
Need help near you? Read our powerflush page first, or see Powerflush in Aylesbury and Powerflush in Aylesford. You can also see Plumbing Repairs in Carrickfergus, since every service page has a free quote form. We pass each request to a local drainage engineer.
