Radiators Not Heating Evenly? Common Causes & Fixes (2025 Guide)

We get calls every single week from homeowners in and around Exeter saying the same thing:

“Why is the top of my radiator hot but the bottom cold?”
“Why are some of my radiators barely warm?”
“Do I need a new boiler — or is it something simpler?”

If your radiators aren’t heating evenly, don’t panic — it’s a super common issue. In this guide, we’ll break down the usual suspects, what you can check yourself, and when it’s time to call in the pros.

1. Cold at the Bottom, Warm at the Top? It’s Probably Sludge

This is a classic sign of sludge buildup — a mix of rust, scale, and debris that settles in the bottom of your radiators over time.

The fix:
You might need a power flush or chemical clean to clear out the system. In some cases, a magnetic filter (like a Magnaclean) can be fitted to catch sludge and prevent future build-up.

If multiple radiators are affected, it’s usually a system-wide issue.

2. Cold at the Top, Warm at the Bottom? Try Bleeding Your Radiators

This one’s easy: you’ve probably got air trapped at the top of the radiator.

The fix:
Grab a radiator key and carefully bleed it until you hear air hissing out, followed by a steady stream of water. Be ready with a towel or small bowl to catch any drips.

Don’t forget to check your boiler pressure afterward — bleeding can lower it slightly.

3. One Radiator Cold While Others Are Fine? Check the Valves

Sometimes, it’s as simple as a stuck or closed valve.

Check that the thermostatic radiator valve (TRV) is open and moving freely.
The lockshield valve on the other side might need adjusting too — this balances the flow across all your radiators.

If the valve’s faulty, we can replace it quickly.

4. Whole System Struggling to Heat Up? Could Be Your Pump

If all your radiators are warm-ish but not really heating properly, it could be down to a faulty or underpowered central heating pump.

Old pumps can get sluggish over time or seize completely.
Newer systems may just need a pump with more power — especially if you’ve added radiators or extended the property.

5. Your System Might Need Rebalancing

Even after bleeding and flushing, you might still find some radiators hotter than others — especially those closer to the boiler.

This usually means the system needs balancing: carefully adjusting all the lockshield valves to ensure every radiator gets its fair share of heat.

It’s a fiddly job, but we do it regularly as part of servicing or after fitting a new boiler.

Bonus: Keep Your Heating System Healthy with Regular Servicing

The best way to avoid uneven heating? Stay on top of your heating system’s health.

We’re rolling out flexible EPM service plans for homeowners in Exeter and the surrounding area — with different levels of cover, and optional add-ons like electrical protection and annual inspections. (We’ll let you know when it’s live.)

This kind of preventative care can save you a lot of hassle down the line — and keep your heating running efficiently.