If you’ve ever walked over to a radiator expecting it to be warm and found it sitting there like a cold metal disappointment, you’re not alone. UK radiators have one job, and half the time they act like they’re on strike.
The good news? Most of the reasons are simple, and the fixes are even simpler.

---

1. The Radiator Has Air Trapped Inside (Most Common Issue)

Let’s start with the classic: air inside the radiator.
This is the easiest fix, the most common cause, and the one nobody believes is their problem until they bleed it.

How to know if this is the problem:

• The top of the radiator is cold
• The bottom is warm
• You hear gurgling, trickling or weird noises
• The whole radiator feels uneven

Basically, if your radiator feels like it’s got two different weather systems happening at once, it’s trapped air.

How to fix it: bleeding your radiator

This takes about 10 seconds and you only need a bleed key (80p from any supermarket or hardware shop).

1. Turn your heating ON
2. Go to the radiator that’s misbehaving
3. Put a towel or small bowl underneath the bleed valve
4. Turn the bleed valve anti-clockwise slowly
5. You’ll hear hissssss (that’s the air leaving)
6. When water dribbles out → tighten it again

The radiator should heat evenly in a few minutes.

Important:

Bleeding radiators can drop your boiler pressure.
After bleeding a couple radiators, check your boiler gauge:

• If it’s below 1 bar → top it up to around 1.2–1.5 bar

If the pressure keeps dropping regularly, you may have a different issue — but for most people, this fixes it instantly.

---

2. Sludge at the Bottom (Radiator Warm Top, Cold Bottom)

This one feels unfair, but it happens in older heating systems or boilers that haven’t been serviced in a while.

How to know if this is the issue:

• The top is hot
• The bottom is cold or lukewarm
• Radiator takes ages to warm up
• You keep bleeding it and nothing changes

This means sludge — a mix of rust, dirt, metal flakes and general heating-system grime that sits at the bottom.

Two ways to fix this:

Option A: DIY radiator flush (cheap method)

It’s a bit messy but it works.

1. Turn off the heating
2. Close both valves
3. Unscrew the radiator (have towels & a bowl ready)
4. Take radiator outside or into your bathtub
5. Flush water through it until it runs clear
6. Reconnect it and bleed it

Most radiators respond amazingly after a flush.

Option B: Power flush (pro method)

A heating engineer uses a machine to push chemicals through your whole system.

Cost: £300–£500
Worth it if multiple radiators aren’t heating properly.

Most UK homes need a power flush every 5–10 years.

---

3. Thermostatic Radiator Valve (TRV) Is Stuck

This little valve on the side of your radiator — the one with numbers on it — is supposed to control heat flow.
Sometimes they just… give up on life and get stuck closed.

How to tell it’s stuck:

• TRV feels loose when you turn it
• Radiator stays cold regardless of the number setting
• Nearby radiators work fine

Easy fix:

You can unstick a TRV pin in 30 seconds:

1. Remove the TRV plastic cap
2. You’ll see a tiny metal pin
3. Push it down gently
4. It should pop back up
5. If it doesn’t, tap around it lightly

If the pin is stuck solid, the valve probably needs replacing (around £20–£40).

---

4. Radiator Balancing (Rarely Explained, But Very Real)

This is one of those things nobody ever tells you about.
When a heating system isn’t balanced, some radiators steal all the heat while others barely get anything.

Signs you need to balance:

• Radiators downstairs get hot first
• Radiators upstairs never fully heat
• The furthest room from the boiler is always cold
• Some radiators are roasting and others are freezing

How balancing works:

You’re basically controlling the flow of hot water so every radiator gets a fair share.

Here’s the simple version:

1. Turn ON all your radiators
2. Find the radiators that get hot the fastest
3. Slightly close the lockshield valve (the plain valve on the other side of the TRV)
4. Do this on the radiators closest to the boiler
5. This forces more hot water to the colder rooms

It sounds complicated but it takes 5–10 minutes and can transform your whole house.

---

5. Your Boiler Pressure Is Too Low

Low pressure = weak flow = radiators stay cold.

Signs of low pressure:

• Radiators lukewarm
• Heating cuts off randomly
• Boiler display shows an error
• Pressure gauge below 1 bar

Fix:

Top up your boiler:

1. Find the filling loop (silver hose under the boiler)
2. Open both valves slowly
3. Watch pressure rise
4. Stop at 1.2–1.5 bar
5. Restart boiler

If pressure keeps dropping, you might have a leak somewhere.

If the radiator is working but the room still doesn’t stay warm, you might need to look at airflow and insulation. I explained what actually works in my guide on how to keep a bedroom warm at night.

---

6. Airlock in the System

An airlock is like a big bubble stuck in the pipework.
It stops hot water from reaching a radiator entirely.

Fix #1: Turn heating on full and bleed all radiators in order

Start from the ground floor and move upward.

Fix #2: Increase pressure to push air through

Temporarily raise boiler pressure to 1.8 bar, run heating, then bleed the offending radiator.

Fix #3: Manual pipe bleed (rarely needed)

This is where you bleed the actual copper pipe, not the radiator — usually a job for a heating engineer.

---

7. The Boiler Flow Temperature Is Too Low

Modern boilers sometimes run in “eco mode”, which is nice for bills but terrible for cold rooms.

Fix:

Increase the flow temperature:

• For radiators: set to 65–75°C
• For hot water: 50–55°C

Cold radiators can sometimes be fixed by adjusting this one simple setting.

---

8. Single Radiator Not Working? Could Be the Valve Itself

If one radiator refuses to heat no matter what, the valve might be blocked.

Fix:

Turn both valves fully open.
If still cold, the valve may need replacing.
A heating engineer can do this quickly and cheaply.

---

9. Radiator Installed Incorrectly (Yes, It Happens)

Some radiators are piped the wrong way round — especially in old houses or DIY jobs.

How to tell:

• The TRV is on the return side instead of the flow
• Radiator pulses or makes a boiling kettle noise
• Heat doesn’t spread evenly

This usually needs an engineer to swap pipes.

---

10. System Pump Weak or Failing

If NONE of your radiators get hot properly unless everything else is turned off, your heating pump might be weak.

Signs:

• Loud humming from boiler cupboard
• Slow heating
• Radiators barely warming
• Heating cuts off early

Fix:

Pump replacement (£200–£350).

Not the most fun, but solves the entire problem instantly.

---

Final Thoughts

Radiators can stop working for loads of reasons, but most of them are simple:

• trapped air
• stuck TRV
• low pressure
• system imbalance
• sludge

Start with the easiest fixes and work your way through.
Most people solve the problem without needing a heating engineer.

You can use this guide every time a radiator starts acting up — it covers every common cause with real fixes that actually work in UK homes.