Why is my radiator cold at the bottom?

If your radiator is hot at the top but cold at the bottom, you've got a textbook case of one of three issues. The good news: you can diagnose which one it is in about five minutes. The other good news: two of the three are fairly cheap to fix.

The three causes (in order of likelihood)

1. Sludge build-up (most common — 80% of cases)

Over time, central heating systems accumulate iron oxide and other debris — known as “sludge” in the trade. It's magnetic, black, and looks a bit like wet coffee grounds when you drain a system.

Sludge settles at the bottom of radiators (because gravity). The water can't flow properly through the cold sludge layer, so the bottom of the radiator stays cold while the top heats up normally.

How to tell:

• The cold area at the bottom covers most or all of the bottom of the radiator
• The cold/hot line is fairly horizontal
• Multiple radiators are affected — not just one
• System is more than 8-10 years old, or has had work done recently

The fix: Power flush the system (around £395 for a typical 3-bed). This is a proper professional job — high-velocity water and chemical dispersant pushed through the system to dislodge and capture the sludge. We finish by adding corrosion inhibitor to prevent it coming back.

2. Air in the system (easy DIY fix)

Air rises, so trapped air would normally cause cold spots at the top of a radiator (the part near the bleed valve). However, air can get trapped in odd places, especially if your system has been recently drained and refilled.

How to tell:

• Cold area is at the TOP of the radiator (most common air-related problem)
• Sometimes you hear gurgling or knocking when the heating turns on
• Recent boiler work, or recently topped up pressure

The fix: Bleed the radiator. Use a radiator key (or a flat-head screwdriver on some models), open the bleed valve at the top of the radiator, and let air hiss out until water starts trickling. Catch it with a cloth or cup. Close the valve. Repeat with all radiators in the house, starting with the lowest floor.

After bleeding, check the boiler pressure gauge. If it's dropped below 1.0 bar, you'll need to top it up via the filling loop (instructions are usually under the boiler).

3. Stuck thermostatic radiator valve (TRV)

Each radiator has two valves — one at each end. One is a thermostatic valve (TRV) with a numbered dial; the other is a lockshield valve (with a plain cap). Both can stick over time, particularly if they've been left in the same position for years.

How to tell:

• Only ONE radiator is affected
• Cold area is at the TOP, not the bottom (TRV is usually on the side, near the top)
• Adjusting the TRV dial doesn't change anything

The fix: Remove the white plastic head of the TRV (just unscrew it). Underneath is a small metal pin — push it down with your finger. It should spring back. If it's stuck, gently work it up and down with a pair of pliers (don't force it). 9 times out of 10, that frees it.

If the pin is properly seized, you need a new TRV head. About £25 for a quality replacement and 10 minutes to fit. We can do it for £85 (call-out + parts) but it's genuinely a DIY job if you're comfortable with basic plumbing.

The five-minute diagnostic

Before doing anything, work out which problem you've got:

1. Walk around the house with the heating on for 30 minutes. Touch every radiator.
2. Note which radiators are affected and where the cold spot is (top, bottom, one side).
3. Match against the three patterns above.

If multiple radiators have cold bottoms — sludge. Power flush.

If radiators have cold tops — air. Bleed them.

If only one radiator is affected and it's cold at the top — stuck TRV. Free the pin.

What if it's none of these?

A small minority of cold-radiator problems aren't any of the above:

• Failed pump — system is gurgling, takes ages to heat up. Not a DIY fix.
• Closed valve — someone (a previous owner, a plumber) closed a lockshield valve and forgot. Worth checking both valves on the cold radiator are fully open.
• System imbalance — radiators furthest from the boiler get less flow than radiators closer. Fixed by “balancing” the system using the lockshield valves. Tricky to do well — usually worth getting a plumber in.

When to call an engineer

• You've bled the radiators but the cold spots persist (sludge or imbalance)
• The boiler keeps cycling on and off (could be sludge restricting flow back to the boiler)
• You've got cold radiators AND noisy/banging pipework (kettling — needs urgent attention)
• The system has been working fine for years and suddenly multiple radiators are cold (something has shifted — pump, blockage, valve)

If you're not sure, a £95 radiator service from us covers diagnosing the issue, bleeding all radiators, balancing the system, and giving you an honest answer on whether you need a power flush or just routine maintenance. We don't upsell power flushes that aren't needed — they're a big job and we'd rather have you happy with a £95 fix than annoyed at a £395 one.