The good news? A radiator staying completely cold usually points to just a handful of issues. And most of them don’t need an engineer or any expensive tools. It’s more about figuring out where the heat flow is stopping.
1. First Check: Is the Radiator Getting Any Flow at All?
This sounds obvious, but it’s the step most people skip. If both radiator pipes are stone cold, it means no hot water is reaching it. Not “a little”, not “weak flow” — literally nothing is entering. That instantly narrows things down to:
- the valve is closed, stuck, or blocked
- the lockshield is too tight
- the system isn’t pressurised enough for flow to reach that circuit
- air trapped somewhere before the radiator
Put your hand on both pipes at the bottom. If both are cold while other radiators are hot, that radiator is isolated for one reason or another — and now you hunt down the cause.
2. Check the TRV (Thermostatic Valve) — The Most Common Culprit
If your radiator has a TRV (the numbered dial), the pin underneath often gets stuck in the shut position after months of not being used. I’ve had this happen at least three times — the head turns normally, but the actual pin underneath isn’t moving, so the radiator gets zero flow.
Pull the TRV head off. You’ll see a little metal pin. Tap it gently with something like the blunt end of a screwdriver. It should spring up and down. If it doesn’t move much, it’s stuck.
Free that pin, put the head back on, and 50% of the time the radiator comes to life within minutes.
3. Check the Lockshield Valve — It Might Be Almost Fully Closed
The lockshield valve (the one under the plastic cap) is what actually controls how much water flows through the radiator. If it’s barely open, the radiator won’t heat — even if everything else is perfect. And because people rarely touch these, they sometimes get knocked during decorating, moving furniture, kids bumping it, etc.
Turn it a quarter turn open. Just a little. Give the system five minutes. If the pipe suddenly starts warming, that was your issue.
I go into more detail about radiator balancing here:
How to Balance Radiators Properly.
4. Your Boiler Pressure Might Be Too Low for the Radiator to Heat
Low pressure can stop hot water reaching certain radiators — usually the ones furthest from the boiler or on higher floors. The boiler still runs, other radiators might heat, but one or two furthest away stay ice cold.
Check your gauge. If it’s under 1 bar, especially around 0.5–0.8, the system simply can’t push water everywhere. Top it up to around 1.2–1.5 and give it five minutes. It’s amazing how often this alone fixes the issue.
5. Air Might Be Trapped in the Pipes Before the Radiator
You can bleed a radiator perfectly, but if air is stuck in the pipe leading TO the radiator, it won’t heat at all. The radiator isn’t the problem — the pipe is.
A trick I’ve used many times: turn off every other radiator in the house except the cold one, then restart the heating. This forces all flow toward that single radiator. If air is lodged somewhere, the increased pressure often pushes it through.
6. Internal Sludge Can Stop Flow Completely
Sludge doesn’t always cause “cold bottom, warm top”. Sometimes it blocks the entry points so badly that the radiator stays fully cold. It’s more common in old systems, or in houses where inhibitor hasn’t been added for years.
If the radiator feels heavy, cold, and slightly damp on the bottom (a weird tell, but true), sludge might be the cause.
Here’s another giveaway: the radiator is cold, but the pipe leading to it gets hot. That means the blockage is inside the radiator itself.
7. The Pump Isn’t Sending Enough Flow to Reach That Radiator
Pumps don’t just fail instantly — they weaken over time. When that happens, radiators closest to the boiler heat normally, and the ones furthest away stay cold or sluggish.
If you’ve noticed slow heating across the system lately, or radiators upstairs behaving differently to downstairs, the circulation pump might be on the weak side.
Not every pump issue means replacement — sometimes it’s speed setting or system imbalance.
8. When Your Radiator Isn’t Heating, Connect the Clues
The easiest way to diagnose the real cause is to connect three clues:
- Are the pipes warm or cold?
- Is the valve stuck or open?
- Is there pressure and flow to other radiators?
If pipes are cold → it’s a flow issue.
If pipes are hot → blockage or internal valve issue.
If everything else heats → local problem at that radiator only.
Once you understand this, you’ll never panic about a cold radiator again.
9. Want a Deeper Dive? Start With the Basics
If you want a complete breakdown of how to warm your home efficiently — not just fix radiators — I’ve put everything into one full guide here:
How to Keep a UK Home Warm for Cheap (Complete Guide)
And if your radiator is heating strangely (hot top, cold bottom), here’s the article for that:
Radiator Hot at Top but Cold at Bottom
