Short answer: Most heating faults announce themselves through one radiator, one room, or one pressure gauge — and the pattern tells you the cause before you touch a tool. Cold at the bottom of a radiator is sludge, cold at the top is air, a whole cold radiator is a flow problem, and falling boiler pressure follows its own short checklist. Find your pattern in the table below, then follow the guide that matches it.
Who this page is for: Homeowners with radiators and a boiler — the guides are UK-led, with US notes for hydronic (hot-water radiator and baseboard) systems where the hardware differs · DIY level: Diagnosis is easy and free; each guide then labels its fix honestly — free check, moderate DIY, or pro-only · Cost: Every diagnosis on this page costs £0 / $0
Safety box — read this before any heating guide on this site: Nothing here involves opening a boiler casing or touching gas parts — never do either; gas work is for a Gas Safe registered engineer (UK) or a licensed technician (US) only. Radiator water is hot enough to scald, so let the system cool before removing anything, and work with the system warm rather than at full temperature when bleeding or balancing. If you smell gas at any point, stop everything: no switches, no flames — get out and call the UK Gas Emergency line (0800 111 999) or, in the US, leave and call your gas utility or 911.
How diagnosis works on this page
Most heating advice online starts with a fix (“bleed your radiators!”, “book a powerflush!”) and hopes it matches your fault. That is backwards, and it is how homeowners end up paying for a system clean when the real fault was a stuck valve pin that frees up in five minutes. Every guide on FaultToFix works the other way round:
- Name the pattern. Not “the heating is broken” — the exact pattern: which radiator, where on it is cold, when it happens, and what the pressure gauge is doing.
- Match it in the table below. Each pattern points at a most likely cause, and just as importantly, rules the others out. A cold bottom and a cold top are opposite faults with opposite fixes.
- Follow the matching guide in order. The guides run cheapest-check-first, so you confirm or eliminate the free causes before anyone talks about parts, flushing, or call-outs.
Symptom → likely cause: the heating overview table
| What you notice | Most likely cause | Where to start |
|---|---|---|
| Radiator hot at the top, cold at the bottom | Sludge (black magnetite) settled in the base — hot water flows over it | Radiator cold at the bottom guide |
| Radiator cold at the top, hot at the bottom | Trapped air — air rises and collects at the top | Bleed that radiator; the method is covered in the radiator noises guide |
| Cold slab in the middle of the radiator | Sludge again, banked up in the middle channels | Radiator cold at the bottom guide |
| One radiator completely cold, pipes cold too | No flow — a closed valve or a stuck TRV pin | Stuck TRV pin guide |
| One whole room always colder than the rest | Balancing, a stuck pin, sludge, or an undersized radiator — in that order | One room always cold guide |
| This radiator lukewarm while others are roasting | Balancing — nearer radiators are taking the flow | One room always cold guide |
| Radiator gurgling or bubbling | Trapped air — or corrosion gas if it keeps coming back after bleeding | Radiator noises guide |
| Several radiators cold at the bottom | Sludge across the whole system, not just one radiator | Radiator cold at the bottom guide — system clean section |
| Boiler pressure drops over days, no leak visible | Micro-leak, failing expansion vessel, or the PRV passing — in that order | Boiler pressure guide |
| Pressure dropped right after bleeding radiators | Normal — bleeding lets pressure out along with the air | Top up via the filling loop; full method in the boiler pressure guide |
If your pattern is not in the table, it is probably a combination — a cold room and a noisy radiator, for example. Start with the radiator pattern, because radiator faults are the ones you can diagnose and mostly fix yourself, and work towards the boiler, not the other way round.
Start with your symptom — the heating guides
Radiator faults
Radiator Cold at the Bottom but Hot at the Top? It’s Sludge — Here’s the Fix
Start here if: one radiator (or several) is hot at the top and cold at the bottom while the heating is running. The two-minute test costs nothing — bleed a little water into a tray and look at the colour. Black or gritty water confirms sludge; clear water sends you elsewhere. The guide covers the full cold-pattern table (bottom vs top vs middle vs whole radiator), flushing a single radiator with a hosepipe step by step, when several affected radiators mean a professional system clean instead (a powerflush typically runs £350–£600), and the inhibitor and magnetic filter that stop the sludge coming back.
One Radiator Not Working? Check the Stuck TRV Pin First (5-Minute Fix)
Start here if: a single radiator stays completely cold — pipes too — while the rest of the house heats normally. The usual culprit is the thermostatic radiator valve (TRV) pin, which seizes down over the summer shutdown and never lets hot water in, however high you turn the head. The guide shows the pin check with the TRV head removed, how to free a stuck pin with penetrating oil and gentle pressure, and what it means if the pin will not move or the valve weeps — the point where replacing the TRV is the honest answer.
One Room Always Cold? It’s Probably the Balancing — Work Through These 4 Causes in Order
Start here if: the house heats, but one room never catches up. This is the diagnosis page none of the task-style guides bother to write: it works through the four real causes in order — valves and settings first, then the stuck pin check, then sludge inside that radiator, then balancing (nearer radiators stealing the flow), with an undersized radiator as the last resort explanation rather than the first guess. Most of the checks are free and take about 20 minutes in total; the balancing fix itself is a lockshield-adjusting job, explained radiator by radiator.
Radiator Gurgling or Bubbling? Air, Sludge or Pump — How to Tell Which
Start here if: the radiators talk to you — gurgling, bubbling, trickling, or ticking as the system heats up. The guide is a sound-based triage: what each noise usually means, how to bleed correctly (radiator order matters, and so does topping the pressure back up afterwards), and the important follow-up question — if bleeding fixes the noise for a week and then it returns, that points at corrosion gas building up inside the system, which is a different problem with a different fix. It also marks clearly where the pump — and therefore a heating engineer — takes over.
Boiler-side faults
Boiler Pressure Keeps Dropping but No Leak Anywhere? Work Through This Checklist
Start here if: you keep topping the pressure up and it keeps falling, but there is no puddle, no drip, and no obvious leak anywhere. This is a diagnosis-only guide on purpose — it sells you nothing and opens nothing. It gives you the pressure log method (record the gauge cold and hot over three heating cycles; the pattern of the drop is itself the biggest clue), then the three safe homeowner checks in order: radiator valves and joints with the kitchen-roll test, the external pressure-relief discharge pipe, and how the gauge behaves while the heating runs. It ends with a hard stop list — the readings and signs that mean the casing stays on and a Gas Safe engineer (UK) or licensed technician (US) takes it from there.
Four free checks before you call anyone
Whatever your pattern, these four checks take under 15 minutes, cost nothing, and between them settle a large share of heating faults — and if you do end up calling an engineer, your answers make their job faster:
- Both valves open? Every radiator has two valves: the TRV (numbered head) at one end and the lockshield (plain cap) at the other. TRV set to 3 or higher, lockshield at least partly open. A radiator with a closed valve is not a heating fault — but it is diagnosed as one every autumn.
- Bleed one radiator and look at the water. Use a bleed key and a tray. Air hissing out explains a cold top. Black or gritty water confirms sludge. Clear water and no air rules both out and points you at flow or balancing instead.
- Read the pressure gauge cold. With the system off and cold, most sealed systems sit around 1 to 1.5 bar — check your boiler manual for its figure. Write the reading down; a single reading means little, but a falling trend across days is the key evidence in the pressure guide.
- Note the pattern, exactly. Which radiators, where on each one, and whether the pipes at the base are hot or cold. “Upstairs back radiator, cold bottom half, pipes hot” gets you to the right guide — and the right fix — immediately.
When heating is NOT a DIY job
Diagnosis is always safe; some fixes are not. Stop and call a professional if:
- Anything involves the boiler casing or gas parts. The casing stays on. Gas Safe (UK) / licensed technician (US) — no exceptions, and no guide on this site will ever tell you otherwise.
- You smell gas. Leave the house, then call the emergency line (UK: 0800 111 999; US: your gas utility or 911). Do not go back in to finish a job.
- The pressure gauge rises sharply when the heating runs — climbing well past its normal hot reading, or the pressure relief valve dripping constantly outside. That is expansion-vessel or PRV territory and it is pro diagnosis.
- Water is leaking near electrics, or a ceiling below a pipe run is bulging or staining fast. Turn the water off at the stopcock and call an engineer or plumber.
- You have no heating at all in cold weather with elderly people, babies, or anyone medically vulnerable in the house. Treat it as urgent — call an engineer rather than working through a checklist in a cold house.
US / UK term box
Reading from the other side of the Atlantic? The faults are the same; some of the words are not.
- Thermostatic radiator valve (TRV) = the numbered dial on a UK radiator. US hydronic systems use thermostatic radiator valves too, but many US homes have simple manual valves or zone controls instead.
- Lockshield = the plain capped valve opposite the TRV; US equivalent is the balancing or return valve on the radiator.
- Bleed key = radiator air-vent key (US: vent key or coin-slot vent, turned by hand or a flat screwdriver).
- Powerflush = a professional machine flush of the whole system (US: power flushing / system flush by a hydronic contractor).
- Boiler (UK) = in US homes the closest equivalent is a boiler on a hydronic system; most US houses heat with a forced-air furnace instead — if you have a furnace and ductwork, the radiator guides here do not apply, but the symptom-first method does.
- Inhibitor = corrosion inhibitor — same idea both sides: a chemical dosed into the system water to slow rust and sludge.
FAQs
Why is one radiator cold when all the others are hot?
Because the fault is almost always at that radiator, not at the boiler. Work in this order: is the TRV turned up and the lockshield open; is the TRV pin stuck down (the classic after-summer fault); where exactly is it cold — bottom means sludge, top means air; and is every other radiator simply taking the flow first, which is a balancing fix. The one room always cold guide works through all four in order.
Should I bleed my radiators every year?
Bleed them when the symptom appears — a radiator cold at the top — rather than by the calendar, and always check the colour of the water that comes out. Black water is early warning of sludge long before a radiator goes cold at the bottom. Remember that bleeding drops the system pressure, so check the gauge afterwards and top up if needed, as described in the boiler pressure guide.
My boiler pressure keeps dropping — is that dangerous?
A slowly falling pressure is usually a small water-side fault — a micro-leak, a passing pressure-relief valve, or a failing expansion vessel — rather than an emergency, but it should not be ignored or just topped up forever. What is never safe is opening the casing to look. Use the pressure log and the three outside-the-casing checks in the boiler pressure guide, and hand the pattern you recorded to a Gas Safe engineer (UK) or licensed technician (US).
Can I clear sludge myself, or do I need a powerflush?
One radiator with a cold bottom is a reasonable DIY job: remove it, hose-flush it outside, refit, and dose the system with inhibitor afterwards — about an hour, as in the radiator cold at the bottom guide. When several radiators show the same pattern, the sludge is system-wide and the honest answer is a professional chemical clean or powerflush rather than flushing radiators one by one.
Where do I start if the heating “works” but the house still feels cold?
Start with the room, not the boiler. Feel the radiator in the cold room and run the pattern checks: valves open, pin moving, cold at the top (bleed it) or the bottom (sludge), and how it compares with the radiators nearest the boiler. A single cold room with a working system is a flow or balancing story far more often than a boiler story — the one room always cold guide is written for exactly this case.
FaultToFix heating guides are written symptom-first: diagnosis before fix, cheapest check first, and a clear line where DIY stops. Sources for safety boundaries: Gas Safe Register (UK) guidance on gas work and boiler servicing.
Last updated: 26 October 2026