What this fault finder does
A radiator that will not heat properly is nearly always one of a small number of faults, and the pattern of where it is cold tells you which. Cold at the top means something quite different from cold at the bottom, and a single cold radiator points somewhere different from a whole house of them.
This fault finder asks which part is cold, how many radiators are affected, what the boiler pressure is doing and what noises the system is making, then ranks the likely causes. Just as usefully, it tells you which of them are reasonable household jobs and which are not.
That division matters. Bleeding a radiator, freeing a stuck valve pin, adjusting a lockshield and topping up a sealed system through the filling loop are all things a careful adult can do with a bleed key and an afternoon. Anything involving the gas supply, the boiler's internals or the flue is not, and in Great Britain it is a criminal offence for anyone not on the Gas Safe Register to carry out gas work.
Describe the symptoms
What the cold pattern tells you
Where the cold is on the radiator is the single most useful observation, because heat distribution across the panel follows directly from what is obstructing the flow.
| Symptom | Usual cause | Why |
|---|---|---|
| Cold top, warm bottom | Trapped air | Air is lighter than water and collects at the highest point |
| Cold bottom, warm top | Sludge | Corrosion debris is heavier than water and settles at the lowest point |
| Cold middle, warm both ends | Sludge across the lower waterways | Flow short-circuits along the top rail |
| Completely cold, others fine | Closed or seized valve | No flow is reaching the radiator at all |
| Lukewarm, never hot | Balancing | It is getting some flow, but less than its share |
| All radiators cold | Pressure, pump, valve or boiler | The fault is upstream of any individual radiator |
Air rises and sludge sinks, and a heating system is no exception. Air collects in the highest radiators, so upstairs rooms go cold at the top first. Sludge settles in the lowest and slowest parts of the circuit, so downstairs radiators are usually the first to run cold along the bottom. A whole floor behaving differently from the other is a stronger signal than any single radiator.
How this fault finder works
Each answer carries weights towards each of the eight faults, and the fault with the highest total is shown first.
Score for a fault = sum of the weights it receives from your seven answers
Match strength = score ÷ the highest score that fault could reach, as a percentage
Faults are ranked by score, and any scoring nothing are hiddenThe weights reflect how specific a symptom is. Cold at the top scores six for trapped air because almost nothing else produces that pattern. Being on an external wall, or a system that started playing up suddenly, scores two or three because it fits several faults at once.
Each result also carries a competence marker: whether it is a household job, one for a registered engineer, or one that starts as a household job and becomes an engineer's if it recurs. That marker is fixed to the fault rather than calculated, because the boundary is a legal and safety one, not a matter of degree.
Worked example: one cold radiator upstairs
A bedroom radiator is cold across the top third and warm underneath. Everything else in the house is fine, the boiler gauge reads about 1.2 bar cold, both valves are open, and there is a faint trickling noise when the heating first comes on. It started suddenly a few days ago.
Entering those observations — cold at the top, just this one radiator, upstairs, pressure normal, both valves open, gurgling, started suddenly — gives:
| Fault | Match | Who should do it |
|---|---|---|
| Air trapped in the radiator | 91% | Household job |
| A valve is closed or partly closed | 38% | Household job |
| Low system pressure | 35% | Household job, engineer if it recurs |
| A pump, diverter valve or boiler fault | 29% | Gas Safe registered engineer |
| A seized thermostatic valve pin | 21% | Household job |
Trapped air, comfortably ahead of everything else, and marked as a household job. The combination that produces that ranking is the cold top plus the gurgle plus the upstairs location: any one alone would be suggestive, all three together are close to conclusive.
The detail worth carrying away is in the last step rather than the diagnosis. Bleeding will fix the radiator in about a minute, and it will also drop the sealed system's pressure. Bleed several radiators in one session without checking the gauge afterwards and the pressure can fall below the point at which the boiler will fire — turning a five-minute job into an evening without heating for no reason other than not looking at the gauge.
Where the line is
Domestic heating spans jobs that need a bleed key and jobs that are criminal offences to attempt unqualified. The boundary is worth being precise about.
Reasonable to do yourself: bleeding radiators, removing and refitting a thermostatic valve head, freeing a stuck valve pin, adjusting lockshield valves to balance the system, topping a sealed system up through the filling loop, resetting the boiler using the controls on the front panel, and changing thermostat batteries.
For a Gas Safe registered engineer: anything inside the boiler casing, anything on the gas supply or the flue, replacing or recharging an expansion vessel, replacing a pump, diverter valve or pressure relief valve, power flushing, and any work on the gas appliance itself. Under the Gas Safety (Installation and Use) Regulations 1998 it is an offence for anyone not competent and registered to carry out work on gas fittings, and that applies to homeowners working on their own property, not only to tradespeople.
Also worth flagging: unvented hot water cylinders are covered separately under Approved Document G and require specific competence. Draining a system and refitting radiators is legal to do yourself but produces the two commonest DIY floods in British homes — an unsecured valve and a forgotten open drain-off.
- You can smell gas. Leave, do not touch switches, and ring the National Gas Emergency Service on 0800 111 999.
- The boiler is leaking water, or water is discharging from a pipe on an outside wall.
- There is black staining or sooting around the boiler, or a carbon monoxide alarm sounds.
- The pressure gauge reads zero, or sits in the red with the system cold.
- Radiators are hot when everything is switched off, which suggests a control fault.
Things that affect the result
- Two faults at once. An old, uninhibited system commonly has both sludge and recurring air, because the corrosion that produces one produces the other.
- How long the heating has been running. A radiator judged five minutes after switch-on will read cold whatever is wrong with it. Give the system twenty to thirty minutes.
- Thermostatic valves doing their job. A radiator in a room that has reached temperature is supposed to go cold. Turn the head fully up before deciding anything is wrong.
- Recent work. Radiators taken off for decorating are a frequent source of closed lockshields and trapped air.
- Radiator size. A radiator that heats fully but never warms the room may simply be too small — that is a sizing question rather than a fault.
- Open vented versus sealed systems. A system with a tank in the loft has no pressure gauge, and the equivalent faults show up as a noisy or empty feed and expansion tank instead.
Common mistakes
- Bleeding a hot system. The water inside is well above boiling point at pressure. Turn the heating off and let it cool.
- Forgetting to check the pressure afterwards. Every bleed drops the pressure of a sealed system, and enough of them will stop the boiler firing.
- Bleeding the same radiator repeatedly. Recurring air is a symptom of corrosion or a leak, not a maintenance routine.
- Leaving the filling loop connected and open. It over-pressurises the system and is a cross-connection with the mains.
- Closing lockshields hard to force flow elsewhere. Balancing works in eighths of a turn with time between each.
- Assuming a cold radiator means a broken boiler. A single cold radiator almost never does.
- Adding chemicals to an old system without advice. Freeing decades of deposits can reveal a leak that the sludge was sealing.
Frequently asked questions
How often should radiators be bled?
There is no schedule. Bleed them when a radiator is cold at the top, and check them at the start of the heating season. What matters more than frequency is the trend: if the same radiators need bleeding every few weeks, air is being produced or drawn in, and that points at internal corrosion or a small leak somewhere on the system rather than at anything you can fix with a bleed key.
What pressure should my boiler be at?
Most sealed domestic systems are designed for roughly 1 to 1.5 bar with the system cold, and it is normal for the reading to rise by half a bar or so when the heating is up to temperature. Check the figure on the boiler itself or in its manual, because it varies by model and by how tall the house is. Consistently below 1 bar and the boiler may refuse to fire; above about 3 bar hot and the pressure relief valve will discharge.
Is a power flush worth the money?
It depends on what is wrong. On a system with genuine sludge — several radiators cold at the bottom, kettling from the boiler, black water when you bleed — a proper flush followed by inhibitor and a magnetic filter can restore performance and extend the life of the boiler. On a system that simply needs balancing, or that has one seized valve, it is an expensive way to fix nothing. Ask what evidence the quote is based on before agreeing.
Why is my radiator cold when the boiler is working fine?
Because most radiator faults are local to the radiator. A closed lockshield, a seized thermostatic valve pin, trapped air or sludge all stop that one radiator heating while the boiler runs perfectly. Whole-system symptoms — every radiator cold, or the heating and hot water behaving oddly together — are the ones that point back at the boiler, pump or diverter valve.
What is a lockshield valve?
The plain valve at the opposite end of the radiator from the control knob, usually hidden under a push-on plastic cap. It sets how much flow that radiator takes and is meant to be adjusted once when the system is balanced and then left alone. Turning it needs a small spanner or the plastic key that came with it.
My radiators are hot but the house is still cold. Is that a fault?
Not necessarily a heating fault. If radiators reach full temperature across the whole panel, they are working as intended and the issue is that the heat output is not matching what the room loses — undersized radiators, poor insulation, draughts, or a room that has been extended since the system was designed. That is a sizing and fabric question rather than a repair.
Can I fit a radiator myself?
Replacing a radiator with one of the same size on existing valves is plumbing rather than gas work and is legal to do yourself. It involves isolating the valves, draining the radiator, and dealing with water that is black, stains permanently and will ruin a carpet. Moving pipework, altering the system or anything on the boiler is a different matter. Nothing about a radiator swap requires the gas supply to be touched — if a job does, it needs a Gas Safe registered engineer.
Is what I enter stored?
No. Your answers are processed entirely in your browser and are never transmitted or retained.
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References
These organisations publish the guidance behind this page.
- Gas Safe Register — the official register of gas engineers, and the legal position on who may carry out gas work
- Health and Safety Executive — the Gas Safety (Installation and Use) Regulations 1998 and guidance on domestic gas safety
- Approved Documents, GOV.UK — Approved Document G covers hot water systems, including unvented cylinders
- Energy Saving Trust — guidance on heating controls, balancing and running a wet central heating system efficiently
- Chartered Institute of Plumbing and Heating Engineering — the professional body for plumbing and heating, and a route to finding a qualified installer
External guidance changes. Check the current position at the source before relying on it for a decision that matters.