BoilerBuddy · symptom guide
No heating from boiler — the four things that cause it.
A boiler that's lit and pumping but no heat reaches the radiators is the system telling you one of four things — the controls aren't asking for heat, the circulating water is short-circuited by a trapped air pocket, the diverter is sending hot water the wrong way, or the system pressure has dropped so far the pump can't push. None of those mean the boiler is broken. The cards below rank them most-likely-first, and the four safe at-home checks come after, with a clear line for when to stop and call out.
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Why this happens
Four causes, ranked by how often we see them on a combi.
If the radiators stay cold and the boiler is firing, the heat is going somewhere — but not into the system. The four common culprits, most-likely-first, are the thermostat and programmer (the controls side), a trapped air pocket that the pump is short-circuiting around (the system side), a diverter valve that has stuck the wrong way (the hot-water side), and a pressure drop below 1.0 bar that has starved the pump (the pressure side). Each has a different physical sign and a different fix; the cards describe each.
Thermostat or programmer isn't asking for heat
A room thermostat turned down to 10 °C, a programmer toggled to 'off' or to a slot that isn't due, a smart-app schedule that never fires the heating, or a TRV on every radiator turned down to zero — any one of these leaves the boiler lit but no call for heat on the system side. Walk to the room thermostat and turn it up to 25 °C, then check the programmer is on 'on' for heating. If a radiator now warms, the cure is a one-line correction to the schedule or a TRV lift; book no engineer.
Safe at home · check the thermostat and programmer first
Run this in the triage →Air locked in the system
An air pocket in the system acts like a blockage — the pump pushes water round but the air shunts it past the radiator, leaving that radiator (or that section of the system) cold across the whole surface. Trapped air usually shows up after a recent refill, a leak-fix, or a new radiator install. Bleed the cold radiator once with a radiator key; if the cold patch returns within days, treat it as a sign, not a fix — magnetite sludge or a recurring leak is the deeper cause. The bleed vs sludge walkthrough at /guides/cold-radiator-bottom walks the call clearly.
Safe at home · bleed once, escalate if it returns
Run this in the triage →Faulty diverter valve
On a combi the diverter switches between hot-water-for-taps and hot-water-for-radiators. If it sticks on the hot-water side, the boiler fires for a hot-tap demand and you get heat at the tap — but never for the radiators. If it sticks halfway, neither side works properly and the boiler may short-cycle on every hot-tap draw. Some diverter faults show up as a tapping or knocking noise from the secondary heat-exchanger on a hot-tap draw, which is the same scale family as kettling. Engineer territory; a new diverter valve or motorised actuator is the usual fix, and the valve is sealed-component work — don't try to free it yourself.
Engineer territory · diverter / actuator
Run this in the triage →Low system pressure
The pressure gauge reads below 1.0 bar when the system is cold — the pump can't push effectively and the radiators stay lukewarm. This is the same gauge-below-1-bar symptom as a separate pressure-loss page, but here it shows up only because the heating has stopped reaching the radiators. Open the filling loop to bring the needle into the green band (1.0–1.5 bar when cold); if the gauge drops again within a day, that is a leak, not a bleed, and the related pressure-loss page takes over from here.
Safe at home · re-pressurise, then escalate if it drops
Run this in the triage →Try this first
Before anything else, rule out the controls and the pressure.
Two of the four causes above are easy to clear from the front of the boiler without opening the casing. Turn the thermostat up to 25 °C, set the programmer to 'on' for heating, and check the gauge reads in the green band. If all three look right and the radiators still don't warm, the cause is on the diverter side or the system side and you are heading for the 'book a registered engineer' line.
8 steps. Yes / no at each. Stops the moment a step says it should.
Know when to stop
Stop if you notice
- A smell of gas, or a yellow, orange or sooty flame.
- A carbon monoxide alarm is sounding, or someone has possible CO symptoms.
- Smoke, scorching, visible damage, overheating or a leak around the boiler.
- The boiler keeps locking out or will not stay lit.
Where the safe-at-home line stops
If any of these apply, stop and call a Gas Safe engineer.
These are the lines that mean the fault is past the safe-at-home boundary. Any of them — leave the boiler in its current state, don't keep testing, and book a registered engineer today, not next week:
- Bleeding the cold radiator brings water back but it goes cold again within days — magnetite sludge on the heat-exchanger surface or a recurring leak on the system side; engineer territory, not a bleed.
- The diverter is making a tapping or knocking noise on every hot-tap draw, or the boiler short-cycles when you open a tap — sealed-component work on the diverter, the secondary heat-exchanger, or the actuator; registered-engineer territory only.
Next step
Run the full triage in about a minute.
Tell BoilerBuddy what your boiler is doing. We'll return a ranked list of likely causes, the safe checks to do first, and the exact line that means "call a Gas Safe registered engineer".
FAQ
The five questions this symptom raises.
Need a qualified engineer?
BoilerBuddy can pass what you've worked out to a local Gas Safe engineer — symptoms, checks, and the bits that point to the likely cause.
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Engineer handover
Send this sheet with you to the visit.
Print or email it to your engineer so they arrive already knowing what your boiler is doing and what you've already tried.