Why does the boiler stop when a bath is being filled?
A combi boiler should normally continue heating water while a bath tap creates enough flow. If it stops before the bath is full, first check whether the water keeps flowing but turns cold, the flow itself falls away, or the boiler shows a fault and locks out.
Those are different symptoms. A hot-water demand that disappears can involve the tap flow or demand sensing. A lockout points to a recorded boiler fault. A stored-water system may simply have used the hot water already available in its cylinder.
First identify the hot-water system
A combi heats mains water as a tap runs and usually has no separate hot-water cylinder. A system or heat-only boiler normally heats stored water in a cylinder. With storage, the boiler may not fire continuously while the tap is open; it reheats the cylinder according to its controls.
Check the handover information or model instructions rather than assuming from the boiler’s shape. If a cylinder has been depleted, recovery time and control settings matter more than tap flow through the boiler.
What “stops” can mean
- The burner symbol disappears but water still flows: the combi may no longer be detecting enough hot-water demand.
- The boiler shows a code or reset message: it has recorded a fault or safety shutdown.
- The water becomes cooler but the boiler still shows demand: the requested flow may exceed the temperature rise the combi can provide.
- The whole display loses power: check the ordinary power supply and arrange qualified help if it repeats.
- A cylinder system runs cool: stored hot water may be depleted or the cylinder may not have reheated.
Bath taps create a high flow
A bath filler is designed to deliver water quickly. Every combi has a model-specific maximum hot-water output. At a higher flow, each litre receives less heat, so the water may be cooler even though the appliance is working normally.
That limitation should cause a temperature change, not unexplained lockout. Try a moderate stable flow and note whether the boiler continues. Do not restrict the tap to a dribble: below the model’s minimum demand flow, the boiler may stop heating altogether.
Check for competing water demand
Another tap, shower, toilet or appliance can reduce the dynamic flow reaching a combi. Worcester Bosch explains that mains-fed hot-water performance depends on pressure and flow while water is actually being used, and multiple outlets share the available supply.
Repeat the observation once, when other outlets and appliances are off. If the boiler then runs normally, the incoming supply or simultaneous demand is part of the pattern. It does not necessarily mean the boiler is faulty.
Watch the display, not the inside
Record the exact tap or flame symbol, temperature and fault code shown before, during and after the stop. Do not reset repeatedly: a code that returns needs diagnosis, and clearing the display can remove useful evidence.
Keep the casing closed. Internal flow sensors, heat exchangers, valves and safety controls are engineer-only components. A visible symptom cannot safely identify one of them as the cause.
Compare another hot outlet
Run a different hot tap separately. If the boiler stops only with the bath tap, the filler, mixer or local pipework may be affecting flow. If it stops at every hot outlet after a similar time, a shared boiler or supply issue is more likely.
If the bath stream alternates between strong and weak, see why hot water may pulse at a tap. That article is already in the writing workflow and helps distinguish one-outlet behaviour from a whole-home pattern.
Could scale or a restriction be involved?
Scale or debris can restrict a tap, pipe or boiler waterway, especially in hard-water areas. Symptoms can include reduced flow, unstable temperature or noise. Those clues are not proof, and chemical descaling or internal cleaning is not a homeowner task.
A plumber can assess the outlet and mains flow. A Gas Safe registered engineer can test the boiler’s hot-water performance against its model specification.
Safe checks to make
- Identify whether the home has a combi or hot-water cylinder.
- Note the bath flow, temperature and time until the stop.
- Photograph the exact display message.
- Repeat once with other water-using appliances off.
- Compare one other hot tap separately.
- Check for visible leaks without removing covers.
Stop the test if water becomes unpredictably hot, a leak appears or the boiler locks out. Do not keep filling and emptying a bath to reproduce the fault.
When to arrange a repair
Book professional diagnosis if the boiler repeatedly loses hot-water demand at a normal bath flow, locks out, displays a recurring code, leaks, makes a new severe noise or fails at several outlets. Give the engineer the system type, tap used, time to failure and exact display sequence.
If there is a gas smell, carbon-monoxide alarm, burning smell, smoke or water near electrics, stop treating it as an ordinary hot-water fault and follow emergency safety advice.
The short answer
A bath tap demands a large, sustained hot-water flow. A combi may deliver cooler water at a very high rate, but it should not repeatedly lock out. Establish whether demand disappears, a fault code appears or a cylinder has simply run out, then compare another outlet and arrange the correct professional check.
Boiler stopping before the bath is full?
Record the flow, timing and display sequence before arranging a diagnosis.