Combi boiler heating works but there is no hot water
Vaillant publishes three diverter valve symptoms and only one is the obvious one. What the split between heating and hot water actually narrows down.
What this means
A combi boiler makes heating and hot water with the same burner, so a fault that affects one and not the other sits after the heat is made rather than before it. Vaillant attributes that pattern to the diverter valve, the component that decides whether hot water goes to the radiators or to the taps, and states that a homeowner may not work on it.
Equipment and model context
Cross-brand symptom guidance
- United Kingdom combination boilers that heat domestic hot water on demand
This applies to combination boilers, which heat domestic hot water on demand. Vaillant notes that system and conventional boilers with a storage cylinder do not have a diverter valve, so on those systems a loss of hot water with working heating is a cylinder, immersion, or zone valve question instead.
What you may notice
- Radiators heat normally and the hot taps run cold or lukewarm.
- Hot water reaches the taps only while the central heating is also running.
- Water at the taps is warm rather than hot, at any flow rate.
Likely causes
- Vaillant describes the diverter valve as opening or closing to direct hot water either to radiators or to taps and showers.
- Vaillant gives three symptoms of a stuck valve: hot taps with radiators that do not come on, lukewarm water because some hot water is passing to the radiators, and hot water at the taps only when the heating is running.
- Baxi's own checklist puts the timer, thermostat, low-pressure cut-off, boiler pressure, and a frozen condensate pipe alongside the diverter valve as things to establish first.
Diagnostic paths
Radiators are hot and the taps are cold
The heating works normally, and no hot water arrives at any tap however long it runs.
This is the pattern Vaillant attributes to the diverter valve failing to move toward the hot water position. Confirm first that the boiler is not showing a fault code and that the hot water setting on the control has not been turned off, then arrange a Gas Safe engineer.
The water is lukewarm rather than cold
Hot taps deliver water that is warm but never reaches temperature, at any flow rate.
Vaillant lists this as a separate diverter valve symptom, caused by the valve sitting part way so some heat continues to reach the radiators. It is a different observation from cold water and worth reporting as such.
Hot water only arrives when the heating is on
The taps run hot during the heating season and stop working once the heating is switched off for the summer.
Vaillant names this pattern explicitly and notes it is most noticeable in warmer months. It suggests the valve is stuck toward the heating position, so hot water reaches the taps only as a by-product.
Neither heating nor hot water is working
The radiators are cold as well, or the boiler is showing a fault code or a pressure warning.
The diverter valve is not implicated, because it only decides where heat goes rather than whether heat is made. Work through Baxi's list instead: the display, any fault code, the timer and thermostat, and the pressure gauge.
| Heating | Hot water | Where the fault sits |
|---|---|---|
| Normal | None at all | After the heat is made, at the diversion stage |
| Normal | Lukewarm at any flow | Valve part way, per Vaillant's second symptom |
| Running | Hot only while heating runs | Valve toward heating, per Vaillant's third symptom |
| None | None | Before the heat is made: gas, pressure, ignition, or controls |
| None | Normal | Heating side controls, timer, or a zone valve |
| Normal | Hot but low flow | Water supply or an outlet, not the boiler |
One burner, two destinations
A combi boiler has a single heat source. When a hot tap opens, a flow sensor tells the control to fire the burner and to send that heat to a plate heat exchanger that warms the incoming mains water. When the heating calls, the same burner sends its heat to the radiator circuit instead.
The component that switches between those two destinations is the diverter valve, and Vaillant describes it as opening or closing to direct hot water either to radiators or to taps and showers. Nothing else in the appliance makes that choice.
That single fact is what makes the symptom so informative. If the burner fires and the radiators get hot, then gas, ignition, the flame signal, the pump, and the pressure are all working. The failure is downstream of all of them.
The two symptoms nobody recognises
The obvious diverter valve failure is hot radiators and cold taps. Vaillant publishes two others that are far less recognisable and are routinely blamed on something else.
The first is lukewarm water. Vaillant attributes it to the valve being stuck part way, so a share of the hot water continues toward the radiators while the rest reaches the tap. Owners tend to read that as a boiler that is undersized or a temperature setting that needs turning up, and neither adjustment fixes it.
The second is hot water that arrives only when the heating is running, which Vaillant says is most noticeable in warmer months. It produces a household that had no complaint all winter and loses hot water in April, and the seasonal timing sends people looking for a cause that has nothing to do with the appliance.
What to establish before calling
Baxi's published sequence starts before the diverter valve. Is the boiler running at all, and is a fault code displayed. Have the controls been changed, including the timer being switched off and the thermostat dial being moved. Does the pressure gauge read around one bar, since a low-pressure cut-off will stop the appliance.
Baxi also asks about a frozen condensate pipe and about a separate water tank where one exists. Those checks cost nothing and they separate a boiler that is not making heat from a boiler that is making heat and sending it to the wrong place.
The distinction matters commercially as well as technically. An engineer told that heating works and hot water does not arrives expecting to look at diversion; an engineer told that nothing works arrives expecting to look at ignition.
Why this is not an owner repair
Vaillant's position is blunt: a homeowner may not carry out this maintenance. Reaching the diverter valve means opening the casing of a gas appliance, and in the United Kingdom work on the appliance is restricted to Gas Safe registered engineers.
What the engineer does is worth knowing so the visit makes sense. Vaillant describes cleaning a stuck valve with a commercial chemical cleaner and refitting it, or replacing it outright where cleaning will not hold.
The other reason to leave it alone is diagnostic. A valve that has stuck is often reporting on the water in the system rather than on itself, and a system carrying debris will stick the replacement too. That question, and what to do about it, belongs to the same visit.
Heat is made once and then sent to one of two places. Which stage still works tells you which side of the diversion point the fault is on.
- Gas and ignitionIf radiators heat, this stage is working
- Burner and primary exchangerShared by heating and hot water alike
- Diverter valveChooses radiators or taps, and the only component that can affect one alone
- Plate heat exchangerTransfers heat to mains water for the taps
What to give the technician
Tell the engineer whether the taps run cold or lukewarm, whether hot water behaves differently while the heating is running, whether the radiators heat normally, what the pressure gauge reads, and whether any fault code has appeared. Add the boiler make and model from the data badge and roughly when the behaviour started.
Safe homeowner checks
- Check the boiler display for a fault code and note it exactly before doing anything else.
- Confirm the hot water setting on the programmer or timer has not been switched off.
- Read the pressure gauge and confirm it sits near one bar, as Baxi's checklist asks.
- Try the taps with the heating switched on and again with it off, and note whether the result differs.
- Check whether the water is cold or lukewarm, because Vaillant treats those as different symptoms.
When to call a technician
- Any suspected diverter valve fault, which Vaillant states a homeowner may not work on.
- A boiler that will not hold pressure or that shows a fault code after resetting.
- Hot water that fails on a system or conventional boiler with a cylinder, where the cause is different equipment.
- Water that is scalding rather than merely hot, which is a temperature control question.
- A smell of gas at any point, where the correct response is to leave and telephone the national emergency line rather than arrange a repair.
Questions people ask about this
Why do I have heating but no hot water?
In a combi boiler both come from the same burner, so a fault affecting one alone sits after the heat is made. Vaillant attributes that pattern to the diverter valve, the component that directs hot water either to the radiators or to the taps.
What is a diverter valve?
Vaillant describes it as the component that opens or closes to direct hot water either to radiators or to taps and showers. It exists only in combi boilers that heat water on demand, not in system or conventional boilers with a storage cylinder.
Why is my hot water only lukewarm?
Vaillant lists lukewarm water as a diverter valve symptom in its own right, caused by the valve sitting part way so some of the hot water continues toward the radiators instead of reaching the tap.
Why did my hot water stop working in summer?
Vaillant describes a stuck valve producing hot water at the taps only while the central heating is running, and notes that this is most noticeable in warmer months when the heating is off.
Can I fix a diverter valve myself?
No. Vaillant states directly that a homeowner may not carry out this maintenance. Reaching the valve means opening the casing of a gas appliance, which is restricted to a Gas Safe registered engineer.
What should I check before calling an engineer?
Baxi's own checklist covers the boiler display and any fault code, the timer and thermostat settings, the pressure gauge reading around one bar, and a frozen condensate pipe. Those separate a boiler making no heat from one sending heat to the wrong place.
Evidence recordHow this page was checked
official manufacturer support article · checked 2026-09-08
Every technical claim above was written from primary documentation held in the HVAC Bench evidence record: Vaillant UK and Baxi technical literature. Where a source limits a definition to certain models, test conditions, or product classes, that limit is repeated here rather than generalised.
- Documentation class
- official manufacturer support article
- Scope of the definition
- Confirm against the exact model manual
- Last checked
- 2026-09-08