Hot-water faults on combi boilers are common because the appliance must switch instantly between space heating and domestic hot water (DHW). Most issues fall into a few categories: hydraulics (diverter valve), heat transfer (plate heat exchanger), water quality (limescale), system pressure, or mains supply limits.
Symptom: heating works, no hot water
If radiators heat normally but taps run cold or lukewarm:
- Check system pressure — Very low pressure can lock out DHW. See our pressure guide.
- Check for a fault code — Note the code and look it up at MyBoiler.com or in the manual at BoilerManuals.com.
- Diverter valve stuck on heating — The valve may not move to send flow through the plate heat exchanger. Often needs engineer replacement or service.
- Plate heat exchanger blocked — Limescale restricts flow; hot water may start warm then cool quickly, or never reach temperature. Common in hard-water areas.
- DHW thermistor or NTC fault — The boiler misreads water temperature and modulates incorrectly.
- Frozen condensate (winter) — Can lock out the whole boiler including DHW. Thaw only if safe to do so; prevention is better (insulated condensate pipe).
Symptom: hot water works, no heating
- Room thermostat or programmer set to off or low.
- Diverter valve stuck on hot water mode.
- Motorised valve or wiring issue on some models.
- Fault code related to pump, air lock, or flow — engineer diagnosis needed.
Symptom: weak shower or slow hot water
Before assuming the boiler is faulty, separate flow rate from temperature:
Temperature OK but flow poor
- Mains supply limit — Combis cannot boost poor incoming flow. Multiple outlets open at once will share the available flow.
- Scaled plate heat exchanger — Restricts passage; professional descale or replacement may be needed.
- Blocked shower head or tap aerator — Simple clean can restore flow.
- Combi undersized for demand — A 24 kW unit may struggle with a large rain shower. See reading combi datasheets and sizing.
Flow OK but not hot enough
- DHW temperature set too low on the boiler front panel — adjust within safe limits (consider scalding risk at kitchen taps).
- High flow shower exceeding boiler capacity — The boiler cannot heat very high flow rates to full temperature. Reduce flow or upgrade output.
- Incoming mains very cold in winter — Same kW must achieve a larger temperature rise; shower may feel cooler.
Symptom: water runs hot then cold (or “cold sandwich”)
- Boiler minimum firing rate too high for the tap flow — common with small basin draws.
- Thermostatic mixer shower fighting with a combi — use a shower designed for combi systems or adjust settings.
- Storage combi or buffer can help in some properties — discuss with your engineer.
Safe checks you can do
- Confirm pressure is in range (1–1.5 bar cold).
- Try hot water at a kitchen tap (less restriction than a shower).
- Run only one hot outlet at a time.
- Check whether the boiler display shows DHW demand (flame symbol or flow indicator).
- Note any fault code before resetting.
Do not open the boiler casing, disturb gas connections, or work on sealed components.
What the engineer will check
- Diverter valve operation and DHW flow switch.
- Plate heat exchanger condition — descale or replace.
- Gas rate and combustion performance under DHW load.
- Mains dynamic pressure and flow at peak demand.
- DHW NTC sensors and wiring.
Prevention
- Limescale inhibitor or water softener on the cold feed in hard-water areas — see choosing guide.
- Annual service including hot-water performance check — BoilerService.com.
- Correct sizing at installation — installation best practice.
Gas appliance repair must be carried out by a Gas Safe registered engineer. This guide is for general information only and does not replace professional diagnosis.