No hot water but heating works usually comes down to four issues: a faulty diverter valve, a blocked plate heat exchanger, a failed flow sensor or thermistor, or — on older boilers — a pilot light or ignition fault.
Discovering you have no hot water when your central heating is running perfectly is a frustrating puzzle. Many homeowners assume that if their radiators are warm, the boiler itself must be completely fine. In reality, a boiler can easily maintain your central heating circuit while completely failing to supply your hot taps and showers.
When you are left with no hot water from boiler outlets, the issue usually comes down to an internal component or sensor failure. Understanding how to diagnose these symptoms accurately helps you understand whether it is a setting issue or a mechanical fault that requires an engineer.
1. A Sticking or Faulty Diverter Valve
If you are dealing with a situation where there is no hot water but heating works, a malfunctioning diverter valve is the single most common culprit on a combi boiler. This internal component acts like a mechanical traffic controller, opening and closing to direct hot water either to your radiators or to your hot taps.
When you open a tap or shower, the valve should automatically prioritise your domestic hot water. If the valve mechanism seizes or becomes fouled with internal system debris, it can get physically stuck in the heating position.
The Tell-Tale Sign: If your central heating working but no hot water is entering your taps, test a radiator. If your radiators suddenly start getting even hotter the moment you turn on a hot tap, the diverter valve is failing to close off the heating circuit properly.
2. Blocked Plate Heat Exchanger
Another frequent mechanical issue that leads to a boiler working but no hot water is a blocked plate heat exchanger. This component transfers heat from the boiler's internal water circuit to the fresh cold mains water feeding your taps.
In hard water areas across South London, this narrow piece of pipework frequently clogs with limescale or black iron oxide sludge. When this restriction occurs, you might notice your hot water cuts out after a few seconds or only runs lukewarm, even though the central heating circuit functions flawlessly.
3. Failed Flow Sensors and Thermistors
Sometimes, your appliance is physically capable of producing heat, but a faulty electronic sensor stops it from recognising your demand.
- The Flow Sensor: When you open a tap, a flow sensor tells the boiler to fire up for hot water. If this sensor fails, you will hear absolute silence from the appliance when the tap is opened, resulting in heating working but no hot water despite the boiler panel looking normal.
- The Thermistor (Temperature Sensor): If the internal temperature sensors degrade, they can send incorrect voltage readings to the main control board. The boiler mistakenly believes the water is already boiling hot and shuts down the burner instantly, leading to a frustrating lack of hot water from your boiler supply.
4. A Lost Pilot Light or Ignition Fault (Older Boilers)
If your home still has an older standard-efficiency or back boiler with a visible pilot light, this small permanent flame is what ignites the main burner on demand. On these older systems, a pilot light that has blown out or won't stay lit will usually stop the appliance firing at all — for both heating and hot water — so it's rarely the cause if your radiators are genuinely still warm.
Where it does become relevant is uneven ignition: a worn ignition electrode or a dirty flame sensor can allow the burner to light reliably at the lower firing rate used for central heating, but fail to sustain ignition at the higher firing rate a combi boiler switches to for instant hot water. If your hot water cuts out, splutters, or never fires up at all while the heating runs normally, this uneven ignition performance is worth having an engineer check alongside the diverter valve.
What About the Reverse Problem?
It is worth noting that systems can occasionally experience the exact opposite symptom. If you have boiler hot water but no heating, the diverter valve may have seized in the domestic hot water position instead.
When you have reliable boiler hot water but no heating inside the house, it can also point to a failed wall thermostat, a broken system programmer, or an unresponsive motorised zone valve on older conventional heating setups.
Simple Troubleshooting Checks Before You Call
Before arranging a professional service visit, perform these simple checks to rule out external control errors:
- Check the Digital Programmer: Ensure your hot water schedule has not been accidentally turned off or set to a different profile on your smart thermostat.
- Inspect the System Pressure: Look at the water gauge on your front panel. If the needle has dropped below 1 bar, it can stop the hot water system from firing. Top it up back into the ideal 1.0 to 1.2 bar operating zone using your filling loop.
- Try Multiple Taps: Check if the lack of hot water is house-wide or isolated to a single faulty mixer shower valve.
Reliable Heating Support Across South London
If you have completed basic checks and still have no hot water, you cannot fix internal mechanical or gas-fed components yourself. Opening a pressurised boiler casing requires specialist legal qualifications.
As a trusted plumber and heating expert, R.W. Miller provides dedicated boiler repair services across South London, including Wandsworth, Bromley, Lambeth, Lewisham, and Greenwich. All gas diagnostics and systemic repairs are personally carried out by Reion Miller, a fully qualified independent professional registered with the Gas Safe Register (Reg. No. 919881).
Our straightforward trade pricing is completely transparent: we charge a fixed £80 diagnostic fee for the first hour to test the internal sensors and isolate the exact fault, with a clear £90 per hour rate thereafter if further labour is required to complete the repair.
If your boiler is working but no hot water is reaching your shower, call Reion Miller directly on 07375 813996 or make an appointment for an independent, local assessment.
