In This Guide
If your central heating system is working fine but there’s no hot water, you’re not alone. This is a common issue in UK homes, especially those with combi boilers like Worcester Bosch, Ideal, Baxi, or Vaillant. System boilers and gas boilers can also experience similar problems, such as issues with the hot water cylinder or pilot light. Modern boilers often display error codes to help with troubleshooting. The good news? Many of the causes are relatively simple to identify, and some even have easy fixes you can try before calling out a professional.
In this guide, the experts at 0800 Homefix explain what it means when your hot water isn’t working but your heating still is, how to fix it depending on your boiler type, and why checking your boiler settings is one of the first steps to take.
How Do Combi Boilers Work?
Combi (combination) boilers heat water directly from the mains when you turn on a hot tap. The boiler heats water on demand using heating elements and an internal thermostat to regulate water temperature, ensuring efficient heating water for your needs. There’s no storage cylinder, which makes them compact and efficient. But it also means the same boiler unit handles both central heating and hot water. In contrast, a system boiler is part of a hot water system that uses a hot water tank to store heated water. If one function stops working while the other continues, it points to a fault in a specific part of the system.
Common Reasons Why Hot Water Stops Working
1. Diverter Valve Fault
If your heating works but hot water doesn’t, a stuck or faulty diverter valve is the most likely culprit. This component switches the flow of heated water between your taps and radiators. This is a classic ‘heating but no hot’ situation, where the boiler provides heating but not hot water.
Symptoms:
- Radiators get hot
- No hot water at taps or showers
- Boiler seems to work only when heating is on
Fix: This generally needs a Gas Safe engineer to repair or replace the valve. If the diverter valve fails, it can prevent the system from providing hot water or heating as needed. Ensuring the valve is functioning correctly is essential for proper boiler operation.
2. Sensor or Thermistor Issues
Your boiler relies on temperature sensors to detect when hot water is needed. These sensors monitor the water temperature and work with the thermostat settings to ensure the correct hot water output. If the domestic hot water (DHW) sensor is faulty, your boiler might not respond when you turn on a tap.
Fix: A professional will need to test and possibly replace the sensor. A faulty sensor may also trigger an error code on the boiler display.
3. Broken Diaphragm or Microswitch
Older boilers may have a diaphragm and microswitch assembly that detects water flow, and these systems often rely on a pilot light or pilot lights for ignition. If these break, or if the pilot light goes out, the boiler won’t ignite for hot water. Additionally, a faulty gas valve can prevent the pilot light from igniting, resulting in no hot water.
Fix: These parts, including the gas valve or pilot light components, can be replaced or repaired by a qualified engineer.
4. Airlocks or Blockages
Air trapped in the system or blocked pipework can prevent hot water from flowing correctly. In some cases, airlocks or blockages may result in only cold water coming from the taps. Additionally, wet patches around pipes or radiators may indicate a leak that is contributing to the blockage.
Fix: Bleed your radiators or flush the system with professional help.
5. Low Water Pressure
If your boiler pressure is below 1.0 bar, it might not be able to function properly. You should check the pressure gauge to confirm if the system has low pressure.
Fix: Top up your boiler to between 1.3 and 1.5 bar using the filling loop. You may need to re-pressurise the boiler, but be careful not to create excess pressure. The pressure relief valve is a safety feature designed to release excess pressure if needed.
6. Faulty Hot Water Tap or Shower Mixer
Sometimes the issue isn’t with the boiler at all but with the tap or mixer valve.
Fix: Check if other taps have hot water. If only one outlet is affected, the tap or valve may need replacing.
7. Frozen Condensate Pipe
In winter, your condensate pipe can freeze and block water flow. Frozen pipes are a common issue during cold weather, and a frozen pipe can block water flow to the boiler.
Fix: Thaw the pipe using warm water (never boiling) and insulate it to prevent future issues.
Error Codes That Might Help
Check your boiler display. These fault codes could indicate a hot water issue:
- Worcester EA: No flame detected (can affect hot water ignition)
- Ideal F1/F2/F4: Ignition and sensor-related errors
- Vaillant F28/F29: Ignition failure or gas supply issue
- Baxi E133/E168: Ignition lockout or PCB error
If you see a fault code, consult your boiler manual or the manufacturer’s instructions to interpret what each code means for your specific model. You may be able to safely reset your boiler to clear minor faults, but always follow the boiler’s manual for the correct procedure.
Always consult your user manual or contact 0800 Homefix for accurate diagnosis.
Troubleshooting Checklist Before You Call a Pro
- Check boiler pressure: Repressurise if below 1.0 bar
- Try another tap: Is the issue isolated?
- Reset the boiler: This clears minor faults
- Listen for ignition: Is your boiler trying to fire up?
- Check power supply: Inspect the fuse box and circuit breaker to ensure power is reaching the boiler.
- Check boiler timer and thermostat settings: Make sure the boiler timer is set correctly and thermostat settings are above the minimum required temperature.
- Use a voltage tester: If you have an electric boiler that is not powering up, use a voltage tester to check for an electrical fault.
If none of the above work, it’s time to call in a professional.
Why Heating Still Works When Hot Water Doesn’t
This simply means your boiler’s heating side is functioning properly, but the hot water system is experiencing boiler issues – these could involve the diverter valve, DHW sensor, or flow detection components. It’s good news in that your whole boiler isn’t broken and the heating is functioning correctly, but the hot water system still needs attention.
When to Call 0800 Homefix
Call us if:
- Your boiler shows repeated error codes
- You’ve topped up pressure and reset but hot water still fails
- You suspect the diverter valve or sensors are at fault
- You hear strange noises or notice leaks
Some boiler faults may involve a safety device, such as a thermocouple, which is designed to shut down the boiler for safety reasons. These issues should only be handled by a professional.
If your boiler is old or keeps breaking down, it may be time to consider a new boiler for improved reliability and efficiency.
For complex or safety-related problems, always call a qualified heating engineer or Gas Safe registered engineer to ensure safe and proper repairs.
Our Gas Safe registered engineerscover all boiler brands and models, and we can usually fix hot water issues in a single visit.
Frequently Asked Questions
Can I still use my heating safely if hot water isn’t working?
Is it dangerous if I have no hot water?
How much does it cost to fix no hot water?
What is an open vented system boiler?
Book Your Boiler Repair with 0800 Homefix
Don’t wait around for lukewarm showers. Our local heating experts and engineers are ready to restore your hot water and peace of mind today. 0800 Homefix repairs all types of hot water systems, including electric boilers, electric water heaters, and hot water tanks.
Call us now or book online for fast, affordable boiler repairs.
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Editorial note: This article is intended as a general homeowner guide. All plumbing work on supply pipes should be carried out by a qualified, WaterSafe-registered plumber. If you suspect a gas leak, leave the property immediately and call the National Gas Emergency Service on 0800 111 999.
