Finding that your boiler provides central heating but not hot water can be particularly frustrating during the colder months. The good news is that working radiators usually mean the boiler can still produce heat, so the problem may be limited to the hot-water controls, water-pressure settings or the components that direct heat around the system. This guide explains what you can check safely and when the fault should be investigated by a Gas Safe registered engineer.
Quick checks you can do now
Before changing settings, confirm exactly what is happening. Run a hot tap for at least 30 seconds and check whether the water remains completely cold, becomes lukewarm or briefly heats before cooling. Try more than one tap because a faulty shower valve, tap cartridge or local mixer can sometimes imitate a boiler problem.
Use this short checklist:
- Check the boiler display: Look for a fault code, warning symbol or flashing pressure reading.
- Confirm the boiler mode: Make sure it has not been placed in heating-only, holiday, standby or frost-protection mode.
- Try another outlet: Test a kitchen or basin tap as well as the shower.
- Check the pressure: On a sealed system, the reading is commonly around 1 to 1.5 bar when the boiler is cool, although the manufacturer’s instructions take priority.
- Review hot-water controls: Check that the timer, programmer or cylinder thermostat is requesting heat where applicable.
- Reset only as instructed: One reset may clear a temporary control fault, but repeated resetting is not a repair.
Do not remove the boiler casing, touch internal wiring or attempt to free internal components yourself. Stop using the appliance and seek professional advice if you smell gas, see scorching, notice water entering electrical parts or a carbon monoxide alarm activates. If you smell gas, follow the national gas emergency guidance and avoid operating electrical switches.
Why is there no hot water when the heating works?
The likely cause depends partly on the type of heating system fitted. A combi boiler heats water on demand when you open a hot tap and normally uses a diverter valve to switch heat away from the radiators and towards domestic hot water. If the boiler does not detect the tap opening, or if the valve fails to move correctly, the radiators may work while the taps stay cold.
A system or regular boiler generally stores hot water in a cylinder. In that arrangement, the boiler may be operating normally for central heating while a programmer, cylinder thermostat, motorised valve or cylinder-related component prevents the stored water from heating. Check whether you have a hot-water cylinder in an airing cupboard before assuming that advice for a combi boiler applies to your home.
Winter can make a developing fault more noticeable. Incoming mains water is colder, so the boiler must transfer more energy to achieve the required tap temperature. Heating is also used more frequently, which can expose a weak actuator, sticking valve or intermittent sensor. Cold weather does not automatically mean the boiler is undersized or needs replacing.
The behaviour of the display can provide useful information. On many combi boilers, opening a hot tap produces a tap symbol, flame symbol or change in operating noise. If there is no response, the boiler may not be detecting water flow. If it fires but the tap stays cold, the issue may involve heat transfer, a diverter valve or another internal component.
Check the boiler pressure safely
Low system pressure is a common issue in sealed heating systems. Although some boilers continue to run at a reduced pressure, others may lock out or behave inconsistently. Check the pressure gauge or digital display while the system is cool. A typical normal range is approximately 1 to 1.5 bar, but always use the figure stated in the boiler manual.
If the pressure is below the recommended range, the manual may explain how to repressurise the system using its approved filling arrangement. Only do this if you recognise the controls, can follow the instructions confidently and there are no obvious leaks. Open valves slowly, stop at the recommended pressure and close them fully afterwards. Never continue filling to compensate for a recurring pressure loss.
Pressure that falls repeatedly can indicate a leak, an issue with the expansion vessel or discharge through a safety valve. Pressure that climbs rapidly towards the upper limit as the heating warms also requires investigation. These faults are not normally solved by topping up the boiler over and over again.
It is also important not to confuse central-heating system pressure with tap-water pressure. The gauge measures water in the sealed heating circuit; it does not show the mains flow through a combi boiler. A weak hot tap may instead point to restricted mains flow, a blocked filter, scale or a plumbing issue. Compare hot and cold flow rates at the same outlet, but avoid dismantling taps or boiler components unless you are competent to do so.
Review thermostat, timer and hot-water settings
Controls are often overlooked, particularly after a power cut, clock change or accidental button press. Start by checking that the boiler is in its normal winter or heating-and-hot-water mode. If it has separate temperature controls, ensure the domestic hot-water setting has not been turned down to minimum. Avoid setting it excessively high, as very hot water increases scalding risk and wastes energy.
On a combi boiler, the room thermostat controls the radiators but usually does not need to request heat for a hot tap to work. Therefore, turning up the room thermostat is unlikely to restore missing tap water. Some combis have an eco or preheat setting; switching off preheat may create a slightly longer wait, but it should not normally result in permanently cold water.
For a home with a hot-water cylinder, check the following:
- The programmer has an active hot-water period or boost selected.
- The displayed time and day are correct.
- The cylinder thermostat has not been turned down accidentally.
- Any visible controller has power and is not showing a fault.
- The boiler reacts when hot water is selected, provided the cylinder is cool enough to require heat.
A cylinder thermostat is commonly set around 60°C to manage hygiene risk while limiting scalding and energy waste. Do not remove its cover or alter wiring. If selecting hot water produces no response despite correct settings, an engineer may need to test the thermostat, programmer, wiring centre or motorised valve.
Common diverter valve and flow-detection faults
A diverter valve is one of the most common explanations for a combi boiler heating working but no hot water. Its job is to route boiler heat either through the radiator circuit or through the domestic hot-water heat exchanger. Internal parts can become stiff, worn or restricted by system debris, while the valve’s motor or actuator can also fail.
A valve stuck in the heating position may leave the radiators working normally while producing cold or lukewarm tap water. A partially moving valve can create inconsistent temperatures or allow radiators to warm when only a hot tap is running. These clues are useful, but they do not confirm the diagnosis because sensors, controls and heat exchangers can produce similar symptoms.
Other common combi boiler possibilities include:
- Flow sensor fault: The boiler does not recognise that a hot tap has opened.
- Restricted plate heat exchanger: Scale or debris reduces heat transfer, sometimes causing water to alternate between hot and cold.
- Temperature sensor fault: Incorrect readings prevent the boiler from controlling hot water properly.
- Low mains flow: A supply or plumbing issue prevents reliable hot-water activation.
- Control or wiring fault: The request for hot water does not reach the relevant components correctly.
Diverter valve diagnosis requires tests inside the appliance and must not be attempted by removing the boiler case. A Gas Safe registered engineer can observe the valve operation, check sensors and compare flow and temperature readings. Depending on the boiler and the condition of the part, the repair may involve replacing an actuator, serviceable cartridge or complete valve assembly.
In hard-water areas, scale can contribute to poor heat transfer through a plate heat exchanger. Typical signs include reduced hot-water flow, fluctuating temperature or acceptable warmth at a slow tap rate but cooler water when the tap is opened further. Flushing or replacing an internal heat exchanger is professional work and should follow the manufacturer’s instructions.
When to arrange a professional boiler check
If the basic settings and pressure are correct but there is still no hot water, an engineer can identify the fault without relying on guesswork. Note the boiler model, pressure reading, displayed fault code and what happens when a tap opens. It is also useful to report whether the problem affects every outlet, whether the radiators heat during hot-water use and whether the water is cold, lukewarm or intermittent.
Homes across Bordon and Whitehill often contain a mixture of combi, system and older regular boilers, so the appropriate checks vary between properties. The same applies in nearby Liphook, Alton and Farnham, including GU35, GU34 and GU9 addresses. A clear description of the system and symptoms helps the attending engineer bring suitable diagnostic equipment and likely parts.
Professional attention is sensible when pressure repeatedly falls, the boiler locks out again after one reset, water leaks from the appliance, hot-water temperature fluctuates sharply or internal components appear to be involved. Repair may be straightforward, and a diverter-related fault does not automatically mean the whole boiler must be replaced. The engineer should assess the appliance’s age, condition, parts availability and repair cost before discussing the available options.
Until the fault is checked, avoid repeated resets, constant repressurising or forcing any visible valve. Keep boiler vents and external terminals unobstructed, retain access to the appliance and continue testing carbon monoxide alarms according to their instructions.
Next steps: For help diagnosing and repairing the fault safely, contact Embassy Gas on (01420) 558993.