Why Is My Heating Working but I Have No Hot Water?

When your radiators are toasty but your taps run cold, it can be confusing and frustrating. The good news is that heating working usually means your boiler has power and is running, but a fault is stopping hot water getting to your taps.

First step: know which boiler you have

The cause of no hot water often depends on the type of boiler in your home. Combi boilers heat water directly from the mains when you turn on a tap, while system and conventional boilers use a hot water cylinder to store heated water.

If you are unsure what you have, look for clues. A single compact unit with no separate cylinder is usually a combi. If you have a separate hot water tank, you most likely have a system or conventional boiler. For more detail, see our types of boilers guide.

Once you know your boiler type, it is easier to work through the most likely causes and decide what you can safely check yourself.

Safe checks before calling an engineer

There are a few simple checks most homeowners can do safely, without removing any boiler covers. These can sometimes resolve the problem or at least give clearer information to a Gas Safe engineer.

• Check your programmer and timers: Make sure the hot water setting is actually on, and that the clock has not reset after a power cut.

• Confirm thermostat settings: Room thermostats and cylinder thermostats should be turned up high enough to call for heat.

• Inspect boiler pressure: On sealed systems, pressure usually needs to be around 1.0 to 1.5 bar when cold.

• Look for error codes: Many modern boilers show fault codes; note these down for the engineer.

Never remove the boiler casing or try to access internal components yourself. The case is usually a safety seal, and only a Gas Safe registered engineer should work inside the boiler.

Common causes on combi boilers

With a combi boiler, the same unit provides both heating and hot water. If your heating works but you have no hot water, it often means the boiler is not correctly diverting heat to your taps.

Diverter valve stuck or failed

The diverter valve is a moving part inside the boiler that directs hot water either to your radiators or to your taps. Over time, it can stick or fail, particularly in hard water areas.

Typical signs include radiators heating up when you run a hot tap, or hot water that comes and goes. This is not a DIY repair. Do not try to free a stuck valve with tools, as you could cause leaks or damage. A Gas Safe engineer can test and replace the valve if needed.

Blocked plate heat exchanger

Combi boilers use a plate heat exchanger to transfer heat from the boiler water to your tap water. Limescale and sludge can clog the narrow passages inside it, reducing hot water flow or temperature.

You might notice hot water that starts warm then runs cold, or temperature that fluctuates when more than one tap is used. Cleaning or replacing a plate heat exchanger involves draining and opening the boiler, so it must be done by a qualified engineer.

Thermostat or sensor faults

Temperature sensors tell the boiler how hot the water is. If a sensor fails or gives incorrect readings, the boiler may not fire for hot water or may shut down too soon.

This kind of fault often shows as error codes on the display. You can safely note the code and reset the boiler once, following the manual. If the problem returns, do not keep resetting; call an engineer to diagnose the sensor or control board.

Common causes on system boilers with a cylinder

If you have a hot water cylinder, your boiler heats the stored water indirectly through a coil. In this setup, heating working but no hot water usually points to controls or cylinder-related issues.

Cylinder thermostat and controls

The cylinder thermostat tells the boiler when the stored water needs heating. If it is set too low, turned off, or has failed, the boiler may never be asked to heat the cylinder.

Check that the cylinder stat is turned up (typically around 60 °C) and firmly clipped to the side of the cylinder. Also confirm that your programmer has the hot water channel set to "on" or "timed" at the periods you need hot water.

Motorised valve problems

Many systems use motorised valves to direct hot water from the boiler either to the radiators or to the cylinder coil. If the hot water valve sticks shut, the heating will work while the cylinder stays cold.

You may notice that the pipe from the boiler to the cylinder coil remains cool when hot water is selected. Do not try to force the manual lever or open the valve head, as this can break the mechanism or cause leaks. An engineer can test the valve and wiring centre.

Low boiler pressure and airlocks

On sealed systems, low pressure can stop the boiler from fully heating the cylinder even if radiators still get some heat. Check the pressure gauge on the boiler front panel.

If it is below the recommended level, you can usually top it up using the filling loop as per the manufacturer instructions. If pressure keeps dropping, or you hear glugging noises from pipes, there could be a leak or airlock that needs professional attention.

Timer and control settings to double-check

It is surprising how often programming or simple control settings lead to no hot water. Before assuming a fault, work through a few basics.

Check the mode on any smart thermostat or app, as holiday or eco settings may have turned hot water down or off. Make sure the boiler itself is not set to "heating only". If you recently had new controls fitted, confirm that the hot water channel has been correctly configured.

Symptoms that need urgent attention

Some signs suggest your boiler or system may be unsafe or at risk of serious damage. Do not ignore these, especially if you also have no hot water.

• Boiler leaking water: Puddles, drips or staining under the boiler or cylinder need immediate investigation.

• Loud banging or knocking: Persistent noises can point to kettling, pump issues or severe circulation problems.

• Repeated lockouts: If the boiler keeps shutting itself down with fault codes, it is protecting itself from a bigger problem.

If you smell gas, see scorch marks, or suspect a carbon monoxide issue, switch the boiler off, ventilate the area and contact the gas emergency number straight away.

What not to do when hot water stops working

It is understandable to want to fix the problem quickly, but some actions can put you at risk or damage your boiler. Never remove the boiler casing; internal gas and combustion components must only be worked on by a Gas Safe engineer.

Do not force stuck valves, taps or levers, and avoid using chemical descalers inside the boiler unless specifically instructed by a professional. Repeatedly resetting a boiler that keeps locking out can also cause further damage and mask an underlying safety issue.

Next steps and getting professional help

If you have checked your controls, pressure and thermostats and still have no hot water, it is time for a proper diagnosis. Many issues such as diverter valve failures, plate heat exchanger blockages and sensor faults can be quickly identified with the right test equipment.

You can read more about fault finding and fixes on our boiler repairs page, which explains common repair options and what to expect from a visit. A prompt repair often prevents a small issue turning into a costly breakdown.

For a safe, thorough diagnosis, contact Westhill Boiler Services and book a visit from a Gas Safe registered engineer. Call 01224548803 to arrange an appointment if your heating works but your hot water still will not come back after basic checks.