Why Do I Have Hot Water but No Heating

A common issue faced by households across the UK is when a boiler provides hot water as normal but fails to deliver central heating. This situation can be frustrating, particularly during colder months when heating is essential. While it may appear puzzling that hot water is available but radiators remain cold, there are several possible reasons behind the problem. Understanding these can help homeowners, landlords, tenants, and property managers decide whether the issue can be resolved with simple checks or if professional assistance is needed.

Understanding How Boilers Provide Heating and Hot Water

Most modern boilers in the UK are either combi, system, or conventional models. A combi boiler provides hot water and heating directly from the unit without the need for a cylinder or tank. System and conventional boilers supply hot water through a cylinder and distribute heating separately to radiators. When only the hot water side of the system is working, it often points to a fault within the heating controls or circulation system rather than a complete boiler failure. Recognising the type of boiler you have is the first step in narrowing down potential causes.

Thermostat or Programmer Issues

One of the most common reasons for having hot water but no heating is a fault with the thermostat or heating programmer. If the thermostat is set too low, the heating system may not switch on because the room temperature is already above the set point. In other cases, the programmer may have incorrect timings set or may have failed altogether. For example, if the hot water schedule is active but the heating schedule is not, you will only receive hot water. Checking and adjusting these settings should be the first step when diagnosing the issue.

Problems with Diverter Valves

In combi boilers, a diverter valve controls whether hot water is directed to the taps or to the radiators. If this valve becomes stuck in the hot water position, the boiler will continue to deliver hot water while the heating system remains inactive. This is a frequent fault in older combi boilers and often requires a Gas Safe engineer to replace the valve. In system or regular boilers, similar issues can arise with motorised valves that control the flow of water between the hot water cylinder and the heating system.

Airlocks and Circulation Problems

Another potential cause is an issue with circulation. Central heating systems rely on pumps to move hot water through pipes and radiators. If the circulation pump has failed or is running at the wrong speed, water will not reach the radiators even though it is being heated. Airlocks in the pipework can also restrict circulation, particularly after recent work on the heating system or a pressure drop. In such cases, radiators may remain cold even though the boiler is functioning correctly.

Pressure and Boiler Settings

For combi and system boilers, system pressure is a critical factor. If the boiler pressure is too low, the heating side of the system may fail to operate while still allowing hot water delivery. This is because hot water demand relies on mains pressure, whereas central heating relies on system pressure within the boiler. A quick check of the pressure gauge should confirm whether this is the cause. In most boilers, a pressure reading between one and one and a half bar is required. If the reading is below this level, topping up the system via the filling loop may resolve the problem.

Faulty Components

In some cases, the problem may lie with specific components such as the boiler’s printed circuit board (PCB), sensors, or wiring. These parts are responsible for ensuring the correct signals are sent between the thermostat, valves, and heating circuits. A fault with these components can prevent heating from activating even though hot water remains available. Diagnosing such faults usually requires professional testing and replacement.

Seasonal and Usage Factors

It is also worth considering how the system has been used. Many households switch off central heating entirely during summer months, relying only on hot water. When the heating is first turned back on in autumn, problems such as seized valves or stuck pumps may become apparent. Running the heating for short periods during warmer months can help prevent these issues by keeping moving parts active.

When to Call a Professional

While some issues such as thermostat settings or low pressure can be resolved by homeowners, many causes of hot water without heating require a Gas Safe engineer. Working on gas appliances without proper training is unsafe and may also void manufacturer warranties or landlord compliance requirements. Landlords in particular have a legal duty to maintain safe heating systems, so arranging professional repairs quickly is essential to meet obligations under the Gas Safety (Installation and Use) Regulations 1998.

Preventing Future Problems

Regular boiler servicing is the most effective way to prevent issues where heating fails while hot water continues to work. An annual service will check valves, pumps, pressure, and safety devices to ensure the system is operating correctly. Bleeding radiators, monitoring pressure, and testing thermostats at the start of the heating season can also help identify problems before they become disruptive. For properties with tenants, clear instructions on using the programmer and thermostat can avoid confusion and unnecessary callouts.

Final Thoughts

Having hot water but no heating is a frustrating but common boiler fault. In many cases, the issue lies with thermostats, diverter valves, circulation pumps, or system pressure. Some checks can be carried out by homeowners, but most faults require professional repair to ensure safe and lasting results. By keeping boilers serviced and carrying out seasonal checks, households and landlords can reduce the risk of being left without heating during the colder months.