My Central Heating Works, But I Have No Hot Water

In UK homes, boilers perform two basic functions: heating the home and supplying hot water to taps and showers. Most UK homes are equipped with central heating systems that provide heating and hot water simultaneously.

Typically, this system should perform well without breakdowns, provided the boiler is serviced regularly and maintenance or repairs are carried out in a timely manner. Despite all necessary precautions, such as regular servicing and timely maintenance, a sudden fault may still occur. One of the most common situations a home experiences is central heating working, but no hot water at the taps or in the shower.

While it is concerning, especially on icy mornings, it may not be a serious issue that can be fixed without calling a Gas Safe-registered engineer. However, at times it may indicate a more serious problem that can only be addressed by a professional heating engineer. This scenario suggests that one component of the central heating system, intended to supply hot water to taps, has failed, while the boiler is operating normally.

In this blog, we will explore the possible reasons for a situation where the heating works but no hot water is available at the taps and in the showers, along with easy solutions.

My Central Heating Works, But I Have No Hot Water – Common Reasons

There may be several reasons for the situation: “heating works but no hot water”. The most common reasons are summarized here:

1.     Faulty Diverter Valve – Combi Boilers

The central heating system serves two purposes: heating and supplying hot water to taps and showers. This dual functionality in Combi boilers is primarily controlled by the "diverter valve". It determines where the heated water is diverted: radiators, taps, or showers. Under normal circumstances, it allows heated water to go to radiators (for heating purposes). When any of the hot water taps or showers are opened, the diverter valve stops supplying heated water to the radiators and routes it to the hot water taps and showers. When the hot water taps and showers are closed, the diverter valve returns to its original position, disconnecting the supply of heated water to them, whereas the supply to radiators is restored. In this way, both functions are performed simultaneously through a diverter valve.

While the diverter valves are designed to perform this specific function, they may occasionally experience a fault. The most common fault in a diverter valve is it being stuck in the wrong position, resulting in a disconnected supply of heated water to taps and showers. Likewise, over time, the diverter valve may degrade and fail to operate as expected.

In both cases, the apparent signs of a faulty diverter valve may include:

·         Radiators continue to heat even when a hot water tap is opened.

·         While heating is still on, taps and showers deliver cold or lukewarm water instead of hot water.

·         No apparent abnormal sounds generated by the boiler, but there is no hot water in the taps and showers.

How To Fix It?

A faulty diverter valve often requires replacement, which can only be performed by a Gas Safe-registered engineer.

2.     Faulty Motorized Valve – System or Conventional Boilers

Unlike Combi Boilers, conventional systems are fitted with a motorized valve that controls the flow of hot water between radiators and the hot water cylinder. The common signs of a faulty or stuck motorized flow control valve include:

·         Water flows only to radiators, limiting heating availability.

·         The hot water cylinder is not receiving hot water, resulting in no hot water at the taps and showers.

How To Fix It?

The best option is to replace it, which can only be done by a professional Gas Safe-registered engineer. Never attempt to replace it yourself; it is risky.

3.     Problems/Issues With Hot Water Thermostat

Boilers typically have two separate temperature controls: one for heating and the other for the hot water supply. The device that is responsible for temperature control is known as a "thermostat". These must be set according to the requirements. Heating settings are typically set above 20°C, whereas a hot water thermostat requires a temperature setting above 50°C.

If, for any reason, the hot water thermostat is set to a very low temperature, there will be no hot water in the taps and showers, while the heating may still be working correctly. The hot water thermostat may also stop working due to prolonged, consistent use. Similarly, it is possible that the setting was disturbed accidentally. Another possible reason is that the boiler was recently reset and the hot water thermostat was set to a very low temperature.

How To Fix It?

·         Reset the boiler and recheck the hot water thermostat temperature setting.

·         Adjust/increase the temperature setting to somewhere between 55 and 60°C.

·         Wait a few minutes and check whether the taps are producing hot water. If the problem persists, the fault may lie elsewhere. In this case, call a Gas Safe-registered engineer to identify the problem and resolve it.

4.     Faulty Cylinder Thermostat In Hot Water Tank Systems

In homes with a separate hot water tank (cylinder), a separate thermostat is installed on the tank. It communicates with the boiler to trigger it when the water stored in the cylinder needs to be heated. If this thermostat fails, the boiler will not heat the stored water, whereas heating may still work independently.

When it happens, check the thermostat settings; it should be set to a temperature above 55°C. Also, ensure the thermostat is not switched off. Check for any loose electrical wires or connections.

How To Fix It?

If the settings are correct and the thermostat is switched on, but it fails to work, there is likely a fault. In this case, it may need to be replaced. Call a qualified gas safe registered engineer to replace it.

5.     Low Boiler Pressure

Modern boilers are designed to operate at a specific pressure, typically between 1.0 and 1.5 bar. If the boiler pressure drops below that level, the boiler's functionality may be compromised. In this case, the boiler may still provide heating, but the hot water supply is often hindered. However, if the boiler pressure drops below 0.5 bar, the boiler often locks out due to the internal safety mechanism.

How To Fix It?

Check the boiler pressure. If it is below 1.0 bar, repressurize the boiler using the filling loop. Top up the pressure somewhere between 1.0 and 1.5 bars. Make sure not to over-pressurize the boiler.

6.     Limescale Build Up – In Hard Water Areas

Limescale buildup is another common cause of heating but no hot water, especially in areas with hard water. Limescale build-up mainly affects the heat exchanger plate in combi boilers. It also affects the sensors and valves. Common symptoms of limescale build-up include weak or intermittent hot water in taps and showers. In more severe cases, the boiler fires up, but the water is not heated effectively.

How To Fix It?

This problem can only be fixed by a qualified Gas Safe registered engineer, as it may involve:

·         Replacement of the heat exchanger.

·         Power flushing of the boiler.

Other Possibilities

·         Frozen Pipes – In extreme cold, outdoor pipes may freeze, causing no hot water at taps and showers. This can easily be fixed by thawing the frozen pipes with warm (not hot) water.

·         Error Codes – The boilers may experience errors that prevent hot water. Most modern boilers display these errors as codes, such as H.01 or E.01. Typically, these codes indicate problems with the gas supply or pilot issues. These types of errors or faults can only be fixed by a Gas Safe-registered engineer.

·         Boiler Needs Resetting – Sometimes, no hot water may indicate the boiler requires a reset. This can be done easily by pressing the reset button for a few seconds. The boiler resets to its default settings and restarts. In most cases, the problem can be fixed, but if the issue reappears, it is time to call a Gas Safe-registered engineer.

Conclusion

The central heating system in homes is designed to operate continuously throughout the winter, including heating rooms and supplying hot water to taps and showers. Under normal circumstances, a well-maintained boiler meets both requirements efficiently. However, at times homes may face a critical situation when the heating is working, but there is no hot water in the taps and showers. There may be several reasons for this issue, but the most common is a faulty or stuck diverter valve.

While some minor issues, such as low boiler pressure, can be fixed easily on your own, serious faults, such as a diverter valve problem, can only be fixed by a qualified boiler engineer, preferably a Gas Safe-registered engineer. Although this is not an emergency, it should be addressed as soon as possible to prevent further damage to the central heating system. However, if there are apparent signs of a gas leak, it needs immediate attention.