Most UK homes rely on a gas combi boiler for both heating and hot water. As well as being effective and economical, they also have a reputation for being extremely reliable. However, as a piece of equipment that is typically in use 24 hours a day seven days a week, it is inevitable that from time to time problems may occur.

One problem that can be particularly frustrating is when the heating is working but there is no hot water, or if there is hot water but the boiler is not heating your home. Read on to learn more about common causes for these problems and how to deal with them.

1. Check that you have electricity, gas and water

The first rule of solving problems with your boiler is to start with the simplest possible causes. Before checking anything else, it is important to make sure that the boiler itself is actually working. Always begin at the boiler control panel. If you have a digital display and nothing is shown, it may be that there is no power to the boiler due to a power cut. Try turning on your lights and sockets to quickly confirm or eliminate a problem with the power supply. You could also check that all the fuses in your fuse box are in the correct position.

In a similar manner, it is also important to make sure you have gas and water. These can be checked by running a tap and turning on another gas appliance such as your cooker.

If you have done all of the above and are happy that all your electricity, gas and water are all working, then it is time to check for any fault codes.

2. Is there a fault code displayed?

Many boiler problems will result in a fault code being displayed in your control panel. If you have the operating manual to hand, this is the best place to look for what a fault code on your boiler means.

If you have a Viessmann boiler, you can also use the convenient online boiler fault code tool.

Remember, boiler fault codes can vary from boiler to boiler. If you can’t find a description for your fault code or if instructed by the manual, you may need to contact a Gas Safe engineer for assistance.

3. Check the boiler controls

If you still have no hot water or no heating after checking the above steps, then you will want to check that the boiler settings are correct. Your boiler will have separate controls to turn on the heating and hot water. It is possible that the hot water or heating may have been set to off by mistake, or the setting has been inadvertently changed. 

Check the timer

Some boilers may have a timer to control the hot water. If your boiler timer controls your hot water, this may be the cause of the problem. Your timer may have been affected by the clocks changing, power supply, back-up batteries failing or even somebody altering the settings without you knowing. Adjusting your timer may resolve the problem.

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Check the thermostat

Your heating temperature is usually controlled by a thermostat. For most boilers this will be a manual dial mounted on the wall in the main living area or hall. Newer boilers may be fitted with digital thermostats. Ensure that the thermostat has not been accidentally set to off. Set the thermostat to a higher temperature with the timer set to active, and see if the boiler fires up. If it fails to respond, there may be a problem with the thermostat. This will require attention from a trained engineer.

4. Check the diverter valve

If you have eliminated any other causes listed above and you still have heating but no hot water or your boiler heating is not working but the water is, then it is likely that you have a problem with the diverter valve. This is is a valve that opens and closes to provide heat to the radiators or to heat your water.

If your heating is working but you have heating but no hot water or vice versa, it is likely to have become stuck. You will need to contact a Gas Safe engineer to solve a problem with your diverter valve.

The boiler is working but I have no hot water or heating

If you have followed steps 1 to 3 above and you have neither hot water nor heating, it is likely that there is a fault with a component that requires attention from a trained engineer. We recommend that you have your boiler inspected as soon as possible.

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