No hot water on a combi boiler where do I start?

No hot water on a combi boiler is a very common and very irritating fault. Like any other appliance, combi boilers can sometimes break down or malfunction.

Combi boilers are a popular type of boiler in the UK, as they are efficient and can provide both heating and hot water on demand. One of the most common problems that homeowners experience with combi boilers is no hot water.

What do you mean by no hot water?

There are two definitions of no hot water. It can be the water coming out of the tap is cold. Or it could be there is literally no water flowing out of the tap when the tap is opened.

This page is mostly concerned with the hot water being cold.

Why might there be no hot water coming from my tap?

• The tap is broken. The mechanism inside the tap can break, check other taps in the house to see if they still work. If they do it’s probably the tap at fault.
• The water has been isolated at the boiler. This is quite common when people try to top up the pressure on their boilers. The valves under the boiler are turned off stopping the flow of water to the hot tap. Check under the boiler to see if the isolation valves are pointing forward.
• Water is isolated at a different stop cock or service valve. Have a look for any other valves to see if they are turned off.

What are the causes of no hot water with a combi boiler?

There are a number of reasons why you might not be getting any hot water from your combi boiler. Some of the most common causes include:

Low boiler pressure

Combi boilers need to have a certain amount of water pressure in order to operate properly. If the pressure is too low, the boiler will not be able to heat the water.

Faulty diverter valve

The diverter valve is responsible for switching the flow of water between the heating and hot water systems. If the diverter valve is faulty, it may prevent the hot water from flowing to the taps.

Blocked heat exchanger

The heat exchanger is the component in the boiler that heats the water. If the heat exchanger is blocked with limescale or other debris, it will not be able to heat the water efficiently.

Frozen condensate pipe

The condensate pipe is the pipe that drains away the waste water from the boiler. If the condensate pipe freezes, it will block the flow of water and prevent the boiler from working properly.

Faulty flow sensor

The flow sensor tells the boiler the hot tap has been opened. If the flow sensor is faulty the boiler will not respond to an open tap and the hot water will stay cold.

When the hot tap is opened the hot water light or symbol should be displayed. When the tap is closed gain the light should go off.

Other boilers don’t have a display so you are relying on listening to the boiler to hear if the boiler responds. Turn off the heating and wait for the noises to end. Now turn a tap on a listen for the boiler to make a noise. If the boiler makes no noise it is very likely the flow sensor is faulty. You will need a gas safe engineer to replace it.

Hot water flow rate too low

If the water flow rate coming out a hot tap is too low you will get no hot water on combi boiler. The boiler has a minimum flow rate required to activate.

If the water is flowing too slow the boiler won’t recognise a demand and won’t activate. It is possible to use a flow cup (like this one) to measure the rate the water is flowing. Compare this to the minimum required for your boiler stated in the manual.

Compare the flow rate of your hot and cold water. If the hot is very slow compared to the cold the boiler could be clogged with limescale or an isolation valve may be partly shut.

If both taps are slow check your stop cock is fully open or contact your water supplier.

Other boiler faults

There are a number of other things that can go wrong with a combi boiler that can cause no hot water, such as a faulty thermostat, temperature sensor, or ignition system.

How to troubleshoot no hot water with a combi boiler

If you are not getting any hot water from your combi boiler, the first thing you should do is check the boiler pressure. The pressure gauge should be in the green zone. If the pressure is too low, you can top it up by following the instructions in your boiler manual.

If the boiler pressure is correct, the next thing to check is the diverter valve. You can usually find the diverter valve under the boiler. If the diverter valve is stuck or faulty, it will need to be replaced by a qualified gas engineer.

If the diverter valve is working correctly, the next thing to check is the heat exchanger. You can usually find the heat exchanger at the front of the boiler. If the heat exchanger is blocked, you can try to clean it yourself using a descaling product. However, if the heat exchanger is badly blocked, it will need to be replaced by a qualified gas engineer.

If the heat exchanger is clean and the diverter valve is working correctly, the problem is likely to be with another component in the boiler. In this case, you should call a qualified gas engineer to diagnose and repair the problem.

How to prevent no hot water with a combi boiler

There are a number of things you can do to help prevent no hot water with your combi boiler:

• Have your boiler serviced regularly: A regular boiler service will help to identify and fix any potential problems before they cause a breakdown.
• Treat your boiler water: Adding a boiler water treatment product to your boiler water can help to prevent limescale build-up and other problems.
• Insulate your pipes: Insulating your pipes can help to prevent them from freezing in cold weather.
• Be aware of the signs of a problem: If you notice any problems with your boiler, such as a drop in pressure, strange noises, or leaks, call a qualified gas engineer to investigate immediately.

What to do if you have no hot water from your combi boiler

If you have no hot water from your combi boiler, the first thing you should do is check the boiler pressure and the diverter valve. If the problem persists, you should call a qualified gas engineer to diagnose and repair the problem.

In the meantime, there are a few things you can do to cope with no hot water:

• Heat water on the hob: You can heat water on the hob and use it to fill a bath or wash dishes.
• Use a kettle: You can use a kettle to boil water for drinking, washing, or cooking.
• Take a cold shower: If you are brave enough, you can take a cold shower. Cold showers have a number of health benefits, such as improving circulation and boosting the immune system.
• Go to a friend’s or family member’s house: If you really need a hot shower or bath, you can always go to a friend’s or family member’s house.

No hot water is a nuisance, but it is usually a problem that can be fixed quickly and easily by a qualified gas engineer