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Combi Boiler Problem: Shower Keeps Going Hot and Cold

If you own a combi boiler and your water keeps going hot and cold when showering, we cover all the possible causes of this problem. 

Combi boilers are the most popular boiler type for small and medium-sized homes because they combine heating and hot water into one compact appliance. They also heat water directly from the mains when you turn on a tap and don’t require separate tanks or cylinders. But when you have a combi boiler installed, you may find that your water keeps going hot and cold when showering without you adjusting the heat setting of your shower. 

If your shower goes hot and cold, you’ll want to get to the bottom of the problem as soon as possible. This article explains why this might be happening and what you can do about it. 

What is causing your shower to go hot and cold?

There’s nothing worse than jumping in a hot shower only to discover that the water keeps going lukewarm or cold and then hot again. So, what’s causing it?

Issue 1: A blockage in your boiler’s heat exchanger

A shower that keeps going hot and cold is usually a sign of a blockage in the hot water heat exchanger inside your combi boiler. Sludge can build up in your heating system, especially if the filters are faulty, and cause your shower to go hot and cold when in use. Combi boilers often suffer from this type of blockage, which can lead to a wildly fluctuating water temperature.

It’s a common problem with combi boilers and is more likely to happen if your engineer didn’t flush your central heating system properly before installing your boiler. 

How to fix it

When your combi boiler’s hot water heat exchanger accumulates sludge, it will need descaling. This job requires taking apart your boiler to descale the heat exchanger directly. As you should never attempt to remove any boiler components or fix this type of issue yourself, you’ll need to contact a Gas Safe registered engineer to remove the heat exchanger and descale it for you. 

If your engineer discovers that the heat exchanger is severely damaged, they will likely recommend a replacement. A new heat exchanger can cost anything between £400 and £600, and you might be better off purchasing a new boiler rather than replacing the heat exchanger unless your boiler is still under warranty. 

Issue 2: You’re using multiple taps at the same time

Another common issue that causes your shower to go hot and cold on a combi boiler is using multiple water outlets at once. So, if you take a shower whilst someone else in your home tries to run a tap in the kitchen, the temperature and pressure of the water will be affected. That means the entire time another water outlet is in use, the temperature of the water will constantly change, preventing you from enjoying a relaxing shower. 

How to fix it

The obvious solution is to not run multiple taps at once. But if you live in a house with several bathrooms or where you often have the shower on at the same time as other hot taps, you might want to consider replacing your combi boiler with a system or regular boiler.  

You can ask a Gas Safe registered engineer to visit you in your home and help you determine which type of boiler will best suit your requirements. 

Issue 3: Your combi boiler might be on its last legs

If your combi boiler is on its way out, this could explain your hot and cold shower problem. You may decide to contact a heating engineer to diagnose and fix any issues, but it probably won’t be worth paying for a repair if your boiler is old and inefficient. 

How to fix it

When was the last time you bought a new boiler? If your combi boiler is old and not meeting your hot water needs, it might be time to look at replacing it. Most boilers last between 10 and 15 years on average with proper maintenance. 

Before you rush out and purchase a new combi boiler, you should get an expert opinion on each boiler type. A Gas Safe registered boiler engineer is an invaluable source of information and can help you choose a boiler type and size that fits your current and future needs. 


The best-case scenario is that your boiler isn’t damaged and your household using multiple water sources at once is to blame for your hot and cold shower problem. If this is the case, all you need to do is make sure no one else in your home uses a hot tap whilst you take your showers. 

If it turns out your boiler’s heat exchanger has a blockage, you must not attempt to fix this yourself. Any repair work on your combi boiler will require the expertise of a Gas Safe registered engineer, meaning you’ll need to spend some of your hard-earned cash. 

A hot and cold shower problem could even be your boiler’s way of telling you that it’s on its last legs and might need replacing. If this is true, we recommend you compare boiler quotes from different local and national companies to get the right boiler at the right price.