Home Improvement

Why is Your Shower Going Hot and Cold on Combi Boiler?

Combi boilers are high-efficiency water heaters and central heating units. They are perfectly able to heat your home and provide hot water, without the need for a separate water tank. But this also comes with one common problem of causing the hot water to go cold or lukewarm and then back to hot again. You won’t notice it unless you’re using your shower, and naturally, the experience will not be pleasant.

In this article, the Fantastic Services team will explain all about this common issue and what you can do about it. This post is for all of you who:

  • Own a combi boiler;
  • Are experiencing water going hot and cold abruptly;
  • Wish to know what to do about it.

So, why is your shower going hot and cold?

The most common reason for this often occurring issue is a blockage in the hot water heat exchanger located inside the boiler. Most probably because of sludge buildup in the heating system. Unfortunately, sludge buildup is common, especially in installations where the fitter didn’t flush the heating system properly.

How to fix the problem

There are two possible options. First, you can try descaling the heat exchanger with a commercial descaler. This will require that you take apart your combi boiler, which is something the Fantastic Services team would never advise (unless you yourself are a certified boiler technician). Or, depending on the severity of the damage, you might need to replace the heat exchanger with a new one altogether.

Is your shower going hot and cold on the combi boiler?

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Another reason for water going hot and cold

It’s natural to blame this issue on the combi boiler, but sometimes this isn’t the case. If the flame indicator of the combi boiler is steady while someone in your household is using the shower, then the cause might be completely different.

One of the main things that can affect combi boiler performance is the number of simultaneously used water outlets. If you own a combi boiler, you might know that you shouldn’t use hot water in the shower and turn on the kitchen tap at the same time, as this will affect the pressure and the temperature of the water. If you do this, then that’s the reason for the water in your shower going hot and cold.

Or, your combi boiler might simply be old. Like everything else, your combi boiler can break down, in which case you will have only cold water. If it’s been more than ten years of usage, maybe it’s time for a replacement.

Do you need professional help with your combi boiler?

Is your combi boiler giving you trouble? Its complex system can develop all kinds of issues and once they present themselves, you have to be ready. Always rely on a professional boiler technician when it comes to problems with your heating system. If you’re in need of expert help, get in touch with the Fantastic Services repairmen team. They will fix your problem, replace parts if needed, and can give you helpful maintenance tips upon request. 

Visit the main website for price rates for our professional boiler technicians!


  • The most common reason for the water in the shower going hot and cold is usually damage in the heat exchanger of the combi boiler;
  • Fixing a damaged heat exchanger  will require you to disassemble the boiler unit and possibly to replace the part;
  • It’s not advisable for an amateur to attempt any repair work on a boiler; 
  • If your boiler is not damaged, the same issue can occur if your household is using multiple water sources.


Has your combi boiler caused your shower to go hot and cold? How did you deal with the problem? Tell us in the comment section below!

Image source: sydeen/shuttestock.com

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