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No Hot Water From Taps? Combi Boiler Troubleshooting Guide

no hot water

Turning on the hot water tap only to have no hot water coming through is one of the most irritating, stressful and tedious things that can happen to you at home. All you want is to wash your face, do the dishes or have a bath. Instead? You’re left with icy cold water and are left feeling thoroughly miserable. Once the initial annoyance and shock has worn off, however, you’ll be left with a more pressing question – why.


Just why isn’t there any hot water coming through your taps when you have a combi boiler? We’ve put together this comprehensive troubleshooting guide to help establish what’s causing your lack of hot water, whether you can fix it yourself or whether you need to get in a qualified gas and heating engineer. So, read on!

What is a Combi Boiler?

combi boiler

First, though, let’s remind ourselves of what a combi boiler is, how it differs from more traditional boiler types, and why this can lead to more issues with hot water. A combi boiler (short for combination boiler) is a smaller boiler that doesn’t require a hot water storage tank unlike conventional boilers. Rather, they get hot water immediately from the mains. Combi boilers have become so popular thanks, amongst other things, to their cost-effectiveness.


Interestingly, combi boilers account for over half of domestic installations per year, so in many ways they’re now the ‘conventional’ type, however for the sake of ease, when we say conventional we’re referring to older boiler types. Because combi boilers provide hot water on demand, rather than building up stores in a storage tank, it’s drastically more noticeable when something goes wrong, as you don’t get any hot water whatsoever.

Why is My Combi Boiler Not Giving Me Hot Water?

There are several reasons why your combi boiler might not be giving you hot water.

Pilot Light Issues

pilot light ssues

The first thing to check is whether your combi boiler is firing or not. If your boiler’s pilot light isn’t lit – on older boiler models you’ll be able to see through the frame whether the flame is lit, whilst newer models will provide you with an error code if it’s not lighting – then your boiler won’t be producing hot water. To rectify this, check your boiler’s manual to see if you can get the pilot light going again. If it’s still not lighting (and therefore your boiler still isn’t firing) after this, then get in touch with an engineer for a repair.

Boiler Pressure Problems

boiler pressure problems

If your boiler’s pilot light is lit, but your boiler still isn’t firing up, then this most likely indicates an issue with your boiler’s pressure; if the boiler pressure is either too low or too high, then it won’t produce any hot water. Low pressure is most commonly fixed by consulting the boiler’s instruction manual, which will often have a guide on how to increase your boiler pressure. 


Another option is to use the flexible filling loop to repressurise your boiler. Find the filling loop on your boiler and ensure that it’s properly attached at both ends. Then, open both valves and look at the boiler pressure gauge; you should see the pressure start to increase. Once the pressure gauge has reached a normal pressure, close the valves on the filling loop.


If your boiler pressure is too high, then bleeding your radiators can often help bring the pressure down. Failing this, again, it’s worth consulting a heating engineer.

Other reasons for your combi boiler not providing hot water also include:

  • Failure in the diverter valve
  • Faulty thermostat
  • An airlock within the combi boiler.

Water Runs Hot Then Cold (Combi Boiler)

water runs hot then cold

If you’ve noticed your combi boiler providing hot water which then goes cold (before becoming hot again) or just remains tepid/lukewarm when you’re trying to get hot water, then this could be a sign that the hot water heat exchanger within the combi boiler is suffering from a partial blockage. Fortunately, this can be relatively easily addressed.

You can either remove the heat exchanger and give it a thorough clean and descale using a descaler from a brand like Fernox, or purchase a new heat exchanger entirely. If you don’t feel comfortable with this kind of work yourself, then you can have a boiler engineer carry out the cleaning/replacement for you.

How Do You Check a Diverter Valve on a Combi Boiler?

checking a diverter valve

The diverter valve is an integral component of any combi boiler. It’s responsible for opening and closing in order to direct hot water to taps, showers and radiators. Signs that your diverter valve is to blame for your hot water problems include when you have to turn on your heating in order to get hot water (likewise if you have hot water but your heating isn’t working), as well as if your hot water is only ever lukewarm.


To check/test your diverter valve without fiddling with the valve within the boiler itself, there’s a simple three-step process you can undertake; firstly, turn on your hot water tap. If no hot water comes out, then the second step is to turn on your heating, before waiting for a few minutes. Thirdly and finally, once you’ve had the heating on for several minutes, re-turn on your hot water tap. If hot water then comes out of the tap, this indicates your diverter valve is at fault.

What Does a Diverter Look Like?

what does a diverter look like

The diverter valve consists of two main components: the cartridge (which, in most cases, is three-way) and the motorised head, which sits atop it (this example here shows both the cartridge and the motor). Diverter valve replacements can cost anywhere between £30 and £200 depending on the brand, build quality and size (some cartridges have more ports than others, for instance).

How Do You Replace a Diverter Valve On a Combi Boiler?

replace a diverter valve on a combi boiler

The simplest (and safest) way of replacing a diverter valve on your combi boiler is to call in an engineer. Diverter valves are housed pretty deep within the boiler unit, and require a significant level of expertise to replace (or repair) properly.


A Gas Safe engineer will be able to replace your faulty diverter valve with the minimum of fuss, and get your hot water back and running in no time whatsoever. Whilst your engineer is replacing your diverter valve, you can also ask that they give a thorough inspection/clean of your boiler’s other key components whilst they’re there.

How Do You Know If You Have an Airlock?

Another common reason your combi boiler won’t provide any hot water is due to an airlock either in your pipes, or within the boiler itself. Airlocks are where liquid gets blocked or stopped as a result of a pocket of vapour (in most cases air, hence the name) occurring in a high point of the pipe – this is because gas has a lower density than liquid, and therefore rises to the high point of the pipe, creating the impassable pocket. Indications you have an airlock are similar to that of a faulty diverter valve, and include:


  • No hot water from taps
  • Cold radiators
  • But it can also include banging/tapping sounds. If you’ve got very noisy pipes or a boiler that’s noisier than usual, an airlock might well be the culprit.

Will an Airlock Clear Itself?

Occasionally it can do, although we’d not recommend leaving it to find out. Instead, what we’d suggest is either getting in touch with a certified boiler engineer, or attempting the steps in the next section.

How Do You Clear an Airlock in a Hot Water Tap?

The easiest way to clean your airlock affecting your hot water supply is to call an engineer. They’ll be able to identify where the airlock is – that’s to say, whether it’s in the pipes around your home, or in the boiler itself – and flush it out both safely and efficiently.

Final Thoughts…

Getting no hot water from your taps really can be a nightmare. One filled with icy showers and a cold home. Fortunately, though, with the help of a qualified boiler engineer, it’s one that you’ll soon wake up from, and that nightmare will give way to peaceful dreams once more. Hopefully, after having read this, you’ll have a clearer idea as to why you’re not getting any hot water through your taps.

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