Is Your Shower Going Hot and Cold? Your Combi Boiler May Be Failing!

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Written By: Experts | Last Updated: April 2023


A combi boiler is a highly efficient gas water heater that supplies your bathroom, shower enclosure and kitchen with hot water on demand. It also circulates warm water to the radiators. So with one of these boilers, you don’t need an expensive hot water storage cylinder in the attic.

Unfortunately, if your combi boiler stops working properly you could start to experience your combi boiler shower going hot and cold. In all likelihood, an aging combi boiler will be the problem.

If you suspect that’s the case, you should ask a Gas Safe engineer to investigate. We recommend you do so because gas, electricity, and heat need to be managed carefully. And, of course, your insurance may not cover you if you troubleshoot the issue yourself and something goes wrong. 


Why Are Gas Combi Boilers Popular in the UK?

Gas combi boilers are heat-efficient because they supply hot water on demand, as opposed to storing it in a cylinder for later use.

Another advantage is you get council pressure directly to your shower and so you don’t need to pay for a separate booster pump. Yet another advantage is you save space, which means more room for the items you want instead of necessary utilities.


Combi Boiler Hot Water Goes Cold

If your shower is going hot and cold, then cold again, it’s natural to blame the combi boiler for the problem. However, if the flame is steady while someone else is showering and reports the water going hot and cold you may have a different issue.

To explain this in detail we need to delve a little into combi boilers and how they work. We’ll keep it simple as we can but there are some things you need to know to understand what’s happening. 

  • Gas-fired combi boilers draw water directly from the mains on-demand, and heat it according to their settings
  • They are on standby between events, making them thermally efficient and cost-effective
  • This means that when we draw hot water from them it is ‘fresh-baked’ and at mains pressure
  • Hence, we only need to heat it once and we don’t need a ‘power shower’ pump to send ‘stale’ hot water from a cylinder in the attic

To achieve this instant heat, the cold water from the Council main passes through a ‘heat exchanger’. This technology transfers up to 90% of the energy from the burning gas to the water molecules. This agitates them and heats them to the temperature we expect.

If that does not happen satisfactorily then we may find the combi boiler shower going hot and cold while the combi boiler appears to function normally. Before you log a call-out there’s more you need to know.

The Combi Boiler is Doing More than Providing a Hot Shower

We were so busy thinking about the combi boiler shower we forgot about the other people in the house! When we are showering someone else may be washing up, while a third person watches TV by the warming radiator. That is quite a lot to expect from a combi boiler without backup hot water in a cylinder in the attic. 

If a combi boiler can’t manage all the hot water demands simultaneously, then it gives hot running water priority. 

Therefore, it may turn the radiators off temporarily, while someone draws water in the kitchen, runs a bath, or takes a shower. However this is different from the problem of a combi boiler shower going hot and cold while a combi boiler appears to be functioning normally according to what you can see.


Why Does My Shower Go Hot and Cold? 

1. Multiple Outlets

You may have noticed the pressure in the garden hosepipe is not as good when someone is filling the bath, or doing the weekly laundry perhaps.

Pretty much the same happens when a combi boiler has to deliver hot water to several outlets at the same time. Remember, it is simply processing (heating) water that comes from the Council main at a given pressure. There are two things you could try if you can’t put up with this. 

  • Fit a mains booster (£300 – £500) that draws water from the mains – as opposed to merely receiving it – when it senses pressure and flow are low.
  • Install an accumulator tank (£500 – £1000) at a suitable point. This stores compressed cold water that rushes out faster than the Council mains could ever do when a tap opens somewhere in the house.

The combi boiler can then deliver faster, but only for as long as the water store lasts. After that, you may have a combi boiler shower going hot and cold because the combi boiler can only pass on what it receives.

2. Your Combi Boiler is On Its Way Out

A combi boiler only does one thing, and that’s heat water with gas. If it breaks down, or the gas supply fails, you have instant cold water.

If you are realising your combi boiler is too small for what you want, give a thought to a regular boiler system before you decide. 

That’s because regular gas boilers have electric immersion heaters in the storage cylinder, meaning a hot / cold shower should be a thing of the past.

As a result, you have a temporary solution to your needs, although this comes at a higher energy cost so get yourself back on main gas as soon as you can!


Shower Going Hot and Cold? Get Advice Today   

A combi boiler should be perfectly adequate in a flat or small house, but beyond that it may struggle in the ways that we described above. 

A registered Gas Safe engineer can provide impartial advice whether you need a conventional boiler with a storage tank instead. If you need assistance with faulty combi boiler, contact us team today!

For more information about your combi boiler, take a look at the following guides: