If you’re thinking about replacing your heat-only or system boiler with a combi, or you’re moving into a house with one, you may be wondering what the pros and cons are of combi boilers are.
Combi boilers are currently the most popular domestic boiler in the UK, but why are they so popular? And are they right for everybody?
In this article, we look at what a combi boiler is, and discuss the pros and cons of this type of system, to help you decide whether a combi boiler is right for you or not.
What is a combi boiler?
Before we get into the pros and cons, it’s important to know what a combi boiler is, and how it differs from the other two types of boilers (heat-only or system boilers).
The clue is in its name…
A ‘combi’ boiler provides a ‘combination’ of hot water and central heating from one compact unit, without the need for external tanks or cylinders.
This is different to a heat-only boiler that also needs a cold water storage tank; a hot water cylinder; and a tank to maintain the water level of the central heating system.
You can read the pros and cons of heat-only boilers here.
It’s also different from a system boiler that requires a cylinder for storing hot water.
You can read the pros and cons of system boilers here.
A combi boiler is the only one that doesn’t store hot water in a cylinder, because it heats water directly from the mains.
With a combi, as soon as you turn on a hot tap, it heats the water instantly on demand. This means that there’s no need for a storage tank.
How do combi boilers work?
Since combi boilers do two jobs (provide instantaneous hot water and central heating) from one unit, they have two independent heat exchangers.
The primary heat exchanger connects to your radiators, and the secondary heat exchanger connects to your hot water supply.
The primary heat exchanger heats the water that is pumped around the radiators in your central heating system. Once this hot water has warmed up the radiators, it recycles back into the boiler.
The recycled central heating water that has travelled through all of the pipes in your heating system is dirty and you obviously wouldn’t want to bathe in it!
So, when you want hot water for a bath, a combi boiler will pause your central heating, and open a diverter valve.
The diverter valve allows the recycled central heating water to enter the secondary plate heat exchanger.
The recycled central heating water is used to heat the sealed pipes which contain your clean mains water. Your cold mains water is heated instantaneously and is delivered to your taps, so you can have a clean hot bath.
You can see from this process that a combi can’t transport hot water to your radiators and to your taps at the same time. Your hot water usually takes priority.
Pros of combi boilers
Now that we understand what a combi boiler is, let’s take a look at the advantages of choosing this type of system:
Pro: Combi boilers are space saving
The first obvious advantage to choosing a combi boiler, is that it’s simple design won’t take up much space in your home.
Instead of needing additional cold water tanks and a hot water cylinder, a combi boiler is its own self-contained unit that will sometimes be small enough to fit inside a kitchen cupboard.
This means that you can free up cupboard and loft space that would otherwise be needed to house tanks or cylinders for a heat-only or system boiler.
Combi boilers are therefore popular in smaller homes where space is at a premium.
Pro: Combi boilers are energy efficient
Modern combi boilers are very efficient thanks to their condensing technology and the changes made by the Boiler Plus Regulations.
All combi boilers must have a minimum ErP (Energy related Products) rating of 92% and must have one of four additional energy saving devices fitted. These include: flue gas heat recovery; weather compensation; load compensation; or smart controls with automation and optimisation functions.
This means that your combi is very efficient and could help you to make savings on your energy bills in addition to helping with our ambitious national target of meeting net zero by 2050.
Combi boilers are economical when compared to heat-only or system boilers. Because a combi supplies hot water on demand, you only ever pay for what you use.
With a heat-only boiler (which stores hot water), hot water tends to be produced twice a day, even if it’s not needed. If it doesn’t get used, this hot water cools down, wasting energy.
To find out why having an efficient boiler is so important, you might like to read our related blogs: ‘What is an energy efficient boiler and why should I want one?’ and how to choose an energy efficient boiler.
Pro: combi boilers provide unlimited hot water
If you’ve ever had a heat-only boiler, there may have been times when you’ve run out of hot water…
You’ve gone to run a bath, but the water’s cold – and you’ve ended up squabbling with family members over who used the last of the hot water!
This situation will occasionally happen with a heat-only or system boiler because they store hot water in a cylinder for use later.
A cylinder can only hold so much hot water, and once it’s been used up, you’ll need to wait a while for the cylinder to refill.
A combi boiler doesn’t have this problem. With no water tank to heat up, combi boilers can provide an unlimited supply of hot water on demand.
A combi heats water as and when you need it, so you’ll never run out of hot water again!
Cons of combi boilers
However not all homes and families are suited to a combi boiler.
Con: Combi boilers struggle to meet high demands for hot water
Since combi boilers work by heating water for a tap instantly on demand, they’ll struggle when multiple taps are turned on simultaneously.
Most combi boilers provide between 9 – 16 litres per minute. Whatever the capacity, it will be halved if two hot water outlets are in use at the same time, i.e. to around 8 litres per minute at best.
Therefore, combi boilers are best suited to homes with relatively small hot water demands at any one time.
So, when you’re thinking about whether a combi is right for you, you should consider how many bathrooms you have, and how you use your hot water.
If everyone takes it in turns to shower in the morning, then a combi would work. But if you’re a household where everyone wants showers at the same time, then a combi won’t be suitable.
In larger homes with multiple bathrooms, a heat-only or a system boiler would be a better choice.
Heat-only and system boilers can meet high demands for hot water because they store large quantities of hot water in a cylinder, which can supply multiple outlets at the same time.
Con: Relies on good mains pressure
Many heat-only boilers use gravity from a tank located in the loft to work effectively. But a combi boiler works differently and takes its water directly from the mains.
Combi boilers need good mains pressure to work effectively and so they’re not ideal for homes in areas with low pressure.
Con: If a combi breaks down there is no heating and hot water
Since a combi boiler provides both your heating and hot water, in the event of a break-down you’ll have neither!
Immersion heaters are incompatible with combi boilers because there’s no hot water tank. This means that if your boiler were to break down, there is no backup system, and you’ll have no heating or hot water until your boiler is repaired.
Our favourite combi boilers
If you’ve decided that a combi might be right for you, you might be wondering which make and model to go with.
The Heating People are not tied to any manufacturer of boilers, we can work with all of them. But we do have our favourites!
We believe that Vaillant; ATAG; Worcester Bosch; and Viessmann offer the best boilers on the market.
All these brands offer outstanding engineering, efficiency, reliability and guarantees. So, whichever you choose, you’ll have a great boiler.
But in no particular order, here are our top 5 combi boilers from these brands:
- ATAG iC Economiser Plus
- Vaillant ecoTEC exclusive with green iQ
- Vaillant ecoTEC Plus
- Worcester Bosch Greenstar 4000
- Worcester Bosch Greenstar 8000
Check out our related blog: The best combi boilers 2022.
For tailored advice about these or any other boilers, contact one of our friendly team to book a free survey.
Conclusion?
Whilst there are many advantages to having a combi boiler, they’re not right for everyone.
Combis are best suited to homes with relatively small hot water demands at any one time. For these households, combis are an excellent, efficient choice.
You might like our related blog: Pros and cons of system boilers.
