Combi Boiler Pros and Cons – What You Need To Know

If you’re reading this page, then chances are you’re thinking about getting a combi boiler. This page covers almost all combi boiler pros and cons. Hopefully, it will help you weigh up all the advantages, and disadvantages of a combi boiler and help make the right decision for you.

What is a combi boiler?

Combination boilers, commonly known as combi boilers are a single unit that provides both heating and hot water. Suitable for small to medium-sized homes, they are the most popular type of boiler in the UK.

Vaillant combi boiler pros and cons



Without the need for a hot water tank, combi boilers take up less space than their traditional counterparts. Making them ideal for homes short on space. Also, space gained from removing old water tanks can be used for additional storage.  Or in some cases, extending bedrooms or bathrooms.

Highly efficient

Combi boilers have to be at least 90% efficient due to legislation. Which is considerably more than traditional boilers. But, that is only taking into account the boiler. Not the cylinder. With traditional boilers, a lot of energy going into heating the hot water tank. Because of legionella disease, the hot water tank has to be kept at 60°c at all times. Meaning, that the boiler is constantly having to maintain that temperature. Poorly insulated, older cylinders lose a lot of residual heat. This can be great for keeping towels warm but, it’s not very efficient!

The more temperature the cylinder loses the more the boiler has to work to keep it warm. Because combi boilers heat hot water instantly, they eliminate the need for a cylinder and save all that extra energy!

Comprehensive warranties

A huge pro that a lot of people forget. Maintaining regular boilers can be very expensive.

Pumps and zone valves will need changing every few years. Any issues with the cylinder may result in a replacement costing well over £600. Unlike traditional boilers, combi boilers have no external moving parts. All parts are inside the boiler. And covered under the manufacturer’s warranty as a result.

Just the warranty can save hundreds of pounds over the lifetime of a boiler. That’s without considering the money saved from smaller gas bills.

Better water quality 

Because cold water is fed directly into the boiler it is less likely to be contaminated. With gravity-fed systems, water is stored in the hot water cylinder and in the cold feed tank. If the lid isn’t secure, rodents can contaminate the water. There are even horror stories of rats falling in cold feed tanks and drowning! Also, problems developing with the cylinder can result in stagnant central heating water contaminating the entire tank.

Selling point

A combi boiler makes a great selling point when the time comes to move. Although a small factor, a combi boiler can make a positive impression on potential buyers. Most people don’t want to be forking out a few thousand points on a new boiler after moving into a new home. Furthermore, a newer, more efficient boiler will upgrade your EPC rating.

Increased hot water pressure 

Due to being mains fed, combi boilers can drastically increase hot water pressure. Provided the incoming water pressure is good enough. Even the smallest combi boilers produce around 9 litres of hot water per minute. To put that into perspective, a gravity-fed hot water system may provide around 4-6 litres a minute. With a bigger combi, it is possible to get a much higher flow rate. As long as the water pressure is there to feed it. Some combi boilers even provide up to 16 litres a minute!

On-demand hot water

Combi boilers instantly heat up the water passing through. Meaning instant hot water, all day, every day. No waiting for the cylinder to warm up after the kids have used all the hot water!

No airlocks

Older gravity-fed systems can develop airlocks that can stop radiators or even entire systems from working. It’s much easier to rid the system of airlocks, due to combi boilers being pressurised,

Availability of parts

Due to combi boilers being a relatively new innovation, most parts are readily available. Whereas with older regular boilers, parts can become difficult to find. And, in some cases, obsolete.

We hope you’re finding ‘combi boiler pros and cons’ helpful, read on to learn about all the disadvantages of a combi boiler.


Not suitable for large households

Due to combi boilers instant hot water capabilities, they struggle to cope with a large demand. Meaning, they’re not suitable for households with more than 1-2 bathrooms, or households with large families. Even larger combi boilers will struggle to supply enough hot water to run two showers at once.

Running cold water whilst a shower or bath is running may result in the hot water reducing. Showers may run down to a trickle, or even go cold. For a house with teenagers, this may result in a lot of problems when everyone is getting ready for school or work.

More to go wrong

As combi boilers contain more internal parts there is a lot more that can go wrong. Also, due to the additional parts and electronics, it can make life more difficult for engineers to diagnose problems with them. Furthermore, parts can be very expensive to buy, and time-consuming to replace.

This isn’t a problem when the boiler is under warranty but once the warranty ends they can be unwanted expenses. A lot of the time resulting in an ‘uneconomical repair’ and a new boiler. Hence why we always recommend getting a good boiler with a long warranty.

Worcester greenstar inside view

Generally, problems with traditional boilers are easier to diagnose and fix.

No immersion heater

When fitted with an immersion heater, the cylinder has a backup heat source. Due to combi boilers not having a cylinder, a breakdown can leave a household without heating and hot water.

Won’t work with pumps or power showers

Incompatible with combi boilers, shower pumps and power showers are designed to be used with stored hot water systems. Installing a combi boiler will mean they have to be removed. If you’re used to enjoying a high-pressure shower fed by a shower pump or power shower, this may be bad news.

When supplied with good water pressure, this isn’t a problem. Combi boilers rely on good water pressure coming in. But, if a property has poor water pressure, this will result in a poor shower.

Shower may need replacing

Some older showers are designed to only work with low-pressure gravity-fed systems. Converting to a combi system may result in the shower needing to be replaced. Not an expense anyone wants after spending a few thousand pounds on a new boiler!


Hopefully, this guide has helped educate you on all combi boiler pros and cons. If you’re interested in buying a new boiler but want more information a few relevant posts are listed below.

How Does A Combi Boiler Work?

Boiler Installation – The Complete Guide

Always use a Gas Safe Engineer for any gas related work in your home.

Dont cut corners with gas

If there is anything you think we have missed, or you have any feedback on this post please don’t hesitate to get in touch by email –

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