A combi boiler combines central heating water heating with on-demand faucet hot water. This eliminates the need for a separate water storage tank and means you can have as much hot water as you need, on demand, 24 hours a day.

When water is requested, either from the central heating thermostat or by turning on a tap or shower, the boiler ignites and begins almost immediately transferring heat into water from the mains and sending it through the system. This is often far more energy efficient than other types of boiler as you’re not heating up and storing large amounts of water in a tank when you might not need it; you only heat and therefore pay for the energy needed to heat the water you use.

If you want to know the exact differences between a combi boiler, heat only boiler or system boiler, read our quick overview here.

How Does A Combi Boiler Work?

Firstly, gas enters the boiler and flows to the burners inside. These burners are activated by messages from the valves in both the central heating and hot water systems that are sent due to the movement of water created by a tap or shower being turned on. Inside the boiler, the requested water from the tap or shower flows over a paddle switch, turning the burners on in the specific part of the boiler that heats water for the taps.

Any surplus heat generated is stored in a heat exchanger, a compartment inside the boiler. When the central heating is in full flow, the surplus heat in the heat exchanger is used to heat the hot water for the taps or heating system.

Both the central heating pipes and the hot water pipes pass through the heat exchanger, allowing them to heat each other if needed. This means both systems can be on and heating at the same time, so you can still have a shower in winter with your radiators running.

If the central heating is activated by the thermostat but no hot water is being requested, the combi boiler’s burners will heat the central heating pipe directly - this allows the hot water for the central heating system to be heated efficiently. If the hot water is needed but there is no central heating required, this will happen in the same way, in reverse (the hot water pipe will be heated but not the central heating pipe).

What Is A Combi Boiler

Plumber and YouTuber Allen Hart shows how to fit our Ideal Logic Max combi boiler and shares his views on one of our most loved products: Watch the Full Ideal Logic Max Review and Installation here.

Is A Combi Boiler Better Than A System Or Heat Only?

There’s no easy answer to that question. A combi boiler has a lot to recommend it, thanks to its space-saving ability and energy efficiency benefits. However, if you have a large home and are likely to have a high demand for hot water at certain times of the day (for example two showers running at once) it might not be the best option. 

If the heating and water system you inherit with a house is a heat only/tank system and you want to switch to a combi boiler it can be an expensive and intrusive job. We recommend carefully weighing up the cost and efficiency benefits it might generate before making a decision.

What are the Advantages of a Combi Boiler? 

The main advantage of combi boilers is they only heat the water you need. Unless you have a reasonably large family or use a lot of hot water, it’s inefficient to have water boiled all the time just in case you need it. That’s why combi boilers should cost less to run than other models, as well as being more environmentally friendly.

Another benefit of combi boilers is they don’t take up much space. Many homes have them in the kitchen, utility room or under the stairs, and there’s no need for a separate cold or hot water tank. That’s a huge plus if you live in a flat or don’t have a loft.

Finally, you’re installing a boiler from scratch, the combi boiler will probably work out cheaper than other models as there’s less equipment and pipework to fit.

Our guide has much more information on the potential advantages and disadvantages of a combi boiler.

Can Combi Boilers Run Two Showers?

Whilst you physically could and you would get some amount of warm water, a standard combi boiler does not deal well with two showers at the same time because the water demand is significant. As when running any two faucets at the same time, you’ll also experience a halved water flow so it might not be the most enjoyable experience.

If you are looking to run two showers at the same time regularly, a system boiler with an unvented cylinder would be recommended instead. Take a look at our guide for more information on whether combi boilers are good for showers

How Do I Know If I Already Have A Combi Boiler?

This is a question we get asked a lot and it’s easy to overlook if you’re not experienced with boilers. If you have a combi boiler, it’s unlikely there is a hot water tank somewhere in the house. 

A good test to see if you have a combi boiler is to run the hot water tap nearest the boiler. If the boiler doesn’t make a sound and you get hot water coming through, most likely there is a tank somewhere in the property. If the boiler immediately springs to life and you hear a reassuring ‘click’ of ignition and then hot water comes through, this is most likely a combi boiler.

Where Is The Best Place To Put A Combi Boiler?

The best place to put any boiler is closest to where your gas pipes enter the house and on an external wall where the boiler can vent and take in air; placing a combi boiler is no exception. This is primarily due to safety as the shorter the exhaust on the boiler, the less risk of any leaks - whilst those are very rare these days, it’s because great care is taken to ensure risks are minimised. The other advantage is cost saving, reducing the need to extend gas pipework and costly exhaust ducting to and from your combi boiler.

Your licensed boiler installer will be able to tell you where the best place is to fit your combi boiler. If you’re looking to have a boiler fitted, get in touch or use the boiler finder to discover which boiler might be right for you.

Can A Combi Boiler Explode?

There are many things which can explode but are exceptionally unlikely to. In the case of modern combi boilers, they can withstand pressures of up to around 30 PSI with standard operating pressure sitting at around 12 PSI. Before ‘exploding’ many other components would likely fail, preventing the boiler from heating or building the pressure any further. So whilst the answer is yes, a combi boiler could explode, this is no more likely than with any other type of boiler and these days is exceptionally rare and on par with being hit by a bus or struck by lightning.

However, to ensure your combi boiler doesn’t explode, malfunction or suddenly stop working, you should get it serviced by a qualified, licensed boiler engineer each year. This keeps you and your family warm, safe and secure.

What Size Combi Boiler Do I Need?

When it comes to finding your ideal boiler size, which is measured in kilowatts (kW) rather than feet or inches, there are several things to consider. The amount of bedrooms, radiators, and baths and showers in your home will all factor into helping you choose your combi boiler.

For a smaller property like a one-bedroom flat with 10 radiators, a 26kW combi boiler like the compact Logic+ Combi would provide plenty of power for your hot water and central heating. In a larger home where there would be more demand between family members for hot water, the stylish Vogue Max Combi would be a better fit — it’s available in sizes up to 40kW.

If you are looking for a combi boiler for a specific house size or number of rooms, please read our guide on combi boiler sizing and definitely check out our boiler finder which asks questions such as how many rooms, bathrooms etc. you have before recommending which boiler might be right for you.