System Boiler vs Combi Boiler: Which Is Right for Your Home?

A new boiler is a decision you will live with for 10 to 15 years, so it is worth getting right. Most homes are choosing between two types: a combi boiler, which heats water on demand with no cylinder, and a system boiler, which heats a stored tank of hot water.

The short answer: Choose a combi if your busiest hot water moment is one shower or tap at a time, your mains water supply is strong, and you want to save space. Choose a system boiler if two or more outlets often run together, you have or plan more than one bathroom, or you want a cylinder for solar or a future heat pump. Bathroom count is only a rough guide. What counts is how much hot water you need at once.

So, in this blog, we’re going to break down the difference between these two types of boilers and provide the  pros and cons of each to help you come to the right choice for your home.

What is a System Boiler?

System boilers take cold water from the mains supply, where it’s then heated by the heat exchanger (which transfers the energy from the gas to the water). The hot water is then moved to a storage cylinder where it’s kept hot until you need it. The heating system is within the boiler system, and that’s where this type of boiler gets its name from. And when you need the hot water, it’s then pulled from the tank.

NOTE: A system boiler does need a cylinder to store the water which has been heated, but it doesn’t need a cold-water tank like conventional boilers.

See our guide to what a combi boiler is and what it does.

What is a Combi Boiler?

A combi boiler is a highly efficient single unit that provides hot water for the water supply and the heating. This type of boiler takes water from the mains and heats it on demand whenever the tap or heating is turned on - meaning there’s no need for a storage cylinder.

The combination boiler gets its name because there’s the combination of two tasks, heating and hot water, all undertaken within the one boiler. And because there’s no need to pre-heat the water and then store it, the combi boiler is very cost effective. For other pros to this type of boiler, see below!

Pros and Cons of a System Boiler

So, let’s have a look at some of the pros of having a system boiler in your home:

• If you need hot water from several outlets at the same time, then a system boiler can cope with that level of demand as the water is already heated and ready to be used by all.
• The system boiler takes up less space than a traditional boiler because it takes water directly from the water supply (meaning it doesn’t need a cold-water tank - winner!).
• If you’re looking for hot water as soon as you turn the tap, then a system boiler can pretty much do that. That’s because a pump is built into the system, allowing the fast delivery of already heated water.
• And lastly, system boilers are compatible with solar panels that can generate power to heat the water in the hot water tank.

Sounds good, right? Now for the cons:

• If you suddenly need a lot of hot water, then you may use all the stored water and then have to wait until it heats up more.
• As the water needs to heat up in advance to when you want to use it, you’ll need to programme a timer for when is best for you. If your plans change, you may find yourself without some hot water.
• If you don’t have much space in your home, then you may struggle to find room for the hot water tank associated with this type of boiler.
• Once the hot water enters the cylinder, the temperature will begin to drop. This means that it’s essential that the cylinder is well insulated to prevent too much loss of heat - which then affects the boiler’s efficiency.
• There's more things that can go wrong with a system boiler so higher maintenance costs are a major con. However, you can reduce these costs by taking out a boiler cover plan. We recommend you do this!
• And finally, a system boiler is unlikely to be suitable for a home where there is a water supply with low pressure, or an older radiator system that may not cope with the higher pressure that comes with a system boiler.

Pros and Cons of a Combi Boiler

Now let’s take a look at the pros of a combi boiler:

• If you need hot water on demand, then the combi boiler is a great fit for your household. While you need to wait for the system boiler to heat and then store the water, the combi heats the water once you turn on the hot water tap.
• They are highly efficient - in fact, there’s a requirement for all new combi installations to have an efficiency rating of at least 92%. This is great news as the energy bills will be much lower with a combi boiler, and you’ll be reducing your carbon footprint in the process too.
• It's easy to install and with no need to fit hot water cylinders, the boiler installation is much quicker with a combi boiler ... saving time and money (WINNER!).
• As the water is coming from the mains, you’ll have high-pressure hot water - though that does depend on the water pressure coming into your home to begin with.
• And finally, with just a single unit to find room for, combI boilers are a great choice for when space is limited.

So now you know all the pros of a combi boiler... here are the not so great things about them:

• A combi boiler will struggle to cope with a high demand for hot water. So, if you have the washing machine on while someone has a shower, then the water may suddenly go cold for one of those outputs... NOT IDEAL AT ALL! If you have more than one bathroom, then a combi will probably not be able to meet your hot water needs so definitely consider that.
• You will have to be a little patient with this type of boiler as once you turn on the hot water tap, the supply needs to be heated and then it’ll be sent to the tap. Now it is only a matter of a few seconds, but it’s not the speed of delivery that you’ll get from a system boiler.
• A combi boiler needs water to come from the mains at a high pressure. If the pressure is low, then the water coming from the taps will also be at a very low pressure which is frustrating.
• If your home has an older plumbing system, it may not be able to cope with the high pressure water flowing from a combi boiler, so it's important you check if your new boiler will require new pipes.

Which is Right for Your Home? A Quick Decision Aid

Step 1: Find Your Home Type

Step 2: Do the Bucket Test (60 Seconds)

A combi needs a strong mains supply. Open your kitchen cold tap fully and time how long it takes to fill a 10-litre bucket. If it fills in about a minute or less (roughly 10 litres a minute or more), a combi is usually workable. If it takes noticeably longer, a combi will likely struggle, and a system boiler is the safer choice. A plumber can measure flow and pressure accurately before you commit.

Step 3: Count Your Busiest Hot Water Moment

A typical mixer shower draws around 8 to 12 litres a minute. A 30 kW combi delivers roughly 12 litres a minute at a normal shower temperature (a 24 kW unit nearer 10, a 35 kW unit nearer 14). In winter, when incoming water is colder, real output is lower. In plain terms, a combi supplies one shower well and a second poorly. If two people often shower at once, or a bath is filled while someone showers, choose a system boiler.

Step 4: Check Space and Plans

Do you have an airing cupboard or similar space for a cylinder? If not, a combi is easier. Do you expect to add a bathroom, install solar, or move to a heat pump? If so, a system boiler with a cylinder keeps those options open.

Your result: if you pass every check and the answer to each is "one at a time", pick a combi. If any single check points the other way (two showers at once, weak mains, an extension planned), pick a system boiler. If you are unsure, we are happy to look at your home and advise before you spend anything.

Read our full comparison of the different types of boilers on our Boiler Types page.

Before you Switch Type

Changing from one type to another is more than swapping the boiler. A system boiler needs room and pipework for a cylinder, and a combi needs good mains and often removes the old tanks and cylinder. Read our checklist of things to consider before installing a new boiler, and once you have chosen a type, work out the output you need in our guide to choosing the right boiler size. If your radiators are old or the system is sludgy, a power flush at the time of installation protects the new boiler.

Repair or Replace Before Choosing a New Type?

If your boiler is under about 10 years old and has a single failed part, a repair is usually the better value. Replacement makes sense when the boiler is 10 to 15 years old, breaks down repeatedly, cannot get parts, or you are changing type anyway.

For repairs, we cover boiler repair in Northampton, Kettering and Wellingborough. If it is time for a new one, our boiler replacement service includes advice on the right type for your home, with local teams for boiler replacement in Northampton, Kettering and Wellingborough.

Not sure which boiler suits your home? We will check your water supply, your hot water use and your space, then give you a clear written quote with no call-out fee. Call 01604 372395 or get in touch here.

Frequently asked questions

Is a system boiler better than a combi boiler?

Neither is better overall. A combi suits small and medium homes with one bathroom and strong mains. A system boiler suits larger homes, two or more bathrooms, and households that use hot water at several outlets at once.

How many bathrooms can a combi boiler supply?

One comfortably. It can supply two if the hot water is rarely used in both at the same time. For three or more bathrooms, a system boiler with a cylinder is usually the better choice.

Can I replace my system boiler with a combi?

Often, yes, if your mains flow and pressure are strong enough and your home has one bathroom or light demand. It removes the cylinder and frees up space, but a plumber should check your supply and pipework first.

Do system boilers need a tank in the loft?

Not a cold water tank, because the expansion vessel is built in. A system boiler does need a hot water cylinder, and if that is a vented cylinder, it is fed from a small tank in the loft.

Which is cheaper to run, a combi or a system boiler?

Efficiency is similar. A combi has no stored water to keep hot, so it is usually cheaper for small households. In a busy home the gap narrows, provided the cylinder is well insulated.

Which is better for low water pressure?

Low mains pressure usually points to a system boiler with a vented cylinder, or a like-for-like regular boiler. A combi needs good mains flow. A plumber can measure your flow and pressure.

For any help or guidance, contact us HERE or give us a call on 01604 372395.