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System Boiler vs Combi Boiler: Pros and Cons

System Boiler vs Combi Boiler

Separating system and combi boilers can be tough when it comes to finding a new boiler. So, in this article we’ll be comparing the pros and cons to help you find the most suitable boiler for your home.

System boilers explained

Most components needed for a system boiler to provide your home with heating and hot water are within the unit itself. Rather than supplying instant hot water, like a combi boiler, system boilers store the hot water in a cylinder or tank, ready to travel to the taps when you need hot water.

System boilers: pros & cons

Let’s take a look at some of the ways your home could benefit from having a system boiler installed.

System Boiler Pros How You’ll Benefit
Simpler installation than conventional boilers As system boilers don’t need as many external parts as conventional boilers, they won’t take as long to install, helping to save you time and money.
Mains water pressure System boilers take their water supply from the mains, meaning that you’ll get stronger water pressure out of your taps.
Quick response to hot water demand With the addition of a pump, system boilers are able to respond quickly to heating and hot water demand.
Don’t need a cold water tank No need for a cold water or feed and expansion tank in the loft means that System boilers take up much less space.
Solar thermal compatibility Should you have solar thermal, or are considering it, then the hot water tank or cylinder can be adapted to heat the water using solar energy.

As well as the many pros, there are a few things to consider before deciding whether a system boiler is the right choice for your home.

System Boiler cons Reason for Consideration
Need more space than a combi System boilers need a tank or cylinder to store hot water unlike combi boilers which provide instant hot water.
Hot water tank insulation

Storing hot water in a tank means that it’s then susceptible to heat loss so sufficient insulation is required to prevent energy wastage.
Size of hot water tank The size of the tank will determine how much hot water is available to your home before it needs refilling.
Hot water isn’t supplied instantly When the hot water tank does run out of hot water, you’ll need to wait for it to fill up before having access to hot water again.


Combi boilers explained

Combi boilers are the most modern boiler type available and take up the least amount of space as there’s no need for any tanks or cylinders.

Everything the combi needs to provide your home with heating and hot water is within the unit itself. By taking its water supply directly from the mains, you’ll have instant access to hot water that’s delivered at a strong pressure.

Combi boilers: pros & cons

From reducing the carbon footprint of your home to saving space around your home, when compared to other boiler types, there are many benefits you’ll be able to enjoy when installing a combi boiler.

Combi Boiler Pros How You’ll Benefit
Highly efficient All new combi installations across the UK have to meet an efficiency rating of at least 92%, which will help to lower the carbon footprint of your home as well as your energy bills.
Compact unit Combi boilers are a single combined unit that can comfortably fit within a kitchen cupboard. Plus, there’s no need for any bulky tanks or cylinders around the home.
Quick and simple installation As there’s no need for the installer to fit any tanks or cylinders, installation will be much quicker when compared to a system or conventional boiler. Helping to save you time and money.
Instant hot water System boilers store hot water in a cylinder, which needs a lot of energy and will sometimes leave you waiting for it to refill before you can get access to hot water. combi boilers, on the other hand, take their water supply directly from the mains, so you’ll have hot water on demand.
Mains water pressure By supplying instant hot water from the mains, you should get a strong flow of water (depending on the strength of the mains water supply in your home).

Before you commit to having a combi boiler installed based on the many benefits we’ve highlighted above, there are a few considerations to take into account, especially if you live in a larger home with 2 or more bathrooms.

Combi Boiler Cons Reason for Consideration
Can’t meet high demand for heating and hot water As combi boilers take their water supply directly from the mains, they struggle to supply water to more than 1 tap or shower at a time. Homes with more than 1 bathroom should consider a system or conventional boiler instead.
Unsuitable for properties with weak mains pressure It’s important to make sure that your home has a strong mains pressure because if it’s weak or inconsistent so will the water out of the taps.
Might not be compatible with old pipework Older heating systems were designed for a weaker supply of water so might not be able to cope with the higher mains pressure.

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Conventional boilers

As well as system and combi boilers, there are also conventional boilers, which also come under many other names too: regular, traditional and heat-only. This is the oldest boiler type available on the market but have really advanced over the years to offer increased performance, reliability and efficiency.

Conventional boilers need more external parts than system and combi boilers as, in addition to a hot water cylinder, they also require a cold water storage tank and feed and expansion tank in the loft.

Cold water fills the cold water storage tank, which then goes down to the hot water cylinder, where it’s then heated. This water can then be used for taps and showers when you need it. The feed and expansion tank is there to maintain the water level as water expands when it’s heated.

Conventional Boiler Pros Conventional Boiler Cons
Can meet high demands for heating and hot water

Need lots of space around the home
Much more efficient now than in the past

Hot water isn’t supplied instantly
The simplest and cheapest option if you’re looking to replace an older conventional heating system

Hot water being stored in a hot water cylinder is susceptible to heat loss
An immersion heater can be fitted to the hot water cylinder as a back-up should the boiler breakdown

Installation can be a long and expensive installation.

System boiler or combi boiler: which is best?

Finding a winner between system and combi boilers ultimately comes down to the heating and hot water demands of your home.

For smaller homes with 1 bathroom, there’s no better choice than a combi boiler. You’ll be supplied with instant access to hot water from a highly efficient unit that should help you to save money on your energy bills.

As beneficial as combi boilers are, they’re unable to supply a strong supply of water to more than 1 tap or shower at a time so, for that reason, if you have 2 bathrooms then you should install a system boiler.


Replacing a system boiler with a combi

When looking to replace a system boiler with a combi, the installer will need to remove the domestic hot water cylinder, convert the piping to suit a combi and then replace the 2 units.

This will take longer than a like-for-like boiler replacement and take up to 2 days to complete.

Replacing a combi boiler with a system

If you’re looking to replace your combi with a system boiler, it’s probably because you need a boiler that can meet the higher demand for heating and hot water.

Replacing the 2 units is a fairly simple process, especially if you’re planning to have the system boiler installed in the same place as the combi. While this would tend to only take a day to complete, system boilers also need a domestic hot water cylinder which will add another day onto the installation.

It’s important to make sure there’s space available for the hot water cylinder, as well as the hot and cold water piping, in a cupboard or loft area.

Get free installation quotes

Getting multiple quotes before hiring an installation to replace your current boiler will give you the greatest chance of finding the best deal. At Boiler Guide, we’ve made this quick and simple to do.

Simply complete our online form, which shouldn’t take too long, letting us know about the work you need, and we’ll match you with up to 3 Gas Safe registered engineers based in your local area.

Each heating engineer will provide a free no-obligation quote for the work which you can then compare.

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