Eco Happy (Solar Expert)

Published February 28, 2024

Picking the right boiler for your home has massive implications for your overall quality of life. Boilers not only give us hot water on demand, but they also provide our indoor heating. Choosing the perfect boiler for your needs can be an intimidating prospect. 

To help you navigate the system boiler vs combi boiler debate, Eco Happy has created an in-depth guide. We will first look at which boiler is better for you. Then we will list the pros and cons of system and combi boilers, consider the role they play in your heating and what costs are associated with them. Finally, we’ll look at which unit to pick based on the size of your home.

System Boiler Vs Combi Boiler: Which Is Better?

The fundamental difference between a system boiler and a combi boiler is their ability to deliver hot water to multiple outlets in your home.

The system boiler makes use of a large hot water storage tank which allows it to meet a higher hot water demand since there is always heated water ready to flow.

Combi boilers, on the other hand, have no storage tank and heat water directly from the mains water supply. They only produce hot water when needed.

As a general rule of thumb when you’re picking between a system boiler or a combi boiler, check what you currently have in your home and stick with that. However, keep in mind that your requirements for your boiler may have changed over time. Perhaps you are planning to expand your home, or you want to grow your family. When life happens, you need to adapt your heating needs accordingly.

Let’s look at the pros and cons of each boiler unit individually.

System boilers


  • System boilers provide a hot water supply to multiple faucets.
  • Once the tank is full and heated, system boilers provide instant hot water.
  • System boilers are perfect for homes with weak mains pressure as the storage cylinder provides additional water pressure.
  • System boilers are fully compatible with solar panels, allowing you to reduce your energy bills.


  • The system boiler’s design means it will need to take up more space in your home for the hot water tank and the associated pipework.
  • Maintenance for system boilers is a bit more involved due to more points of failure present.
  • While the hot water you need is stored until ready for use, once the storage cylinder has been drained, you will need to wait for the system to refill and warm before you will have access to more hot water.

Additional risk factors of system boilers

Due to the more complex nature of the design of system boilers, it’s important to note some important key elements of a system boiler installation.

  • To keep your hot water storage cylinder functioning optimally, you will need to make sure that your boiler is sufficiently insulated to keep the water cylinder warm and to cut down on energy costs.
  • The risk of a burst or leaking boiler means that the location of your hot water storage is important.

Combi boilers

Combi boilers are the most common type of boiler found in homes across the UK and they’re much more efficient than conventional boilers. They are a simple fuss-free solution to your hot water needs. Combi boilers do not use a cold water tank like traditional boilers and the unit only heats water as it is needed.

Combi boilers have their advantages and disadvantages, so let’s take a closer look. 


  • Combi boilers require much less space than other boiler types.
  • There is much less maintenance required as there are no additional storage tanks.
  • You have instant hot water as they heat water directly from your mains water line.
  • The warranty that comes with your combi boiler will cover your entire water system, saving you potential additional expenses should something go wrong with your boiler.
  • Easy to install.
  • Lower running costs save you a lot of money on your energy bill.


  • There is a few seconds delay in providing water after you open the tap as the unit needs to heat the water first.
  • The unit will struggle to supply sufficient hot water to multiple taps at the same time.
  • Not suitable for homes with low mains water pressure.

The Best Boiler for Underfloor Heating – System Or Combi Boiler

Both system and combi boilers work great for underfloor heating. That said, system boilers have a slight advantage as they are more powerful than combi boilers.

Depending on the boiler type and the setup in your home, additional adjustments may be required for them to work optimally.

For example, most combi boilers would require the installation of a two-port valve. This will allow the boiler to supply underfloor and central heating. The valve controls the flow of water from the boiler to the other areas of your home that need access to hot water.

The Best Value Option

The best value boiler comes down to your home heating needs. Combi boilers offer overall great value when it comes to running costs and installation costs. However, if you have a larger household with a high hot water demand, you may find that the initial savings on a combi boiler aren’t worth it. 

Constantly having to heat water directly from the mains could mean higher heating bills for your household over time. This is where the system boiler outshines the combination boiler.

Let’s compare how system boilers and combi boilers measure up against each other in terms of their value. We will look at the initial installation costs first and then compare the two boiler types’ running costs to see their impact on energy bills and how they can help you save money.

System boilers

System boilers can meet the heating needs of larger homes. This inevitably means there will be more initial expenses tied to getting the boiler installed in your home. However, the boiler shows its value in its long-term usage. 

Installation costs

A system boiler exists in two parts: the boiler itself and a hot water storage tank. The hot water storage unit will add to the fees and the time required to complete the installation of a new or replacement boiler.

System boiler installation costs can range between £1,500 and £4,500.

Running costs

System boilers are a cost-effective solution for a household with high hot water consumption. However, since hot water is stored at length, it will eventually start cooling down. This means that unused water will need to be reheated. If the extra stored water is not regularly used, it will add to your overall energy bill unnecessarily.

As there are more points of failure present on a system boiler, there are potentially higher maintenance costs for the device in the long run as well.

Combi boilers

Combi boilers bring a lot to the table concerning value. These compact units shine in single-bathroom households and they’re simple to maintain and cheap to run. However, if you push these units to their maximum with excessive water demand, the value they bring can quickly plummet.

Let’s take a look at what costs are attached to using a combi boiler in your home.

Installation costs

Combi boilers are a single-unit system that is quick and relatively simple to install. The average cost of a new unit as well as the installation can run between £1,500 and £3,000.

Running costs

Since combi boilers only heat water when needed, they’re much cheaper to run than system boilers. Depending on the make and model, you can save up to £200 annually on your energy bills as compared to a system boiler.

Additionally, if the boiler breaks down, there is no additional hot water cylinder that could cost you extra for repairs.

Which Boiler Is Best For Your Needs?

The size of your home and the number of bathrooms both play an essential role in determining which unit you need.

A system boiler is a must-have for larger homes with more than two bathrooms. The hot water storage tank adds much-needed capacity for daily usage. It also adds additional water pressure to the plumbing of larger homes.

Combi boilers, on the other hand, are the ideal heating solutions for small to average-sized homes. A combi boiler saves space, is low-cost, has high energy efficiency and can still potentially service multiple bathrooms.

Modern combi boilers come in higher-capacity models with up to 54 kWh of output, which can deliver more than 20 litres of flow per minute. Unfortunately, the running costs will increase as well.

With this in mind, let’s talk about what type of boiler would best suit your particular property.

Pick the right boiler for the size of your home

As noted at the start of this guide, you can use the current boiler setup of your home as a gauge for which unit will be the best for you. Below you will find a concise list of what boiler to use in what size home to best meet the water and heating needs.

System boiler

  • Average-sized home with more than 3 bathrooms
  • Large-sized homes with 2 or more bathrooms

Combi boiler

  • Small homes with up to 2 bathrooms
  • Average-sized homes with up to 2 bathrooms
  • Large-sized homes with 1 bathroom


Which boiler is more eco-friendly?

Modern boilers are required by UK law to run at a minimum of 92% energy efficiency. At their baseline, both boiler types are much more eco-friendly than old conventional boilers. System boilers are fully compatible with solar energy solutions, while combi boilers only heat water on demand, meaning they can use less energy overall.

Which boiler is easier to install?

A variety of factors can influence installation costs. Generally speaking, a combi boiler is easier to install since it only consists of the boiler unit itself. On the other hand, system boilers require an additional storage cylinder which will add time and costs to an install.


In general, we find that combi boilers cover a much wider range of applications in UK homes. The country is growing in size, with smaller households being more common today than in the past thanks to densely populated urban areas. 

This trend is only likely to grow, making combi boilers the perfect choice for most homes due to their cheap and efficient delivery of hot water and heating.

When we look at bigger households with multiple family members or tenants, a combi boiler simply cannot compete against a system boiler and its much larger capacity.

To pick the perfect boiler for you and your family, always start by checking your current boiler installation. Then, measure if your current boiler is meeting your needs concerning its capacity and cost.

From here, you can consult a heating engineer for their expert opinion. Find the absolute best boiler deals by contacting Eco Happy today.

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