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Last updated: 28th March, 2024


System Boilers vs Combi Boilers

System Boilers vs Combi Boilers

In this article we will discuss the topic of different boiler types, specifically system boilers vs combi boilers, explaining how each new heating system type works as well as their pros and cons.

What Is A System Boiler?

A system boiler links with a hot water storage cylinder to provide a home with central heating and hot water.

It does this by taking cold water directly from the mains (instead of a separate feed tank) and heating it via an internal heat exchanger, this transfers gas energy to water.

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The hot water is then moved to the hot water cylinder for storage.

Unlike some regular boilers, such as oil or heat only boilers, that use bulky external components, with a system boiler the majority of components are integrated within the appliance.

The only external part is the aforementioned hot water storage cylinder; This is a much smaller unit than some of the other hot water storage tank systems used by regular boilers, these tend to eat up loft space.

What Is A Combi Boiler?

Combi (combination) boilers also link a home’s central heating system and water heating, however they are a single unit, with all components integrated within the boiler.

The combi signals a sensor upon hot water demand, then takes water directly from the mains and the heat exchanger heats it for distribution to water outlets. This means there's no accompanying cylinder or hot water tank.

Comparing System vs Combi:


System Boiler Pros

Combi Boiler Pros

  • Even though a system boiler requires a small external cylinder, they are still a far more streamlined set up than most traditional heating systems due to them sourcing their water supply from the mains.

  • Being a single piece boiler unit, combi boilers are incredibly compact and space saving as no external components for storing hot water are required.

  • System Boilers can cope with the demand of several outlets i.e multiple taps needing hot water at the same time.

  • Combis are known to be incredibly efficient with many getting an ErP energy rating of A in the 90’s in terms of percentage score.

  • System boilers are often compatible with solar panels, a clean alternative energy source to save money. Also due to their energy efficient multiple outlet capabilities at one time, system set ups are considered more eco-friendly.

  • Combis source their water supply from the mains and retain a lot of this water pressure. Improved jet strength of water flow means less likelihood of curve trapped or pooling water.

  • Due to an internal pump, and a banked body of hot water, system boilers can produce hot water on demand quicker than heat only boilers for example.

  • Combination boilers tend to need less repairs due to them having less individual or external parts.


System Boilers Cons

Combi Boilers Cons

  • As the water needs to heat up in advance, a schedule must be created to do so. This means a change of plans could affect hot water production.

  • A combi may struggle with a higher water demand of multiple bathrooms due to no prior storage capability.

  • If a system's pre heated hot water is quickly used up, you will have to wait for more water to be heated before use.

  • Homes with old plumbing foundations may not be able to cope with the flow pressure of a modern combi.

  • A system boiler requires an external cylinder as a place to store the hot water produced.

  • A combi boiler requires good mains pressure, if the mains pressure is low the resulting flow rate will be low.

  • A system’s cylinder must be insulated, otherwise as hot water enters it will drop in temperature, increasing energy bills.

  • Speed of flow although may be quicker than the majority of boiler types, it will still take a few seconds for the water to heat up.

System Boiler Vs Combi Boiler Price

As with any appliance, new boiler and installation prices can vary due to a number of contributing factors including brand, model, merchant and engineer rates.

In general, the average price for new system boilers (including installation rates) in 2022 is around £2000 with the equivalent combi boiler system prices in the £1975 region.

So not too much to separate the two in terms of initial cost, financial impact shouldn't hold sway over practicality in the case of these differing options.   

Which Should I Buy?

The decision of whether to buy a new system boiler or new combi boiler should be dependent on the particular requirements you or your home need. Both as previously discussed aren't a million miles away in terms of finance with the combi being the slightly cheaper option.

If saving space is your number one priority, again both options are economical in terms of proxemics, with the combi being the most compact as a single unit.

The system boiler however has a marginally quicker speed of flow, and both boiler types are at their most optimum with modern plumbing infrastructure.

The choice may simply come down to a system boiler being more effective for larger homes while the combi is suited to smaller properties; both are fantastic modern heating system options whose pros outway their cons especially in relation to older conventional boilers.

If you’ve liked what you've been reading and want an old boiler replaced, or just fancy an upgrade, get a free quote on a wide selection of boilers with gas safe registered installation here at iHeat.

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Frequently Asked Questions

Should I Change Regular Boiler to System Boiler?

Switching from a regular boiler to a system boiler can be beneficial if you're looking for a more efficient heating system with a faster response time and reduced space requirements (since system boilers don't need an expansion or feed tank in the loft). System boilers are also easier to install than regular boilers and can provide a more consistent water pressure. Consider this upgrade if you have a medium to large-sized property with more than one bathroom, as system boilers can cope better with higher hot water demands.

Should I Upgrade to a System Boiler?

Upgrading to a system boiler might be a good decision if:

  • You experience low water pressure with your current setup.
  • You need to improve hot water supply to multiple bathrooms.
  • You wish to remove cold water tanks from the loft to free up space or reduce the risk of freezing in winter.
  • You're looking for a more energy-efficient heating system.

What are the Disadvantages of a Combi Boiler?

Disadvantages of a combi boiler include:

  • Limited hot water supply: Combi boilers might struggle to supply enough hot water simultaneously to multiple outlets, making them less suitable for homes with multiple bathrooms.
  • Water pressure dependency: The performance of a combi boiler is directly related to the mains water pressure. If the mains pressure is low, the output from showers and taps may be unsatisfactory.
  • No backup hot water: If the combi boiler breaks down, you won't have access to hot water, unlike systems with a hot water cylinder that might retain hot water for a short period.
  • Not always compatible with old radiators: In some older heating systems, the high water pressure from a combi boiler can highlight weaknesses in the system, such as leaks.

Do System Boilers Run Out of Hot Water?

System boilers are connected to a hot water cylinder that stores a significant amount of hot water ready for use. While it's unlikely to "run out" of hot water under normal usage, the cylinder can be depleted if there's excessive demand. However, it will refill and reheat water continuously, so any shortage is temporary.

Does a System Boiler Need a Hot Water Cylinder?

Yes, a system boiler requires a hot water cylinder to store hot water for domestic use. This setup allows the boiler to provide hot water more efficiently to multiple taps or showers at once, making it suitable for homes with higher hot water demand.

What Boiler Do I Need for a 4 Bed House?

For a 4 bedroom house, especially with multiple bathrooms, a system boiler connected to a suitable hot water cylinder or a larger combi boiler is often recommended. The exact size and type depend on the number of bathrooms, the household's water usage, and whether the property has space for a hot water cylinder. Typically, a boiler with an output of 24-35 kW is suitable for such homes.

How Many Radiators Does a 30kW Boiler Have?

A 30kW boiler can comfortably support around 15-20 average-sized radiators. The exact number can vary based on factors such as the size and type of radiators, insulation quality of the home, and specific heating requirements. It's crucial to consult with a heating engineer to ensure your boiler's capacity matches your home's heating needs accurately.

Stephen Day profile photo
Article by
Stephen Day | Co-founder
Gas Safe registered and FGAS certified engineer with over 20 years experience in the heating and cooling industry.