According to Uswitch, around 80% of UK homes had combi boilers in 2023.
But does this mean combi boiler is the right choice for everyone?
Choosing between a combi and a system boiler is one of the most important heating decisions you’ll make for your home.
Well, in this guide we’ll go through the differences between combi and system boilers and by the end, you’ll know exactly which one suits your home best.
Combi Boilers Explained
Combi boiler on teal wall with copper and PVC pipes.
What Is a Combi Boiler?
A combi (or combination) boiler is a single unit that handles both your heating and hot water needs. These space-saving systems are the most common choice in UK homes, especially in flats and smaller houses.
What makes them special? They don’t need separate water tanks or cylinders, giving you valuable storage space.
Looking to install a new combi boiler? Fitz Plumbing and Heating offers boiler installation all over London.
How Does a Combi Boiler Work?
Think of a combi boiler as an on-demand service. When you turn on a hot tap, cold water flows directly from the mains into the boiler, where it’s heated instantly and sent to your taps or radiators.
It’s rather clever, no water is stored, so you only heat what you actually use.
Pros of a Combi Boiler
Great space-saver: Fits neatly in a kitchen cupboard without any extra tanks.
Money-saving efficiency: Only heats water when you need it, keeping your energy bills down.
Always fresh water: No stored water means no stagnation concerns.
Quicker installation: Fewer pipes mean less disruption during fitting.
Good value: Units start from £500, with average installation totalling around £2,750.
Modern technology: Top models like the Vaillant ecoTEC Plus offer quiet operation and 10-year warranties.
Cons of a Combi Boiler
Struggles with multitasking: Try running two showers at once and you’ll quickly see the limitations!
Not ideal for larger homes: Can be overwhelmed by high hot water demands.
Depends on good mains pressure: If your water pressure is weak, performance suffers.
No backup system: If the system fails, you lose both heating and hot water immediately.
Recommended Combi Boiler Sizes
Choosing the right size combi boiler is crucial for efficient performance. Here’s a quick guide:
- Up to 10 radiators: 24-27 kW (ideal for 1-2 bedroom flats/apartments)
- 10-15 radiators: 28-34 kW (perfect for 2-3 bedroom medium-sized houses)
- 15-20 radiators: 32-40 kW (suitable for larger properties)
System Boilers Explained
System boiler with hot water cylinder, white pipes, and stone-walled utility room setup.
What Is a System Boiler?
A system boiler sits between old-school conventional boilers and modern combis. It heats your radiators directly (like a combi) but also stores hot water in a cylinder for when you need it.
This gives you the best of both worlds, direct heating and a ready supply of hot water.
How Does a System Boiler Work?
System boilers take water straight from the mains (no cold water storage tank needed) and heat it. Some go directly to your radiators, while the rest are stored in a hot water cylinder for your taps and showers.
Most homeowners set their system to warm up the cylinder overnight. This ensures plenty of hot water is ready for morning routines when demand is highest.
[ALSO READ]: 6 Mistakes to Avoid When Installing a Boiler as a Homeowner
Pros of a System Boiler
Great for busy households: Can handle multiple showers and taps running at once.
Reliable hot water: The stored supply means consistent temperature and pressure.
Family-friendly: Perfect if you have two or more bathrooms.
Modern controls: Easy-to-use digital interfaces for programming.
Built-in components: The pump is integrated into the boiler for a neater installation.
Consistent performance: No temperature drops when someone turns on a tap elsewhere.
Cons of a System Boiler
Takes more space: You’ll need room for both the boiler and hot water cylinder.
Limited hot water supply: Once the cylinder’s empty, you’ll wait for more to heat up.
Higher initial cost: Prices range from £580-£3,500 for the boiler, with an average installation of around £2,790.
More complex: Additional components mean more potential maintenance points.
Recommended System Boiler Sizes
System boilers are sized differently from combis as they’re designed for larger properties:
- Up to 10 radiators: 12-15 kW
- 10-15 radiators: 18-24 kW
- 15-20 radiators: 30-40 kW
For a 4+ bedroom large house with up to 28 radiators, consider a 35-42 kW system.
Here is a guide from Valliant about boiler sizes.
Combi vs System Boiler
| Features | Combi Boiler | System Boiler |
| Space Needed | Just the boiler unit, is very compact | Boiler plus hot water cylinder |
| Hot Water Supply | 11-16 litres per minute (on demand) | Stored in the cylinder (typically 120-300 litres) |
| Best For | 1-3 people, 1 bathroom homes | 4+ people, multiple bathroom homes |
| Multiple Users | Struggles when used simultaneously | Handles multiple users with ease |
| Energy Efficiency | Very efficient – no stored water heat loss | Good, with some heat loss from the cylinder |
| Installation | Simpler, fewer components | More complex with a cylinder setup |
| Typical Cost Range | £500-£3,000 (boiler only)
£1,500-£4,500 (with installation) |
£580-£3,500 (boiler only)
£1,700-£4,000 (with installation) |
| Average Total Cost | £2,750 | £2,790 |
| Water Pressure | Depends entirely on the mains pressure | More consistent delivery |
| Suitable Property | Flats, small to medium houses | Medium to large houses |
Final Thoughts
Your choice ultimately depends on household size, water usage, and available space. Combis excel in smaller homes with fewer residents. System boilers shine in larger households where multiple people need hot water simultaneously.
The best option isn’t about following trends, it’s about matching your specific needs. Today’s modern boilers offer improved efficiency regardless of type, helping reduce energy bills while we transition toward greener heating solutions.
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