What is a Combi Boiler?

What is a Combi Boiler?
A combi boiler (short for combination boiler) is a type of boiler that combines two functions in one compact unit:
- Heating your home (via radiators or underfloor heating).
- Providing hot water on demand (to taps, showers, etc.).
Unlike traditional boilers, combi boilers don’t require a separate hot water storage tank or a cold water storage tank. They heat water directly from the mains supply whenever you need it.
How Does a Combi Boiler Work?
For heating your home, the boiler sends hot water through your radiators or underfloor heating system. A built-in control system ensures the boiler prioritizes hot water for taps or showers over central heating when both are needed at the same time.

Key Features of a Combi Boiler:
- Compact Design: Ideal for smaller homes or spaces where installing separate tanks isn’t feasible.
- On-Demand Hot Water: Delivers hot water instantly without waiting for a storage tank to heat up.
- Energy Efficiency: Since water is heated only when required, there’s less energy waste compared to systems that keep water heated in a storage tank.
- Simple Installation: Requires fewer components (no storage tanks), making installation quicker and less disruptive.
Advantages and Disadvantages of Combi Boilers
Advantages
- Space Efficiency:
No need for a hot water storage cylinder or cold water tank in the loft, freeing up valuable space. - Energy Efficiency:
Only heats water when needed, reducing energy waste.
Many models are over 90% efficient, meaning lower energy bills. - Quick Installation:
With fewer components compared to traditional systems, installation is faster and usually cheaper. - Convenience:
Instant hot water without waiting for a tank to fill or heat up. - Cost Savings:
Lower initial setup costs compared to traditional systems.
Disadvantages
- Hot Water Flow Limitations:
The flow rate may drop if multiple taps/showers are used simultaneously, making it less ideal for larger households with high water demand. - Dependence on Water Pressure:
Requires good mains water pressure for optimal performance. Homes with low water pressure might need a pump or pressure-boosting system. - Not Ideal for Large Homes:
Homes with multiple bathrooms or high heating needs may benefit more from a system boiler with a storage tank. - No Backup Hot Water:
Since there’s no storage tank, if the boiler breaks down, you won’t have access to any stored hot water.
Key Factors to Consider When Choosing a Combi Boiler
- Size and Output:
kW Rating: The higher the kW rating, the more powerful the boiler.
Choose a boiler suitable for your home’s heating and hot water demands. - Water Pressure:
Ensure your mains water pressure is adequate for a combi boiler. - Fuel Type:
Consider the availability and cost of fuel in your area. - Energy Efficiency:
Look for models with high efficiency ratings or those recommended by energy-saving programs. - Smart Features:
Modern combi boilers often come with smart thermostats or remote control options. - Brand and Warranty:
Opt for reputable brands with good warranties and customer support. - Installation and Maintenance Costs:
Factor in the costs of professional installation and regular servicing.
Combi boilers are popular in modern homes and are particularly suitable for smaller to medium-sized properties. They offer convenience, energy efficiency, and a compact solution for heating and hot water needs.
Ideal Homes for a Combi Boiler
- Flats and Smaller Houses: Perfect due to their compact design.
- Medium-Sized Homes: Work well if there’s moderate hot water demand.
- Homes Without a Loft: A great solution when a loft isn’t available for a cold water tank.
Combi Boiler Installation by North Somerset Heating
We are Gas Safe registered Heating Engineers that install combi boilers through North Somerset. We can advice you if a combi boiler is what your home needs. Click here for an online boiler quote!