At PlumbJax, we love combination (combi) boilers for their efficiency and space-saving benefits. However, many homeowners hesitate to consider a combi boiler due to common misconceptions. One of the biggest myths is that combi boilers are only suitable for small flats and not for larger family homes. The truth is, a combi boiler can be an excellent choice in many situations—but not all. Here’s what you need to know to determine whether a combi boiler is right for you.
What is a Combination Boiler?
A combination boiler provides both central heating and hot water from a single unit, without the need for a separate water tank or cylinder. Unlike conventional systems that store hot water, a combi heats water on demand, making it highly efficient. There is no hot water cylinder that you heat up just in case you want to use it. When you open the hot tap, the boiler kicks in and starts to heat the water. When you turn the tap off, the boiler stops—so no water is being heated unnecessarily.
The Importance of Sizing a Combi Boiler
One of the most common mistakes in the UK is oversizing boilers. Many homeowners assume that a larger boiler will provide better performance, but this often leads to inefficiency, higher energy bills, and unnecessary wear on the system.
At PlumbJax, we insist on inspecting your home before specifying the boiler that will suit your environment and needs. Our expert team:
- Conducts a site survey
- Measures water flow rates
- Checks boiler space and flue clearances
- Ensures gas pipe sizes are suitable
This thorough assessment ensures that the boiler we recommend is neither too large nor too small, but perfectly suited to your home’s heating and hot water needs.
How is a Combi Boiler Sized?
Combi boilers are sized based on two key factors:
1. Hot Water Demand
The most important factor when choosing a combi boiler is the amount of water coming into the property from the water supplier.
- Low water flow rates: Below 9 litres per minute
- Excellent flow rates: 18 litres per minute and above
- Most homes: Between 11 and 15 litres per minute
2. Flow Rate Requirements
Homes with multiple taps and outlets require higher flow rates. However, it’s important to assess whether these outlets will be used simultaneously.
Other Considerations:
1. Unvented or Pressurized Hot Water Cylinder Considerations
An unvented hot water cylinder is also connected to the incoming mains water supply. If a property has poor mains pressure, an unvented cylinder will not improve the situation.
2. Should I Get a Bigger Combi Boiler?
- If the boiler’s flow rate exceeds the mains water supply, it won’t reach its full potential.
- If the boiler is too powerful, it may overheat water too quickly, leading to excessive temperature swings.
- An oversized boiler may cycle on and off too frequently, leading to inefficiency.
Combi Boiler Sizing Guide:
- 24-26kW: Provides about 10 litres of hot water per minute
- 28-32kW: Provides 12-14 litres per minute
- 40kW+: Provides 16+ litres per minute
Gas Pipe Size Considerations
Combi boilers require larger gas pipes than traditional systems. A GasSafe Engineer will assess whether the gas pipe size is adequate and may need to upgrade it if necessary.
When is a Combi Boiler a Great Option?
Combi boilers are ideal if:
- You Have One or Two Bathrooms – They deliver hot water on demand, making them perfect for moderate water use.
- You Want to Save Space – They eliminate the need for storage tanks.
- You Want Energy Efficiency – They only heat water when needed, reducing energy waste.
- You Have Good Mains Water Pressure – They rely on strong, consistent water pressure to function effectively.
When a Combi Boiler Might Not Be the Best Choice
Combi boilers may not be suitable if:
- You Have a Large Home with Multiple Bathrooms – A system boiler may be better for high water demand.
- You Have Low Mains Water Pressure – A storage cylinder may be preferable.
- You Need Hot Water for Multiple Uses at Once – Combi boilers struggle with simultaneous high-demand use.
Final Thoughts
Combi boilers offer energy efficiency, space savings, and convenience, but they are not a one-size-fits-all solution. Selecting the right size is essential for optimal performance.
If you’re considering a new boiler, PlumbJax can help. Our expert team will assess your needs and recommend the most suitable heating solution.
Get in touch with us today to discuss your options!
