Selecting a new boiler is one of the most important investments you’ll make in your home. To ensure you have reliable heating and hot water without excessive energy bills, this new boiler guide breaks down the three main types and explains how your property’s size and hot water demand should dictate your final choice.
Combi vs system boiler pros and cons
The main difference between the three boiler types is how they handle your domestic hot water (DHW).
1. The combi boiler (combination)
A Combi (or combination) boiler is a single, compact unit that heats water directly from the mains when you turn on a tap.
- Pros: It is the most space-saving option as it requires no separate hot water cylinder or cold water storage tank, making it ideal for smaller homes and flats. It also offers highly efficient, instantaneous hot water on demand.
- Cons: The primary drawback is its inability to maintain a strong flow of hot water to multiple taps or showers being used at the same time. Performance is also heavily reliant on your home’s incoming mains water pressure.
2. The system boiler
A system boiler takes water directly from the mains but stores it in a separate hot water cylinder, usually in an airing cupboard.
- Pros: System boilers are perfectly suited for medium-to-large homes and those with multiple bathrooms, as the stored hot water allows for a strong, consistent flow to several outlets simultaneously. They are easier to install than conventional units as they don’t require a cold water tank in the loft.
- Cons: They require space for the separate cylinder, and there may be some minimal heat loss from the stored water (though modern cylinders are very well insulated).
3. The conventional boiler (regular/heat-only)
Also known as a regular or heat-only boiler, this is the most traditional system. It requires two external components: a hot water cylinder and a cold water storage tank, typically located in the loft.
- Pros: It is excellent for very high hot water demand—often the best boiler for larger houses—and is also compatible with older central heating systems that cannot handle the high pressure of a combi or system boiler. It also works well in areas with very low mains pressure.
- Cons: It is the bulkiest option, requiring significant space for the boiler, cylinder, and loft tank. The installation is often the most complex and costly if you are installing a brand new system.
What size boiler do I need?
Boiler size is measured by its maximum heating output (kW). Crucially, the sizing logic is different for combi boilers than for system/conventional boilers.
Sizing a combi boiler
For a Combi boiler, the kW rating is mainly determined by the required hot water flow rate.
- Small homes (1-2 bedrooms, 1 bathroom): You typically need an output of 24–27 kW.
- Medium-to-large homes (3-4 bedrooms, 2-3 bathrooms): Look for a boiler in the 28–34 kW range.
- Very large homes (5+ bedrooms): You would need a high-output model of 35–43 kW, though a System boiler is often a safer choice for this level of demand.
Sizing a system or conventional boiler
For system and conventional boilers, the kW output focuses primarily on heating your radiators, as the hot water is handled by the cylinder.
- Small homes (up to 10 radiators): 12–18 kW is generally sufficient.
- Medium homes (10–15 radiators): A range of 18–26 kW is recommended.
- Large homes (15–20+ radiators): You will need a powerful unit in the 27–40 kW range.
Conventional boiler replacement considerations
If you are replacing an existing conventional boiler, you have three primary paths, each with different installation complexities and costs:
- Conventional to conventional (like-for-like): This is the easiest and most cost-effective installation, as it uses the existing cylinder, tank, and pipework. Choose this if you have very high water demand or an older heating system.
- Conventional to system: This involves removing the cold water tank from the loft while retaining the hot water cylinder. It’s an ideal upgrade for retaining high hot water capacity while freeing up loft space, but requires some pipework modification.
- Conventional to combi: This is the most complex installation, as it requires the removal of both the cold water tank and the hot water cylinder, followed by extensive pipework changes. This option is only recommended if you are certain your household’s hot water demand is low enough for a Combi to handle.
Ultimately, while this guide provides strong recommendations, you should always consult a Gas Safe-registered heating engineer. They will perform a professional heat-loss calculation specific to your home’s insulation, windows, and layout to ensure the exact boiler size you install is perfectly matched to your needs.
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