If you’re in the market for a boiler there are two popular types to choose from – combination or system. Both options have pros and cons, but weighing them up can help you choose the best boiler for your home.
How does a combination boiler work?
Commonly referred to as a ‘combi’, a combination boiler is a compact unit. It heats and supplies hot water straight from the mains.
Unlike a system boiler there is no storage tank needed. Instead, when you require heating or hot water a gas burner in the combi is ignited. Heating hot water and delivering it immediately. The unit is typically wall-mounted and so is ideal for small houses or apartments.
How does a system boiler work?
System boilers keep hot water in a separate cylinder, often in a pressurised system. So instead of being compact, they’re made of two parts.
The boiler heats the hot water in the cylinder. It then pumps this water around the house for your heating and hot water needs. The cylinder is usually kept in an airing cupboard, loft, garage, or utility room. You therefore need a larger space to store a system boiler and so they’re more common in larger buildings.
What are the advantages of a combi boiler?
- Instant hot water
Because combi boilers draw straight from the mains there’s no waiting for a tank to refill. Making it a more time efficient option.
- Less space needed
No separate tank or loft space needed. They’re compact and so are perfect for smaller homes and apartments. There’s no need to have a separate space to fit a bulky cylinder.
- Simpler installation
A combi boiler has fewer components. This makes installation simpler and more cost-effective.
- Energy efficiency
There’s no reserve so a combi boiler only heats water you use. The water stored in a cylinder for a system boiler may lose heat if not properly insulated. Making the combi boiler the more energy efficient choice.
What are the disadvantages of a combi boiler?
- Limited flow rate
Because there’s no storage tank, combi boilers heat water on demand. As a result, using showers, baths, and taps at the same time can put a strain on the unit. In larger homes with multiple bathrooms a limited flow rate may be an inconvenience.
- Mains pressure dependent
Combi boilers rely on mains pressure. This means they work at a lower internal pressure, to protect its sealed components from bursting. But your boiler unit having low pressure can reduce its performance.
- No hot‑water reserve
Unlike a system boiler there is no reserve. Therefore, if the boiler fails, you lose both heating and hot water.
- Not ideal for large households with multiple bathrooms
Heating water on demand without storage can lead to big drops in pressure and temperature when multiple taps or showers are in use. And so combi boilers aren’t suitable for a household where multiple bathrooms are used simultaneously.
What are the advantages of a system boiler?
- Higher flow capacity
The reserve cylinder lets a system boiler supply multiple outlets at once. This keeps the temperature and pressure steady.
- Works well with low mains pressure
As the sealed system maintains pressure internally, this unit isn’t dependent on the fluctuating mains supply. The cylinder reserve means water can be delivered at a consistent temperature and pressure.
- Better suited for large homes or properties with multiple bathrooms
System units maintain pressure and temperature while supplying hot water to multiple bathrooms or taps. This is different from combi boilers. Making them the more ergonomic choice for simultaneous water use.
- Provides a hot‑water reserve
Useful for occasional high‑demand periods. If you have a busy house, party or some guests staying over, a system boiler is a good choice for any host.
What are the disadvantages of a system boiler?
- Requires a cylinder
The cylinder, required as part of a system unit, takes up space and adds to installation cost. It requires additional pipework and possibly a pump.
- Heat loss from the cylinder
Storing the water in the cylinder can lead to heat loss, making them less energy efficient. Even insulated cylinders lose some heat when idle.
- Potential for “cold start”
After a long period of inactivity, the cylinder may need reheating. Consequently, hot water and heating isn’t as instantaneous as when delivered via a combi boiler.
Installation
- Choose a certified installer
A gas-safe registered engineer should handle the fitting, whether it’s a combi or a system unit. An emergency gas engineer can fit a boiler for you right away.
- Size the boiler correctly
Oversizing leads to inefficiency; undersizing causes inadequate heating.
- Regular servicing
Annual checks keep efficiency high and extend lifespan.
Final thoughts
To summarise a combi boiler is compact, efficient and perfect for smaller homes. System boilers are bigger, but they offer consistent heating. This is great for homes with several bathrooms or when heating and water are used at the same time. Ultimately the right choice hinges on your home’s layout, daily water usage and long term plans.