1. The Boiler Itself

A system boiler, also known as a sealed system boiler, is a highly efficient heating system. It is typically installed in the home to provide both central heating and hot water. The system boiler is connected directly to the central heating system and uses water from the mains supply.

2. The Hot Water Cylinder

Unlike a combi boiler that heats water directly when needed, a system boiler heats water and stores it in a hot water cylinder (or tank). This tank holds a reservoir of hot water for immediate use, making system boilers ideal for homes with higher hot water demands (like larger families or multi-bathroom households). The water is stored at a set temperature and is ready to be used at any time.

3. The Heating Process

Here’s how the heating process works:

• Water Supply: Cold water from the mains supply flows into the system boiler.
• Heating: The system boiler heats the water using either gas or electricity. It then pumps the hot water into the hot water cylinder.
• Hot Water Storage: The hot water is stored in the cylinder until it’s required, typically at a temperature of around 60-65°C.

The boiler continues to operate and maintain the water temperature in the cylinder, ensuring hot water is readily available when needed.

4. The Distribution of Heat

When you turn on the hot tap or shower, the system boiler sends hot water from the cylinder through the pipes and into your taps or appliances. This hot water flows instantly, meaning you don’t have to wait for it to heat up, as it’s already stored and ready to go.

At the same time, if the central heating is on, the system boiler also heats the water circulating through the radiators, keeping your home warm.

5. Sealed System

A key feature of system boilers is that they are part of a sealed system. This means there’s no need for a cold water tank (usually located in the attic) or an expansion tank. The system is sealed to maintain the pressure of the water in the system, which helps ensure consistent heating and hot water. The system is pressurized, and an automatic pressure release valve ensures it doesn’t get too high or low.

6. Pressure & Expansion

The sealed nature of the system boiler means there’s no need for venting. Instead, the system operates under a set pressure, which is maintained by a pressure vessel or an expansion vessel. The expansion vessel prevents any issues related to thermal expansion and ensures that the pressure is constant.

7. The Role of the Pump

The pump within the system boiler ensures that water is circulated throughout the heating system and hot water cylinder. This pump ensures that both heating and hot water are delivered efficiently and consistently throughout the system.

8. Efficiency and Hot Water Demand

Because the system boiler stores hot water, it is especially beneficial for larger homes with a high demand for hot water. The boiler can provide multiple outlets of hot water simultaneously, such as multiple showers, taps, and even appliances like washing machines. However, it’s important to note that if the stored water is used up, you may need to wait for it to heat back up.