A sealed system boiler heats water within a closed loop of pipes, without requiring a separate cold water tank or open vent pipe in the loft.
Understanding Sealed Boiler Systems
Unlike older, open-vented heating systems that rely on gravity feed from a tank in the loft, a sealed system is a closed circuit. This means the water circulating through your boiler, pipes, and radiators is contained entirely within the system. This design offers several advantages, including typically higher water pressure at your taps (for hot water) and eliminating the need for roof space tanks.
Key Components of a Sealed System
A sealed system boiler setup typically includes:
- The Boiler: Heats the water.
- Circulation Pump: Moves the heated water around the system.
- Radiators: Where heat is emitted into rooms.
- Pipework: Connects all components in a continuous loop.
- Expansion Vessel: Manages pressure changes.
- Pressure Relief Valve: A safety mechanism to release excess pressure.
- Pressure Gauge: Shows the current system pressure.
Managing Pressure with the Expansion Vessel
One of the most critical differences in a sealed system is how it handles pressure changes caused by heating the water. As water heats up, it expands. In an open-vented system, this expansion is accommodated by the open vent pipe leading to the header tank.
In a sealed system, there is no open vent. Instead, sealed systems have an expansion vessel that absorbs the pressure as the water in the system expands due to the heat in the system. The expansion vessel typically contains a diaphragm or partition with air or nitrogen on one side and system water on the other. When the water heats and expands, it pushes against the diaphragm, compressing the air/gas on the other side and accommodating the increased volume without letting the system pressure rise dangerously high. When the water cools, it contracts, and the air/gas pressure pushes the water back out of the vessel into the system, maintaining adequate pressure.
System Operation Cycle
- Demand for Heat: When you turn on your heating or hot water, the boiler fires up.
- Water Heating: The boiler heats the water passing through its heat exchanger.
- Circulation: The circulation pump pushes the heated water through the pipes and into the radiators (or heat exchanger for hot water).
- Heat Emission: Radiators release heat into the rooms.
- Pressure Management: As the water heats and expands, the expansion vessel absorbs the excess volume to keep the system pressure stable.
- Return: Cooled water returns to the boiler to be reheated, completing the cycle.
- Safety: If, for any reason, the pressure does exceed safe limits (e.g., due to a faulty expansion vessel or overfilling), the pressure relief valve will open to discharge water and lower the pressure.
Why Choose a Sealed System?
- Higher Pressure: Can often provide better hot water pressure directly from the boiler (especially with combi boilers).
- No Tanks in Loft: Frees up loft space and avoids the risk of freezing tanks in winter.
- Tidier Installation: Generally simpler pipework without vent and feed pipes running to the loft.
Understanding the role of the expansion vessel is key to grasping how these efficient and modern heating systems function safely within a closed environment.
| Component | Primary Function |
|---|---|
| Boiler | Heats water |
| Circulation Pump | Moves water through the system |
| Radiators | Emit heat into rooms |
| Expansion Vessel | Absorbs water expansion to control pressure |
| Pressure Gauge | Displays system pressure |
| Pressure Relief Valve | Safety device to release excess pressure |
