A pressurised central heating system, often referred to as an unvented system, works by taking water directly from the mains water supply and distributing it at high pressure for both heating radiators and providing domestic hot water. This approach offers significant benefits over older, conventional systems.
Pressurised systems heat mains water directly (in the sense that it enters the system from the mains, not from a low-pressure tank in the loft), rather than drawing from a cold water storage tank typically found in conventional systems. This design inherently leads to a reduction in wastage and saves space compared to these conventional setups.
However, it's important to understand how it differs from a combi boiler. While both use mains water directly, a pressurised system is different to the combi boiler as water for domestic hot water is heated via a water cylinder, rather than the boiler itself instantly heating the water on demand. The boiler heats water for the radiators, and also heats the water stored within the special unvented hot water cylinder.
Key Components and Process
A typical pressurised system consists of:
- A Boiler: Heats the water for both the radiators (central heating loop) and provides heat to the unvented cylinder for domestic hot water.
- An Unvented Hot Water Cylinder: A robust cylinder designed to hold and heat mains-pressure water for taps and showers. It receives heat from the boiler via a coil inside the cylinder.
- Radiators: Heat the rooms in the property, circulated by a pump.
- Pipework: Carries the heated water throughout the system.
- Expansion Vessel: A crucial safety component that accommodates the expansion of hot water within the sealed, high-pressure system.
- Safety Valves: Additional safety features to release pressure if it becomes too high.
The process generally follows these steps:
- Mains water enters the property under pressure and feeds directly into both the boiler (for the heating circuit) and the unvented hot water cylinder.
- The boiler heats water, which is pumped around the radiator circuit to warm the house.
- Simultaneously, the boiler can heat water that flows through a coil inside the unvented cylinder. This transfers heat to the stored domestic water.
- When a hot water tap or shower is turned on, the mains pressure pushes the hot water out of the cylinder.
- The expansion vessel and safety valves manage the system's pressure as the water heats and expands.
Advantages Over Conventional Systems
Pressurised systems offer several key advantages:
- Stronger Water Flow: Because hot water is delivered directly from the mains supply, taps and showers typically have much better pressure and flow rates compared to conventional systems that rely on gravity from a tank in the loft.
- No Header Tanks: Eliminating the need for cold water storage tanks and feed/expansion tanks in the loft saves valuable space.
- Reduced Wastage: With no open tanks in the loft, there's less risk of evaporation or overflow issues, contributing to a reduction in wastage.
- Improved Efficiency: While the mechanism is different from a combi, the sealed nature can contribute to overall system efficiency by preventing airlocks.
Pressurised vs. Combi Boilers: The Hot Water Difference
The primary distinction highlighted by the reference lies in how domestic hot water is provided:
| Feature | Pressurised (Unvented) System | Combi Boiler |
|---|---|---|
| Hot Water | Heated and stored in a cylinder | Heated instantaneously by the boiler |
| Hot Water Flow | High pressure, good for multiple outlets | High pressure, but flow can drop if multiple outlets are used simultaneously |
| Cylinder | Requires a special unvented cylinder | Does not require a cylinder |
| Tanks | No header tanks in the loft | No header tanks in the loft |
In summary, a pressurised system uses mains pressure throughout, heating radiator water directly via the boiler and domestic hot water via a cylinder heated by the boiler. It's a space-saving, efficient option that provides robust water pressure.
