A central heating header tank, also known as a feed and expansion tank, plays a crucial dual role in traditional vented heating systems by ensuring a constant supply of cold water to the system and safely accommodating the thermal expansion of heated water.
Understanding the Header Tank's Dual Role
In all vented hot water and central heating systems, a header tank serves as a vital component for both feeding the system and managing pressure changes. Perched in the loft or at the highest point of the system, it acts as a reservoir for cold water and a safety buffer for expanded hot water.
The Cold Water Feed Mechanism
The primary function of a header tank is to consistently supply cold water to your central heating system and often, to the hot water cylinder.
- Gravity-Fed Supply: The tank is typically connected to the mains cold water supply via a ballcock (or float valve), which automatically refills the tank as water levels drop. Due to its elevated position, gravity naturally pushes water from the tank down through a cold feed pipe into the heating system. This ensures a steady, low-pressure supply of water to fill radiators and the boiler circuit.
- System Top-Up: As water is consumed or lost slowly over time (e.g., through minor evaporation or very small leaks), the header tank automatically replenishes the system, maintaining the correct operating level without manual intervention.
Managing Thermal Expansion
Water expands when heated, and in a sealed system, this expansion would lead to dangerous pressure build-up. The header tank provides a safe, open-vented space to manage this.
- Open Vent Pipe: A crucial open vent pipe (sometimes called an expansion pipe) rises from the boiler or hot water cylinder and terminates above the water level inside the header tank.
- Safety Valve: When the water in the heating system or hot water cylinder heats up, it expands. This expanded water simply rises up the open vent pipe and flows back into the header tank, safely dispersing the excess volume without increasing system pressure.
- Overflow Protection: The tank also features an overflow pipe which serves as a critical safety feature. Should the ballcock fail and the tank overfill, or if excessive expansion occurs, water will exit through this pipe, usually leading outside, preventing spills within the property and indicating a problem.
Key Components of a Header Tank System
Understanding the different parts connected to a header tank clarifies its operation:
| Component | Function |
|---|---|
| Header Tank | The main plastic or metal tank, located in the loft, serving as a reservoir and expansion point. |
| Ballcock/Float Valve | Automatically controls the inflow of mains cold water to maintain the tank's water level. |
| Cold Feed Pipe | Carries cold water from the header tank down to supply the central heating system and hot water cylinder. |
| Open Vent Pipe | Allows expanding hot water from the system to return safely to the header tank, preventing pressure build-up. |
| Overflow Pipe | A safety outlet that directs excess water away from the tank (usually outside) if it overfills. |
Why Vented Systems Utilize Header Tanks
Vented (or open-vented) central heating systems rely on header tanks because they offer a simple, robust, and inherently safe method of managing water supply and expansion. Unlike modern unvented systems that use sealed expansion vessels and pressure relief valves, vented systems operate at atmospheric pressure at the top, making them less prone to catastrophic pressure failures. This design is highly reliable and has been a standard in homes for decades.
Common Issues and Maintenance
While header tanks are generally low-maintenance, issues can arise:
- Continuously Running Overflow: This often indicates a faulty ballcock valve, causing the tank to constantly refill and spill. It wastes water and can cause damp issues.
- Low System Pressure: If the tank's water level is too low or the cold feed pipe is blocked, the heating system may not fill adequately, leading to poor heating performance or airlocks.
- Frozen Pipework: In colder climates, especially in lofts, the pipes leading to and from the header tank can freeze if not properly insulated, preventing water flow.
Regular checks, particularly ensuring loft insulation doesn't obstruct tank ventilation but adequately covers pipes, can prevent many common problems. For more detailed information on heating system maintenance, you can consult resources like Energy Saving Trust or a qualified heating engineer.
