SEALED SYSTEMS
MARK MOGEY, HEAD OF OEM & ROI SALES AT ALTECNIC, OFFERS AN EXPERT OVERVIEW OF SEALED SYSTEMS AND THEIR COMPONENTS…
Getting back to basics with sealed systems
Mark Mogey, Head of OEM & ROI Sales.
Sealed systems are now seen as the ‘norm’ for domestic heating systems, with older heating systems usually having an open vented boiler with a feed and expansion vessel. Sealed systems offer better running costs, ease in installation and long-term savings compared to an open-vented system. As a result, it is highly likely that most systems will be converted to a sealed system style. SEALED SYSTEMS A sealed system is closed to the atmosphere and eliminates the feed and expansion tank as well as associated pipework, thus reducing the risk of contamination, corrosion and noise in the system. For a sealed system to operate, it must include all of the following: • Expansion vessel • Safety relief valve (or pressure relief valve) • Pressure gauge • Filling loop Each component has a significant role in ensuring that the sealed heating system functions in the most efficient way. EXPANSION IN THE SYSTEM Lack of expansion leads to unnecessary wear on the pump and other components. Systems may reach critical pressures (usually three bar) and discharge via the safety valve. Not only is this a waste
of water but it also leads to homeowners calling out engineers or even topping up the system themselves. Constant introduction of fresh water also adds air to the circuit, and this can cause corrosion. Expansion vessels allow for a higher volume of water to be stored. When water is cold, the pre-charge pressure forces the diaphragm against the tank towards the inlet. As the temperature increases, the expanded water volume pushes against the diaphragm, creating additional volume for the water to enter. When the temperature decreases, the pre-charge pressure forces the water from the tank and back into the main heating system. This maintains a constant pressure within the heating system, helping to reduce energy consumption. It is extremely important for expansion vessels to be sized correctly to operate efficiently and avoid damage. More information on expansion vessel sizing can be found in the CIBSE Domestic Heating Guide. The maintenance of an expansion vessel is also important for a system. An expansion vessel’s pre-charge gas pressure should be checked regularly as part of a planned maintenance programme. Maintenance is made easier when the design and quality of the expansion vessel is prioritised. For example, when a nitrogen- filled vessel is selected for a system rather than a compressed air-filled vessel, the life span of the vessel is significantly increased.
This is because oxygen within a vessel can cause the membrane of the vessel to corrode, and any air through the membrane can reduce the pre-charge pressure. With a nitrogen-filled vessel (like the Reflex range of expansion vessels), this issue is eliminated. THE IMPORTANCE OF A FILLING LOOP Although filling loops are a key element of a sealed system, they are not to be connected permanently to a boiler. The filling loop is installed between the mains cold water supply inlet and the central heating return pipe in the boiler. After the system has been filled and pressurised, the service valves must be closed, the system isolated and the flexible hose removed and stored until it is used again. To meet Water Regulations requirements, the filling loop must be WRAS approved and made up of the following: • WRAS-approved backflow prevention device (a double-check valve conforming to BS EN 13959 suitable for protection against a fluid category three risk) • Temporary connecting pipe (a removable flexible hose) • Service valve (two isolating ball valves) • Four-bar pressure gauge (optional) The Altecnic ALT-ST0035 filling loop consists of a double-check valve,
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