A sealed central heating system is one that is primarily filled with water and then sealed of containing the water inside. They are filled, usually with a cold mains connection through a temporary hose connection and then closed off. The water now stays within the system and is not under the influence of any outside pressure. Abbey Wood Plumbers service and repair these systems on a regular basis.An example of one of these systems is a combination boiler. The temporary filling hose used top fill is always removed as it could be a point of entry for any chemical fluids such as cleaners. The water regulation authority require that this should not happen as it could form a contamination issue. Therefore the hose must be removed and not just connected and turned off!The Difference from a vented system. As stated, there is no influence by outside pressure on a sealed system as it is kept in a closed loop. A vented system has a tank in the loft ususally that will control the volume of water in the system as required (due to leaks, evaporation etc) via a float valve in the tank called a feed and expansion tank. A Thamesmead Plumber is trained to work on both systems. The water in any heating system will expand as it heats so therefore, as it is in a sealed not vented circuit, the expansion of the water is contained by an expansion vessel, sometimes called a bladder that may be fitted in the boiler itself. The sealed system has its uses for instance if you live in a small flat where space is limited, this system without a tank is ideal. Although this is a very high pressure system it is fitted with safety equipment such as:Pressure relief valve in case system pressure rises above the normal running pressure.Expansion relief valve in case the expansion vessel fails.Non return valve to prevent backflow of hot water through to the cold water line.Temperature limiting valve to prevent dangerous increase in operating temperature.