Discounted Heating are here to help you choose the correct boiler for your home. There are several types of wet central heating system used in the UK. The most suitable for you will depend firstly on whether it is a replacement or new installation. From then on it is dependent on the relative merits of each system type. The majority of the systems currently installed in the UK to date are 'Open vented' systems. This means that water is fed into the system from a tank in the loft. However, sealed systems are becoming very popular, particularly with the advent of the combination boiler, as they eliminate the water feed tanks in the loft and reduce installation time.
The main four system types are shown below.
1.Open vented system, with pumped central heating and gravity domestic hot water Traditional open vented system with gravity domestic hot water heating and pumped central heating. In the traditional open-vented system design, the system is fed with water and kept under pressure via gravity-fed water from a tank in the loft. The hot water cylinder is heated simply via a gravity hot water circuit from the boiler, and central heating via a pumped circuit from the boiler. 
2.Open vented system, with fully pumped central heating and domestic hot water The standard open vented system with both domestic hot water and central heating from a single pumped circuit from the boiler (hence the system is "fully" pumped) Over the last 20 years the fully pumped, open vented system has become the preferred option for most installations be it newbuild or replacement. Increased control over domestic hot water heating and quicker heat up lead to better system performance and efficiency. 
3.Sealed system, with fully pumped central heating and domestic hot water The sealed system is increasing in popularity due to the elimination of the system water feed tank and open vent pipework in the loft. The system is fed and pressurised with water direct from the mains, then sealed. This simplifies installation and reduces maintenance requirements in the loft and can have installation cost savings. A conventional tank-fed indirect hot water cylinder can be used, as shown in diagram 1. However, if a mains pressure unvented domestic hot water cylinder is used then all tanks/pipework are eliminated from the loft, as shown in diagram 4. This eliminates the risk of freezing pipes in the loft, eliminates maintenance requirements in the loft and has further installation cost savings. The customer also clearly benefits from the provision of increased flowrate mains pressure domestic hot water to all outlets. 
4.Sealed system, with fully pumped central heating and instantaneous domestic hot water, using a combination boiler. This system has increased in popularity due to the elimination of all water tanks in the loft, and also the domestic hot water cylinder. Hot water is heated instantaneously, on demand by the boiler. The customer can realise benefits due to elimination of the hot water cylinder (more cupboard space), elimination of the possibility of freezing pipes in the loft and mains pressure hot water and showering. 
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