Different types of Boiler
Combination or combi boilers, combine the Central Heating (CH) with the domestic hot water (DHW) in one unit without the need for water storage tanks. They are able to supply the heating system in a large house and also hot water on demand, when DHW is run off, the combi boiler stops pumping water to the heating circuit and diverts all the boilers power to instantly heating DHW. Some combis have small internal water storage vessels combining the energy of the stored water and the gas or oil burner to give faster DHW at the taps or increase the DHW flowrate. Combi boilers are rated by the DHW flowrate. The kW ratings for domestic units are 24kW to 42kW, giving approximate flowrates of 9 litres per minute to 25 litres per minute. The high flowrate models are more suited to houses with more than one bathroom.
Boiler performance is based on the efficiency of heat transfer and highly dependent on boiler size/output, System design and installation is critical.
A condensing boiler is a water heating device designed to recover energy normally discharged to the atmosphere through the flue. It can do this through the use of a secondary heat exchanger which most commonly uses residual heat in the flue gas to heat the cooler returning water stream or by having a primary heat exchanger with sufficient surface for condensing to easily take place. The best term for boilers designed to condense on the primary heat exchanger may be "fully condensing." Condensing boilers are now largely replacing earlier, conventional designs in powering domestic central heating systems
Condensing boiler manufacturers claim that up to 98% thermal efficiency can be achieved, compared to 70%-80% with conventional boiler designs.
Typical models offer efficiencies around 90%, which brings most brands of condensing gas boiler in to the highest available categories for energy efficiency. In the UK, this is a SEDBUK (Seasonal Efficiency of Domestic Boilers in the UK)
A system boiler (also known as a closed or sealed system boiler) works on the principle of stored hot water. This means that you will need an airing cupboard or other space for the hot water storage tank.
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