This type of boiler combines 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 flow rate. Combi boilers are rated by the DHW flow rate. 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 flow rate models are more suited to houses with more than one bathroom
If your boiler is more than 15 years old, it is likely to be a regular or heat onlyl type of boiler designed to operate with an average hot water temperature of 60 to 70°C. They often have a cast iron heat exchanger and use atmospheric burners, which draw the air required for combustion from around the boiler by natural convection. They tend to be larger than boilers of more modern design.
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.
Direct weather compensation control
To achieve more savings, the temperature of the water can be regulated according to outside temperature. In milder weather, the flow temperature is reduced, thus saving energy. This is done through the use of a compensator linked to internal and external thermostats. This form of control is particularly useful in condensing boilers as lower return water temperatures can be achieved, thus ensuring that maximum condensation occurs within the boiler and increasing efficiency.
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