The basic description of a boiler is that it is a ‘closed vessel in which water or other fluid is heated in a controlled manner’. The heated or vaporized fluid leaves the boiler for use in various processes, including heating applications, boiler-based power generation, washing and meal preparation.
In a gas boiler heating system, natural gas enters your home from the gas main pipe in the street, through a gas governor and meter and into the property. All the energy that will warm up your home is stored, in chemical form, inside the gas.
The boiler burns the gas to make hot jets that are fired upon a heat exchanger containing cool water. The heat exchanger which is usually stainless steel or copper, bends back and forth several times through the gas jets so that it picks up the maximum amount of from the jets. The heat from the gas is transferred to the water.
Unlike the name would suggest, ‘Boilers’ do not boil water. Traditionally heating systems in the UK have been designed so that the boiler produces water at approximately 82°C, returning to the boiler from the system at 71°C. However, nowadays boilers produce water at the lower approximate temperatures of 75°C flow and 55°C return, this giving the modern boiler chance to condense.
From April 2005 all central heating boiler installations fell under the control of building regulations and oil-fired central-heating boilers in April 2007. The legislation states that all gas boilers fitted in both new and existing dwellings must be condensing boilers with either an ‘A’ or ‘B’ efficiency rating (A= greater than 90%, B= 86%-90%).
Unlike a traditional boiler, a secondary heat exchanger is used in a Condensing boiler to extract more heat out of the flue gases.
This means that water in the flue gases from the boiler, condenses in a stainless steel heat exchanger and drains away at the bottom of the boiler, latent heat is given up when a liquid condenses.
The efficiency of a condensing boiler can be 15% more than a conventional boiler with figure about 93% to 96% quoted by manufacturers.
When the flue gases are cooled after the secondary heat exchanger, the temperature is reduced so that the natural buoyancy is minimised, therefore, meaning that a fan is required to remove the flue gases.
Some designs split the heat exchanger into two parts and use an ordinary non-condensing copper heat exchanger for the first, and an aluminium condensing heat exchanger for the second. Aluminium doesn’t react well with water so it is lined with copper to stop corrosion.
Generally, you should expect a condensing boiler to be ‘A’ rated (which means a stated efficiency greater than 90%).