Combination boilers are condensing boilers that generate hot water on demand and central heating for a home. Boilers are rated by BTU (British Thermal Unit), and certain calculations are used to determine how powerful a boiler needs to be to perform at optimum efficiency within a home. BTU is defined as the amount of energy needed to heat one pound of water by one degree Fahrenheit. Because boilers are available in a variety of sizes, it is important to calculate the average BTU required for a home of a given size.

Measure the length, width and height of each room. Multiply these together to produce a cubic foot measurement and jot down the number for each room on paper.

Calculate heat loss/variables for each room. Lounge and dining rooms should be multiplied by five, bedrooms by four and common areas and kitchens by five. Add 15 per cent to the calculations for rooms facing north and 20 per cent for rooms facing south. Deduct 10 per cent of the total for all rooms with double glazed windows.

Add calculations for all rooms together, which should now produce an estimated BTU output required to heat the house.

Add 20 per cent to the total estimated BTU calculation to account for water circulation in radiators. Add a further 10 per cent to the calculation to account for any other heat loss, such as poor insulation in certain areas of the home.

Establish the pressure in the main water line to your home, which will be required to calculate water pressure needed for the boiler to heat water on demand. Call a plumber to calculate the main's water pressure accurately as water pressure varies between different boiler specifications.