What is the heated floor area?

Give the area you actually heat — leaving out an unheated garage, cellar or loft. The indicative output of the heat source follows from it.

Is it a house or a flat?

A house loses heat through all its walls and the roof; a flat usually only through the external wall. At the same floor area, that difference matters.

What condition is the insulation in?

Of all the questions, this one affects the estimated output the most. Choose the option that fits most closely — we will refine it on site.

How many storeys do you heat?

Count the living storeys, including the loft if it is heated. Do not count an unheated cellar or garage.