Three years after a young Swiss couple purchased a single-family house built in 1942 the oil furnace had to be replaced. This was the motivation to completely renovate the house, including a 13 m2 drain-back solar system (fig. 6) with an 800 L storage tank.

The renovation also included adding roof, wall and basement ceiling insulation (220,

200 and 80 mm). The windows were replaced with units glazed with a Ug of 0.55. The oil burner was replaced with a wood pellet stove and a ventilation system with 80% heat recovery was added. As a result, the solar system is able to cover much of the space heating in spring and fall and all the domestic water heating demand in summer. The annual primary energy demand for space and water heating was drastically cut from 230 to 47 kWh/m2, or an 80% reduction. This means instead of burning 3,500 litres of heating oil per year and they burn only 1 tons of wood pellets [6].