Last autumn I installed a SolarVenti SV14 on my weekend house in the mountains (in a height of 920 m).
The effect is amazing.
Throughout the entire winter the temperature never dropped below 2˚ C - and now that it is spring the temperature in the house is permanently around 10-12˚ C  (6-8˚ C higher than during the same period last year). When we arrive in the weekend house and turn on the heat, a room temperature of 22˚ C is reached easily and fast. I am very satisfied with the product.
Zoran Ogrinc, Pohorje, Slovenia