Do you have a humidifier on the heating system? We had the same problem, but was a simple adjustment to the humidifier (turning it down a bit). If the house is very tight an air exchanger might be a good idea for health reasons as well as condensation. Condensation can promote mold, and you don not want that!!! As a very simple test, crack a window, (I mean just a hair, paper thin!). Wipe off the condensation on your windows, and see if your condensation reappears over the next 24-48 hours. You would need to do this when the temperature is below 20 as your stated in your message.