An air to air exchanger draws outside air in, and exhausts inside air. They go through a heat exchanger to try and retain some of the heat from the exhaust air. You would know if you had one it is a seperate unit.
Newer houses are so airtight that they trap moisture. All the efficiencies in insulation and vapuor barriers have actually created a humidity problem. It's kind of funny bacause now you take your warm moist air and pump it outside, and draw in cold dry air. The heat exchange is not very efficient so you are cooling down your house, which means the furnace has to work more, which sort of defeats the point of all the insulation.
In the end it's still better than leaky walls.
Your windows are always going to be colder than the walls and you will get some frost build up because of that but it is a sign that the air in the house is fairly humid. An average family pumps 7 litres of water into the air every day, add in tropical plants and you can see where this goes.
Anyway it's not really something to be worried about. Just watch where that moisture is accumulating and do what you can to prevent damage. The window frames tend to pay for that frost build up and it can seep into the trim and damage the drywall.
Just opening a couple windows for short periods in the day will do wonders.
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