This is my understanding and experince. They take the warm air from your ceiling which has been heated by the sun's ray's and move the air around your house. I would recomend that you have the in line heater option with it, because if the air temp drops in your ceiling space you get the cold air coming down through the vents. I belive that all the systems are much the same HRV, Moisturemaster, DVS, the other and most cost effcient that I know is Home Tech. My parents have just put one in after having a DVS at there last two homes. The DVS was going to cost just over the 4K mark for a three bedroom house the Home Tech was $2,300.00 installed. Works well.
They are not a heating option for your house, as on full heat they work on 4Kw only. You will need some other form of heating, heat pump's, gas, fireplace, etc...
New homes with alumium windows have vent holes, as this was a major problem when they first came out many years ago.
I hope this helps.
Regards Mal.