Ironically, you have to keep rooms quite hot to have anything to recover - and you're committing quite a few kWh to run the ventilators. If your house is new enough to seal properly and you really do want to be hot indoors, it's definitely more economical. Some big "if"s there. But skipping the heat and ventilating naturally is free :-) Can't beat that. It wouldn't be a practical plan in places that regularly get serious cold, but Britain is not one of those places, barring a few Scots mountaintops.