My husband I have just renovated a small house and made it very energy efficient and air tight. We decided to use ordinary vent fans on a timer to ventilate the house. These fans seem to be sucking out a lot of the warm air. There is no way to add a central ERV with ducting in this small house and it would be too big and expensive. I saw there are spot ERVs available for around $400.00. Are they worth the price? Will we get any substantial energy savings from them compared to our direct vent on a timer?