The above picture shows an insulated return duct connected to the return plenum drawing exterior air. The thought behind this setup is to draw fresh air into the house acting similar to a Heat Recovery Ventilator ( HRV ). The air is not conditioned, heated or cooled, the result being the furnace and/or air conditioner must work harder to condition the outside and in house air mixture, thus creating a loss of efficiency, increasing heating or cooling costs. For example, cold air is drawn into the duct at zero degrees Celsius, the return air temperature will be lowered considerably in comparison to the house air, the furnace must then work harder to increase the temperature to the required setting, thus increasing the energy cost. The same reasoning can be associated to air conditioning, bringing in hot humid air mixing it with in house air. The solution to this problem, if the owner perceives more fresh air is required is to install a HRV, Heat recovery Ventilator.