DIY Heat Recovery Ventilator ?

[Matt]

Either way, you're still paying to heat that air. The air you draw from outside will take a specific amount of energy to heat to room temperature, and that energy will come from your furnance/fireplace/heat pump/cookstove... whatever. Another minor problem is that it will take more fan than you think to push much air volume through any amount of dryer hose -- that stuff is fairly resistant to air-flow with all it's ridges and bends.