Due to our houses are built so tight, houses have to breath.Energy recovery ventilation is the process of exchanging the stale existing air by outdoor fresh air. There are two kinds of recovery ventilation , Heat Recovery Ventilation (HRV) and Energy Recovery Ventilation ( ERV ).HRV is the process of using the heat in stale exhaust air to preheat incoming fresh air IN winter allowing an exchange of heat through conduction.ERV is the process of using the cool in the stale exhaust air to cool down the incoming fresh air in Summer allowing an exchange through conduction. It is very essential to deliver a fresh air to a healthy indoor environment. In summer the outdoor fresh air enters ERV system and cooled down by the heat exchanger and then distributed to our rooms and the returned air will be exhausted outside. The same thing happens in winter while the cooled outdoor fresh air enters the HRV system and its warmed up by the heat exchanger and then distributed to our rooms and the returned air will be exhausted outside. Usually the ERV system has two pumps, one for the intake of the fresh air and the second one is to pump out the returned air. In between there is the heat exchanger which is considered the brain of the ERV system. Heat exchanger is a device used primarily to transfer or exchange of heat from one medium to another medium. Basically , the exchange of heat via conduction happens when the kinetic energy of the particles of outdoor air is increased or decreased based on the process of exchange.The efficiency of an ERV system is the ratio of energy transferred between the two air streams compared with the total energy transported through the heat exchanger.Typical efficiencies range from 55% to 75%, but some extremely efficient models are rated as high as 93% efficiency.In conclusion the main function of ERV/HRV is to remove the pressured air in your house and replace it with a fresh air from outside. ERVs are used in summer and the HRVs are used in winter. The heat exchanger main function is to cool or heat the fresh air from outside based on the climate. The new models of the ERVs are very efficient.