Combination systems use both exhaust fans and supply fans. If the supply and exhaust flows are within 10 percent of each other this is called a balanced ventilation system and the house has a neutral pressure.
Combination systems are often integrated devices, sometimes with a heat exchanger or heat recovery wheel. The supply and exhaust airstreams are typically of equal flow. Combination systems can also consist of a mixture of supply fans and exhaust fans. It may be as simple as a quiet continuous bathroom exhaust fan matched to an outdoor air connection that introduces air into the return air plenum of a continuously-operating central heating/cooling system air handler.
It should be noted that ventilation systems utilizing constant operation of the central heating/cooling system air handler can use a very significant amount of electricity on an annual basis.
(Excerpted from the CEC Title 24 Residential Compliance Manual)