A heat recovery ventilator is provided which includes a heat exchanger having discrete inlet and exhaust passageways extending therethrough for providing heat transfer between respective fluids flowing along the inlet and the exhaust passages. The inlet passageway provides fluid communication between a supply inlet plenum and a supply discharge plenum. The outlet passageway provides fluid communication between an exhaust inlet plenum and an exhaust discharge plenum. The supply inlet plenum has a supply port for admitting supply air into the supply inlet plenum. The exhaust discharge plenum has an exhaust port for discharging air from the exhaust discharge plenum. A transfer port extends between the supply inlet plenum and the exhaust discharge plenum for providing fluid communication therebetween. A flow diverter is associated with the transfer port and is movable between a venting configuration closing the transfer port to allow fluid flow through the exhaust discharge port and a defrost configuration closing the exhaust discharge port to open the transfer port.