Abstract: A building ventilator to provide a supply of air into a building comprises an external inlet port and a separate external outlet port and an internal inlet port and a separate internal outlet port to allow respective airstreams to flow into and out of the ventilator respectively at an exterior and at an interior of the building; a heat exchanger mounted in the flow paths between the external and internal inlet and outlet ports. The ventilator is configured to drive an airflow from the external inlet port through the heat exchanger and out of the ventilator via the internal outlet port; and to drive an airflow from the internal inlet port through the heat exchanger and out of the ventilator via the external outlet port. The ventilator comprises a heating element mounted in the air flow path between the external inlet port and the heat exchanger.