A new high-efficiency home heating system includes an air-to-air heat exchanger which uses energy from outgoing stale

Question:

A new high-efficiency home heating system includes an air-to-air heat exchanger which uses energy from outgoing stale air to heat the fresh incoming air. If the outside ambient temperature is -10°C and the relative humidity is 30%, how much water will have to be added to the incoming air, if it flows in at the rate of 1 m3/s and must eventually be conditioned to 20°C and 40% relative humidity?