The stale air from inside is at about 21°c and the outside air is at about 5°c in winter. This outgoing stale air is sent over a heat exchanger before discharging it to outside. This causes the heat exchanger to rise in temperature to 20°c. The incoming fresh air at 5°c is sent over the other side of the heat exchanger, without mixing the two air streams, and it causes the incoming fresh air to rise to 19°c. This is then blown throughout the house. Without the HRV system, your house would be constantly filled with 5°c air from outside. This then needs to be heated up by the radiators and causes reduced comfort levels.