ADM provided the BAXI system and have been extremely helpful throughout. A comprehensive plan was drawn, amendments were easy. The system alleviated the need for any trickle vents or extractor fans. It basically supplies fresh air to all dry rooms (lounge, bedrooms etc) and extracts air from all wet/hot rooms (kitchen, bathrooms) - see diagram below. It incorporates a heat exchanger to transfer heat from the outgoing air to the incoming. This does not justify its cost on pure economic grounds but for quality of air (life?) it is superb, particularly if you have any asthma sufferers in your family. Highly recommended.