The Akor HRI is a high volume French system with a 70% efficiency. It uses flexible ducting which makes the installation easier. It is also one of most economical systems available.

The Ubbink range of high efficiency units uses rigid ducting (although semi-rigid or flexible can be used). The Ubbink range is made & tested in the Netherlands, and is probably the most efficient range of heat recovery units on the market (quoting from Ubbink).