u can send me pictures on my personal email robert.panchal at yahoo dot com and i will help u i am an electrical engineer;it depends on which make u have ;there are two units one master and another is slave , master has controller mounted PCB module and it senses temperature and interacts with controller; 'HRV and some companies are charging very high  , just sharing some more information "in true sense unit which is designed to recover heat and allows external fresh air to circulate is a perfect one and also filters should be easily removable for enduser to clean regularly which in current system doesnt allow