SDG #252 Heat loss energy savings for home ventilation

Extract fans and trickle ventilation keep a house healthy, but they quietly dump heat outside all winter. This episode starts with the arithmetic rather than the assumptions: airflow rate, temperature differential and the specific heat capacity of air are used to put a real wattage figure on continuous ventilation, which then converts into a running cost and a payback window for heat recovery. That framing is the useful part, because it shows how quickly a modest airflow at a large indoor/outdoor delta becomes a meaningful load, and where a recovery or smarter control strategy actually earns its keep versus where it is just gadgetry.

The hardware half covers a custom smart control board taken through PCB fabrication and SMD assembly, with the board arriving populated and ready for bring-up. Bring-up is where the interesting failures appear: the RS485 link does not behave, and the fault is chased with an oscilloscope on the differential pair rather than guesswork in firmware. Attention lands on the 75176B transceiver, driver enable behaviour and bus termination, the classic places where a half duplex link falls over.

The repair itself is a good demonstration of technique. Two Metcal irons used together let the offending part be lifted cleanly without cooking the surrounding assembly, which is far more controlled than trying to flood a small SOIC with hot air next to connectors and plastics. Serial decode on the scope then confirms the corrected traffic, with the captured frames compared against the expected register and data tables.

For anyone building their own ventilation controller, the takeaways are practical: verify the physical layer before blaming code, and design the enable and termination scheme properly the first time.