A week or so ago I asked about a way to program my HVAC fan to come on periodically throughout the night and /u/polarc offered some fantastic advice on how to wire it up.
I ended up finally getting all my various pieces in this week, including a bulk order of some spade connectors as well as a pack of 10 diodes (https://www.amazon.com/gp/product/B00UOCVXXW/ref=oh_aui_detailpage_o01_s00?ie=UTF8&psc=1). I ended up getting a 12VDC powered digital programmable relay, versus a 24vac one primarily because I have a ton of 12V switching power supplies sitting around. Purchased this one specifically (https://www.amazon.com/gp/product/B012FSL2GK/ref=oh_aui_detailpage_o01_s01?ie=UTF8&psc=1)
For the install itself, I mounted the timer to the HVAC return duct so it was out of the way and in an easy to locate spot. I also had a spare outlet right there to plug in my 12V adapter. Wiring consisted of some spare thermostat cabling.
Not much to the wiring, just connected to R and G for the fan trigger. As /u/polarc pointed out, I needed to have a diode on the original thermostat line to prevent back feeding 24V from the manual fan controller forcing the AC to come on.
The timer relay is currently set to turn the fan on for 30 minutes at three times spread out overnight. Tested it to make sure it all functions (as well as the original thermostat) and all looks good. I plan on seeing how it makes the upstairs feel overnight and make adjustments as needed. While the bare soldered diode has a lot of exposed bare wire right now, I plan on going back in there eventually and putting some heat shrink tubing just so nothing odd shorts out or bridges against it.
Overall pretty happy that with about 20 bucks in parts I finally have a way to program my HVAC fan separately of my thermostat, without having to move to a wifi/smart unit.
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