I need more cooling to one bedroom. I'm installing a blower in the duct and I want it to turn on when the Heat Pump blower is on. I have a relay that I would like to activate from the thermostat blower and common wires. When the relay would be activate, it's contacts would complete a separate 120VAC circuit to the inline blower. Is it acceptable to splice into the thermostat wiring for this type of application?

morning everyone, I've been reading like crazy on this topic, and keep getting conflicting info. What would you guys do? We have a 100 year old farmhouse. Central air was retrofitted who knows how long ago, but we have a HE A/C unit and air handler. First floor cools exceptionally well, 2nd floor not so much. There are returns in each room of the 1st floor, but not a single one on the 2nd floor, aside from the staircase. My grandpa suggested using a wall cavity on the 2nd floor as a return, and I've identified a section that opens all the way to the basement. The only registers on the 2nd floor are in 2 of our 3 bedrooms (more on that below) and in the bathroom. In the 3rd bedroom, we couldn't figure out why it was so cold in the winter and so hot in the summer. There's a register in the floor. I took the grate off, and found that there is no ducting. It's like they put the "register" in place to make it look like it was connected. Luckily, I can run a duct to it from below as it lines up with a duct in the kitchen. How would you guys go about cooling the 2nd floor more efficiently? We've got fans running, and that helps, but as it gets hotter here in Cincinnati our bill is already climbing. Really appreciate any feedback or ideas. Side note, our siding is about to be replaced and the house Tyvek wrapped and air sealed at the same time. That should make a sizable dent right?