The X10 Powerflash module could send a signal if triggered when your HVAC fan is on.
An easier approach would be to use a boaster fan that runs automatically. ie It has a built in sensor that turns it on and off with the air flow.
A quick Google search gave me this
boaster fan on sale. Check around. This model should work nicely for you and appears to be on sale right now at more than one place.
Home Depot, while currently out online, might be the best "brick & mortar" place to pick one up.
Check this link.If aesthetics are important (ie. A dangling cord is too tacky.) stick on molding to hold the cord is cheap and can be painted to match your wall. (I've only seen it in white.)
If you really don't want your solution to be visible, they make in line duct boasters.
(Don't let the wife know about this option as they are a much bigger pain to install.
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