Exhaust Vent Booster

I’ve never seen this setup. It is a standard Armstrong Ultra SX Cat 1 furnace. The installation is in the crawl space, with a horizontal vent out the side of the wall. There is a booster fan at the vent termination that is wired into the inducer fan circuit. Seems to be working fine, but I have my doubts that the manufacturer would approve of the installation. Any thoughts? I couldn’t find any info on the Allied or Armstrong web sites that was helpful.

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