Product review (2 stars): Can only be used vertically, poor customer service
There is a small flap inside the blocking door which opens when your dryer is on so that some of the hot air goes outdoors when set to vent indoors, and if you have the product placed horizontally, it doesn't close back up after the dryer has stopped, which allows cold air to come into the house from the vent.  So evidently this cannot be used horizontally, only vertically.  I guess when it is vertical, the small flap sits down and closes.  When I called Dundas Jafine to ask questions about the product, I was treated like an idiot by the customer service representative.  Will not purchase anything from this manufacturer again due to their customer service representative's demeanor.