Hello ginger and thank you for your question.
The Cadet Com-Pak Twin Plus 16-1/4 in. x 12 in. 4,000-Watt 240 Volt Fan-Forced In-Wall Electric Heater (Model # CSTC402TA) can only be installed “horizontally” as shown (with the arrows in the wall can pointing upwards).
The OWNER’S GUIDE can be found here: http://www.homedepot.com/catalog/pd...
You will need to follow the specific installation instructions for Com-Pak Twin Plus (Model CST) for this product.
Best wishes on your project.