You may want to ask this on the electrical forum. I had several lights capped off until I found the perfect lights. The chandelier over our staircase weighs 200 pounds and no electrical box would support it. I ended up putting in a chandelier lift and bypassed the box altogether. Metal boxes hold more weight than plastic boxes. Do not use plastic ones if you do not have your fixture or know the weight of the fixtures. The strongest residential electrical box supports up to 50 pounds.

You could mount one to the surface and feed it with wiremold, as long as you can be sure to get the mounting screws directly into a ceiling joist. There are also fan boxes and braces that can be installed through a box size opening. You would then fish wires across the ceiling to get power. If there is an attic above it is very easy. A finished floor above takes some more work and will cause some wall and possibly ceiling damage.

The box should be a metal one and securely mounted to the ceiling structure through its back, not a side bracket. There are some plastic boxes that are made for lighter weight fans, but I would look for a metal box if I weren't installing it myself.

I would not recommend fans with metal blades, they will rust and typically require ceilings over 10 feet because they are not considered safe in residential applications. You are best off with fans that have special UV protective ABS plastic blades or thicker solid wood blades that are rated for outdoor wet or damp locations. Wet rated fans have the highest tolerance for abuse from the elements and are the best choice, but a covered porch like yours can use a damp rated or wet rated fan. Also, in outdoor locations it is critical to consider the amount of airflow that the fan produces because open outdoor locations do not allow the air to escape on the sides and the breeze you feel from a fan is a lot less than what you feel in an enclosed area indoors. So you need more powerful fans than those listed above. The Donaire fan recommended below has a wind speed factor of about 3.8 MPH, which is over 50% more breeze than the Westinghouse fan above. The ultimate outdoor fan is made of marine grade 316 stainless steel and has ABS blades. It is the Donaire by Matthews Fan Company and is available in brushed nickel or bronze stainless steel. Here are some pictures of the fan taken from the product detail pages using the Room Settings Viewer at Hansen Wholesale. This feature allows you to add your own room setting so you can see how any fan on the website looks in your own setting, so you may want to give that a try. Donaire in Brushed Nickel Donaire in Bronze There are many other high quality outdoor ceiling fans available from Hansen Wholesale. To add your photo to use the room settings viewer email some pictures to: DesignService@hansenwholesale.com with the subject "Room Settings Viewer - Houzz" Response provided by: Hansen Wholesale https://www.hansenwholesale.com 1-800-201-1193