Is this fan only for new construction?

The 936 is an old work fan brace assembly, that includes a support bar, and cannot be mounted to a 2x4 cross brace. The 294 fan brace kit, can be mounted to the side of a joist/truss.

I think this would be used for existing construction just as often as for new construction. I fed it through a 5 inch hole from below in existing construction. If you get it in good and tight, and your joists are firm and not rotten or something, it should be as strong as some screws holding a regular fan box.

this is only a remodel box... NOT new construction...

" I can and am willing to use one of those between the beam old construction ceiling fan boxes with brackets since I can feed that through the hole and tighten down between the beams. " Sounds like you got the right idea on what this product was meant to do. If you can feed it thru the hole and space it between two beams (joists), tighten it down with bracket centered over hole, you've got it licked. Good luck! P.S. By the looks of the date of your question, you probably have the issue resolved by now.

I have installed over 50 ceiling fans. At the first I used a 2X4 but let me assure you I would never use a 2X4 now. Those brace kits will hold 10 times better & are much easier to install. Just make sure you use a metal junction box. Forget what the instructions said to use a board. Ask yourself a question, which will hold more weight wood or metal. What do you think? Their just trying to save you money. Good luck.