How to patch hole in furnace pipes?
There are a several things you need to do.

Remove the fan from the heating duct. This will entail properly wire-nutting the wires and terminating them in a junction box that is covered but left accesible per code. If the fan is on it's own circuit (unlikely) you would want to shut down that breaker. Importantly, you want to mark the new junction box and any breakers/switches/thermostats so a future owner will understand what was done.

You'll want to close off the 8-1/2 inch hole so that it is airtight. Assuming that the inner surface around the hole is smooth (sheet metal, OSB or similar sheathing) I would cut a plywood square larger than the hole (11x11) and mount it on the interior surface with a high quality construction adhesive and secure it with either appropriately sized sheet metal or wood screws. I would then run a heavy bead of silicone caulk around the edge of the plywood to ensure there is no leakage of air. If you are attaching the plywood directly to the ductwork ideally you'll want a solid surface on the outside of the sheet metal to insure good contact. You might be able to add another piece of plywood on the exterior to accomplish that. In that situation you'll also have the option to patch it with a square of sheet metal instead of the plywood using sheet metal screws and applying silicone caulk to the seams.

Lastly you'll probably want to add some insulation and finish the exterior appropriately.