My house is built on a slab. Within the slab are "channels" which act as ducts for the HVAC system.
I assume it is partially lined with metal but based on photos from an HVAC-cleaner, it's hard to tell how far that metal goes from the registers.
Long term, I plan on re-routing all of my HVAC outside of this slab ductwork as it is extremely dirty, probably contaminated with all kinds of mold and insects, and essentially a big mess. This house was built over 60 years ago.
In the mean-time, I am about to re-do a single room's floor. It is currently concrete (carpet had been ripped up) but is going to be replaced by porcelain tile. This specific room is really close to the furnace (with a direct wall-vent a previous owner "cut in") so I am not concerned about its temperature.
I want to fill in, or otherwise seal off, the slab-duct and register in this room so I can tile over it. Long term, I want to do quit using all of my in-slab ducts.
Can I simply seal off the duct at this register? Or do I need to fill the entire duct in the slab in this room? What material (concrete/cement? specific type?) should I use in either case?