No, it is not even close to being acceptable. You cannot have penetrations from the garage into the house. Which is why a fire door and fire rated wall is code required. Penetrations into the fire wall is against code because the code wants to prevent deaths from carbon monoxide poisoning and fire.

Your HVAC tech is an idiot-- which is, unfortunately, an accurate description for the vast majority of HVAC techs. The code does not allow what is proposed. If you want to condition the garage install a ductless mini-split.

Thanks for the replies but where I live in florida we don't have fire walls and doors. My entire attic, including the attic over my garage is all open. There wouldn't be any penatrations from garage to house. Simply adding 2 ducts off the plenum into the garage with no return. They wouldn't be open all the time. Just when I am working in the garage. Also, we don't park in the garage and there are no chemicals either. It's just a man cave. Thanks again.

The HVAC guy evidently isn’t the only idiot in FL. “FL man” is a meme for a reason.

Learn better science dude. Its surprising you‘ve lasted this long. Codes are national and international.

Randy,

I don't know what passes for a building code in Florida, but here's what the 2018 International Residential Code (which is the core of most state's building codes) has to say about ducts in garages:

R302.5.2 Duct Penetration

Ducts in the garage and ducts penetrating the wall or ceiling separating the dwelling from the garage shall be constructed of a minimum no. 26 gauge (0.48 mm) sheet steel or other approved material and shall not have openings into the garage (emphasis added.)

Your HVAC tech is an idiot. You don't want to join him in that category, do you? Don't tie your home's ducted HVAC system to supply your garage.

If you remove the overhead garage door and infill with wall, then that would allow you to have the supply vents (be aware that a space without return air will not perform with optimal comfort anyway). Maybe one Florida storm you slide the bar over and bring in the car or motorcycle to wax, then after a drunken night in your man cave you decide to show off for your buddies and rev up the hot rod or bike. There is a reason this particular section of code has been around since codes have been around. Usually codes start as reactive to real events. And the walls are fire separation walls, not fire rated, but regardless of someone's terminology the intent is to reduce risk to you and your family.

This isn't so hard. The easiest: If you blow air out of the house (HVAC vent in the garage), that creates negative pressure that sucks air into the house. Think of summertime in Florida. The amount of air blown through an HVAC duct into the garage will be offset by an equal amount of hot and humid outside air sucked into the house through the normal places air enters non-tight houses: exhaust ducts for bathrooms and kitchens and even backpressure on water heater stacks if in the house, holes in walls (electrical switches and sockets, pipe openings), around and through windows, and the like.

A second easy one: there may not be a firewall between the home and the garage but it should be sealed from air flow. Why? An opening would allow car exhaust to enter the house. Turn on a bathroom fan, start the car, and air will be sucked through the duct and into the house. Definitely against code restrictions anywhere in the US.

Get a new HVAC tech - the guy is a know-nothing idiot, as others has said.