Is this a condensing boiler with PVC supply and exhaust pipes? You might need to raise the boiler so the ignition point is 18" above the floor. Ask the local code official.

You would want an air intake opening and an exhaust opening so you don't affect air pressure in the boiler room and suck combustion fumes into the garage. You also don't want to have air intake low in the wall which could introduce gas fumes into boiler room. Is there a fire door on the boiler room?

boiker has air in take and exhaust Already. I’m saying just something to allow waste heat into the garage

If you add any kind of opening between the garage and the boiler room, the boiler room would then be considered part of the garage space (2015 IRC G2408.2). That raises the issue of the height of the ignition source and the design of the combustion & exhaust air, so the first thing to determine is the type of boiler and how it is installed.

All they had was a passive vent maybe 12” x 10” installed midway on wall between boiler room and garage. I had always heard having a low vent and high vent but they went in the middle and only one.

"a passive vent ... on wall between boiler room and garage"

Is the combustion air opening in the shared interior wall between the spaces rather than in an exterior wall?

Where is the access to the boiler room? Post a diagram.

What is the fuel?

That door is access to boiler room from garage. Air intake is pulled in from outside fresh air. Exhaust is pushed outside to fresh air.

If the opening to the right of the door is not open to the garage and the long 6"+/- rigid metal duct that terminates below the one that looks like a flexible dryer or bathroom exhaust duct, then adding a fan in an opening from the boiler room to the garage should only require the existing gas boiler ignition source to be 18" off the floor if it isn't already and if it's not listed as "flammable-vapor resistant".

The concern regarding a boiler in a single family attached garage (other than it being hit by a car) is the possibility of flammable vapor near the floor igniting.

But the final word on this will be from the local building official since these rules are often amended locally.

In in any case, in a single family dwelling the shared wall of a boiler room and garage isn't required to be fire-resistance rated nor is the door assembly.

The opening to the right was a passive vent to the garage.