My first thought is “oh no” which melted to Boilers are not like HVAC units in that you are not bringing air in to the house from the garage.
Is the space big enough that combustion air is not an issue?
many areas require 18 inches above the floor but some do not .
Check pipe penetrations or any for that matter which lead into the home .
Why are garages (both attached and detached) fire hazards?
Oil or gasoline can drip from cars. These fluids may collect unnoticed and eventually ignite.
Flammable liquids, such as gasoline, oil and paint, are commonly stored in garages. Some other examples are brake fluid, degreaser, motor oil, varnish, lighter fluid, and fluids containing solvents, such as paint thinner. These chemicals are flammable in their fluid form, and some may create explosive vapors.
Heaters and boilers, which are frequently installed in garages, create sparks that can ignite fumes or fluids. Car batteries, too, will spark under certain conditions.
Mechanical or electrical building projects are often undertaken in the garage. Fires can easily start while a careless occupant is welding near flammable materials.