Turn on Measure, then click two points on the drawing.
Drag to pan, scroll or pinch to zoom, or use the buttons. With the drawing focused, the arrow keys pan, plus and minus zoom, and 0 fits the sheet. Every tag, marker and dimension is a focusable target: press Tab to walk them and Enter to select one.
A mechanical plan is an architectural plan with the air system laid over it. The first thing to do is separate the two. Everything drawn faintly is the architectural background: the walls, the door openings, the grid and the room tags, copied from A-101 and screened back so it cannot be mistaken for mechanical work. It is there so you can say where a duct is, and nothing on it is the mechanical contractor's scope. If the background disagrees with A-101, A-101 wins.
Everything drawn at full weight is the mechanical work. Start at the equipment. RTU-1 is a packaged rooftop unit sitting on the roof above Mechanical 110, drawn dashed on this plan because it is above the level being shown. Its tag sends you to the equipment schedule at the bottom of the sheet, which gives the supply air quantity, the outside air quantity, the cooling tonnage, the heating output and the electrical characteristics. A tag on a plan and a row in a schedule are always the same piece of equipment; the plan says where, the schedule says what.
Follow the air from there. A duct is drawn as two parallel lines the true width of the duct, with the size written along it as width by height in inches. The riser leaves the roof unit, drops through the roof curb and lands in Corridor 114, where the trunk splits east and west. Each trunk gets smaller as it gives air away, and the sizes on this sheet step down accordingly: the riser is the biggest, the trunks are next, and the branches into each room are the smallest. If a size looks wrong for the air it carries, you have found either a mistake or a duct you have mis-traced.
Every branch ends at an air terminal. A square with a diagonal cross is a supply diffuser; a square with parallel bars is a return or exhaust grille. Each carries a tag and the air quantity in cubic feet per minute right on the plan, and the same tag appears in the air terminal schedule with its room and its neck size. Reading a diffuser is a three step move: find the symbol, read the tag and the CFM beside it, then confirm the tag in the schedule.
The thermostats are the last piece. Each one is a small circle with a T, mounted on a wall in the room it controls, and it is the reason a room's air quantity is what it is. The legend at the top left names every symbol used, and the general notes state the rules that are not drawn: coordinate with the structure, balance to the quantities shown, damper every branch, and seal the joints.