Mechanical, plumbing and fire protection drawings

Mechanical, plumbing and fire protection systems make a building habitable and safe: they heat, cool and ventilate it, bring in clean water and carry away wastewater and stormwater, and suppress fires. On many projects they account for a quarter to a third of the construction cost, and they compete for space in ceilings, shafts and walls with structure, electrical systems and each other. Their drawings rely heavily on symbols, abbreviations, single-line representations, schedules and diagrams, so they look very different from architectural drawings. This lesson explains how to read HVAC plans, equipment schedules, piping plans, plumbing plans and riser diagrams, fire protection drawings and control diagrams. CON-230 covers how these systems work and are installed; here the focus is on reading the drawings and spotting coordination issues.

The mechanical legend and general notes

Mechanical, plumbing and fire protection drawings each begin with a legend that defines dozens of symbols and abbreviations: duct fittings, diffusers and grilles, dampers, valves of many types, pumps, piping services, plumbing fixtures, drains, cleanouts and sprinkler heads. Learning the legend is the first step. Abbreviations for piping services are especially important, because many pipes look alike on plans and are distinguished only by their labels. Common examples include:

General notes on mechanical sheets cover items such as coordination responsibilities, access to equipment and valves, seismic restraint, firestopping of penetrations, testing, balancing and commissioning, and the contractor's obligation to provide offsets and fittings needed to avoid conflicts. That last point matters: MEP drawings are often diagrammatic, showing the intent and approximate routing rather than the exact path of every duct and pipe.

Diagrammatic drawings and coordination

Unlike architectural and structural drawings, many MEP plans are explicitly diagrammatic. They show what is connected to what, the sizes and approximate routes, but not every elbow, offset and hanger. A note such as "Drawings are diagrammatic and indicate general arrangement; provide all fittings and offsets required" is common. The trades then produce coordination drawings, increasingly from a combined 3-D model, that fix the exact routes of ducts, pipes, conduits and sprinkler mains in the ceiling space and resolve clashes (Lesson 11). Reading MEP drawings therefore means understanding the system intent and checking whether it can physically fit.

HVAC plans

HVAC (heating, ventilation and air conditioning) plans show the air distribution system for each floor. Ductwork is drawn in one of two ways:

• Single-line: each duct is a single line with its size noted, used on small-scale plans and schematic layouts.
• Double-line: each duct is drawn with two lines showing its actual width, used where space is tight and on coordination drawings.

Duct sizes are given as width × depth for rectangular ducts, with the first number usually the dimension seen in plan, or as a diameter for round ducts. Supply, return and exhaust ducts are distinguished by labels, line types or hatching in section. Air terminals, such as supply diffusers, return grilles and exhaust grilles, are drawn with symbols and tagged with a type and the design airflow. Variable air volume (VAV) boxes, fan coil units, dampers (balancing, motorized, fire and smoke dampers), access doors, thermostats and sensors are also shown.

US mechanical drawings use imperial units: duct sizes in inches (such as 24 × 16 or 12" Ø), airflow in cubic feet per minute (CFM), water flow in gallons per minute (GPM), static pressure in inches of water column (in. w.c.) and cooling capacity in tons (1 ton = 12,000 Btu/h) or Btu/h.

Equipment schedules

Mechanical equipment is described in schedules on the drawings: air handling units (AHU), rooftop units (RTU), boilers, chillers, pumps, fans, VAV boxes, fan coils, unit heaters, heat exchangers, humidifiers and more. Each row gives the equipment tag, location, service, capacity, airflow or water flow, pressure, electrical characteristics (voltage, phase, full-load amps or kilowatts), weight, the basis-of-design manufacturer and model, and remarks. Schedules link to the electrical drawings, which must provide the right power; to the structural drawings, which must carry the weight and provide supports and openings; and to the architectural drawings, which must provide access, clearances and screens. Comparing the weight and electrical data in the mechanical schedules with the structural and electrical drawings is a routine and valuable check.

Hydronic and refrigerant piping

Heating and cooling water systems (hydronic systems) and refrigerant piping are shown on piping plans, usually single-line, with service labels, pipe sizes and flow arrows. Valves, strainers, balancing valves, expansion tanks, air separators and pumps are shown with symbols. Mechanical room plans and sections at large scale show the boiler or chiller plant, pumps and headers in detail. Piping schematics or flow diagrams show the whole system without scale, making it easier to understand how the parts connect, and they are invaluable for commissioning.

Plumbing plans

Plumbing drawings show the domestic water, sanitary drainage and vent, and storm drainage systems:

• Domestic water: cold water from the service entrance and meter, through backflow preventers, to fixtures; hot water from heaters, often with a recirculation loop; pipe sizes and valves.
• Sanitary drainage and vent: gravity drain pipes from fixtures to the building sewer, sized by fixture units and sloped, with vents that admit air and protect trap seals, and cleanouts at changes of direction and the ends of runs.
• Storm drainage: roof drains and overflow drains or scuppers, rainwater leaders down through the building, area drains and the connection to the site storm system.
• Fixtures: water closets, urinals, lavatories, sinks, showers, floor drains, drinking fountains and emergency fixtures, located on the plans and listed in a plumbing fixture schedule giving make, model, mounting height, trim and connection sizes.

Underground plumbing below the slab is usually shown on a separate plan, because it must be installed, inspected and tested before the slab is poured. Getting underground plumbing right is critical: once the slab is placed, moving a drain is expensive and disruptive.

Riser diagrams, often drawn in isometric (Lesson 4), show the vertical arrangement of drains, vents and water lines through the building, with pipe sizes, fixture connections and cleanouts. They make it much easier to understand a multi-story plumbing system than plans alone.

Plumbing in the United States is governed by the plumbing code adopted locally, usually the International Plumbing Code or the Uniform Plumbing Code. Minimum slopes for drain pipes 2 1/2 in and smaller are typically 1/4 in per foot (about 2 %), with 1/8 in per foot allowed for larger pipes.

Fire protection drawings

Fire protection drawings show sprinkler systems, standpipes, fire pumps and related equipment. On many projects, the engineer's drawings show only the design intent: the hazard classification of each area, the water supply, the location of the fire service entrance, risers, standpipes and fire department connection, and performance requirements. The sprinkler contractor's designer then lays out every head and pipe and performs hydraulic calculations, producing shop drawings that are reviewed by the engineer and, often, by the authority having jurisdiction and the owner's insurer.

Key elements on fire protection drawings include sprinkler heads (by type: pendent, upright, sidewall, concealed, dry), branch lines and cross mains with sizes, the sprinkler riser with its control valve, flow switch, tamper switch and alarm valve, the fire department connection on the exterior, standpipe hose connections in exit stairs, and the fire pump if one is needed. Hazard classifications (such as light hazard for offices and ordinary hazard for storage rooms and mechanical rooms) determine head spacing and water demand.

In the United States, the International Building Code and International Fire Code, as adopted locally, require sprinklers in many buildings and reference NFPA 13 (installation of sprinkler systems), NFPA 13R and 13D for residential occupancies, NFPA 14 (standpipes) and NFPA 20 (fire pumps). Insurers may impose additional requirements.

Controls and sequences

Building automation and controls drawings show control diagrams for each system, with sensors, actuators, controllers and points lists, and the written sequence of operations that describes how each system should operate. Controls are often designed in detail by the controls contractor, but the engineer's diagrams and sequences define what the system must do. They are essential for commissioning and are often the last part of the mechanical work to be completed.

Reading MEP drawings for coordination

A construction manager reading MEP drawings is looking for whether the systems will fit, will be supported, will be powered and will be accessible:

• Ceiling space: compare duct sizes, pipe sizes and sprinkler mains with the depth between the ceiling and the underside of the structure.
• Shafts and openings: check that shafts are big enough and that openings in slabs, walls and roofs are shown on the structural and architectural drawings.
• Rated assemblies: identify every penetration of rated walls and floors that needs fire dampers or firestopping.
• Equipment: confirm weights, curbs, housekeeping pads, clearances and access for maintenance.
• Power: confirm that every motor and piece of equipment appears on the electrical drawings with matching characteristics.
• Sequencing: identify underground and in-slab work that must be done early.