Mechanical estimating

Mechanical work, meaning plumbing, heating, ventilation, air conditioning, fire protection and their controls, is usually the largest single subcontract on a commercial or institutional building. On a hospital or laboratory it can approach a third of the construction cost. General contractors almost always subcontract it, but that does not mean the general contractor's estimator can ignore how it is priced. On bid day the estimator must judge whether the mechanical quotes are complete and reasonable, carry a sensible number when only one quote arrives, and catch the scope gaps between mechanical, electrical, controls and general trades that cause so many disputes. On negotiated work and design-build projects, the general contractor may need to prepare or check a detailed mechanical estimate itself.

This lesson explains how mechanical contractors build their estimates, how a general contractor can check them, and where the gaps usually are. The numbers in the examples are illustrative; real labor units and prices come from a mechanical contractor's own history, published labor-unit manuals and supplier quotes.

What mechanical work includes

In MasterFormat, mechanical work falls mainly under Division 21 (fire suppression), Division 22 (plumbing), Division 23 (heating, ventilating and air conditioning) and parts of Division 25 (integrated automation). In the United States, plumbing work follows the plumbing code adopted locally (often the International Plumbing Code or the Uniform Plumbing Code), mechanical work the International Mechanical Code or Uniform Mechanical Code, and fire sprinklers NFPA 13; state and local licensing governs who may install plumbing and mechanical systems.

How a mechanical contractor builds an estimate

A mechanical contractor's estimate is built from a detailed takeoff by system. Each fixture, piece of equipment, length of pipe or duct and fitting is counted from the drawings, then priced with material costs from supplier price lists (with the contractor's negotiated discounts) and with labor units: standard hours per fixture, per meter or foot of pipe by size and material, per fitting, per joint or per kilogram or pound of ductwork. Labor units are adjusted for job conditions in the same way as general contractor productivity (Lesson 3): height, access, congestion, occupied buildings and so on.

Mechanical estimators commonly use dedicated software tied to manufacturers' price databases and labor-unit libraries, with digital takeoff from drawings or models. Their estimates typically separate labor hours from material, which allows a general contractor to check the implied crew size and duration against the schedule.

Ductwork by weight

Sheet metal contractors traditionally price fabricated ductwork by weight, because both the material and much of the fabrication and installation labor scale with the amount of metal. The weight of a rectangular duct run is its perimeter times its length times the weight per unit area of the sheet gauge used, plus an allowance for seams, joints, reinforcing and waste. Round and spiral duct, flexible duct, and fittings such as elbows and transitions are often priced by piece or length. Terminals (diffusers, grilles, registers), dampers, fire dampers and access doors are counted and priced individually.

# Weight and cost of rectangular galvanized duct runs (illustrative weights and rates)
runs = [  # width in, height in, length ft, sheet weight lb/ft2 by gauge
    (36, 20, 105, 1.406),   # main trunk, 20 gauge
    (24, 16, 160, 1.156),   # 22 gauge
    (16, 12, 245, 1.156),
    (12, 8, 360, 0.906),    # 24 gauge
]
allowance = 0.18                 # seams, joints, reinforcing, waste
total = 0.0
for w, h, L, ws in runs:
    lb = 2 * (w + h) / 12 * L * ws * (1 + allowance)
    total += lb
    print(f"{w} x {h} in, {L:3d} ft: {lb:7.0f} lb")
rate_fab, rate_install = 1.86, 2.86   # $ per lb fabricated, $ per lb installed labor
print(f"Total {total:,.0f} lb; fabricated ${total * rate_fab:,.0f}; "
      f"installation ${total * rate_install:,.0f}")

36 x 20 in, 105 ft:    1626 lb
24 x 16 in, 160 ft:    1455 lb
16 x 12 in, 245 ft:    1560 lb
12 x 8 in, 360 ft:    1283 lb
Total 5,923 lb; fabricated $11,018; installation $16,941

The output gives the weight of each run and the total cost of fabrication and installation. A general contractor reviewing a sheet metal quote can use the same arithmetic to check whether the quoted weight is plausible for the drawings, and can compare cost per unit of floor area with similar projects.

Plumbing by fixtures

Plumbing estimates start from the fixture schedule: water closets, urinals, lavatories, sinks, showers, drinking fountains, floor drains, mop sinks, hose bibbs and so on. Each fixture brings with it a carrier or support, trim, connections, and a share of the waste, vent and water piping. For a conceptual check, an estimator can price plumbing as a cost per fixture, including the associated rough-in piping, plus the main lines, water heaters, pumps, storm drainage and gas service priced separately. For a detailed estimate, piping is taken off by length, size and material as shown above.

Fire protection

Sprinkler systems are usually designed by the sprinkler contractor (design-assist or delegated design) from performance criteria in the specifications and the applicable standard, so the contractor's estimate includes design, hydraulic calculations and approvals as well as installation. Sprinkler work is commonly checked by heads and by area: the number of heads depends on the hazard classification and the maximum coverage per head, typically about 20 m² (225 ft²) for light hazard occupancies with standard spray heads, less for ordinary hazard. Estimators must also check whether the scope includes the fire pump, the backflow preventer, the fire department connection, standpipes, dry systems in unheated areas, and the water service to the building, which may belong to the site services contractor.

HVAC equipment, controls and commissioning

Major HVAC equipment is priced from supplier quotes, which usually include the equipment, factory start-up and warranty, but not freight to site, rigging, setting, curbs, vibration isolation, electrical connections or controls interfaces. Crane lifts for rooftop units must be coordinated with the general contractor's crane plan or priced as separate mobile crane days. Building automation is usually priced by control points: each sensor, actuator or monitored status is a point, and the controls contractor's price includes devices, wiring (often), panels, programming and graphics. The division of control wiring between the electrical and controls trades is one of the most common scope disputes.

Testing, adjusting and balancing (TAB) of air and water systems is often carried by the mechanical contractor through an independent balancing firm, and commissioning may be carried by the owner through a separate commissioning authority. The estimate must reflect who pays for what, including the contractor's own labor to support functional testing and to correct deficiencies.

Where the gaps are

Mechanical scope gaps tend to appear at interfaces. The following list is a useful checklist when writing mechanical scope sheets and leveling quotes (Lesson 7).

Checking mechanical prices on bid day

With the takeoff methods above, a general contractor can check mechanical quotes without redoing them. Useful checks include cost per unit of floor area compared with recent similar projects (adjusted for location and time, Lesson 12), mechanical cost as a share of total construction cost, cost per fixture and per sprinkler head, implied labor hours compared with the schedule (a quote implying 30,000 hours on a 12-month mechanical schedule means an average crew of about 17 people), and the presence of major equipment at prices consistent with supplier budgets. A quote far below the others is not good news until the estimator understands why.