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The average mechanical installation labor in a commercial building can account for 35 to 55 percent of the total. It's usually the single biggest cost element.
There are seven variables, which are difficult or impossible to control, when assembly is done on-site. Some of these items involve labor indirectly. They look like inconveniences and physical restrictions, but their effect is to eat up time on the job dealing with them rather than actually performing the estimated work.
Job Location
The farther the job is from the contractor's shop, the longer the on-site work will take if all assembly is done on-site. Parts and components must be delivered, not always at the same time for one reason or another. Some get misplaced. Inevitably, some critical items are left behind and someone must drive back to get them, and then return, usually in rush hour traffic. The fewer trips to the building site by crewmembers, the better. The contractor has better control when the actual assembly of systems and subsystems is performed in the confines of the controlled shop environment.
Physical access on the job. When the mechanical room is below grade, or when it's 50-feet up, there are problems of access. At best they can be dangerous. When part of the job is performed at ground-level, and components are stored in a staging area, and the installation is a ladder and an elevator up, getting everything and everyone coordinated takes extra time. Communication gets complicated, and sometimes miscommunication causes a crewmember to be sitting up there, or down there, waiting for a part or an extra pair of hands which are waiting somewhere else.
Distance from resources
If the mechanical room is two or three hundred feet form the power and light sources, crewmembers are going to be continually going back and forth making and remaking connections. They'll experience delays as damaged welding leads are discovered and replaced. It's an axiom: the farther your resources are away from where they're needed, the more accidents and delays you'll experience.
Weather-related delays
They are bound to happen. They're caused by snow and cold, water and mud, heat and humidity, hail, tornadoes and hurricanes. And there's no way to control those elements. The best way to control the problem is to fabricate assemblies under roof where the elements can be controlled to minimize the need for field fabrication.
Material handling logistics
Unless the job is prefabricated, individual parts and materials must be transported to a job site, staged there, brought to the mechanical room in the right sequence, and assembled piece-by-piece. As a general rule, the more loose parts you can assemble in the shop, the lower your final costs will be.
Contention with other trades
In the beehive of a construction site everyone tends to look out for their own. If one crew can establish a "beach head" in the mechanical room before your crew does, working around them can be very time-consuming as well as frustrating. Under the best of conditions, there's a certain amount of waiting while someone else occupies space you need to be in. The problem gets compounded if more than one contractor needs to use resources such as electricity or elevators or a fork lift while another is using the same resources.
When a prefabricated unit is scheduled to be delivered to a job site, the general contractor can schedule a day, usually well in advance, when the mechanical room and its resources become the sole province of the mechanical contracting crew.
Poor scheduling and planning
Many contractors are not great planners, there is just not enough time in the day anymore. Around a construction site, things are always changing. Paths get blocked, space becomes cramped, access is cut off, crews contend for the same space at the same time. Just as you were about to move equipment and components into a mechanical room with block and tackle, the windows are installed and you are told to take the long way around. These are more than annoyances when you have to spend several extra hours moving materials. When the mechanical equipment is prefabricated, it becomes a major item on the contractor's schedule. It must be accommodated in the logical progression of construction and installation. No longer is it permissible to ask the pipe fitters to "take the long way around".
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