The thermostat was in the wrong zone
Comfort complaints in an older office building almost always arrive as an equipment accusation. The rooftop unit is fifteen years old, half the floor is uncomfortable, and the obvious purchase is a new rooftop unit. It is the most expensive way to find out the problem was never the rooftop unit.
What this building actually had was a distribution system that no longer matched its floor plan. The owner acknowledges in the contract that multiple interior remodels were performed over the years without modifying the original HVAC design. Walls moved; the terminal units and the thermostats did not. Up to ten thermostats were found controlling areas no longer served by the terminal unit they were wired to, which means the box was reading one room and conditioning another. No amount of new tonnage upstream corrects that. A new rooftop unit delivers correctly conditioned air into the same distribution system, which then sends it to the same wrong places, at a higher cost and with the complaints intact.
So the fix is at the terminal level. Four single-duct VAV boxes and five parallel fan-powered boxes with electric reheat were added, with existing ductwork modified to receive them and a thermostat at each box for local control. The fan-powered boxes with reheat exist because a zone that goes to minimum airflow on a cooling-only system has no way to stop getting colder — the box gives that zone its own fan and its own heat instead of borrowing warmth from a neighbor's overcooling. Ceilings come out and go back in to reach the plenum, and every new and existing box on both floors is rebalanced as one system.
The rooftops were still replaced, and the specification reflects the new zoning rather than the old. Each 50-ton cooling-only unit moves 20,000 CFM at 2.0 inches of external static, with four compressors on four-step capacity control down to 25 percent turndown, an enthalpy economizer and low-ambient operation to 25 degrees. Turndown is the number that matters once zoning is corrected, because a properly zoned building spends most of its year at part load.
Redundancy for the server room
A server room does not follow the building. Its load is roughly constant across a 24-hour day and a 12-month year, it does not care about occupied-hours schedules, and it is the one space where an economizer changeover or a Saturday shutdown for rooftop service is not an inconvenience but an outage.
That is the argument for giving it a dedicated machine rather than a better branch off the building system. The room gets a 5-ton Liebert split-DX air handler, cooling only, with a variable speed EC fan, a condensate pump and a new rooftop condensing unit with refrigerant piping routed up through the building. It is added for redundancy, so the building system still reaches the space; the split-DX is a second path rather than a replacement.
Two details are what make it a server room machine instead of a small comfort system. The scroll compressor carries 40 to 100 percent digital capacity control, which lets it hold a steady room at partial load instead of short-cycling the way a fixed-capacity compressor does when the racks are half loaded. And the outdoor unit is rated to run down to 30 below zero, because the room needs cooling in January as much as in July, and a standard comfort condensing unit will not start at those ambients.
The unit and its condensing unit are fed from a new electrical panel installed in the server room. That is a small line item with a large operational consequence: the cooling for the room is on a circuit that can be identified, isolated and worked on without anyone tracing a panel schedule in a building that has been remodeled repeatedly.
