Packaged rooftop units, replaced without repeating the last mistake

The nameplate on the unit you have tells you what somebody selected, at some point, for a building that may since have changed its glazing, its lighting, its occupancy and its use. It does not tell you what the building needs.

Before we quote a replacement we trend supply and return temperatures, take a full static pressure traverse, and run a fresh block load. It is common to find units moving 20 to 30 percent less air than their schedule claims, which means the building has been under-served for years and nobody has been able to say why.

Sometimes that survey tells us a unit does not need replacing at all. We would rather say so than sell a machine that fixes nothing.

Replacement units rarely match the footprint of what came off the roof, which means a curb adapter. A generic adapter fills the gap and leaves an abrupt transition immediately at the unit discharge, exactly where velocity is highest and a bad fitting costs the most.

We draw and cut adapters in our own shop for each specific curb and each specific unit, with proper transitions. On the Central Park project that, plus filter rack correction, was most of the reason total external static pressure fell from an average of 2.6 in. w.c. to 1.1 in. w.c.

We also check the structure. Rooftop equipment in Colorado has to account for the ground snow load and, more importantly, for drift loading against taller adjacent construction, which is where curbs and framing actually get overloaded.

A rooftop unit spends almost none of its life at full load. IEER, the integrated energy efficiency ratio, weights performance at 100, 75, 50 and 25 percent capacity, which is much closer to how the machine will actually run than a single full-load EER figure.

The economizer matters at least as much. In climate zone 5B, free cooling is available for a very large number of hours, and ASHRAE 90.1 requires economizers on most units above 33,000 Btu/h. We have opened seven out of nine units on a campus and found the economizer seized, disconnected or set to a changeover point that never occurs.

Ours get a differential enthalpy changeover, calibrated sensors, fault detection enabled, and the setpoint recorded in the maintenance log so that next year somebody can check it.

Most of our rooftop work happens on Saturdays. We plan crane picks from a single set-up point per weekend, protect the roof membrane, sequence the units so any given tenant loses at most a Saturday, and are off the roof before Monday.

Long-lead equipment is ordered against a confirmed programme, not a hopeful one. If a unit will take fourteen weeks, we say fourteen weeks at proposal stage.