Worked cases · Chicago

An office floor on one packaged VAV rooftop

The DOE Medium Office plan — four perimeter zones and a core — on a 90.1 System 5 rooftop with boxes, an economiser, demand-controlled ventilation and a supply fan holding duct static.

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Lakeside is one 1 660 m² floor from the DOE prototype: four perimeter zones and a core, DOE occupancy shape, 26.7 / 15.6 °C setback, optimum start, a 0.012 kg/kg ceiling. One packaged DX VAV rooftop serves it — an outside-air box with an economiser and a 62.1-pinned DCV floor, six inverter modules on a 7 m³/s coil, a 60 kW electric warm-up heater, a supply fan holding 150 Pa of duct static, a two-fan condenser with head-pressure control — with a VAV box on every zone.

Three sizing lessons, all measured: three modules on 3 m³/s held none of the zones (the floor peaks near 70 kW at 90.1's densities and 32 m² of aperture); 6 m³/s left north and east short a dozen hours a week when every box's peak coincided; and boxes must be sized to each zone's peak with headroom rather than area share — south 1.8 m³/s against north 1.2.

On Chicago's year the core is held to the hour, the perimeter zones read 16–25 unmet cooling hours each, and 122 MWh of the 125 MWh of heating is reheat — the boxes' at their minimums and the ceiling's. That is what a VAV rooftop in a cold climate does, and the page says so.

What it reads on Chicago

Cooling / heating	107 MWh / 125 MWh (122 of it reheat)
Electricity	165 MWh
Zones	core held to the hour; perimeter 16–25 unmet cooling hours each
Deck	13 °C reset to 18 °C from the critical zone (90.1 §6.5.3.5)

What it taught

A box at its bare peak is short whenever the deck sits above its floor; size boxes to each zone's peak with headroom.
A VAV box with a cooling load opens on the neutral deck on heating hours rather than parking at its minimum — or the core sits at 26 °C for 455 hours.
A VAV handler makes its winter deck with its economiser, but only while some box is cooling; a cold deck otherwise is reheat for nothing.

The machines

Example packaged DX VAV rooftop, 7 m³/s — open in the builder

Sources

DOE Commercial Prototype Building Models — Medium Office
ASHRAE 90.1 Appendix G System 5; §6.5.3.5 reset
PVGIS TMY, Chicago

Readings of the shipped model on the building's own PVGIS typical meteorological year; engineering estimates from catalog data and published correlations. The building runs on every plan, including Free, a few times a day.