Patrick Henry Building Renovation 2020

A tri-deck multizone on the existing curbs

Patrick Henry had rooftop multizone units serving 54 zones and no air conditioning. The textbook way to add cooling to a building like that is to convert it to variable air volume: new air handlers, a VAV box in the ceiling above every zone, and new ductwork to connect them. It works, and it means rebuilding the distribution in every room of a building that was also getting new plumbing, sprinklers, ceilings and floors. The project kept the multizone layout instead and changed the kind of multizone.

What was wrong with the old multizone

A conventional multizone unit has two decks, one hot and one cold, with a pair of dampers per zone that blend the two streams. It gives every zone its own temperature from one unit, and it does it by heating and cooling air at the same time and mixing them. Any zone that wants a mild temperature is burning gas and running a chiller to get it. Energy codes have restricted that arrangement for decades.

The third deck

A tri-deck unit adds a neutral deck that carries mixed air past both coils untouched. Each zone damper blends neutral air with cold air, or neutral air with hot air, and never hot with cold. A zone that needs nothing gets neutral air, and no energy is spent on it. The five new units are tri-deck machines with a chilled water cold deck, a gas-fired hot deck and the neutral deck between them, with direct-drive fans on ECM motors and MERV 8 filtration, and they were built to fit the existing roof curbs. Each of the 54 zones kept its own duct and gained a modulating damper and a new wall thermostat on direct digital control.

Where the cooling comes from

One 180-ton air-cooled chiller with a variable primary pump package feeds all five cold decks through piping run across the roof. Because that piping is outdoors, the loop is charged with 30 percent glycol. The chiller runs a 40 to 50 degree leaving water range, and variable speed drives on the compressors and pumps let it follow the load down through spring and fall.

Four numbers that stop a roofing crew

Five roof sections were torn off and replaced with fully adhered 75-mil EPDM over tapered insulation averaging R-25. Fully adhered means the membrane is glued to the board below it, and adhesive is sensitive to weather in ways a mechanically fastened sheet is not. The agreement defined a weather day in advance with four thresholds: a rain forecast above 35 percent, air temperature under 48 degrees, wind over 12 miles an hour, or less than 7 degrees between the air temperature and the dew point.

Each number guards against a specific failure. Rain matters most on a tear-off, because an open deck has nothing over the classrooms below. Bonding adhesive has to flash off its solvent before the sheet is mated, and below the high 40s it does so slowly and unevenly. Wind lifts a wide sheet while it is being positioned and blows dust onto wet adhesive. The dew point spread is the subtle one: when the roof surface is within a few degrees of the dew point, moisture condenses on the adhesive film as it cools, and the membrane is bonded to water instead of insulation. That roof looks perfect the day it is finished and blisters the next summer.

Writing the thresholds into the agreement took the judgment call away from a superintendent with a deadline. Days lost to those four conditions were added to the end of the schedule, so nobody had a reason to glue a roof on a morning it should not have been glued.