How comfortable you feel in a building is to a large extent the result of the temperature of the air. If it’s too warm, you feel sluggish and sleepy: kids fall asleep in classrooms and staff gets less done. But air that’s too cold is also disruptive. Shivering is distracting and at some point it’s hard to type.
But temperature is only one aspect of the indoor environment. Air quality matters, too. McCord Hall is going to be a very comfortable and healthy building – based on air temperature and quality.
“McCord Hall will be probably the healthiest and most efficient building on campus,” said Mike Nixon, W. P. Carey’s director of facilities and space planning. “The indoor air quality will surpass any other building at ASU, and once we get used to it we won’t be happy anywhere else.”
McCord Hall has an under floor displaced air system. The floors are actually two feet above the slab. Running through that space, called the plenum, are cable trays and conduits for electricity – and fresh cooled air. Fans in each wing push air down from the air handlers on the roof to the plenums under the floors, and up through grates at our feet. Understandably, the plenums are sealed so that they are airtight.
Among the many advantages of the system is that more fresh air will circulate through McCord Hall than buildings cooled and heated by the older conventional systems. The building occupants should feel more alert, and since we will be breathing less recycled air, perhaps we’ll share fewer colds!
How it works
DPR’s Lew Laws explains that a conventional system distributes air cooled to 55 degrees through overhead ducts. At 55 degrees, he said, the air will fall to the floor – simple physics. The air will pick up heat from the load, which is defined as anything that generates heat in the space — including people and computers. Once warmed, the air will rise again, to the return ducts which move it back through the system to be re-cooled and re-circulated.
In McCord Hall, the air handlers — those large silver structures on the roof — will cool the air to 65 degrees. It doesn’t need to be 55 degrees because it will arrive at the room under the floor instead of overhead: it doesn’t need to be cold enough to fall. Propelled at a low velocity by the fans, air will travel up through diffusers in the floor, where it will encounter the load: us, and our computers. It picks up our heat, and – like conventional systems – it rises to the return ducts in the ceiling. Sensors will detect air temperature, and will adjust the diffusers in the floor to let in more or less air.
Lew says that McCord Hall was designed with an under floor displaced air system for three reasons: 1) air quality; 2) flexibility; and 3) aesthetics.
McCord Hall’s under floor system circulates warmer air than the conventional systems to achieve the same comfort level for people. Because the air in the system starts at 65 degrees instead of 55, we can provide more fresh air for the same amount of energy used as a conventional system.
In Arizona, 10 percent of the air in most buildings is fresh from outdoors; in California the code requires 20 percent. McCord Hall will run at 20 – 30 percent, and in some seasons the indoor air will be 100 percent fresh. Lew said that the number of days we’ll be able to run on 100 percent outdoor air is greater because we have more 65 degree days in Arizona than 55. (On those few days when it’s very cold outside, heaters will warm the air in the system.)
As for aesthetics, the system reduces the need for overhead ducts, which means ceilings can be exposed, giving the architects more design freedom.
A chilling tale
The way ASU produces air conditioning is itself an interesting story.
In the heart of campus is the Central Plant, a large industrial-looking building between Murdoch Hall and Life Sciences. Lew explained this is where water is chilled to about 42 or 43 degrees, at night when institutional electricity rates are low. That’s important, because 90 percent of the funds spent on HVAC systems is for the operation of the chillers.
The chilled water is stored in six 1 million gallon tanks under the northwest corner of the SRC field. (Interesting point: a huge drainage pipe, laid just before we broke ground for McCord Hall, lies under the south end of the field. So, the fields are probably safe from development for a very long time!)
During the day, cold water is drawn from the tanks in response to demand from the cooling systems in our buildings. Chilled water is pumped from the tanks (which are literally next door) to the roof of McCord Hall, where the air handlers are located. The water flows through coils of pipes, and as air from the returns and from outside circulates around the coils it loses heat. Fans force the cooled air down into the building to the plenums, through the diffusers into the rooms and back to the return ducts which deliver the now warmer air to the air handler.
Everyone involved in the McCord Hall project expects the HVAC system to perform very well. According to Mike Nixon, “if it does what we believe it will, every new building on campus will be cooled this way.”