Ceiling space is that portion of a building between the ceiling of one space and the floor of the space above. This space is usually reserved for HVAC ductwork, lights, conduit, and piping. Modern building designers strive to minimize the size of this zone because it not only adds to the construction cost but also increases the size of the building envelope, and consequently the magnitude of the load on the HVAC system. Most buildings can manage with 2 or 3 ft of ceiling space; older buildings often have 5 or 6 ft above the ceiling.
Strictly speaking, the ceiling space is not a conditioned zone. HVAC designers usually assume, though, that the exterior walls fronting this dead zone are part of the overall building load and include them in the calculations for equipment sizing and the quantity of conditioned air supplied to each space. They also include heat emitted by ceiling-mounted lights and their ballasts, as part of the building load. In the summer these two heat sources can be a considerable portion of the load on the air-conditioning equipment.
Some organizations have experimented with an interstitial space between floors. This is equivalent to the ceiling space, but it’s large enough to walk in. The rationale is that, if all electrical, plumbing, and mechanical equipment is located in this space then it’s easier to remodel the space below with a minimum of disruption to the regular routine. This is important in busy hospitals or service to critical spaces in other occupancies that cannot be moved or shut down while construction work is underway.
The interstitial space is usually ventilated, as an energy-conservation measure. Given that the ceiling is sufficiently airtight, this forced evacuation of air removes the heat emanating from lights before it enters the conditioned zones. This ventilation also prevents the wall and roof loading from contributing to the building load.
Such a scheme can be just as effective in existing buildings that have the ventilation fan interlocked with a thermostat. In the summer, the space is evacuated before it starts to heat up the occupied spaces, and in the winter the system retains the heat from lights, which partially offsets the losses through the perimeter areas.
Most ceiling spaces are too porous to be ventilated without drawing a lot of conditioned air out of the occupied areas below. In such circumstances, it’s advisable to not use the ceiling space as a return-air plenum. This effectively makes it into a conditioned zone. The alternative is a ducted return scheme, which isolates the return from all thermal influences in the ceiling area.
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Excerpt from Energy Conservation Projects.