TABLE OF CONTENTS In all but the earliest fully ducted UFAD installations there is some amount of supply air flowing through the underfloor plenum in direct contact with the concrete floor slab. Control strategies should carefully consider thermal storage in the slab and other components (e.g., floor panels). UFAD systems are very stable in operation with only gradual (usually unnoticeable to occupants) changes in supply temperature over time, unless the supply air is isolated from the mass.
Energy and operating cost savings can be achieved using a thermal storage strategy in the concrete slab. In temperate climates, cool night-time air can be brought into the underfloor plenum where it effectively cools the slab overnight. During the following day's cooling operation, higher supply air temperatures can be used to meet the cooling demand, thereby reducing refrigeration loads for at least part of the day. This 24-hour thermal storage strategy benefits from lower off-peak utility rates and extends the hours of economizer operation. For this strategy to be successful, the following issues must be addressed:
Heating night setback must be used.
Since a precooled slab will be at its coolest temperature in the morning, the design, capacity, and response rate of the heating system, if needed, will be particularly important under morning startup conditions. Ultimately, the most important consideration is to optimize the control of the slab precooling system such that morning warm-up is minimized while still taking maximum advantage of the potential mechanical cooling reduction during the following day.
Enthalpy-based...
