Exploring HVAC system sizing under uncertainty

• Propose a new HVAC system design paradigm under uncertainty.
• Use multi-year weather years as an alternative of current design day methods.
• Support risk-based sizing to meet stakeholder specified guarantees.
• Sensitivity analysis to drive system size reductions.

In current practice, HVAC systems are sized based on standardized procedures that were mostly developed by ASHRAE. The standard approach only implicitly deals with uncertainty in peak system demand through the selection of an appropriate design day and the choice of a safety factor. Although this method works satisfactorily in most cases, it offers no support to a system designer who wants to track the risk associated with an undersized system. The opposite, i.e. avoiding that the system is needlessly oversized deserves even more attention given the fact that current practice of “defensive sizing” leads to oversized systems which leads to wasted capital investment and systems that operate far away from the optimum efficiency loads. This paper explores a new framework to guide the use of uncertainty analysis (UA) and sensitivity analysis (SA) in HVAC system sizing. UA will replace the safety factor with quantified margins based on comprehensive quantification of different sources of uncertainty. A probabilistic-based SA is then used to identify the important individual factors or groups of factors that contribute to uncertainty, providing means of risk management by applying better quality assurance methods or negotiating performance contracts.