100 10 MAT – RAT (°F) Temperature (°F) 90 80 70 60 15% OA 5 y = 0.48x – 0.05 0 Dampers –5 Modulating to Maintain SAT Ideal Operation –10 50 40 5/1 15 Outside Air Supply Air Mixed Air Return Air –15 –30 5/3 5/5 5/7 5/9 5/11 5/13 5/15 5/17 5/19 5/21 5/23 Figure 2: Time series charts of trend data can be confusing. this activity as they can provide input on the feasibility of the recommendations. The recommendations may range from simple fixes such as adjusting controls setpoints to complex fixes such as replacing damper assemblies or installing relief air systems. Besides addressing maintenance issues, some recommendations may also correct issues left over from the original design and con- 100% OA –20 –10 0 OAT – RAT (°F) 10 20 30 Figure 3: X-Y charts can give an immediate understanding of performance. In this case, the system is delivering approximately 50% outdoor air (the slope of the curve fit) in all three operating modes. Its actual performance is far from expected. Further investigation found that both the outdoor air and return air dampers were stuck in a fixed position due to debris and rust. struction, such as improper sensor placement and suboptimal controls sequences. Other common recommendations include: www.info.hotims.com/44630-21 March 2013 ASHRAE Journal 39