<p>Nanofluids are widely reported to enhance convective heat transfer, yet their adoption in circulating thermal systems remains extremely limited. A central reason is that most studies evaluate nanofluids based on short-term Nusselt number enhancement, while neglecting degradation mechanisms, hydraulic penalties, and uncertainty that govern long-term system behavior. As a result, formulations optimized for peak thermal performance frequently underperform (or even reduce overall efficiency) when deployed under sustained circulation. In this work, we introduce a degradation-aware, system-level screening framework that reframes nanofluid evaluation from instantaneous enhancement to deployable performance. Using a curated dataset of water-based nanofluids, including single-particle, hybrid, and surface-modified formulations, candidates are assessed through a cost-aware tri-objective screening framework combining thermo-hydraulic system benefit, circulation-induced degradation resistance, and hydraulic penalties, with material and preparation cost incorporated as feasibility constraints where reliable data are available. Circulation exposure is introduced as a unifying physical variable to organize plausible aggregation-driven degradation states across operating histories using exposure-based conservative envelopes and representative admissible trajectories, while fouling effects are incorporated through conservative performance bounds rather than explicit kinetic assumptions. An exergy-based analysis is employed to distinguish apparent heat transfer enhancement from net system-level benefit, and uncertainty in thermophysical properties is propagated using Monte Carlo methods to evaluate ranking robustness under realistic variability. The screening analysis indicates the following physical trend: Nanofluids delivering the largest Nusselt number gains often experience rapid increases in hydraulic losses, leading to declining or negative exergy performance with continued operation. In contrast, formulations exhibiting moderate enhancement, strong resistance to aggregation, and controlled viscosity growth are more likely to retain superior system-level performance over extended circulation. These findings demonstrate that deployable nanofluids occupy a narrow operating window defined not by maximum enhancement, but by balanced transport behavior under degradation and uncertainty. By explicitly linking circulation-induced stability indicators to system-level efficiency within a screening framework, the proposed framework provides actionable guidance for selecting and designing nanofluids that deliver sustained performance in practical thermal systems. The framework is intended for comparative screening and deployability assessment rather than prediction of the exact long-term evolution of nanofluid behavior.</p>