Ever wondered what “upstream” and “downstream” really mean in your HVAC system? Here’s a simple breakdown: **Upstream** refers to the section **before** a component—where air or water is approaching it—while **downstream** is the section **after** the component—where air or water has already passed through. For example, when air flows through a heating coil, the air **before the coil** is upstream, and the air **after the coil** is downstream. Understanding this is crucial when installing sensors like freezestats, temperature sensors, or pressure switches. Correct placement ensures optimal protection, accurate readings, and better system performance.