Mike PleziaInside service technician, Clark Associates · Commercial Equipment, HVAC, and Refrigeration professional

Reading the Ladder Like a Story #hvac #schematics #tradetips

28 Aug 2026 · 1 min

Watch on YouTube

1:12

In this short instructional video, the speaker introduces ladder diagrams to beginners in the trades, framing the diagram as a story that is read left‑to‑right and top‑to‑bottom. He explains the two vertical power rails (L1 on the left, L2 on the right) and the horizontal rungs that form individual circuits. The key structural rule is that switches, thermostats, pressure and limit devices are placed on the left side of each rung, while the actuating devices (contactor coils, relays) sit on the right side next to L2. The left‑hand conditions must be satisfied before power can reach the right‑hand device. The speaker then offers a practical troubleshooting workflow: start at the top rung, follow the left‑to‑right path, and locate where the “story” stops when a unit fails to run, rather than beginning the diagnosis at the compressor. The tone is conversational and supportive, encouraging viewers to share the knowledge with others.

Chapters

Hey, are you new to the trades? Here's something you may not know about ladder diagrams. A ladder diagram looks intimidating until you realize it's just a story told left to right, top to bottom. Picture two vertical rails: L1 on the left, L2 on the right. Every horizontal line connecting them is a rung, and each rung is its own little circuit. L1 to L2. Left to right, top to bottom. That's the whole reading order. Top rung first, left to right, then down to the next. Here's the piece that makes it click. On every rung, the switches and safety devices— your thermostat call, your pressure switches, your limit switches— sit on the left side. The thing that actually does work, like a contactor coil or a relay, sits on the right, next to L2. The left side has to earn the right side. Every condition on the left must be satisfied before that coil on the right ever gets power. Next time a unit won't run, don't stare at the compressor first. Start at L1 and walk the rung until you find where the story stops. Now that you know, you can help someone else know.