You read an HVAC wiring diagram in layers: first the format — a ladder diagram shows the circuit's logic as rungs between power lines, a pictorial shows the parts as they sit in the unit — then the symbols, then circuit by circuit, from source through switches to load.
Goodman's GVXC20 manual warns wiring is subject to change, so the diagram on the unit is the one to read — and the tracing happens under OSHA's electrical rules.
Ladder vs pictorial diagrams
This guide reads an HVAC wiring diagram through two drawings, because they answer two different questions.
The pictorial is the location drawing: components appear roughly where they physically sit in the cabinet, with the wiring drawn between them, so you can see where a part is and how a wire routes to it.
The ladder diagram is the logic drawing: the circuit is laid out as horizontal rungs between two vertical lines, and each rung reads from the source, through the contacts and switches in its path, to the load those contacts control.
The ladder is where hvac schematic reading does its work.
Because a load only energizes when every contact in its rung's path is closed, the drawing states the operating logic outright: contacts in a series path are conditions that must all be true, and alternative paths around a contact are the ways the circuit can still complete.
Reading a ladder diagram hvac-style is exactly that trace — pick a load, walk backwards through its contacts, and you are left with the list of things that must be closed for it to run.
Whichever drawing you have, the one on the equipment is the one to read.
Goodman's GVXC20 installation manual warns that wiring is subject to change and to always refer to the wiring diagram on the unit itself for the most up-to-date wiring — so a diagram from an older manual or a different model may not match the machine in front of you.
Diagram reading is one skill inside the larger technical set — the HVAC technician skills guide maps the whole set, from meter work to superheat and subcooling.
Common symbols
Symbols are the vocabulary, and the legend on the diagram is the dictionary — read the legend before the circuit, because it maps each symbol on that specific drawing to its component.
Look for the labels on the drawing in front of you too: matching a label on the drawing to the terminal in the cabinet is how you know the part in your hand is the part on the page.
Group the symbols you meet on control schematics into a handful of working types.
There are the loads — the things that do work, such as motors, igniters and the coils of valves and relays.
There are the contacts that feed them: thermostat contacts, pressure-actuated switches, temperature-actuated safety limits, and the relay contacts that carry one circuit's action into another.
And there are the parts that condition power, such as transformers and capacitors.
What each symbol means on the drawing in your hand comes from that drawing's legend and its equipment manual — read a symbol from memory instead of from the legend and the trace can go wrong from there.
Two habits make the vocabulary stick.
First, read the legend every time, even on equipment you think you know.
Second, when you cannot identify a symbol, stop and look it up in that unit's manual rather than guessing.
Tracing a low-voltage control circuit
The low-voltage control circuit is the tracing exercise this section walks.
Goodman's GVXC20 manual, for one, says the unit can be used with a 24 VAC single-stage thermostat — its low-voltage control circuit — when its communicating thermostat is not in use.
That figure belongs to that model — other equipment may differ, so read that unit's diagram and manual rather than trusting a remembered voltage.
The trace itself is the method from the ladder section, applied: pick the load that is not running, find its rung, and walk back through every contact between the load and the source.
Each contact in the series path is a condition the circuit needs.
The diagram tells you what should be closed; your meter tells you what actually is.
The gap between the two is the diagnosis.
Before any probe goes into the cabinet, the safety rules gate the work.
Under OSHA's general-industry electrical rule, 29 CFR 1910.333, live parts an employee may be exposed to must be de-energized before the employee works on or near them, unless the employer can demonstrate that de-energizing introduces additional or increased hazards or is infeasible due to equipment design or operational limitations.
Parts that operate at less than 50 volts to ground need not be de-energized where there is no added burn or arc risk — a conditional carve-out, not a blanket rule.
The general-industry rules cover service and maintenance work in existing buildings; new construction falls under OSHA's construction standards instead.
OSHA's lockout/tagout standard, 29 CFR 1910.147, covers the control of hazardous energy during servicing and maintenance of machines and equipment.
Cord-and-plug equipment is excluded when unplugging controls the hazard and the plug stays under the servicing employee's exclusive control — two conditions, not a general license to work live.
Working a circuit live is the exception, not the default
Reading a furnace board sequence
Read a furnace board's diagram for what the board gathers in one component: look for the terminals it exposes, the safety inputs it takes in and the loads it switches.
The reading method does not change with the packaging: find the load that is not running, locate it on the drawing, and walk back through the contacts and safety devices in its path.
As you walk a heating load's path, check the steps the drawing puts ahead of the load — the thermostat's call and the safety controls, such as limits and pressure-actuated switches.
What the ladder drawing gives you on a board is the order of dependency: a load energizes only when everything ahead of it in its path has closed, so the rung shows what the board requires before it allows that load to run.
That is why the drawing beats guesswork on a no-heat call — instead of replacing the part you suspect, you can measure along the path and find the contact that is open when the drawing says it should be closed.
Don't look for the exact sequence in a generic routine — read it from that unit's diagram and its documented sequence of operation.
Wiring is subject to change, per the Goodman warning above, so the diagram mounted on the unit is the current one.
Use the board's terminal designations to find your test points, and let the drawing, not habit, decide what you expect the meter to show.
Practice approach for apprentices
Diagram reading is a repetition skill, and the repetitions do not need a broken system.
Work up through four habits, in order.
Read the legend first on every diagram, until it is reflex.
Copy a diagram by hand — drawing the rungs yourself forces you to notice which contacts sit in which path.
Then trace out loud: pick a load and walk its path back to the source, naming each contact and what closes it.
Finally, predict before you measure: write down what the drawing says should be open and closed, then verify with the meter under the de-energize rules covered above.
If you are an apprentice, ask for these repetitions on the job — the HVAC apprentices guide covers the role, the route in, and what the years look like.
The skill also slots directly into the diagnostic sequence: reading the diagram is what tells a tech which circuit to test first.
That sequence — power and controls, then airflow, then the refrigerant circuit — is the subject of our guide to how HVAC techs troubleshoot a system.
And for the role itself — the duties, where the work happens and where it leads — see our HVAC technician guide.
Keep the two drawings paired in your head: the pictorial to find the part, the ladder to reason about it.
Techs who can do both stop swapping parts and start proving failures.
Career information, not legal advice: the OSHA electrical and lockout/tagout rules described here are the federal general-industry versions — confirm what applies to your jobsite with OSHA and your employer's safety program.