Voltage, Current, Resistance

Three variables govern every electrical measurement you will ever take. They’re everywhere; they show up in every spec, every troubleshooting procedure, every meter setting. They also happen to have exact analogues in hydronics, which means if you understand pressure, flow rate, and pipe restriction, you already understand voltage, current, and resistance. The electrical version is just a translation.

This chapter is about making that translation explicit and precise enough that you stop thinking in analogies and start thinking directly in electrical terms.

The hydronic translation

Picture the heating side of a residential hydronic boiler. A circulator puts pressure on the supply side. Water flows through the pipe, delivering heat to radiators, and returns cool. If you pinch the supply line closed, flow stops even though the circulator is still running. If you open a bigger path, more water flows. The relationship between how hard the circulator pushes, how much water moves per minute, and how restricted the path is — that’s exactly the relationship between voltage, current, and resistance.

The analogy isn’t approximate — the underlying math is literally the same equation, Ohm’s Law, which the next chapter covers. For now, internalize the intuition: voltage pushes, current flows, resistance opposes.

Voltage

Voltage is the electrical potential difference between two points. “Potential difference” is a technical way of saying pressure — it’s how hard the system wants to push electrons from one point to the other. You always measure voltage between two points, never at a single point in isolation. Asking “what’s the voltage?” without specifying across what is like asking “what’s the pressure?” without specifying between which two plumbing tees.

On an HVAC system you’ll encounter several standard voltage levels:

• 120 VAC is residential line voltage — what the wall outlet provides. Motors, heaters, and line-voltage loads run on this.
• 240 VAC is higher-voltage line for heavier loads — air-conditioning condensers, electric furnaces, electric water heaters.
• 24 VAC is the control voltage generated by the transformer. Almost every decision in a residential system happens at 24V.
• Millivolts (mV) appear in thermocouple and thermopile systems — a standing-pilot thermocouple puts out around 30 mV, a thermopile around 750 mV.
• Microamps (µA) isn’t a voltage — it’s a current unit — but flame-signal readings are the smallest electrical quantities you’ll routinely measure in this work, around 1–10 µA DC.

Those unit prefixes matter. A millivolt is one-thousandth of a volt. A microvolt is one-millionth. Getting the units wrong on a meter reading off by three orders of magnitude is exactly the kind of mistake that sends a new tech down a wrong diagnostic path.

Current

Current is the rate of electron flow — how much charge moves past a point per unit of time. It’s measured in amperes, abbreviated A, with the fractional units milliamps (mA = 1/1000 A) and microamps (µA = 1/1,000,000 A) appearing for smaller signals.

Two things to understand about current measurements: first, to measure current in the traditional way with a meter, you have to put the meter in series with the circuit — that means breaking the circuit open and inserting the meter so current flows through it. This is awkward on live equipment. Which is why clamp meters exist — they measure the magnetic field around a conductor and report current without breaking the circuit. You’ll use both.

Second: current is what actually does work and what actually causes damage. Voltage is how hard the system is willing to push; current is how much is moving. A high-voltage, low-current circuit can zap you and hurt; a low-voltage, high-current circuit can start a fire if it’s shorted. Fuses and breakers are sized by current because current is what damages components when it runs away.

Resistance

Resistance is how much a component opposes current flow. It’s measured in ohms, with the larger unit kilohm (kΩ = 1,000 ohms) and the smaller unit (rarely used) milliohm. Every component has resistance; most wiring has very little.

Resistance is the measurement that proves a component is electrically “alive” when it’s disconnected from power. A solenoid coil has a specific resistance spec — say, 50 ohms for a gas valve coil. Measure it with a DMM set to ohms: if it reads close to 50, the coil is fine. If it reads OL (overload, meaning open circuit), the coil winding has broken. If it reads much lower than spec, the winding has shorted.

This kind of “bench” resistance testing is one of the most reliable diagnostic tools in the book. It tells you whether a component is electrically capable of doing its job before you start puzzling over why it isn’t.

A few useful reference values:

Unit prefixes in practice

HVAC readings span twelve orders of magnitude — from microamps of flame signal to tens of amps of motor current, from millivolts of thermocouple output to hundreds of volts of line power. Fluency with unit prefixes is table-stakes for doing the job well.

A useful habit: always read the full unit out loud when recording a measurement. Say “six-point-eight microamps DC” or “twenty-four volts AC,” not just “six-point-eight” or “twenty-four.” The prefix is where the meaning lives. A meter reading of “6.8” on µA DC means a healthy flame. The same reading on V DC means a blown modcon supply. The same reading on A AC means a big motor running. Same number, three completely different calls.

The hydronic callback

Worth closing with the same analogy we opened with, now that the vocabulary is in place. On a hydronic heating loop, if the boiler isn’t warming the zones, you have a limited number of diagnostic possibilities: the circulator isn’t running, the valves aren’t open, the pipes are restricted, or there’s no water. Every one of those is a direct electrical analogue: no voltage (the circulator stopped), switches are open (valves closed), resistance is high (restriction), or the circuit is open (no water — no path for flow).

That mapping becomes useful once you stop thinking of it as an analogy and start thinking of electrical troubleshooting as exactly the same diagnostic discipline you already know from hydronics, just operating on electron flow instead of water flow. The tools are different. The mental model is the same.

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Check your understanding

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01Voltage is measured between two points. What happens if you try to 'measure voltage at a single point' without specifying a reference?

AThe meter will read zeroBThe measurement is meaningless — voltage is always relative to another reference pointCThe meter reads atmospheric electrical potentialDThe measurement defaults to 'volts above ground'

02Why do flame-rectification readings use microamps (µA) instead of volts?

ABecause the signal is a current, not a voltageBBecause volts are harder to measureCBecause microamps give larger numbersDMicroamps and volts are interchangeable in this context

03A gas valve coil has a spec of 60 Ω. Your DMM reads OL (open line) across its terminals. What does this indicate?

AThe coil is operating within specBThe coil resistance is too lowCThe coil winding is broken internally — the component has failed openDThe meter leads are bad

Before you close the chapter

You should now have the vocabulary to name what a meter measures, the intuition for what each quantity physically represents, and the unit prefixes needed to read numbers correctly. The next chapter introduces Ohm’s Law, which connects the three variables into a single equation that governs every electrical measurement you’ll ever validate.