You've hit on a common misconception and a point of debate in low-voltage AC systems like those used in HVAC controls. Here's a breakdown of why you're right, and why plumbers might still do what you described

Why You Are Correct:
Floating Secondary

You are absolutely correct that the secondary winding of a typical 24VAC control transformer is, by design, electrically floating. This means neither side of the secondary is intentionally connected to ground. In a perfectly isolated system, measuring the voltage from either secondary terminal to ground should yield little to nothing. The DVM may measure a small voltage due to capacitance between the primary and secondary windings or capacitive coupling to nearby grounded objects, but this is not a defined, intentional voltage.

AC, Not DC

Unlike a DC circuit where you definitively have positive (+) and negative (-) polarities, an AC circuit is alternating. The polarity switches back and forth multiple times a second (60 times in the US). Therefore, using the terms "hot" and "common" in the same way you would for mains AC (120VAC) or a DC system is misleading.

Functionally Equivalent

In a typical 24VAC control circuit, it doesn't intrinsically matter which side of the transformer you connect to which point in the control system. The circuit will function the same regardless of how you wire it, as long as the correct connections are made.

Why Plumbers Might Do It (And Why It Can Be Problematic)


Habit and Misunderstanding

A lot of HVAC technicians (and even some electricians) carry over habits from working with mains AC. They might incorrectly assume that there should be a defined "hot" and "neutral" in the 24VAC system, even though there isn't.2.

Troubleshooting "Leakage"

In some installations, there can be slight leakage currents that result in a measurable voltage between one side of the 24VAC secondary and ground. These leakage currents are often caused by faulty components or insulation breakdown. If a plumber finds a significant voltage, it can be a sign that something is wrong and needs further investigation. This is probably the biggest driver of the practice.3.

Potential for Increased Noise

While there is no intended connection to ground on the secondary, a voltage can be induced via a common mode induced on a long unshielded control wire.4.

Inconsistent Practices

If a system is worked on by multiple technicians over time, marking one side of the secondary as "hot" or "common" can lead to inconsistent wiring practices. This can make future troubleshooting more difficult.

The Risks of Treating 24VAC Like Mains AC



Creating Ground Loops

Intentionally grounding one side of the 24VAC secondary can create ground loops, which can introduce noise and interference into the control system, leading to erratic behavior or even damage.

Misdiagnosis

Looking for a "hot" and "common" on the 24VAC side can lead to misdiagnosis of problems. A small voltage reading to ground doesn't necessarily mean there's a fault with the transformer itself.

False Sense of Security

It doesn't actually improve safety in this application.

Best Practices



Isolate and Test

The best approach is to ensure the 24VAC system remains electrically isolated. Test the insulation resistance between the secondary windings and the transformer core/ground to confirm isolation.

Focus on Circuit Continuity

When troubleshooting, focus on checking for continuity in the 24VAC circuits, shorts to ground (if suspected), and proper voltage levels at each component.

Avoid Grounding the Secondary

Unless explicitly recommended by the equipment manufacturer (which is rare), avoid intentionally grounding either side of the 24VAC secondary.

Proper Wiring Practices

Use proper wiring practices, including secure connections and correct wire sizes.

Shielded Cable

Use shielded cable to mitigate the affects of common mode noise induced on the control wiring.

Documentation

Document any changes or repairs made to the system to help with future troubleshooting.

In Conclusion

You are correct that marking a 24VAC transformer secondary as "hot" and "common" is generally misleading and unnecessary. The practice is rooted in misconceptions about how low-voltage AC systems work and can even introduce problems. The key is to understand the principles of operation and follow best practices for troubleshooting and maintaining HVAC control systems.

Final Thought: It's understandable why a plumber might do this, but education and proper troubleshooting techniques are essential for avoiding potential problems. Flag for review