Okay, let's break down what you're seeing and what it likely means. Your concern about a residual voltage when the thermostat is off is valid. Here's a detailed analysis and troubleshooting steps

Understanding the 3-Wire Thermostat Configuration:
Line (Black)

Incoming 240V power.

Load (Black)

Outgoing power to the heater when the thermostat calls for heat.

Red (Jumper)

The "center tap" or common point. One leg of the 240V supply is connected directly to this. Both the red line in and red load out are connected to this point. The thermostat switch only controls the black wire.

Analyzing Your Voltage Readings (Thermostat OFF - Not Calling for Heat)



Red Load to Ground: 120V (Expected) This is exactly what you should expect. The red wire is effectively "always hot" with one leg of the 240V supply. Black Load to Ground: ~23V (Potentially Problematic) This is where the issue lies. This voltage SHOULD be near zero. Red Load to Black Load: 63V (Potentially Problematic) This further suggests that there is a voltage present on the black load wire when it should not be.

What's Likely Happening

The most likely cause of voltage on the black load wire is

capacitive coupling or inductive coupling, also known as "phantom voltage" or "ghost voltage." This happens when wires run in close proximity (like inside the same cable or conduit). The live (red) wire can induce a small voltage into the disconnected (black) wire through capacitance or inductance between the wires.However, 23V and 63V is high for phantom voltage in a typical installation. Here's a breakdown of the possible reasons why it may be so high:1. High-Impedance Meter

Digital multimeters have very high impedance. This means they draw very little current when taking a voltage reading. Even a tiny current flow can create a noticeable voltage reading with a high-impedance meter. This doesn't necessarily mean there's significant power flowing or that a heater would actually operate.2.

Loose Connection/High Resistance Connection

A loose connection somewhere in the circuit (either in the thermostat, wiring to the heater, or even back at the breaker panel) could create a small voltage drop when the circuit is supposed to be off.3.

Incorrect Wiring

Double-check your wiring at the thermostat, the heater, and even the breaker panel. A mis-wired connection can cause unexpected voltage readings.4.

Breaker Issue

A failing or "leaky" breaker could potentially allow a small amount of current to flow even when it's in the "off" position.5.

Neutral Wire Sharing: Are these circuits sharing a neutral with other circuits? This could cause a small amount of current to flow through the black wire, especially if there are loads on the other circuits.

Troubleshooting Steps


Load Test with a Resistor

This is the most important step. A high-impedance meter can fool you. Attach a small resistor (e.g., 10k Ohm, 1/2 Watt) between the black load wire and ground. Then, measure the voltage again with your multimeter. If the voltage drops to near zero, it's likely phantom voltage. If it remains at a significant voltage, it suggests a more serious leakage issue.2.

Inspect All Connections



Thermostat

Carefully examine the connections at the thermostat. Make sure the wires are securely attached and that there are no loose strands. Verify the wiring diagram matches the actual wiring.

Heater

Check the wiring connections inside the heater's junction box.

Junction Boxes

Inspect any other junction boxes along the circuit run.

Breaker Panel



(CAUTION: Work at the breaker panel can be dangerous. If you are not comfortable and knowledgeable about electrical work, hire a qualified electrician.) Check the connections at the breaker. Make sure the wires are securely attached and that the breaker is properly seated in the panel.3. Check for Shared Neutrals

If these heater circuits share a neutral with other circuits, disconnect the neutral for this circuit at the breaker panel (with the breaker OFF, of course!) and re-measure the voltage on the black load wire. If the voltage drops significantly, a shared neutral may be contributing to the issue.4.

Isolate the Circuit

Turn off the breaker for the heater circuit. Disconnect both the black and red wires at the breaker. Then, measure the voltage between each of those wires and ground. If you STILL read voltage with the wires disconnected from the breaker, you have a serious wiring issue (potentially a short circuit somewhere in the wall) and you need to call a qualified electrician immediately.5.

Test the Breaker

With the breaker OFF, use an ohmmeter to check for resistance between the load side of the breaker and the neutral bar. There should be infinite resistance (an open circuit). Any low resistance reading indicates a fault in the breaker or downstream wiring.6.

Consider a Different Meter

Although unlikely, try using a different multimeter to rule out a faulty meter. Analog meters are less susceptible to phantom voltage readings, but digital meters are generally more accurate for actual voltage measurement.

Safety Considerations



Turn off the breaker before working on any electrical wiring. If you are not comfortable working with electricity, hire a qualified electrician. Use a voltage tester to verify that the circuit is de-energized before touching any wires.What to do if it's just Phantom Voltage

If the load test with the resistor proves it's just phantom voltage, then it likely won't be enough to cause the heater to operate continuously. However, it can still be a nuisance and, in some cases, cause flickering or buzzing in sensitive devices. Here are a few options:

Ignore It (Maybe)

If the heater operates correctly when the thermostat is calling for heat and is completely off when the thermostat is off, you might be able to ignore it, especially if the voltage drops to near zero under a small load. But keep monitoring it for any changes.

Capacitor across the Load

A small capacitor (e.g., 0.01 uF, 600V) connected across the heater terminals can help bleed off the phantom voltage. This is a common solution for dealing with phantom voltage in lighting circuits.

Shielded Cable

If you're running new wiring, using shielded cable (e.g., BX or MC cable) can reduce capacitive and inductive coupling.

Important Note:

While phantom voltage can be harmless, it's essential to rule out any potential wiring problems or safety hazards. Don't simply assume it's phantom voltage without thorough testing and inspection.Let me know if you have further questions. Good luck! Flag for review