installed mysa but the heater won't turn off, it just runs constantly no matter what temperature i set

Diagnosis

The most likely cause for a Mysa Smart Thermostat failing to cut power to the heating element is a "hard short" or a bypassed relay. In a standard installation, the Mysa acts as a switch between the Line (incoming power) and the Load (outgoing power to the heater). If the heater runs regardless of the set temperature or the "Off" command in the app, the electrical current is bypassing the internal relay and flowing directly from the Line wire to the Load wire. This typically occurs during installation if the copper conductors are stripped too long, allowing the bare ends of the Line and Load wires to touch behind the wall plate or within the terminal block. Another common culprit is a "stray strand" of copper from a multi-strand wire that has bridged the gap between the L and 1 terminals on the backplate. You can confirm this by turning the thermostat to "Off" in the app; if the baseboard remains hot to the touch after 10 minutes, the circuit is physically closed regardless of the software state. In rare cases, this can be caused by a failed internal relay that has welded shut, though wiring errors are far more frequent.

How to Fix It

To resolve this, you must isolate the electrical connections and ensure there is no physical bridge between the power source and the heater.

1. Immediately flip the circuit breaker to the OFF position. Do not attempt to diagnose this with live power, as baseboard heaters draw significant amperage.
2. Gently pull the Mysa faceplate away from the wall mounting plate.
3. Inspect the wiring terminals. Look specifically at the L (Line) and 1 (Load) terminals. If you see any copper wires extending more than 1/8th of an inch past the terminal screw, they are too long.
4. Loosen the terminal screws and pull the wires slightly out. Trim the excess copper using a wire stripper so that only the necessary amount of wire is inserted into the clamp.
5. Ensure that the insulation of the wire extends right up to the terminal block, leaving no exposed copper visible between the two terminals.
6. Check the back of the electrical box. Ensure that the Line and Load wires are not pressed firmly against each other or against a metal junction box, which could cause a pinch-induced short.
7. Re-tighten the terminal screws firmly. Use a torque-appropriate screwdriver to ensure the wires are seated but not crushed.
8. Snap the faceplate back on and restore power at the breaker. Set the temperature 5 degrees below the current room temperature to verify the heater shuts off.

If That Didn't Work

• Inspect the Heater Junction Box: If the wiring at the thermostat is clean, the short may be located at the heater end. Open the junction box on the baseboard heater itself. Check for charred wires or a "bridge" where the hot and load wires may have melted together due to a loose connection causing high resistance heat.
• Test for Relay Failure: To determine if the Mysa unit itself is defective (welded relay), disconnect the Load wire from the thermostat entirely. If the heater stops running, the short was at the thermostat. If the heater continues to run even with the Load wire disconnected from the Mysa, you have a direct short further down the line in your home's wiring.
• Verify Voltage Compatibility: Ensure you haven't installed a 120V Mysa model on a 240V circuit (or vice versa). While this usually results in a blown fuse or a dead unit, incorrect voltage can occasionally cause erratic relay behavior or immediate component failure.
• Check for Parallel Wiring: If you have multiple heaters on one circuit, ensure they aren't wired in a way that bypasses the thermostat. A "jump" wire installed by a previous homeowner to keep a secondary heater running constantly will override any smart control you install.

Heads Up

• Fire Hazard: A heater that cannot be turned off is a critical fire risk. If you cannot find the short immediately, keep the circuit breaker OFF until a licensed electrician can inspect the line.
• Avoid Over-tightening: When securing wires to the backplate, do not over-torque the screws. Over-tightening can crack the plastic housing or shear the terminal, which can lead to intermittent sparking.
• Wire Gauge: Ensure you are using the correct gauge wire (typically 12 AWG for 20A circuits). Using undersized wire can cause the wire to overheat, melting the insulation and creating the very short-circuit you are trying to fix.
• App Calibration: Once the heater is functioning, give the Mysa 24 hours to calibrate its internal temperature sensor before adjusting your schedules, as it needs to learn the thermal mass of your room.