Understanding and Resolving the H4 Error Code

Last updated: June 22, 2026

The H4 error code indicates that your Mysa thermostat's internal relay is stuck in the closed position, causing your heater to run continuously even when the thermostat is set to a lower temperature or turned off.

Immediate Safety Steps

Turn off the breaker immediately. When you see an H4 error, your heater will continue running and cannot be controlled by the thermostat's temperature settings or on/off commands. While your heaters have built-in overheat protection that would cut power at the heater itself if temperatures rise to dangerous levels, the continuous operation will result in uncomfortable room temperatures and high electricity bills. Keep the breaker off until you receive a replacement unit.

Diagnosing the Issue: Click Test

Before requesting a replacement, perform this Click Test to determine whether the issue is with the thermostat hardware or your home's wiring.

  1. Manually adjust the temperature setpoint 3–4 times, raising it well above and then lowering it well below the room temperature

  2. Listen carefully for clicking sounds from the thermostat

What the results mean:

  • I hear a click, but the heat stays on: This indicates a wiring fault in your home (like a melted wire nut or short) that is bypassing the Mysa. Stop use immediately and contact an electrician, as this is a safety hazard outside of Mysa's control.

  • I hear no click at all: The relay is likely stuck or welded shut, indicating a potential hardware fault. This confirms a hardware failure and you should proceed with a warranty replacement request.

  • The clicking started working and the heat is now turning off: This may have been a temporary software state. Monitor the thermostat for recurring issues.

What Causes H4 Errors

H4 errors typically result from one or more of the following installation or environmental issues:

  1. Over-wattage: Mysa supports up to 3,800W at 240V, 3,200W at 208V, or 1,900W at 120V.. Exceeding these limits will damage the relay over time

  2. Voltage mismatch: Ensure your heater's voltage rating matches your circuit. For example, a 208V heater on a 240V circuit pulls much more wattage than labeled, which will eventually fuse the relay.

  3. Installation surges: If your breaker tripped during the very first setup, a wiring cross may have stressed the relay from the start

Early Warning Signs

Before a full H4 error occurs, you may notice your heater running past the set temperature while still responding to manual on/off commands through the app. For example, the room may reach 75°F when set to 60°F, but the heater turns off when you manually switch the thermostat to off. This indicates early relay degradation and means an H4 error is likely imminent.

If you experience this behavior, try manually cycling the temperature setpoint way up and then way down multiple times. This may temporarily restore normal relay clicking and proper on/off operation by resolving temporary software states that cause the relay to stick.

If the issue persists, turn off the breaker and contact support.

Your heaters should have manufacturer labels showing the exact wattage. Mysa does not currently offer models that support loads higher than 3,800 W at 240 V, 3,200 W at 208 V, or 1,900 W at 120 V. If your heating load exceeds this limit, you will need to split the load across multiple thermostats or use a different control solution.

  • Photo of your purchase receipt or order number

  • Serial number, which can be found in two locations: (1) inside the faceplate when you unscrew and remove it from the wall, or (2) in the booklet that came with the thermostat

  • The name of the Mysa as shown in the app (e.g., "Back Bedroom", "Living Room")

You do not need to return the defective unit - please recycle it where possible.

If the underlying cause of the H4 error (such as over-wattage or wiring issues) is not addressed, the replacement unit may also fail. We cannot provide a second replacement if the new unit fails due to the same environmental conditions.

If the issue repeats with a replacement unit, it strongly suggests a problem with the home's circuitry that an electrician should investigate.

Note that lowering the thermostat's set temperature will not prevent H4 errors if the underlying cause is over-wattage or wiring issues. The wattage produced and voltage remain constant whenever the heater is active, regardless of the temperature setting.

Consider having a qualified electrician verify your wattage and wiring before installing the replacement unit to prevent future failures.