Wiring Guide — Mysa for In-Floor Heating

Wiring Guide

This guide provides detailed wiring instructions for Mysa for In-Floor Heating based on the official installation manual. Follow the diagram that matches your system voltage and refer to the floor sensor connection section before restoring power.

Warning Mysa for In-Floor Heating connects to high-voltage (120/240 VAC) wiring. To avoid personal injury or death, turn off your circuit breakers and follow the proper safety precautions before proceeding. If you are unsure about handling electrical wiring, consult a qualified electrician.

Before You Start

1. Turn off power at the circuit breaker that protects your in-floor heating circuit. Place the breaker in the OFF position.
2. Confirm the heater is off — turn your existing thermostat all the way up, wait 5–10 minutes, then feel the floor to make sure no heat is coming through.
3. Use a non-contact voltage tester to verify no power is present at the thermostat wires.
4. Remove your existing thermostat from the wall.
5. Identify your system voltage: 120V or 240V.
6. Verify your floor heating system's total wattage is within Mysa's limits (see below).

Important If you cannot confidently identify the heating circuit's corresponding breaker, or are unsure whether power to your heater is off, stop work and call an electrician to complete the installation.

Tip Complete the Pre-Installation Checklist before beginning — it covers voltage verification, wattage limits, GFCI considerations, and floor sensor planning.

Compatibility

Mysa for In-Floor Heating works with most 120/240 VAC, 60 Hz electric in-floor heating systems, including heating mats and heating cables.

Mysa is not compatible with:

• 24V systems — including forced air, variable speed, heat pump, or fuel types such as natural gas or oil
• Hydronic (hot water) in-floor heating systems

Important Use this thermostat with copper wire only. Do not use it with aluminum wire.

Maximum Load Limits

Your floor heating system's wattage must not exceed these limits:

Check your heating mat or cable's wattage rating — it's usually printed on the product label, cold lead tag, or in the manufacturer's documentation. If your system exceeds these limits, Mysa cannot safely control it.

What's in the Box

• Mysa Smart Thermostat
• 2x Mounting Screws
• 4x Wire Nuts
• 1x In-Floor Temperature Sensor (10K Ohm at 25°C, 15 ft / 3 m lead)

Tools you'll need: Phillips screwdriver, flat head screwdriver, wire stripper (optional), pliers (optional).

Understanding Line vs Load

Before wiring, identify the two cables coming into your electrical box:

• Line cable — delivers power from the electrical panel (breaker) to the thermostat. Connect this to Mysa's line terminals only.
• Load cable — delivers power to the heater. Connect this to Mysa's load terminals only.

Tip If you're unsure which cable is line and which is load, a licensed electrician can identify them quickly using a multimeter.

Mysa Wire Leads

The Mysa for In-Floor Heating base plate has four color-coded power wire leads, plus connections for ground and the floor sensor:

Use the 4 wire nuts included with Mysa to connect the power leads to your house wiring. Do not reuse old wire nuts from your previous thermostat.

Prepare Mysa for Install

1. Loosen the screw on the bottom of the front plate — don't remove it completely.
2. Remove the front plate to access the mounting screw holes and wire leads on the base plate.

Wiring for 120V Systems

Identify your cables before wiring — confirm which is the panel (line) cable and which is the heater (load) cable.

Connect your Mysa using the supplied wire nuts:

1. Mysa L (black) ← Supply wire L (usually black) coming from the electrical panel
2. Mysa N (white) ← Neutral wire N (usually white) coming from the electrical panel
3. Mysa LOAD 1 (black) ← Load wire (usually black) going to the heater
4. Mysa LOAD 2 (white) ← Load wire (usually white) going to the heater
5. Ground ← House ground wire (green or bare copper) — connect the supply ground to the Mysa ground

Note The supply cable should include a ground wire (green or bare copper). Connect this ground wire securely.

Wiring for 240V Systems

Most electric in-floor heating systems in North America use 240V. You will typically see two hot wires coming from the breaker — there is no neutral wire on a 240V circuit.

Connect your Mysa using the supplied wire nuts:

1. Mysa L (black) ← Supply wire L1 (usually black) coming from the electrical panel
2. Mysa N (white) ← Supply wire L2 (usually red or black) coming from the electrical panel
3. Mysa LOAD 1 (black) ← Load wire (usually black) going to the heater
4. Mysa LOAD 2 (white) ← Load wire (usually red or white) going to the heater
5. Ground ← House ground wire (green or bare copper)

Note On a 240V circuit, both L1 and L2 carry voltage. The Mysa N terminal connects to the second hot wire (L2) — there is no neutral.

Floor Sensor Connection

Mysa for In-Floor Heating includes a 10K Ohm floor temperature sensor (15 ft / 3 m lead). The sensor must be connected for the thermostat to operate.

Attaching the Sensor

1. Route the sensor cable from the floor, through the wall, and into the electrical box.
2. Connect the two sensor wires to the Sensor terminals on the Mysa base plate.
3. The sensor wires are not polarized — either wire can go to either terminal.

Sensor Placement

• Position the sensor tip between heating cables or on top of the heating mat — not directly on a cable.
• Place the sensor within the heated area, centered between cable runs if possible.
• Use the conduit tube provided by your heating system manufacturer if available.

Using a Third-Party Sensor

Mysa works with any floor sensor in the 5K to 40K Ohm resistance range. If you're using a sensor from a previous thermostat or a different brand, connect it the same way and calibrate it in the Mysa app under Settings > Floor Sensor.

Tip If you're installing a new floor, consider running a backup sensor alongside the primary one. Floor sensors can fail over time, and replacing one under tile or flooring is expensive and disruptive. Leave the backup wires capped in the wall box until needed.

After Wiring

1. Double-check all connections match the correct diagram for your voltage.
2. Ensure no bare wire is exposed outside the wire nuts.
3. Verify the floor sensor is connected to the sensor terminals.
4. Carefully fold wires into the gang box — make sure they are not pinched.
5. Secure the Mysa base plate to the wall using the mounting screws.
6. Attach the front plate back onto the base plate and tighten the screw.

Power On and Test

1. Restore power at the breaker — the thermostat display should light up.
2. If it doesn't light up, turn off power immediately and recheck all connections.

Testing the GFCI

Mysa for In-Floor Heating has a built-in GFCI with three physical controls on the thermostat:

• TEST button — manually tests the GFCI's ability to respond to a fault. Pressing this should activate the FAULT light and disconnect heat until you power-cycle the Mysa or press RESET.
• RESET button — restores normal operation after a GFCI trip or test.
• FAULT light — illuminates when a ground fault is detected or the TEST button is pressed.

Important After installation, press the TEST button to verify the GFCI is working correctly. The FAULT light should turn on and the heater should disconnect. Then press RESET to restore normal operation.

Warning If the GFCI can no longer safely detect a current, it will disconnect the heater load and the FAULT light will activate indefinitely. If this occurs immediately after installation, contact technical support at help.getmysa.com.

Built-In GFCI Protection

Mysa for In-Floor Heating has a built-in Class A GFCI (Ground Fault Circuit Interrupter). In the event of a ground fault, the GFCI will trip and quickly stop the flow of electricity to prevent serious injury.

A GFCI does not protect against circuit overloads, short circuits, or shocks — it specifically detects current flowing through an unintended path (like a human body) to ground.

Important Do not install Mysa for In-Floor Heating on a circuit that is already GFCI-protected (e.g., a GFCI breaker or GFCI outlet). The two GFCI devices will conflict, causing constant nuisance trips and preventing your floor heater from operating. If your current circuit has GFCI protection, have an electrician install a standard (non-GFCI) breaker.

Error Codes

If you see an error code after powering on:

Get Support

If your wiring doesn't match these descriptions, or you're unsure about any connection, contact our team with a photo of your wiring.

Email: install@getmysa.com
Support: help.getmysa.com