Installing the Mysa Power C-Wire Adapter

The Mysa Power C-Wire Adapter provides continuous power to your Mysa Smart Thermostat (formerly Mysa for Central HVAC) when your system doesn't have a C-wire. Nearly 50% of homes need this adapter.

Tip For custom, step-by-step instructions with diagrams for your specific system, use the C-Wire Adapter Installation Guide.

Before You Begin

Warning Turn off power to your HVAC system at the breaker before touching any wires.

What you'll need:

Mysa Power C-Wire Adapter
Small flathead screwdriver (for adapter terminals)
Phillips screwdriver (for control board terminals)
Wire labels (included)

Do You Need the Adapter?

Remove your thermostat faceplate and check for a wire connected to the C terminal:

C-wire present: You don't need the adapter
No C-wire but spare wire in wall: Connect the spare to C at both the thermostat and control board
No C-wire and no spare wire: Install the Mysa Power C-Wire Adapter

Installation Steps

1. Locate Your Control Board

Access your furnace, air handler, boiler, or zone valve panel where the thermostat wires connect.

2. Label and Photograph Wires

Label your existing wires using the included stickers. Take a photo before disconnecting anything.

3. Connect the Adapter

Connection varies by system type. The adapter has clearly labeled terminals: R, C, and W/Y/OB for the control board side, and R, C for the thermostat side.

Furnace or Boiler (heat only):

Control board R → Adapter R
Control board C → Adapter C
Control board W → Adapter W/Y
Adapter R, C → Thermostat R, C

Furnace + AC:

Control board R → Adapter R
Control board C → Adapter C
Control board W → Adapter W/Y
Y wires remain connected to control board
Adapter R, C → Thermostat R, C

Heat Pump:

Control board R → Adapter R
Control board C → Adapter C
Control board O/B → Adapter W/Y
Y wires remain connected to control board
Adapter R, C → Thermostat R, C

Zone Valve Systems: Install the adapter at the zone valve, not at the boiler. Each zone with a Mysa thermostat needs its own adapter.

Boilers with T/T Terminals (no 24V power): Use the Rext terminal with an external 24V transformer. One transformer can power up to 3 Mysa + adapter combinations.

4. Mount the Adapter

Secure the adapter inside or near your furnace using the included adhesive pad or a zip tie.

5. Restore Power and Test

Turn the breaker back on. The thermostat should power on and display the Pairing Mode icon (looks like a Bluetooth symbol).

Test heating and cooling to confirm proper operation.

Caution Don't test cooling if outdoor temperature is below 12°C (54°F) — this can damage the compressor.

Troubleshooting

If the thermostat doesn't power on:

Verify all connections match the diagrams
Check the 3–5A fuse on your furnace control board
Confirm 24V AC between R and C with a multimeter

Need Help?

Installing the C-Wire Adapter can be tricky. Our installation specialists can walk you through it.

Email: install@getmysa.com
Online guide: C-Wire Adapter Installation Guide
PDF: Mysa Power C-Wire Adapter User Guide