Installing the Mysa Power C-Wire Adapter on a Cooling-Only (AC-Only) System

Last updated: November 11, 2026

Installing the Mysa Power C-Wire Adapter on a Cooling-Only (AC-Only) System

Some central air conditioning systems have no furnace or heat call at all — just R, Y, and G wires at the thermostat. If your system is cooling-only and doesn't have a C-wire, the Mysa Power C-Wire Adapter is wired slightly differently than on a heating system. This guide walks through that specific setup.

Tip For general adapter installation steps, see Installing the Mysa Power C-Wire Adapter. For custom, step-by-step instructions with diagrams for your specific system, use the C-Wire Adapter Installation Guide.

Is Your System Cooling-Only?

Remove your old thermostat faceplate and check the wiring. Your system is likely cooling-only if:

  • Only R, Y, and G wires are connected (a C wire may or may not be present)

  • There is no W wire, and this thermostat doesn't control a furnace or any other heating equipment

  • Your system is a standalone central air conditioner, with no heat pump, gas furnace, or electric furnace on this circuit

Warning If you also see an O or B wire, you likely have a heat pump rather than a cooling-only system. Heat pumps are wired differently — see the Heat Pump section of Installing the Mysa Power C-Wire Adapter instead.

Before You Begin

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

Wiring the Adapter for a Cooling-Only System

Because a cooling-only system has no heat call, the adapter's W/Y lead connects to Y at your HVAC control board instead of W. The adapter relay is then triggered by the cooling call:

  • Control board R → Adapter R

  • Control board C → Adapter C

  • Control board Y → Adapter W/Y

  • Adapter R, C → Thermostat R, C

  • Control board G → Thermostat G (wired directly; it doesn't pass through the adapter)

Mount the Adapter

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

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 cooling to confirm proper operation.

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

Troubleshooting

If the thermostat doesn't power on:

  1. Verify all connections match the diagram above

  2. Check the 3–5A fuse on your control board

  3. Confirm 24V AC between R and C with a multimeter

Need Help?

If your wiring doesn't match R, Y, and G only — for example if you also see W, O/B, AUX, or multiple transformers — this guide doesn't apply. Follow Installing the Mysa Power C-Wire Adapter instead, or reach out to our installation specialists.