Wiring Reference Guide
This article explains wire functions, terminal labels, and system types for Mysa Smart Thermostat (formerly Mysa for Central HVAC). Use it to plan and verify your wiring before installation.
Tip The Mysa app provides dynamic, step-by-step wiring guidance tailored to your specific system. Download it from the App Store or Google Play before you start.
For hands-on installation steps, see Installation Overview.
Why Wiring Accuracy Matters
Each wire connects to a specific control in your HVAC system. Correct wiring ensures:
- Continuous power to your thermostat
- Safe, stable operation
- Correct heating, cooling, and fan responses
If anything looks unfamiliar, contact an HVAC professional or email install@getmysa.com.
Terminal Reference
Mysa Smart Thermostat uses standard 24V HVAC terminal labels. Wire colors vary — always rely on terminal labels, not colors.
| Terminal | Function | Typical Color | Notes |
|---|---|---|---|
| R / Rh / Rc | 24V power | Red | See Rh/Rc switch section below |
| C | Common | Blue or Brown | Required — provides continuous power |
| W / W1 / W2 | Heating stages | White | W1 = Stage 1; W2 = Stage 2 |
| Y / Y1 / Y2 | Cooling stages | Yellow | Y1 = Stage 1; Y2 = Stage 2 |
| G | Fan | Green | Thermostat-controlled fan |
| O/B | Reversing valve | Orange (O) / Blue (B) | Heat pumps only — switches heating/cooling |
| AUX/E | Auxiliary heat | Varies | Backup heat for heat pumps |
| S1 / S2 | Sensors | Varies | Future support — floor sensors, outdoor heat pump sensors (not yet available) |
Tip Trace wires to your control board to confirm connections if labels are unclear.
Single vs Dual Transformer Systems
Most homes have a single transformer powering both heating and cooling. Some systems — particularly homes with a boiler for heat and separate AC — have two transformers with separate Rh (heating) and Rc (cooling) wires.
Mysa Smart Thermostat has a switch on the back of the main display to handle both:
- Single transformer (most systems): Leave switch CLOSED (default, to the right). This internally connects Rh and Rc.
- Dual transformers (Rh and Rc wires): Set switch to OPEN (to the left).
Important If your old thermostat had both Rh and Rc wires connected, you must set the switch to OPEN.
Warning With dual transformer systems, never connect C-wires from different transformers together — this can damage your equipment. Use an isolation relay or contact an HVAC professional.
Common Wiring Confusions
W vs AUX: On heat pumps, the W terminal is often auxiliary heat. If your terminal shows "W2/AUX" or "AUX/E", it's backup heat.
W1 vs O/B: These sometimes share a terminal. Heat pumps use O/B (reversing valve, usually orange). Conventional systems use W (heating, usually white).
O vs B: Both are reversing valve wires — you'll only have one. O energizes in cooling mode; B energizes in heating mode. Check your system documentation. Mysa defaults to O (most common).
B vs C: On some heat pumps with an O wire, the B wire can be repurposed as a C-wire. Confirm connections at both the thermostat and control board.
Unused Wires
Your old thermostat may have wires Mysa doesn't use (T, L, M, VR). Cap these with wire nuts and tuck them into the wall.
Accessory Terminals
Some thermostats have accessory terminals for humidifiers, dehumidifiers, or ventilators. If your current thermostat has these terminals but nothing is connected, you can install Mysa Smart Thermostat without issue.
If you do have accessories wired to your thermostat, Mysa Smart Thermostat may not be the best fit — it doesn't support accessory control. Sign up at getmysa.com to be notified about our upcoming Smart Thermostat Pro, which will include accessory support.
C-Wire Options
Mysa Smart Thermostat requires a C-wire for continuous power. Nearly 50% of homes need the Mysa Power C-Wire Adapter.
Option 1: Use an existing C-wire — connect to C terminal at both thermostat and control board.
Option 2: Repurpose a spare wire — connect an unused wire to C at both ends.
Option 3: Install the Mysa Power C-Wire Adapter — required when no C-wire exists.
See Understanding Your C-Wire and the Mysa Power C-Wire Adapter for details.
Zone Valves and Boilers
Zone valve systems: Each zone may have its own transformer. Don't tie C-wires together across zones. Install the C-Wire Adapter at the zone valve, not the boiler.
Boilers with T/T terminals: These are dry contacts with no 24V power. Use the Mysa Power C-Wire Adapter or an external 24V transformer.
Troubleshooting: Thermostat Won't Power On
- Check the breaker and furnace door switch
- Verify C-wire is connected at both ends
- Inspect the 3–5A fuse on your furnace control board
- Confirm 24V AC between R and C with a multimeter
Need Help?
- Installation questions: install@getmysa.com
- User Guides: Mysa Smart Thermostat Guide (PDF) | C-Wire Adapter Guide (PDF)