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Mysa C-Wire Power Adapter

Provides reliable 24V power to your Mysa thermostat when no C-wire is available. Installs at your furnace, boiler, zone equipment or air handler.
$25.00

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  • Mysa C-Wire Power Adapter × 1 $25.00
$25.00

Many homes don't have a C-wire at the thermostat, and running new wire through walls is a hassle. The Mysa C-Wire Power Adapter provides continuous 24 VAC power to your Mysa Smart Thermostat by repurposing your existing wiring. Designed to handle the trickiest installations, including two-wire heat-only and cool-only systems where you have just R and W (or R and Y) at the thermostat.
checkGas and oil furnaces
checkAir conditioners and heat pumps
checkHot water boilers with hydronic systems
check24V zone valves and zone valve control boards
check24V relay configurations
checkMillivolt/dry contact configurations (many fireplaces)

Works with Mysa Smart Thermostat only

Installation happens at your furnace, boiler, air handler, or zone valve panel. Our online guide creates custom instructions based on your exact wiring setup—with 11 different diagrams covering most configurations.

Standard installation (furnace/air handler):

  1. Turn OFF power at the circuit breaker
  2. Locate control board
  3. Label existing thermostat wires using included stickers
  4. Photo your wiring for reference
  5. Reroute wires through adapter following your custom guide
  6. Mount adapter using included adhesive pad
  7. Restore power and verify thermostat displays

Boiler and zone valve systems: For hydronic systems with zone valves, install the adapter at the zone valve—not the boiler aquastat. Multi-zone setups need one adapter per thermostat.

External transformer option: Older boilers or systems without adequate 24V power can use the Rext terminal with a standard 24 VAC transformer (sold separately, ~$15–20 at hardware stores). One transformer powers up to three Mysa + adapter combinations.

Model: MYSA CWA V2-0

Input Voltage: 20–30 VAC

Current Rating: 8A (2A max on W/Y/OB)

Operating Temperature: 0°C to 50°C (32°F to 122°F)

Relay Type: Isolated dry contact

External Power: Rext terminal for 24 VAC transformer

Wire Harness Length: ~12" (30 cm)

Ingress Protection: IPX0

Dimensions: [placeholder - to be added]

Colour: White

Our return policy is simple. High utility bills will never return. But if you do need to send your Mysa back, that's simple too - free shipping both ways and expert support at every step.


You expect a smart thermostat to last. After a million units and years of engineering refinement, we know ours do. That's why every Mysa comes with 5-year coverage - more than double the 2-year industry standard.

What's in the box

Features

boltReliable 24V Power
buildNo New Wiring
houseTwo-Wire Ready
quick_referenceCustom Install Guide
package_2Hardware Included
ecoLess E-Waste

Control from Anywhere

"The 4 best smart thermostats of 2025"

~ Wirecutter / New York Times

"Works as expected…finally got rid of the junky 1980’s thermostat that only heated to the max temp"

~ Derek F.

"Does a better job than the other smart thermostats I've used and the app gives you all the information you need to make the best use of your home heating dollars."

~ Ian V.

"The step by step guide really help to make things easier for first time installer like me.”

~ Chris L.

"Mysa support team is awesome; Leigh helped with the installation and made the whole process smooth.”

~ Anmol A.

"Reliable, beautiful, and clearly improved”

~ La Presse

What others asked before buying

Can I install this myself?

Yes. Installation requires basic comfort with electrical wiring at your furnace or boiler. Our online guide creates custom instructions for your exact setup with 11 different wiring diagrams. If you're uncomfortable working with HVAC wiring, hire a qualified technician—typically $75–150.

Do I need the C-Wire Power Adapter?

Over 55% of compatibility checker submissions need the adapter. Homes built before 2000 and hydronic heating systems are most common.

What's included?

The adapter, wire connector for external transformer setups, adhesive mounting pad, wire label stickers, and installation guide.

4.6 34 reviews

  • Frederic Verified

    Canada

    Very good HVAC smart thermostat

    I really like the Mysa HVAC smart thermostat and is fonctionality. Easy to use, everything is intuitive and the app is inline with that. I love being able to dim the led and control everything from distance.

    Review for Mysa Smart Thermostat

  • Claude L. Verified

    Canada

    My Fourth Mysa

    Mysa thermostats are wonderful. We have a ski condo where we have two baseboard thermostats (V1) and those work flawlessly. In addition, we also have a mini-split AC controlled by another Mysa thermostat. Now that Mysa has introduced a new thermostats for HVAC 24 volt system, we purchased it and are super pleased with it. One app to control them all!

    Review for Mysa Smart Thermostat

  • Matt C. Verified

    United States

    Great Smart Thermostat

    Struggled to find a smart thermostat for radiant heat only. They made sure my setup would work with their product before I purchased. Mysa Smart Thermostat works perfectly with my Home Assistant setup. Small issue initially getting the Matter pair codes, but their support was able to resolve.

    Review for Mysa Smart Thermostat

  • Richard E. Verified

    United States

    Needed tech support help.Devon was great.

    Had trouble with first thermostat.Tech support was very helpful and sent me a replacement. Worked perfectly. Definitely adds to comfort while saving g money.

    Review for Mysa Smart Thermostat

  • Bret a.K. Verified

    United States

    The Mysa app makes this system awesome!

    With the app I can keep my house cool without an air conditioner, and help save money. I love the Mysa system.

    Review for Mysa Smart Thermostat

  • Andy S. Verified

    Canada

    Controlling baseboard heaters using low voltage relay

    Controlling baseboard heaters using low voltage relay. I didn't know if I could do this with an existing relay setup. By adding the controller and the C-wire power adapter it works perfectly.

    Review for Mysa Smart Thermostat

  • roger b. Verified

    United States

    One of the things I like is the communication your firm promotes

    One of the things I like is the communication your firm promotes. You cannot find that with many suppliers. I now have two units and will get two more soon. We have two homes and I like the ability to manage each one separately. Nest (Google) failed me. Thanks, Mysa. Roger

    Review for Mysa Smart Thermostat

  • Andrew W. Verified

    Bolivia

    Superb customer support that gives me great confidence in the company

    I was installing this thermostat to replace an existing two wire thermostat that ran a relay switch to a group of baseboards. It is not a normal install and the support from Mysa was nothing short of superb. They sent complete descriptions and diagrams on how to hook up, all within a few hours of my inquiry. Hard to find such good service these days.

    Review for Mysa Smart Thermostat