Purpose

Use this article when a customer has one Mysa thermostat controlling equipment powered by two separate 24 V transformers, such as hydronic in-floor heat on Rh/W and ducted air conditioning on Rc/Y/G.

Key rule

In a dual-transformer setup, do not tie the two transformer circuits together. The Rh and Rc circuits must remain isolated, and only one transformer common should be connected to Mysa’s C terminal.

Mysa is flexible: the thermostat can be powered from either the heating transformer or the cooling transformer. Which transformer powers Mysa depends on which transformer the connected C wire returns to.

How Mysa is powered

Option A: Cooling transformer powers Mysa

Use this when the cooling transformer feeding Rc has enough spare capacity for Mysa’s approximate 3 VA load and there is an available C wire returning to the cooling transformer.

• Rc powers the thermostat through the connected cooling-side C.

• Rc also powers cooling/fan calls such as Y1, Y2, and G.

• Rh is not powering the thermostat in this wiring arrangement; it only feeds heating calls such as W1 and W2.

• There is no C return path for Rh in this arrangement.

Option B: Heating transformer powers Mysa

Use this when a C wire is available back to the heating transformer, such as a hydronic system with R, W, and C at the thermostat location.

• Rh powers the thermostat through the connected heating-side C.

• Rh also powers heating calls such as W1 and W2.

• Rc only powers cooling/fan calls such as Y1, Y2, and G.

Important: connect only one C wire

If both systems provide a common wire, connect only one C wire to Mysa: either the heating-side C or the cooling-side C. Cap off and safely isolate the unused C wire. Do not connect both commons together at the thermostat.

Backplate/jumper guidance

For two-transformer systems, ensure the backplate jumper/switch is set to keep Rh and Rc isolated. Use the dual-transformer / separated Rh-Rc position shown in the Mysa installation materials.

Transformer capacity

Mysa adds approximately 3 VA of load to whichever transformer is connected to C.

• If Mysa is powered from the heating transformer, add this load to the hydronic/heating transformer capacity calculation.

• If Mysa is powered from the cooling transformer, add this load to the cooling transformer capacity calculation.

• If several Mysa thermostats are already powered by the same hydronic transformer, confirm the transformer has enough capacity. Upgrade the transformer or add an approved dedicated auxiliary transformer if total load exceeds available VA.

Example: hydronic heat plus ducted AC

Customer has three hydronic wires at the thermostat: R, W, and C, plus separate cooling wires from a ducted AC system.

• Connect hydronic R to Rh.

• Connect hydronic W to W1.

• Connect hydronic C to C if choosing the heating transformer to power Mysa.

• Connect cooling power to Rc.

• Connect cooling/fan control wires to the applicable cooling terminals, such as Y1, Y2, and/or G.

• If the cooling system also has a C wire at the wall, cap it off and leave it unused when the hydronic C is connected.

Support response guidance

When replying to a customer, avoid saying Mysa must always draw power from either cooling or heating. The accurate answer is:

Mysa can be powered by either transformer in a dual-transformer setup. The transformer connected to the C wire used at Mysa is the transformer that powers the thermostat. The other transformer remains isolated and only powers its own call circuits.

When to escalate

• The customer is unsure which transformer a C wire returns to.

• Both C wires have already been tied together.

• The system uses non-standard controls, zone boards, or relay panels.

• The transformer VA capacity is unknown or appears insufficient.

• The installation is for a VIP/customer-sensitive retrofit and needs human wiring confirmation.