Universal Thermostat Wiring Guide – MRCOOL

Before You Start: Key Things to Know

When switching the Universal system to Heat Mode, it may take a few minutes to start blowing warm air.
The air handler has a 5-minute shutoff delay after the condenser stops. This is normal — it's not broken.
Always turn off the power at the breaker before wiring anything.

Step 1: Match the Wiring – Thermostat to Air Handler to Condenser

You want to make sure your wiring is consistent across all three components.

Basic Wiring Setup

Thermostat	Air Handler	Condenser
R	R	R
C	C	C
B or O/B	B	B
Y	(splice only) or Y	Y
G	G
W1(Only with heat strips)	W1, D(Only with heat strips)	D (Only with heat strips)

Note:

Y (cooling signal) only goes from the thermostat to the condenser. You can splice it at the air handler or use the Y terminal (it won’t do anything there).
B or O/B is the reversing valve, used for heat pump mode. Some thermostats call it “O/B” — that’s fine.
If your thermostat doesn’t have a C wire, just connect the C wire between the air handler and the condenser only — do not connect to the thermostat.

Step 2: Understanding Each Wire

Wire	Name	Function
R	Power	24V hot
C	Common	24V return
Y	Compressor	Signals cooling
G	Fan	Turns the fan on
W1	Heat Strip	Activates auxiliary heat
B (O/B)	Reversing Valve	Switches cooling ↔ heating
D	Data	Sends heat strip call from the air handler to the condenser

Step 3: Thermostat Programming Settings

In your thermostat’s settings, make sure:

It’s set to “Heat Pump” system type.
The reversing valve (O/B) is set to energize in heating mode (MrCool systems use O logic).
If you have a smart thermostat, this option may be in the advanced settings under equipment configuration.

Step 4: If You’re Using Heat Strips

If you're adding electric heat strips to the system, follow these critical steps:

Power Supply:

Breaker power should go to the heat strips, not directly to the air handler.
The heat strips power the air handler when installed properly.

Plug Connection:

Look for the Molex plug (white plastic connector) on the heat strip’s wiring.
Plug it into the matching Molex port on the air handler.

IMPORTANT: If this plug is missing or not connected, your heat strips can overheat or be damaged.

Thermostat Wiring for Heat Strips:

W/W1/AUX at thermostat → W1 at air handler
D at air handler → D at condenser

If your condenser does not have a D terminal, connect that wire to W1 instead. If your air handler does not have a D terminal, connect the D wire to Y.

Terminal Layout Cheat Sheet

Here’s how the terminals are laid out (left to right):

Condenser: Y B D R C (G optional)

Air Handler: D B W1 R C G

Final Checklist Before Powering On

System set to Heat Pump in thermostat
Reversing valve (O/B) set to energize in heating
Heat strips have power from the breaker and a Molex plug connected
All terminals are matched and securely connected
C wire used properly (or skipped at the thermostat if not needed)
D wire routed properly for heat strip control

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