Measured °F→°C setpoint mapping used by US Fahrenheit remotes — Mitsubishi AC 144-bit (MSZ-GS09NA)

[vertoforce]

While building an IR blaster setup in Home Assistant for my Mitsubishi mini split, I reverse-engineered how its US Fahrenheit remote encodes temperature setpoints on the wire — documenting the mapping here since I couldn't find it published anywhere.

US Mitsubishi remotes display Fahrenheit but transmit half-degree Celsius in the IR frame, and the conversion isn't math — rounding (F − 32) × 5/9 to half degrees is wrong at most setpoints. It's a lookup table.

The pattern: half-degree steps per 1°F, with three full-degree jumps so 28 Fahrenheit values cover the unit's 16.0–31.0°C range:

• 61–67°F: C = 16 + (F − 61) / 2
• 68°F → 20.0, 69°F → 21.0
• 69–87°F: C = 21 + (F − 69) / 2
• 88°F → 31.0

Encoding (byte 7 of the frame): low nibble = °C − 16, bit 0x10 = +0.5°C. So 70°F → 21.5°C → 0x15.

Full table, captured from an MSZ-GS09NA remote:

°F	°C	byte7		°F	°C	byte7
61	16.0	0x00		75	24.0	0x08
62	16.5	0x10		76	24.5	0x18
63	17.0	0x01		77	25.0	0x09
64	17.5	0x11		78	25.5	0x19
65	18.0	0x02		79	26.0	0x0a
66	18.5	0x12		80	26.5	0x1a
67	19.0	0x03		81	27.0	0x0b
68	20.0	0x04		82	27.5	0x1b
69	21.0	0x05		83	28.0	0x0c
70	21.5	0x15		84	28.5	0x1c
71	22.0	0x06		85	29.0	0x0d
72	22.5	0x16		86	29.5	0x1d
73	23.0	0x07		87	30.0	0x0e
74	23.5	0x17		88	31.0	0x0f

Only measured on the GS09NA, but other US MSZ models potentially match.

[crankyoldgit]

We only have a Celcius setting/reading capability for that protocol MitsubishiAC protocol.
If you are using the generic common AC interface, and using °F, yeah. It will do a substandard job of conversion.
I suggest people use their own lookup table (like you've provided to pair the °C to °F etc and use the Celcius methods of setting the temp to get accurate results.