Temperature Encoding

Temperature Encoding

Rules

1. All temperatures on the wire are Celsius.
2. Display units (°F vs °C) are controlled by Setup / Scale (0x01 / 0x03) and apply only at the UI.
3. Resolution is 0.5 °C via a packed single byte.
4. Convert using the guide conversion table (0.5 °C steps). When F→C has duplicate F entries, use the first table row (e.g. 64 °F → 17.5 °C, not 18 °C).

Packed temperature byte

Bits	Meaning
7	Sign: 0 = positive, 1 = negative
6	Half degree: 0 = .0, 1 = .5
5–0	Integer magnitude 0–63

Null on write

Condition	Encoding
Do not change field	Entire byte 0x00
Null heat/cool	Positive sign, no half bit, magnitude 0

Valid ranges (setpoints — guide)

Field	Typical valid magnitude region
Heat setpoint	~4–32 °C (bits 5–0)
Cool setpoint	~10–37 °C
Outdoor sensor value	0–55; negatives limited (notes ODT negatives limited to 40 magnitude)

Out-of-range writes → NACK 0x10.

Algorithms

encode(celsius):
 negative = celsius < 0
 mag = abs(celsius)
 half = 0x40 if (mag % 1) >= 0.5 else 0
 return floor(mag) + half + (0x80 if negative else 0)

decode(byte):
 value = byte & 0x3F
 if byte & 0x40: value += 0.5
 if byte & 0x80: value = -value
 return value

scrypted-aprilaire: convertTemperatureToByte / convertByteToTemperature in src/AprilaireClient.ts.

Documented examples

Physical	°C	Byte	Notes
70 °F heat	21.0	0x15	Guide write example
80 °F cool	26.5	0x5A	Guide write example
71 °F room	21.5	0x55	Guide COS example
110 °F heat (invalid)	43.5	0x6B	Guide OOR → NACK 0x10

Away setpoint indices (not packed temps)

Away heat/cool use small integer indices, not the packed temperature byte — see Domain Scheduling

Full conversion table

See Temperature Conversion Table for the complete °C ↔ °F appendix.