Digital thermostat mounted on a concrete wall

Generic photo: Alexander Mass · Pexels License

Wiring a humidifier, dehumidifier or ventilator to the thermostat's accessory terminals (U1, HUM/DEHUM, ACC)

Many pro and smart thermostats have accessory outputs. Some are dry contacts (just a switch, no power of their own, like Honeywell VisionPRO 8000 U1/U1); others are powered 24 V outputs referenced to the thermostat's own R/C. Wiring a dry contact as if it were powered (or the reverse) is the usual reason an accessory never turns on, or a transformer gets back-fed. Identify which kind the thermostat has, how the accessory is powered, and whether it needs a field jumper from R.

Single source ~45 min moderateBrands: Honeywell Home / Resideo, Google Nest, Lennox, All

Before you start

Turn off power to the HVAC equipment and to any separately powered accessory transformer before landing wires.
Never tie two transformers together through a dry contact jumper; follow the accessory and thermostat diagrams.
Steam humidifiers and ventilators can have their own 120/240 V supply; only the low-voltage control wiring is thermostat work.

Diagnostic steps

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