A thermostat under heading 9032.10 is defined as an instrument that senses temperature and automatically switches or modulates a circuit or valve to maintain a set temperature — its control action is temperature-driven and typically single-variable. A multi-variable process controller that uses temperature as one of several inputs, or that incorporates programmable logic beyond simple set-point switching, is more likely to classify under 9032.89 as an automatic regulating or controlling instrument. The functional simplicity of the sensing-and-switching mechanism is the key classification criterion.

CBP has generally held that a programmable or networked thermostat retains its classification under 9032.10 when its essential function remains automatic temperature regulation, even if it incorporates wireless connectivity or a digital interface. The communications capability is considered ancillary. However, if the device is marketed and designed primarily as a home-automation hub that incidentally controls temperature, reclassification risk increases; a binding ruling is advisable for complex smart-home platforms.