The HLD1 Thermostat is primarily designed for use as a O/A duct mount high limit fresh air thermostat, typically used to break a 0-10v dc Y economy cycle signal controlling the position of damper motors. Or to override the position of 2 position damper motors. The output relay is voltage free permitting use on either 240, 24 AC circuits or DC control signal circuitry. Switch point is adjustable with it's status displayed via red / green LED indicators. The HLD1 thermostat is ideally suited for mounting in the fresh air intake duct as long as (at least minimum) air flow is always maintained during system operation.