The ACI Bullet/Button Series Thermistor and RTD Sensors are designed specifically for HVAC system temperature control. The button and bullet style sensors come in thermistor or RTD sensor curves. The A/*BP Stainless Steel Thermistor and RTD Sensor is for under-insulation mounting and direct contact to the pipe. Clean off any insulation or rust to promote solid contact, and secure with tie wraps so the probe is parallel to the pipe.