Temperature difference sensor for hvac systems

A system comprising:

• an evaporator coil comprising:
• an inlet for flow of refrigerant into the evaporator coil, an outlet for flow of refrigerant out of the evaporator coil, and a superheat sensor, wherein the superheat sensor comprises:
• a first thermistor positioned to sense a saturated suction temperature of the refrigerant flowing in a first portion of the evaporator coil;
• a second thermistor positioned to sense a superheated vapor temperature of the refrigerant flowing in a second portion of the evaporator coil, the second thermistor coupled electronically in series with the first thermistor;
• and
• a signal output for transmitting a superheat signal from the superheat sensor, wherein the signal output is coupled electronically to a first terminal of the first thermistor and a second terminal of the second thermistor;
• and
• a controller comprising an input/output interface configured to receive the superheat signal from the superheat sensor, wherein the controller is configured to determine a temperature difference between the saturated suction temperature and the superheated vapor temperature based on the superheat signal.

A system comprising:

• a condenser coil comprising:
• a first inlet for flow of refrigerant into the condenser coil, a first outlet for flow of the refrigerant out of the condenser coil, and a subcool sensor, wherein the sub cool sensor comprises:
• a first thermistor positioned to sense a first temperature of the refrigerant flowing in a first portion of the condenser coil;
• a second thermistor positioned to sense a second temperature of the refrigerant flowing in a second portion of the condenser coil, the second thermistor coupled electronically in series with the first thermistor;
• and
• a first signal output for transmitting a subcool signal from the subcool sensor, wherein the first signal output is coupled electronically to a first terminal of the first thermistor and a second terminal of the second thermistor;
• and
• an evaporator coil comprising:
• a third inlet for flow of the refrigerant into the evaporator coil, a fourth outlet for flow of the refrigerant out of the evaporator coil, and a superheat sensor, wherein the superheat sensor comprises:
• a third thermistor positioned to sense a third temperature of the refrigerant flowing in a third portion of the evaporator coil;
• a fourth thermistor positioned to sense a fourth temperature of the refrigerant flowing in a fourth portion of the evaporator coil, the third thermistor coupled electronically in series with the fourth thermistor;
• and
• a second signal output for transmitting a superheat signal from the superheat sensor, wherein the second signal output is coupled electronically to a third terminal of the third thermistor and a fourth terminal of the fourth thermistor;
• and
• a controller comprising an input/output interface configured to receive the subcool signal from the subcool sensor and the superheat signal from the superheat sensor, wherein the controller is configured to:
• determine a subcool temperature difference between the first temperature and the second temperature based on the subcool signal;
• and
• determine a second temperature difference between the third temperature and the fourth temperature based on the superheat signal.