System and method of operating a variable speed HVAC system

An HVAC system comprising:

• a single speed-system controller;
• an interface device in communication with the single speed-system controller, the interface device comprising:
• a processor;
• a memory in communication with the processor;
• and
• a temperature sensor in communication with the processor, the temperature sensor configured to measure a current space temperature;
• wherein the processor is configured to
• (a) receive a first conditioning signal from the single-speed system controller, wherein the single-speed system controller is configured to receive a user selected set point temperature from a user and to transmit the first conditioning signal from the single-speed system controller to the interface device, wherein the first conditioning signal that is transmitted from the single-speed system controller to the interface device does not include the user selected set point temperature;
• (b) determine a temperature differential when the first conditioning signal is received;
• (c) transmit the temperature differential;
• and
• (d) receive a second conditioning signal from the single-speed system controller.

An interface device comprising:

• a processor;
• a memory in communication with the processor;
• and
• a temperature sensor in communication with the processor, the temperature sensor configured to measure a current space temperature;
• wherein the processor is configured to operate a program stored in memory, the program configured to:
• (a) receive a first conditioning signal from a single-speed system controller, wherein the single-speed system controller is configured to receive a user selected set point temperature from a user and to transmit the first conditioning signal from the single-speed system controller to the interface device, wherein the first conditioning signal that is transmitted from the single-speed system controller to the interface device does not include the user selected set point temperature;
• (b) determine a temperature differential when the first conditioning signal is received;
• (c) transmit the temperature differential;
• and
• (d) receive a second conditioning signal from the single-speed system controller.