Method and apparatus for monitoring a refrigeration-cycle system

1. A method of monitoring operation of a refrigerant-cycle system, the method comprising:

measuring a first temperature of air at a first location downstream of an evaporator using a first temperature sensor;

measuring a second temperature of air a second location upstream of the evaporator using a second temperature sensor;

determining a humidity of air; and

using a processing system, determining a temperature drop across the evaporator based on the first temperature, the second temperature, and the air humidity, wherein the temperature drop across the evaporator is a function of the air humidity.

2. The method of claim 1 further comprising, using the processing system:

determining an expected temperature drop across the evaporator; and

determining that airflow through the evaporator is restricted in response to the temperature drop across the evaporator being less than the expected temperature drop across the evaporator.

3. The method of claim 1 further comprising, using the processing system, determining whether airflow through the evaporator is restricted based on the temperature drop across the evaporator.

4. The method of claim 3 further comprising, using the processing system:

correcting the first temperature based on the air humidity to produce a first corrected temperature;

correcting the second temperature based on the air humidity to produce a second corrected temperature; and

determining the temperature drop across the evaporator based on the first corrected temperature and the second corrected temperature.

5. The method of claim 4 further comprising, using the processing system, determining the temperature drop across the evaporator based on a difference between the first corrected temperature and the second corrected temperature.

6. The method of claim 1 wherein determining the air humidity of air includes measuring humidity of air outside of a building using a humidity sensor.

7. The method of claim 1 wherein determining the air humidity of air includes measuring humidity of air at a third location upstream of the evaporator using a humidity sensor.

8. A monitoring system for monitoring operation of a refrigerant-cycle system, the monitoring system comprising:

a first temperature sensor measuring a first temperature of air at a first location downstream of an evaporator;

a second temperature sensor measuring a second temperature of air a second location upstream of the evaporator; and

a processing system that determines a temperature drop across the evaporator based on the first temperature, the second temperature, and a humidity of air and that determines whether airflow through the evaporator is restricted based on the temperature drop across the evaporator, wherein the temperature drop across the evaporator is a function of the air humidity.

9. The monitoring system of claim 8 wherein the processing system determines an expected temperature drop across the evaporator and determines that airflow through the evaporator is restricted in response to the temperature drop across the evaporator being less than the expected temperature drop across the evaporator.

10. The monitoring system of claim 8 wherein the processing system determines whether airflow through the evaporator is restricted based on the temperature drop across the evaporator.

11. The monitoring system of claim 10 wherein the processing system:

corrects the first temperature based on the air humidity to produce a first corrected temperature;

corrects the second temperature based on the air humidity to produce a second corrected temperature; and

determines the temperature drop across the evaporator based on the first corrected temperature and the second corrected temperature.

12. The monitoring system of claim 11 wherein the processing system determines the temperature drop across the evaporator based on a difference between the first corrected temperature and the second corrected temperature.

13. The monitoring system of claim 8 further comprising a humidity sensor that measures the air humidity of air outside of a building.

14. The monitoring system of claim 8 further comprising a humidity sensor that measures the air humidity of air at a third location upstream of the evaporator.