Methods and systems for monitoring an air filter of an HVAC system

1. A method of monitoring a status of an air filter of an HVAC system, wherein the HVAC system has a number of controllable HVAC components, the method comprising:

placing the controllable HVAC components into a predetermined filter check state;

while the controllable HVAC components are in the predetermined filter check state:

measuring a first differential pressure value across the air filter at a first time;

determining a status of the air filter based at least in part on the measured first differential pressure value;

placing at least one of the controllable HVAC components into a different state other than the predetermined filter check state including activating one or more HVAC components that were not previously activated in the predetermined filter check state, and operating the HVAC system while the at least one of the controllable HVAC components is in the different state;

after the HVAC system is operated with the at least one of the controllable HVAC components in the different state, placing each of the controllable HVAC components back into the predetermined filter check state; and

while the controllable HVAC components are in their predetermined filter check state:

measuring a second differential pressure value across the air filter at a second time; and

determining the status of the air filter based at least in part on the measured second differential pressure value.

2. The method of claim 1 , wherein the number of controllable HVAC components includes a fan and a heating element, and wherein the predetermined filter check state of the fan is “on” and the predetermined filter check state of the heating element is “off”.

3. The method of claim 2 , wherein placing at least one of the controllable HVAC components into a different state other than the predetermined filter check state includes turning the heating element “on” or changing a speed setting of the fan.

4. The method of claim 1 , wherein the number of controllable HVAC components includes a fan and a cooling element, and wherein the predetermined filter check state of the fan is “on” and the predetermined filter check state of the cooling element is “off”.

5. The method of claim 4 , wherein placing at least one of the controllable HVAC components into a different state other than the predetermined filter check state includes turning the cooling element “on” or changing a speed setting of the fan.

6. The method of claim 1 , wherein the number of controllable HVAC components includes a fan with a first non-zero fan speed and a second non-zero fan speed, and wherein the predetermined filter check state of the fan is at the first non-zero fan speed.

7. The method of claim 6 , wherein placing at least one of the controllable HVAC components into a different state other than the predetermined filter check state includes changing the fan to the second non-zero fan speed.

8. The method of claim 1 , wherein the number of controllable HVAC components includes a number of zone control dampers, and wherein the predetermined filter check state of the number of zone control dampers is “open”.

9. The method of claim 8 , wherein placing at least one of the controllable HVAC components into a different state other than the predetermined filter check state includes closing at least one of the number of zone control dampers.

10. The method of claim 1 , wherein the status of the air filter is determined by comparing the measured differential pressure values against a threshold differential pressure value.

11. The method of claim 10 , wherein the threshold differential pressure value is determined by:

while the controllable HVAC components are in the predetermined filter check state:

measuring a calibration differential pressure across a clean air filter;

determining the threshold differential pressure value based at least in part on the calibration differential pressure value.

12. The method of claim 10 , wherein the threshold differential pressure value is determined by:

while the controllable HVAC components are in the predetermined filter check state:

measuring a calibration differential pressure across an air filter that is at least partially blocked;

determining the threshold differential pressure value based at least in part on the calibration differential pressure value.

13. The method of claim 1 , further comprising displaying the status of the air filter on a display of an HVAC controller.

14. An HVAC controller for controlling an HVAC system having a number of controllable HVAC components and an air filter, the HVAC controller comprising:

a controller;

a memory coupled to the controller;

a display coupled to the controller;

wherein the controller is configured to place each of the controllable HVAC components into a predetermined filter check state at a first time, and while the controllable HVAC components are in the predetermined filter check state, receive a measure related to an amount of flow restriction presented by the air filter of the HVAC system, and then determine a status of the air filter based at least in part on the received measure related to an amount of flow restriction presented by the air filter, and then placing at least one of the controllable HVAC components into a different state other than the predetermined filter check state, wherein one or more components of the HVAC system are activated that were not previously activated while in the predetermined filter check state, and operating the HVAC system while the at least one of the controllable HVAC components is in the different state, the controller further configured to return the HVAC components to the predetermined filter check state at a second time; and

the controller configured to display the status of the air filter on the display.

15. The HVAC controller of claim 14 , wherein the measure related to an amount of flow restriction presented by the air filter of the HVAC system is a differential pressure across the air filter.

16. The HVAC controller of claim 14 , wherein the measure related to an amount of flow restriction presented by the air filter of the HVAC system is an air flow through the filter.

17. The HVAC controller of claim 14 , wherein the predetermined filter check state corresponds to a fan only mode.

18. The HVAC controller of claim 17 , wherein the number of controllable HVAC components includes a number of zone control dampers, and wherein the predetermined filter check state of the number of zone control dampers is “open”.

19. The HVAC controller of claim 14 , wherein the controller is configured to repeatedly place each of the controllable HVAC components into the predetermined filter check state, and while the controllable HVAC components are in the predetermined filter check state, receive a measure related to an amount of flow restriction presented by the air filter of the HVAC system, and then determine a status of the air filter based at least in part on the received measure related to an amount of flow restriction presented by the air filter.

20. A method of monitoring a status of an air filter of an HVAC system of a building, wherein the HVAC system includes a number of controllable HVAC components, the method comprising:

operating the HVAC system in a normal operating mode to control a temperature in the building;

occasionally entering an air filter monitoring mode, wherein the air filter monitoring mode corresponds to a fan only mode of the HVAC system;

while in the air filter monitoring mode, receiving a measure related to an amount of flow restriction presented by the air filter, and determining a status of the air filter based at least in part on the received measure related to the amount of flow restriction presented by the air filter; and

displaying the status of the air filter on a display.