Furnace for a rooftop unit

1. A heating, ventilating, and air conditioning (HVAC) system, comprising:

a furnace comprising a primary heat exchanger and a secondary heat exchanger, wherein the primary heat exchanger and the secondary heat exchanger are configured to form a heat exchange relationship between an airflow through the furnace and an exhaust gas flowing through the primary heat exchanger and the secondary heat exchanger, and wherein the primary heat exchanger is positioned upstream of the secondary heat exchanger with respect to a flow of the exhaust gas;

a burner of the furnace configured to generate the exhaust gas and direct the exhaust gas to the primary heat exchanger;

a sensor configured to monitor an ambient temperature; and

a control system configured to receive feedback from the sensor indicative of the ambient temperature, compare the feedback indicative of the ambient temperature to a temperature threshold, operate the furnace in a first operating mode when the ambient temperature exceeds the temperature threshold, and operate the furnace in a second operating mode when the ambient temperature is at or below the temperature threshold, wherein the furnace operates above a condensation temperature of the exhaust gas when in the second operating mode, such that the exhaust gas does not condense when the furnace operates in the second operating mode.

2. The HVAC system of claim 1 , comprising a first collector of the furnace positioned between an outlet of the primary heat exchanger and an inlet of the secondary heat exchanger, wherein the first collector is configured to direct the exhaust gas exiting the primary heat exchanger into tubes of the secondary heat exchanger.

3. The HVAC system of claim 2 , comprising a second collector of the furnace positioned at an outlet of the secondary heat exchanger, wherein the second collector is configured to direct the exhaust gas exiting the secondary heat exchanger to an exhaust outlet when the furnace operates in both the first operating mode and the second operating mode, and wherein the second collector is configured to direct condensate to a condensate drainage system when the furnace operates in the first operating mode.

4. The HVAC system of claim 1 , wherein the primary heat exchanger comprises a first plurality of tubes configured to flow the exhaust gas, and wherein the secondary heat exchanger comprises a second plurality of tubes configured to flow the exhaust gas.

5. The HVAC system of claim 4 , wherein each tube of the first plurality of tubes comprises a first diameter that is greater than a second diameter of each tube of the second plurality of tubes.

6. The HVAC system of claim 5 , wherein the second diameter is between one tenth and one fourth of the first diameter.

7. The HVAC system of claim 4 , wherein the first plurality of tubes are supported by a support beam that is coupled to a cabinet of the HVAC system.

8. The HVAC system of claim 4 , wherein each tube of the first plurality of tubes comprises a rating between 10,000 British Thermal Units per hour (Btuh) and 50,000 Btuh.

9. The HVAC system of claim 8 , wherein the secondary heat exchanger comprises a rating between 50,000 Btuh and 400,000 Btuh.

10. The HVAC system of claim 9 , wherein the secondary heat exchanger comprises a fin coil heat exchanger.

11. The HVAC system of claim 1 , comprising a barrier disposed in the furnace between the burner and the primary heat exchanger and the secondary heat exchanger, wherein the barrier defines a flow path of the airflow through the furnace.

12. The HVAC system of claim 1 , wherein the furnace operates at an efficiency of greater than 90% when operating in the first operating mode.

13. The HVAC system of claim 12 , wherein the furnace operates at an efficiency between 80% and 90% when operating in the second operating mode.

14. The HVAC system of claim 13 , wherein the furnace is configured to generate substantially the same heat output in the first operating mode and the second operating mode.

15. The HVAC system of claim 1 , comprising an additional sensor disposed in a condensate drainage system of the furnace, wherein the additional sensor is configured to monitor a temperature of condensate produced when the furnace operates in the first operating mode.

16. One or more tangible, non-transitory machine-readable media comprising processor-executable instructions to:

receive feedback from a first sensor indicative of ambient temperature;

compare the feedback indicative of the ambient temperature to a temperature threshold;

operate a furnace of a heating, ventilating, and air conditioning (HVAC) system in a first operating mode when the ambient temperature exceeds the temperature threshold; and

operate the furnace of the HVAC system in a second operating mode when the ambient temperature is at or below the temperature threshold, wherein the furnace operates above a condensation temperature of an exhaust gas when in the second operating mode, such that the exhaust gas does not condense when the furnace operates in the second operating mode.

17. The one or more tangible, non-transitory machine-readable media of claim 16 , wherein the processor-executable instructions receive feedback from a second sensor configured to monitor an additional temperature of condensate produced by the furnace when operating in the first operating mode.

18. The one or more tangible, non-transitory machine-readable media of claim 17 , wherein the processor-executable instructions switch from the first operating mode to the second operating mode when the feedback from the second sensor indicates that the temperature of condensate falls below an additional temperature threshold.

19. The one or more tangible, non-transitory machine-readable media of claim 16 , wherein the processor-executable instructions increase a flow rate of fuel to a burner of the furnace, increase a flow rate of oxidant to the burner of the furnace, or both, to switch from the first operating mode to the second operating mode.

20. The one or more tangible, non-transitory machine-readable media of claim 16 , wherein the temperature threshold is between 0° C. and 5° C.

21. A method of operating a furnace of a heating, ventilating, and air condition (HVAC) system, comprising:

receiving feedback from a sensor indicative of ambient temperature;

comparing the feedback indicative of the ambient temperature to a temperature threshold;

operating a furnace of a heating, ventilating, and air conditioning (HVAC) system in a first operating mode when the ambient temperature exceeds the temperature threshold; and

operating the furnace of the HVAC system in a second operating mode when the ambient temperature is at or below the temperature threshold, wherein the furnace operates above a condensation temperature of an exhaust gas when in the second operating mode, such that the exhaust gas does not condense when the furnace operates in the second operating mode.

22. The method of claim 21 , wherein operating the furnace of the HVAC system in the first operating mode comprises operating the furnace at an efficiency greater than 90%, and wherein the furnace operates below the condensation temperature of the exhaust gas to generate condensate when operating in the first operating mode.

23. The method of claim 21 , wherein operating the furnace of the HVAC system in the second operating mode comprises operating the furnace at an efficiency between 80% and 90%.

24. The method of claim 21 , comprising increasing a flow rate of fuel to a burner of the furnace and/or increasing a flow rate of oxidant to the burner of the furnace when the ambient temperature is at or below the temperature threshold to operate the furnace in the second operating mode.

25. The method of claim 21 , wherein operating the furnace of the HVAC system in the first operating mode and operating the furnace of the HVAC system in the second operating mode each provide heated air to a building at substantially the same temperature.