Furnace with primary and secondary heat exchangers

Abstract

A furnace that includes a heater configured to heat a fluid and discharge a heated fluid flow, a first heat exchanger at least partially positioned in a air duct and in fluid communication with the heater to receive the heated fluid flow, and a second heat exchanger at least partially positioned in the air duct and in fluid communication with the first heat exchanger so that the heated fluid flow flows through the second heat exchanger after flowing through the first heat exchanger. The first heat exchanger and the second heat exchanger are arranged in the air duct so that the flow of air travels through the first heat exchanger to transfer heat between the heated fluid flow and the flow of air, and then through the second heat exchanger after the first heat exchanger to transfer heat between the heated fluid flow and the flow of air.

1 . A furnace configured to heat a flow of air and discharge the heated flow of air into a conditioned space, the furnace comprising:
 an air duct including an air inlet and an air outlet, the air duct defining an air flow path from the air inlet to the air outlet, the air duct configured to direct the flow of air along the air flow path from the air inlet to the air outlet;   a heater configured to heat a fluid and discharge a heated fluid flow;   a first heat exchanger at least partially positioned in the air duct and in fluid communication with the heater to receive the heated fluid flow; and   a second heat exchanger at least partially positioned in the air duct and in fluid communication with the first heat exchanger so that the heated fluid flow flows through the second heat exchanger after flowing through the first heat exchanger,   wherein the first heat exchanger and the second heat exchanger are arranged in the air duct so that the flow of air travels through the first heat exchanger to transfer heat between the heated fluid flow and the flow of air, and then through the second heat exchanger after the first heat exchanger to transfer heat between the heated fluid flow and the flow of air.   
     
     
         2 . The furnace of  claim 1 , wherein a majority of the flow of air entering the air inlet is outdoor air. 
     
     
         3 . The furnace of  claim 1 , wherein the flow of air is discharged from the air outlet into a building. 
     
     
         4 . The furnace of  claim 1 , wherein the first heat exchanger is positioned outdoors. 
     
     
         5 . The furnace of  claim 1 , wherein the second heat exchanger is positioned outdoors. 
     
     
         6 . The furnace of  claim 1 , further comprising at least one condensate reservoir fluidly coupled to the second heat exchanger and positioned to receive condensate from the heated fluid flow in the second heat exchanger. 
     
     
         7 . The furnace of  claim 1 , further comprising an air handling unit including a motor and a centrifugal fan, the motor is configured to rotate the centrifugal fan and the centrifugal fan is configured to direct the flow of air along the air flow path. 
     
     
         8 . The furnace of  claim 1 , wherein the heater includes at least one burner. 
     
     
         9 . The furnace of  claim 8 , wherein the at least one burner is an inshot burner, and wherein the first heat exchanger includes a plurality of tubes in which the heated fluid flows. 
     
     
         10 . The furnace of  claim 9 , wherein the second heat exchanger includes a plurality of tubes and a plurality of fins, and wherein the heated fluid flows through the plurality of tubes of the second heat exchanger and the flow of air flows through the plurality of fins. 
     
     
         11 . The furnace of  claim 1 , wherein the first and the second heat exchangers are positioned outdoors, and wherein the air inlet is positioned outdoors. 
     
     
         12 . A method of operating a furnace comprising:
 heating a fluid with a heater;   directing the heated fluid through a first heat exchanger;   directing a flow of air through the first heat exchanger;   transferring heat from the heated fluid to the flow of air in the first heat exchanger;   directing the heated fluid through a second heat exchanger after directing the heated fluid through the first heat exchanger;   directing the flow of air through the second heat exchanger after directing the flow of air through the first heat exchanger; and   transferring heat from the heated fluid to the flow of air in the second heat exchanger.   
     
     
         13 . The method of  claim 12 , further comprising inhibiting freezing of condensate from the flow of air in the second heat exchanger by transferring heat from the heated fluid to the flow of air in the first heat exchanger prior to directing the flow of air through the second heat exchanger. 
     
     
         14 . The method of  claim 12 , further comprising moving outdoor air into the first heat exchanger prior to directing the flow of air through the first heat exchanger. 
     
     
         15 . The method of  claim 12 , further comprising discharging the flow of air into a building after directing the flow of air through the second heat exchanger. 
     
     
         16 . The method of  claim 12 , wherein the flow of air through the second heat exchanger is warmer than the flow of air through the first heat exchanger. 
     
     
         17 . The method of  claim 12 , wherein the fluid in the first heat exchanger is warmer than the fluid in the second heat exchanger. 
     
     
         18 . The method of  claim 12 , further comprising positioning the first heat exchanger outdoors. 
     
     
         19 . The method of  claim 12 , further comprising positioning the second heat exchanger outdoors. 
     
     
         20 . The method of  claim 12 , further comprising fluidly coupling at least one condensate reservoir to the second heat exchanger to receive condensate from the heated fluid in the second heat exchanger. 
     
     
         21 . The method of  claim 12 , wherein heating the fluid with a heater includes igniting a gas in a burner. 
     
     
         22 . The method of  claim 12 , wherein transferring heat from the heated fluid to the flow of air in the first heat exchanger includes raising a temperature of the flow of air from a first temperature less than or equal to 0 degrees Celsius to a second temperature above 0 degrees Celsius. 
     
     
         23 . The method of  claim 12 , wherein the heated fluid has a dew point temperature in the second heat exchanger, wherein transferring heat from the heated fluid to the flow of air in the second heat exchanger includes lowering a temperature of the heated fluid from a first temperature above the dew point temperature to a second temperature below the dew point temperature.