Infern-O-therm Stack Economizer

With the rising concerns over fuel cost and environmental issues, Infern-O-Therm Corp has developed a water heating system using the flu exhaust gas to pre-heat incoming water in direct fired water heating tanks.

The “Patent Pending” design incorporates a heat exchanger bolted to the exhaust stack with the incoming water circulating through the exchanger and then entering the water tank at an elevated temperature. While the tank is filling, the 50-55 degree incoming water circulates through the stack economizer, taking the heat from the exhaust gas and increasing the water temperature prior to being circulated into the tank. Tests have shown an in crease in incoming water temp from 50 to 82 degrees F. at a flow rate of 47 GPM (400 burner). Stack temperature was reduced to an environmentally friendly 180 degrees.


Included with System

  • Insulated stack heat exchanger with supply and return ports
  • Water circulation pump
  • 150# slip on flange (to be welded to stack)
  • thermometer
  • electric control panel (NEMA 4)
  • hose and fittings ( length to be determined)


  • During the filling of the tank, incoming water flows through the stack heat exchanger, raising the temperature of the water. The pre-heated water then flows through the existing spray bar in the tank allowing the direct fired system to heat the water as in normal operation. When the tank is full and the burner fires to maintain the tank temperature during the evening or down times, the circulation pump flows water from the hot tank through the heat exchanger and back into the tank.
  • This system can be installed on new tanks during fabrication or as a “retro-fit kit” for existing Infern-O-Therm and competitor units.

Heat Rating

  • The BTU of the system will vary depending on the burner size rating and the length of the fire-tube installed in the tank. In a optimum situation an HW-15-400 Infern-O-Therm unit with a 465 degree gross stack temp, the economizer raised the incoming water temp 32 degrees above incoming water at a flow rate of 47 GPM. This rates the economizer at 850,000 BTU or 6 gallons per hour of fuel oil.


6 GPH of # 2 fuel @ $ 3.50 per gallon = $ 21.00 per hour saved

6 GPH x 10 hours per day = 60 gallons per day @ $ 3.50 per gallon = $ 210.00 per day saved

300 gallons per week saved @ $ 3.50 per gallon = $ 1,050.00 per week saved