Hiscox’s gas furnace uses an air injector to draw fuel gas into an annular mixing region. The air and gas then travel together through a tube to the furnace. The animation separates these paths so the injector is easy to follow. The calculator compares fuel heat input with a stated heat duty using an assumed overall heat-transfer efficiency.
Gas Furnace Interactive Calculator
Enter the required useful heat rate, an assumed fuel-to-useful-heat efficiency and the available fuel heat input. The calculation compares energy rates; it does not select an injector or set combustion conditions.
Equation Used
Efficiency is the fraction of fuel heat input reaching the chosen useful heating duty on the same operating basis. It is an entered assumption, not a measured rating for this historical furnace. Annual Fuel Utilization Efficiency (AFUE) is a seasonal metric and is not used here. No air-flow, gas-pressure or flame-stability calculation is included.
Air drives the injector
In figure 1626, Hiscox identifies the air jet as the driving stream. It draws gas into an annular nozzle, mixes the streams and forces the mixture through the delivery tube. The picture shows a compact cylindrical furnace connected to that injector.
The enlarged injector section is schematic. Blue markers follow the central air passage. Gold markers enter the surrounding gas region and join the air downstream of the nozzle. Orange markers follow the mixed stream toward the furnace. Wall outlines keep the separate passages visible.
The furnace chamber is shown as a simplified section to explain delivery and heating. Its internal dimensions, workpiece arrangement and exhaust details are not given by the historical exterior drawing. The markers show direction, not measured velocity, mixture ratio or combustion temperature.
A way to deliver a mixed stream
The injector combines a driving air stream with an induced gas stream before the mixture reaches the heating chamber. This explains the distinction between a mixer, its delivery pipe and the furnace that receives the mixture.
The illustration is useful for studying that arrangement. It does not specify a modern gas appliance, fuel conversion or burner installation. Actual equipment needs compatible fuel controls, combustion safeguards and a properly designed exhaust system.
Compare heat input with a defined useful duty
Let Qin be the fuel heat input rate and η the fraction reaching the useful duty. Then Quseful = ηQin. If the required useful heat is Qduty, the corresponding input under that assumption is Qrequired = Qduty/η.
The heat-duty margin is Quseful − Qduty. A negative result means the assumed useful output is below the entered duty. Heat outside the useful duty is Qin − Quseful; this combines all unutilized energy without estimating individual exhaust, wall or other losses.
All four results are energy rates in BTU/hr. Use the same fuel heating-value basis and operating interval when determining efficiency and input. The selected efficiency range supplies comparison values, not a performance range established for the historical furnace.
AFUE describes seasonal energy efficiency for heating appliances. It is not the efficiency input in this calculation. The injector drawing also does not provide the flow and geometry data needed to predict entrainment or burner capacity.
Example: 100,000 BTU/hr input at 70% efficiency
With fuel heat input 100,000 BTU/hr and assumed efficiency 70%, useful heat is 70,000 BTU/hr. For a 62,000 BTU/hr duty, the margin is 8,000 BTU/hr. The corresponding input needed for that duty is about 88,571 BTU/hr.
The remaining 30,000 BTU/hr lies outside the useful duty. It is not a separately calculated flue loss. Raising the assumed efficiency to 80% at the same input gives 80,000 BTU/hr useful heat and an 18,000 BTU/hr margin.
Changing an assumption is a comparison, not a way to adjust the actual furnace. A positive margin alone does not establish adequate warm-up time, temperature uniformity, burner modulation or process quality.
Heat balance and injector performance are different questions
The energy comparison needs input power, useful duty and efficiency. An injector analysis additionally needs gas properties, inlet conditions, nozzle and mixing-tube geometry, and downstream resistance. None of those injector characteristics can be inferred from the heat-duty margin.
Keep the drawing and the result in their stated roles: the animation explains the stream arrangement, while the calculator compares assumed heat rates. It provides no fuel-to-air setting, gas-pressure adjustment, ignition sequence or operating limit.
Frequently asked questions
Which stream drives this injector?
The air stream. Hiscox describes the air injector drawing gas into an annular nozzle and sending the mixture through a tube to the furnace.
Does the drawing specify the nozzle dimensions?
No. The enlarged section explains the passages. It is not a dimensioned reproduction or a fabrication drawing.
Is the efficiency input AFUE?
No. It is an assumed fuel-to-useful-heat efficiency for the stated duty. AFUE is a seasonal heating-appliance metric.
What does a negative margin mean?
The estimated useful heat is below the required useful heat entered. It does not diagnose why a real furnace is underperforming.
Does heat outside the useful duty equal the flue loss?
Not necessarily. It combines all heat that does not reach the selected duty; the individual losses are not calculated.
Can the calculator size the injector or choose a gas pressure?
No. It contains no entrainment, pressure-drop, mixture-ratio or combustion model.
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