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Calculate CFM using the Temperature Rise from the electric heat strips. Use measured Voltage, Total measured amperage of the Air Handler, and Measured Temperature rise from the Return Air to the Supply Air near the unit. It will also provide the output BTUH of the operating electric strips too! (Use whatever fan speed you are trying to measure)
Calculate CFM using the Temperature Rise of the gas furnace. You can use actual measured BTUH output, Calculated BTUH from clocking the meter, or Estimated BTUH from unit efficiency ie. 75,000 input @ 80% = 60,000 output. (Use whatever fan speed you are trying to measure)
Estimate CFM or Velocity based on the Area in square feet of the ductwork. For example a 12" x 12" (1 square foot) duct moving 400 CFM will move air at approximately 400 FPM while a 12" x 24" (2 square foot) duct moving the same 400 CFM will move air at approximately 200 FPM.
Using Watts Law this calculator can be used to identify; P (Watts), I (Amps), or E (Volts) when any of the other 2 are known.
Using Ohms Law this calculator can be used to identify. E (Voltage), I (Amps) or R (Ohms) when any of the other 2 are known.
This calculator can be used to calculate the area in square feet when the height inches and width inches of rectangular duct is known. It is particularly handy to provide the area in square feet of round duct as well.
1" filters are rated to catch dust at 300 FPM therefore Return air filter grilles utilizing 1" filters need to be sized to move the proper CFM at the proper 300 FPM.
Residential Return Grilles without filters are rated to move air at 500 FPM to keep noise levels down and provide minimal resistance. This calculator can also be used to calculate the Filter size of extended pleat filters i.e. 3" 4" 5" as they are rated to catch particulate at 500 FPM.
Operating run capacitance can be achieved by measuring the actual running Amperage of the Start winding, and the measured Voltage across the Run Capacitor (C and H). This can be used to compare to uf (micro farad) and tolerance (5% 10% etc.) rating stamped on the Run capacitor ie 35 uf +/- 10%.
This calculator does not replace good ACCA manual D or SMACNA principles. It does however provide basic duct sizes and CFMs based on these principles. This calculator can be used to estimate CFM or duct size using round Metal or Flexible â€œproperly installed" typical duct runs. Extended runs, multiple turns, poor installation can all affect resulting airflow.
For those jobs when you really need to know if the HVAC system is really doing all it is capable of, here is the solution. By using a psychrometer to obtain the Wet Bulb Supply and Return temperatures of an operating air condition system, and applying the proper CFM of the system, the total heat that the system is removing can be calculated. i.e. if a 3 ton system is removing 34,897 BTUH it is probably doing fine, but if it a 4 ton doing the same, it is probably not. (Nominal 12,000 BTUH per ton)
While Local and National code prevail, this quick reference chart based on NEC Table 310-16 is handy to identify the typical amperage capacity of wire used in the residential HVAC industry.
Quickly identify the pressure in PSIA and PSIG of a Liquid line by imputing the Voltage DC in, and Voltage DC out, of the Transducer.
Quickly identify the pressure in PSIA and PSIG of a Suction line by imputing the Voltage DC in, and Voltage DC out, of the Transducer.
Target temperature difference between the Return air and the Supply air that a system
should attain on high cool based on the heat and humidity load. Finding a greater drop
than shown can indicate low or inadequate system airflow
Thermistors are resistors that change resistance with temperature. NTC Thermistors are
inversely proportional meaning when temperature goes up resistance (ohms) goes
down. These 5 thermistors (5K, 10K, 15K, 20K, and 50K) are typically found in high efficiency
When searching for the proper run capacitor to fill a need on a job it may be beneficial to know that capacitors in Parallel add capacitance.
When keeping the Voltage the same ie 370 or 440 on the replacement capacitors the micro-farads of the two capacitors add up.
Example a 35 uf 440 volt capacitor wired in parallel with a 20 uf 440 volt capacitor add up to a 55 uf 440 volt capacitor.
When wiring two capacitors with the same Voltage rating in series the capacitance is reduced to less than the smallest capacitor micro-farad being used.
For example if you needed a 3uf 440 volt capacitor on a job but you only have a 5 uf 440 volt and a 7.5 uf 440 volt capacitor in your inventory, then wiring them in series would provide you with a 3 uf 440 volt capacitor. These two formulas are the science behind many of the multi tap Run capacitors being sold as a "Universal capacitor"