CFM with Electric Heat
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)
CFM with Gas Heat
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)
CVA Calculator
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.
PIE Calculator single phase (Watts Law)
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.
EIR Calculator single phase (Ohms Law)
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.
Square foot Calculator
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.
Filter Grille Sizer @ 300 FPM
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.
Non Filter Grille Sizer @ 500 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
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%.
Duct CFM @.08 Supply and .05 Return
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 “properly installed" typical duct runs. Extended runs, multiple turns, poor installation can all affect resulting airflow.
Total Heat Removed
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)
Typical Wire Ampacities
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.
Pressure 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.
Estimated AC Temperature Drop
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
Thermistor Temperature/ Ohms
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
HVAC equipment.
Capacitors in Parallel
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.
Capacitors in Series
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"
Psychrometric Calculator
This calculator has the ability to quickly pinpoint the variables when the current Dry Bulb, Wet Bulb, and Elevation are inputted. Psychrometric charts are great, but having the info processed quickly on a jobsite is very valuable!
3 Phase Imbalance
Three-phase imbalance means that inconsistent amplitude of three-phase current (or voltage) in electrical power system and that amplitude difference surpasses the specified range. Unbalanced current of transformer refers to current difference of three-phase transformer windings.
Mixed Air Temperature, MAT->POA
If you need to set the fresh air damper on a rooftop system and you know the percentage you need to satisfy the requirement the Outdoor Air Percentage calculator can be used. By inserting the Return Air Temp ex 70 the Outside Air Temp ex 95 and the current Mixed Air Temperature entering the Evaporator coil ex 72.3 then the percentage of Outdoor Air can be calculated to be approx. 10.1%By opening and closing the outdoor air damper you will change the Mixed Air Temperature thus changing the Outdoor Air Percentage to what you may require. Patience is key with this calculator.
Mixed Air Temperature, POA->MAT
When using a fresh air damper to establish or identify quantities of ventilation air the Mixed Air Formula can be used. By inserting the desired percentage of Ventilation air needed, ex. 10%, then insert the measured Return Air Temp ex 70, and the Outdoor Air Temp ex 95 the Mixed Air Temperature entering the evaporator coil should be approximately 72.5 degrees. Patience is key with this calculator.
R454B PT Chart
When checking Superheat take the Dew Point pressure from the Suction line, converted to temp, and subtract it from the Suction line Temperature. When checking Subcooling take the Bubble Point pressure from the liquid line, converted to temp, and subtract the liquid line Temperature from it.