According to the Bureau of Labor Statistics in the United States, the HVAC industry is expected to experience an 8% increase in job growth by 2030 and at least 38,500 openings for installers and mechanics each year. So if you are planning to start a career as an HVAC technician, you have the green signal now.
However, to get started as an HVAC technician, you must know the functioning of the HVAC system and learn formulas and other technical things. This is because the HVAC system is complex and not easy to understand. However, to get a better understanding of how this system works, you need to follow certain formulas and equations related to HVAC. Thus you will learn how your heating and cooling system operates.
So learning the formulas is the most crucial thing because only then can you detect the HVAC problem or even know if the HVAC system is performing well. This article has a list of HVAC calculation formulas. Let’s learn about them.
Types of HVAC Formulas
In this section, we will cover three important HVAC and other useful formulas that help HVAC technicians determine airflow, temperature, and humidity levels in buildings.
1. Electrical formulas
To install HVAC systems or detect or get a better understanding of a problem, the HVAC technician must know some electrical formulas such as mathematical equations to calculate various electrical properties and values for HVAC work.
Some basic examples include the amount of current flowing through a circuit, the voltage across an HVAC component, or the power dissipated by a resistor.
Many of these formulas listed below can be combined with other mathematical concepts to solve more problems quickly.
You will find some of the most common formulas for HVAC work.
Common HVAC Electrical Terms
- V/E = voltage, or emf
- R = resistance, or load
- P = power
- I = amperage, or current
- VA = volts x amps
- MFD (microfarad) = 1 Farad/1,000,000
- Coulomb = charge transported by a constant current of one ampere in one second
- Farads (F) is the standard unit of measure for electric capacity and voltage.
- Load factor or U factor (U) represents how much power a device uses compared to its rated wattage when it works at full capacity with no losses in between current and voltage.
Ohm’s law
It states that the voltage across a conductor is directly proportional to the current flowing through it, where all temperatures and physical conditions remain constant.
Mathematically, Ohm’s law is written as
V=IR
To calculate the current, the same formula can be rewritten as follows:
I= V/R
To calculate resistance, the same formula can be rewritten as follows:
R=V/I
Where V is the voltage, I is the current, and R is the resistance.
Wattage formula
The wattage formula determines how much power is needed for a task. It also includes the voltage and current of an electrical circuit to calculate the wattage.
So in HVAC systems, the wattage formula helps you calculate an electrical device’s power requirements.
There are multiple ways to calculate this:
W = A X V
W = V X I
Three-phase motor voltage imbalance
The refrigerant in the air conditioning unit absorbs heat from the house as part of the cooling process. But if the AC overheats, it causes problems. Mostly, compressor overheating occurs due to a voltage imbalance between the motor terminals of an engine’s compressor.
Percent unbalance = (largest unbalance divided by average volts) x 100
How to gather the necessary data to run this formula?
1. Measure the line voltage between the phases of the compressor’s motor terminals.
Let’s assume the voltage reading for the lines between the phases:
Line 1 to Line 2= 216 V
Line 2 to Line 3= 230 V
Line 3 to Line 1= 214 V
2. Determine the average of the readings.
216+230+214 = 660/3 = an average of 220 volts.
3. Determine the imbalance for each phase by comparing the difference between the voltage of each phase to the average voltage.
You will get a positive number as a result. So here are the calculations for the numbers we’re working on:
Line 1 to Line 2 = 220 – 216 = 4 V
Line 2 to Line 3 = 230 – 220 = 10 V
Line 3 to Line 1 = 220 – 214 = 6 V
4. The largest imbalance value is found in the last step but is taken and divided by the average volts found in step two. Finally, multiply by 100 to create a percentage.
Percent unbalance = (10/220) x 100
Percent unbalance = (0.045454545) x 100
Percent unbalance = 4.5454545%
5. Now, square the unbalanced percentage and multiply the value by two to determine the percentage increase in winding temperature.
This step helps the technician to determine the actual impact of this imbalance on the temperature of the motor.
The formula looks like this:
Percent temperature rise = 2 X (4.5454545)²
Percent temperature rise = 2 X (20.661152066116)
Percent temperature rise = 41.322304132232
The result shows that a small imbalance in voltage can increase the temperature highly. This is why it is recommended that the technicians examine the overheating compressor issues with extra care.
2. Work and horsepower formulas
The formula to calculate horsepower is:
Horsepower = Torque X RPM/5.252
1HP = 1 lb x 33,000 ft / 1 min
HP = 745.7 watts
Other basic definitions and formulas include:
Force and Work
Work= Force X Displacement
Power
Power = Work / Time or Force x Displacement / Time
Kilowatt (KW) = 3413 British Thermal Units (BTU)
Metric HP = 735.5 watts
Ton of refrigeration
This refers to the quantity of heat required to melt a ton (2000 lbs.) of ice at 32°F.
288,000 BTU/24 hr
12,000 BTU/hr
Approximately 2 inches in Hg. (Mercury) = 1 psi
WORK = Force (energy exerted) X Distance
E.g.: A 160 lb. man climbs a flight of stairs 100 ft. high
Work = 160 lb. X 100 ft.
Work = 16,000 ft.-lb.
Air consistency
Dry Air = 78% nitrogen + 21% oxygen + 1% other gases
Wet Air = Same as dry air + water vapor
Specific Density of Air = 1 / 13.33 = .075 lbs. per cubic foot
Standard Air = .24 Specific Hear (BTUs needed to raise one lb. 1 degree)
Raising one pound of standard air one degree Fahrenheit requires .24 BTUs
Heat/humidity
Relative Humidity = Moisture present / Moisture air can hold
Specific Humidity = Mass of water vapor / Total mass of moist air parcel
Dew Point = When wet bulb equals dry bulb.
Dew Point Temperature (in degrees Celsius) = observed temperature (in degrees Celsius) – ((100 – relative humidity percentage) / 5)
The formula to determine the dew point temperature can also be expressed as
Td = T – ((100 – RH) / 5)
This formula is an accurate approximation to be used only when the relative humidity value is above 50%.
Determining heat in conditions other than standard air
Total Heat (BTU/hr.) = 4.5 x cubic feet per minute (CFM) x Δh (std. air)
Sensible Heat (BTU/hr) = 1.1 x CFM x Δt (std. air)
Latent Heat (BTU/hr) = 0.69 x CFM x Δgr. (std. air)
Other important formulas
Total heat formula to calculate cooling, humidifying, or dehumidifying.
Total Heat (BTU/hr) = Specific Density X 60 min./hr. X CFM X DH
= 0.75 X 60 X CFM X △H
= 4.5 x CFM X △H
Return Air Grilles – Net free area = about 75%
The grille is a part of the ductwork which lies facing across the duct opening. It directs the airflow in a specific direction and prevents debris.
This is found in a rectangular shape that contains multiple parallel openings through which air can be delivered to a ventilated space.
3 Phase Voltage Unbalance = Wattage per sq ft = .0926 x wattage / mass²
CFM = 2.119 x liters per second
GPM = 15.85 x liters per second
Pounds per Square Inch (PSI) = ft. water / 2.31 = inch of mercury(HG) / 2.03 = inch of water / 27.7 = 0.145 x kilopascal (kPa)
Gas laws
Boyle’s law: P1 V1 = P2 V2
Charles’ law: P1/T1 = P2/T2
General gas law: P1 V1/ T1 = P2 V2/ T2
Where P = Pressure (absolute), T= Temperature (absolute), V= Volume
Why Do You Need HVAC Calculations Formulas?
As an HVAC technician, you may think it is important to learn the formulas, and only then can you do the work efficiently. Yes, that’s true. But there are other important reasons to know complete load calculations.
In this section, let’s discuss them.
To ensure the indoor conditions are normal: When you know the formulas, you can quickly check if the HVAC system works correctly by doing simple math. For example, suppose you know how much air must be circulated in a room, the temperature, and humidity levels. In that case, you will know the correct calculation and provide a comfortable environment for people.
Save money on energy bills: By knowing how much air must be circulated in a room by checking if the fans and heaters are working correctly, you can help your customers cut energy bills.
It is mandatory: Certain certifications must be completed to get licensed as an HVAC professional.
Apart from these reasons, HVAC load calculation is important because it helps you diagnose and fix problems. In some places in the US States, the summer of 2021 was the hottest on record in the past seven years. Since the HVAC systems were used for longer, there was an increase in malfunctions.
So there will be surprising breakdowns, and when you are called for repair, you must be able to diagnose the problem quickly. This is why you must maintain the correct temperature and humidity levels in the HVAC systems, do the load calculations, and inform the customer how efficiently the system can work.
Frequently Asked Questions
What kind of math is in HVAC?
Basically, formulas calculate the airflow and temperature in an air conditioner or heating system. The formulas used to calculate these are based on fundamental mathematical principles. In particular, these calculations used by the technicians help to determine the airflow, temperature, and humidity levels in buildings.
Do HVAC technicians need to be good at maths?
There is no straightforward answer to this question because the skills and abilities required for HVAC technicians vary based on the field of work. However, some basic maths skills are important, so they come in handy when working with the HVAC air conditioning and heating systems. Apart from having strong mathematical skills, you must have excellent problem-solving skills.
How to calculate HVAC BTU?
To calculate HVAC BTU, you can use the following formula:
BTUH = CFM x ΔT x 1.08
How is HVAC capacity calculated?
To calculate how much air conditioning you need:
- Divide the sq ft of the house by 500.
- Then multiply the number by 12,000 BTUs.
- Now calculate the heat generated by windows, workers, or other areas of your house and add that number.
You will get to know your HVAC capacity.
Why should you do the heat load calculation?
One of the most important HVAC calculations is heat load calculations. It is imperative to perform a heat load calculation to know how the air is controlled and the temperature is regulated. The purpose of this calculation is to determine the rate of heat transfer by calculating the thermal resistance.
The thermal resistance here will depend on the air density, which will depend on the atmospheric pressure and temperature.
To determine the heating or cooling needed to maintain the desired temperature in a building, you must know the heat load value. The resultant value can be used to make decisions about heating and cooling systems in a house, insulation levels, and other building safety measures.
Conclusion
As an HVAC professional, understanding the heating, ventilation, and air conditioning (HVAC system) is significant because it will help you better communicate your problems or needs.
Though there are multiple tools to assist you in completing the calculations in your day-to-day operations, knowing the HVAC formulas can help you understand the problem. Also, you derive solutions to those calculations quickly.
The above list will be helpful for you in your typical day-to-day work. You can also print these formulas and use them as a cheat sheet.
When you have sound knowledge about HVAC systems, their problems, formulas, and other related things, you will be more successful as an HVAC technician, and your HVAC business will grow as well.