Some important Calculations in HVAC

             In this article we will see some important Calculations, in HVAC.

The following calculations are very useful if your hvac system is not set correctly. Most homes have their heating and air conditioning systems installed incorrectly. This can result in higher energy bills. If you don’t know what type of system your home has, you may want to call an experienced technician before doing any work yourself.

The first calculation that you should do is take the outdoor temperature and find out how often it changes throughout the day. We will use this information to calculate the number of coils in our ductwork. The second calculation we need to do is determine how much heat we lose through windows in our house. Windows are the biggest offenders when it comes to heat loss. You’ll need to make sure they are properly sealed. One way to test them is to put a thermometer in each window and measure the difference between the two.

Now that you have these measurements, you can figure out the amount of BTUs per hour that your home loses through the vents. Subtracting the average daily indoor temperature from the average daily outdoor temperature, divide that by 60 (hours in a day) and multiply by the square footage of your home. This gives you the total BTU output per hour. Divide that by 12 to find the total BTU output over a month. Multiply this number by 0.80 to get the total annual BTU output for the whole year. Now that you have that number, you can determine the size of the furnace or air conditioner needed.

You might have noticed that I didn't mention the humidity level in those calculations. That's because both the temperature and the humidity level affect the amount of heat your AC unit produces. So, the best thing to do would be to add them together. To add the two together, you simply subtract the outdoor temperature minus the indoor temperature. Then, you'll take that value and multiply it by 1.5. Lastly, you'll raise it to the power of 5/12. Now, you have the average BTUs produced per hour.

The difference between temperature set point and actual indoor air temperature - This difference can vary from 0-10 degrees F/0-5C. Temperature sensors are generally accurate within this range.

The difference between outdoor temperature and ambient humidity level - This difference can vary anywhere from 10-100 percent based on whether the space is naturally ventilated or not. Therefore, if a space is artificially ventilated (or if its natural ventilation is blocked), then it may require additional conditioning airflow.

The difference between desired room temperature and conditioned temperatures - This difference is usually less than 10% of the total temperature swing.

The difference between maximum and minimum indoor air temperature - This temperature differential is typically 25F/15C, but it can go up to 35F/20C. With proper planning, we should aim for a maximum indoor air temperature that is no higher than 30F/18C. However, some growers have reported success increasing a space's maximum temperature up to 50F/28C.

The difference between outdoor and conditioned temperature - Depending on whether the space is exposed to direct sunlight or any other heat sources, this can vary anywhere from 5-40F/3-24C. In order to properly compensate for these differences, we need to understand how much cooling is required to reduce the overall temperature by those numbers.

What percentage of the entire temperature swing is being used to cool? - This number will help determine what type of system is needed to provide adequate cooling without causing discomfort.

Airflow Rate – Calculate this based off the CFM (cubic feet per minute) rating of your air handler. To calculate the airflow rate using the cubic feet per minute (CFM), multiply the number of CFMs by 1.6. Example: If the unit has 50 CFM’s, then use 50 x 1.6 80 CFM.

BTU/hour – This can be calculated by multiplying the number of hours that the blower runs each hour by the horsepower rating. Example: If the blower runs 12 hours a day, and it has 500 hp, then you would divide 500 by 12, which equals 4166 BTUs/hr.

CFM/hp – This can be calculated using the following equation: CFM/HP CFM * 0.25. In other words, if you have 400 CFM's at your blower’s rated speed, then you would multiply 400 by 0.25, which equals 100 CFM/Hp.

BTU/sq ft – This can be calculated in two ways. One way is to divide the total square footage of the room by the total energy needed per hour. The other method is to multiply the square footage by the BTU per hour. Example: A 10x10 space requires 1000 BTU per hour. Therefore, you would multiply 1000 by 10, which equals 10,000 BTU/sq ft.

CFM/BTU – This is the calculation that shows how much flow a fan uses in terms of CFM. You can easily find this information online. Example: A 4-ton cooling system uses 40 CFM.

BTU/min/sq ft – This is the amount of BTU used per minute in the average square foot of a room. Example: A 5-ton cooling system uses 25 BTU per minute.

Heat Loss/Heat Gain Ratio (H/C)

The heat loss/gain ratio is simply a way to calculate how much a building can sustainably lose or gain heat. The formula to determine this is: H/C P × U × A / G × B where P is the perimeter of the building, U is its use factor, A is its area, G is its glass factor, and B is its wind factor.

Ventilation Rate

A ventilation rate of 10 cubic meters per second (m3/s) is generally considered adequate for human comfort. This calculation assumes that occupants are breathing at a rate of 12 air changes per hour (ACH).

Air Changes Per Hour (ACH)

Air changes per hour is the number of times the air has changed while moving through the room. For example, if you have a fresh air supply coming into your space from outside and then it’s being exhausted out of the same window, then you would have one change in air. If you had two windows open in the summer time, then you would have 2 changes in air, etc.

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