In this part of Full HVAC Course, We will continue the Heat Load Calculation series, In this we will see about the purpose of heat load calculations in hvac.

Heat Load Calculation

Heat Load Calculation HVAC

The purpose of calculating heat loads and determining if a system's capacity can handle the amount of heat entering the building is to ensure that enough cooling capacity is provided to effectively reduce the temperature inside the conditioned space to safe levels. A properly designed HVAC system should be able to maintain a specified minimum indoor air temperature (IAQ) in a controlled space, and provide adequate ventilation to remove excess heat build-up caused by occupant activity.

For example, in a typical office environment, a 15' x 20' space containing 10 occupants, each weighing 170 lbs. would have a total heat flow rate of 70,000 Btu/hr. Thus, the cooling equipment would need to provide a total airflow of at least 400 CFM (or 1.5 times the volume of occupants per hour). The total heat load of the office space would then be calculated as follows:

Total Heat Flux Total Air Flow X Volume Occupied X Temperature Differential between IAQ and Outdoor Ambient Temperature (*Note*: If the occupancy level changes, the volume occupied should be recalculated based upon the current conditions.)

*Note*: If the IAQ is higher than the outdoor ambient temperature, the IAQ temperature should be subtracted from the exterior temperature.

The sum of the heat fluxes entering the space equals the heat load to the system. Therefore, the capacity of the HVAC system must equal or exceed the heat load to the conditioned space. The following steps should be taken to determine the correct size of the cooling system(s):

Step 1: Calculate the total heat flow entering the space.

Step 2: Determine the number of occupants.

Step 3: Determine the desired comfort level.

In some cases, the total heat flow may exceed the maximum capacity of the cooling equipment. In these situations, additional cooling capacity may be added by increasing the fan speed or adding a second cooling coil. However, before doing this, make sure that the added cooling capacity does not surpass the total heat load in the space. If the added cooling capacity exceeds the heat load, the system becomes overcooled and may cause discomfort to occupants due to excessive humidity build-up, and even damage to furniture and personal belongings. These calculations assume that the building uses forced-air heating systems to maintain the indoor temperature.

It is worthwhile to note that the above method assumes that the cooling system is operating at its full capacity. In practice, this seldom occurs. Many HVAC systems operate at less than 50% of their potential capacity. Furthermore, most buildings have significant seasonal variations in both the heat load and the actual use of the space. Therefore, a more accurate determination of the system’s capacity can be obtained using information collected from previous weather observations, and actual space usage surveys.

When calculating the heat load for a specific space, it is necessary to consider several factors including:

• Type of building

• Yearly average temperature

The Heat load calculation (HLC) is a measurement of the amount of air needed to cool a space at a given temperature. It can be calculated using the following formula:

Air Volume x CFM x Temp /0.60 BTUs per hour.

This equation is used for calculating how much BTU’s are required to cool any given area based upon the size of the room and its volume. For example, if the room is 10 feet wide x 15 feet long x 8 feet tall, then the formula would look like this:

10 feet x 15 feet x 8 feet 480 square feet.

Then the number of cubic feet of air required to cool this space would be calculated by multiplying the number of square feet by 0.8cubic feet :

480 square feet x.08 37.6 cubic feet of air.

However, the total amount of air needed to supply cooling to a specific space may not be enough to provide adequate comfort level. In order to determine what additional amount of air is needed, two things need to be considered; first, the current conditions in the space, and second, the desired conditions.

In the case of a small room that requires additional cooling, the air flow rate can be increased by adding fans. If this does not solve the problem, then the desired conditions should be set in mind. For example, if a room is kept below 70 degrees F, then the air flow rate should be increased in order to obtain comfortable temperature.

For larger spaces where comfort levels are not an issue, such as offices or warehouses, the equipment and energy use will have little effect on the actual HLC. However, in cases where the room is being used as a greenhouse or a food storage facility, the comfort level becomes an aspect that needs to be considered.

2. Calculating the Temperature Difference between Existing and Desired Conditions

An accurate temperature difference is critical before making any changes to the existing equipment or the installation of new equipment.

The purpose of heat load calculations is to determine the amount of cooling capacity required to remove excess heat generated by various equipment and systems in the facility. This is done by taking the total heat produced by these systems and dividing it by the annual operating hours of each system.

Here is a thumb rule to calculate the heat load, this is just a thumb rule.. To calculate the actual load, we need to keep in mind various factors such as orientation of the building, walls exposed, u values of walls, windows, skylights, electric wattage, lighting, people, infiltration etc and many more.. we will see that in future posts.. so here is a thumb rule..

The heat load is the amount of heat generated by your HVAC system, expressed in BTUs per hour (BTU/hr). A high heat load generally means that air conditioning is working harder than necessary. Conversely, a low heat load indicates that your air conditioner may not be doing enough cooling. You should check your cooling capacity frequently because a large change over time may indicate a problem.

To calculate the heat load of your home, multiply the square footage of your house x the number of people living in your home. When calculating the heat load of a room, subtract the temperature outside that room from the desired temperature inside that room. To find out how much AC your home needs, divide the calculated heat load by 4. Multiply that result by 1,000 to get the approximate cost of running your AC unit for one year at the current prices.

Example: If a house is 2,000 sq ft. and has a family of four living there, then its heat load would be 16,000 BTUs/hr. That's a lot! So we'll figure out what the heat load of the room is first. It's 80 degrees outside, so if you want the room to maintain 75 degrees, you need to turn on the air conditioner at full blast and keep it there until you feel comfortable again. Let's say the difference between the room's internal temperature and external temperature is 10 degrees F., meaning the room's heat load is 12,000 BTUs/hour. Now let's multiply that by the square footage of the room...12,000 X 2,000 24,000 BTUs/h. Subtracting the room's temperature from 75 the room's heat load: 24,000 - 7519,000 BTUs/Hr.

Now divide that by 4. The result is 5,125 BTUs/hr.

So 19,000 BTUs/hours / 4 5,125 BTUs hr/hr.

That's the heat load of the house, and to determine how many BTUs your house uses each year, just multiply the result by 1,000. In this case, the answer is 50,625 BTUs/year.

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