Heating Ventilation and Air Conditioning Full Course - HVAC Course (Part 5)

 

This is the 5th part of the full HVAC course. In this part we will see about Heat Load Calculations.

Heat Load Calculation HVAC (Heating Ventilation Air Conditioning) Thumb Rule

The HVAC system has two functions: to heat or cool your home, and to provide ventilation or fresh air to your environment. It works by using either hot water or electricity to move the air through your house. When selecting a home heating or cooling system, you should always consider how much energy the HVAC unit uses. Energy use is measured in British Thermal Units per hour (Btu/h), which is just a measure of energy used. To calculate Btus/h use this simple equation:

Energy Use (Cooling Capacity) x (Cooling Efficiency) + (Heating Capacity) x (Heating Efficiency).

Example 1: If your home has a cooling capacity of 60 tons and your cooling efficiency is 90%, then your energy use would be 0.6 tons x.9 5.4 tons/hour. This means that your HVAC system runs at 5.4 tons/h.

Example 2: If your home has no cooling capacity but you have an electric heater that can run continuously at 100 watts, then your energy use would only be 100 watts x 24 hours 2400 watts/hour. That’s about 4.7 tons/hr!

Heat load calculation in HVAC can be done in three ways, the first way is to do by "Thumb Rule". This way of calculation of heat load will give an approximate result, by analysis from ASHRAE. The second method is the manual method of doing heat load calculation. Manual method can be done using, E-20 manual, which is given by Carrier company format or General Air conditioner type or format. The third and most accurate and widely used way of doing heat load calculations is by using a software such as HAP(Hourly Analysis Program) by carrier. This gives the exact result based on the appropriate inputs.

By Thumb Rule Calculation :

* TR = Area x (0.07) --- For small load conditioner of buildings (such as walking bays etc).

* TR = Area x (0.08) --- For medium load conditioner of buildings (such as hospitals, banks etc).

* TR = Area x (0.09) --- For high load conditioner of buildings (such as supermarkets, malls etc).

For example,

Let us consider this simple bedroom of dimensions

TR =  Area x (0.08)

      = (5 x 4) x (0.08)

TR = 1.6

1 Ton = 400 CFM

1.6 x 400 = 640 CFM

Now let us consider this example and try to find the heat load for the image shown below.

Now let us tabulate all the given values so that we can refer it in an easier way.

For kitchen, here the solution goes :

CFM = (volume x ACH)/1.7 ---- here ACH is Air Changes per Hour

         = ((l x b x h ) x ACH) / 1.7

         = (( 4 x 3 x 2.8) x 8) / 1.7

CFM = 158.12 cfm.

1 Ton = 400 cfm

therefore, kitchen TR = (158.12 / 400)

                                   = 0.395 TR.

In this way, we can use the first method of heat load calculation to get an approximate heat load value.

The heat load calculations are based upon room size, number of lights, wattage per light, average daily temperature, & how many hours of sunlight the room gets...

It would then calculate the amount of air that should be moved through the room (CFM), and multiplied by the square root of the time in order to get the cubic feet of air per minute...

This would give us the total CFM's needed to maintain a certain set temp...

A handy little rule of thumb if you are trying to determine how much cooling is needed to maintain a particular temperature in a space. If you have a thermostat that has a 1 degree accuracy, then you can determine the correct setting simply by dividing the desired temperature by ten degrees. So, if you need 68 degrees Fahrenheit, divide 68 by 10, and you should be able to find what you are looking for.

The heat load calculation is done using the following formula. (U) (H) x (C) / (T). In this equation U the heating value of the air, H the heating capacity of the equipment, C the cubic feet of conditioned space, and T the temperature set point that you want your room to reach.

        This calculation should be used as a starting point to estimate your annual heating energy usage and costs. As always, we recommend that you consult your local building inspector if you have any questions about local codes and standards. In addition, please remember that a properly installed HVAC system can improve indoor quality, comfort, safety, workplace productivity, and human health. A poorly designed or installed system may actually increase air pollutant levels, reduce thermal comfort, and lower overall workplace performance.

Access Previous parts of this course, if you have missed it, by clicking Below..

Part 1 -- Click Here!!

Part 2 -- Click Here!!

Part 3 -- Click Here!!

Part 4 -- Click Here!!