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Heating Ventilation and Air Conditioning Full Course - HVAC Course (Part 36)


                From this part of Full HVAC onwards, we will see about Load Calculations.

Heat Load Calculation

Survey of Building

The purpose of building surveys is to provide information about the physical characteristics of the building including size, materials, construction techniques, and environmental conditions. Building surveys are used for determining the amount of heating and cooling (H&C) required to maintain comfortable temperatures inside the building under varying climatic conditions.

The Building Survey consists of two parts; (1) Mechanical Load Calculations & (2) Thermal Load Calculations. In general, mechanical loads consist of heating, cooling, ventilation, and air conditioning. These loads require electrical power to operate and are measured at the point of use (POU). On the other hand, thermal loads depend upon the building envelope, which may include insulation, windows, doors, and roofing materials. They consume electricity to maintain their temperature set points and are also measured at POU. Together, these two measurements provide the basis for calculating energy consumption.

Sources of Heat inside the Room

Heat in HVAC systems is defined as the temperature difference between inside and outside air. In general, heat is added to the system to make the living space comfortable; however, excess heat may cause discomfort to occupants. If not properly managed, heat can damage building materials and components, including insulation, ductwork, electronic devices, and furniture. Managing heat in HVAC systems requires understanding the type of heat produced by different types of equipment and how they affect people.

Solar Heat Gain By Windows

The solar radiation enters our house through windows, skylights, etc., and passes through air before reaching the roof where it heats (or cools) the building's interior. The amount of solar gain is determined by the size and shape of the window, type of glass, shading material, orientation towards the sun, and the distance between the inside surface of the glass and the outside temperature. The rate at which solar heat gains the house increases as the angle of incidence of sunlight changes. In winter, a south-facing wall receives less solar heat than a north-facing wall, because its surface is warmer, while a north-facing wall absorbs more heat than a south-facing wall due to its colder surface. On average, the solar gain varies from about 80%–85% in summer and 10%–20% in winter.

The amount of solar radiation gain that a house receives varies greatly depending on how much glass is used. A house with no windows at all receives almost nil solar radiation gain. However, if a house has all the windows completely covered with opaque blinds, then the solar radiation gain will decrease dramatically to levels lower than what we get during winter.

Solar gain happens when sunlight enters a window and causes light and heat to enter a building. In a home environment, light and heat enters through windows. A window's total solar gain (TSG) is determined by the size of its glass area and the amount of glass surrounding that area. The larger the glass area, the greater the TSG. If the surrounding glass is transparent, then the total solar gain from the window will increase. However, if the surrounding glass is opaque, then only a portion of the sun's rays will pass through the window. You can calculate how much light enters a room based upon these factors.


Insulation helps keep heat inside your home instead of outside. When you insulate your windows, it works to prevent sunlight from entering the house. Insulating materials can either block the flow of air or keep warm air inside your home. Air barriers help keep cold air out of the home while keeping hot air inside. Double-pane windows and single-pane windows both provide insulation, but double-paned windows have better thermal performance than single-pane windows.


Some types of windows allow only a certain amount of sunlight to enter a home. These include triple-glazed units that have three panes of glass sandwiched together. Triplex units are similar except they have four panes of glass.

Window treatment

There are several different treatments designed to make your windows energy-efficient. Some of them may not work for homes located in areas where temperatures regularly drop below freezing.

The Solar Heat Gain Factor (SHGF)

The solar heat gain factor is determined by the amount of sunlight coming through the window(s). In order to determine the SHGF, find your latitude and then multiply by 0.86. For example, if your location is at 40 degrees North Latitude, the SHGF would be calculated by multiplying 0.40 x 0.86 0.34. You should consider adding some shading material to reduce the SHGF further. A shaded area will allow less direct sunlight to penetrate and will help lower the temperature inside your home.

Window Glazing Material

When looking for shades to block out the sun's rays, note their U value ratings. Materials with low U values tend to have better insulation qualities than high-U materials. U values measure how much heat penetrates a given material. Low-U materials keep air temperatures cooler, while high-U materials retain more heat. Insulation will prevent heat loss from the building envelope and thus maintain thermal comfort.

Shade Covers

Shades not only protect the house from excess heat gain, they can also provide relief from cold weather conditions. If you live in a region where wintertime temperatures drop below freezing, keeping windows covered with shade covers may help to increase comfort levels and reduce drafts.

Solar Heat Gained by walls

Thicker walls block more solar radiation. Thin walls allow solar radiation to pass through. Thicker walls mean more mass and therefore more thermal inertia. More thermal inertia means that the building will take longer to warm up after being exposed to sunlight. If the building takes longer to warm up, it may result in a lower temperature inside the building.

Walls can have a huge effect on how much solar gain your home receives. Most homes do not have enough windows to allow for adequate natural sunlight to penetrate deep into your interior space, so most houses rely on electric lighting and window air conditioners to keep your home comfortable. These devices increase the temperature inside your house, causing your body to lose heat at a faster rate than you would if you had ample amounts of natural sunlight entering your home. Therefore, adding more insulation to your wall surfaces may help reduce this loss of heat.

Heat Gained by Partitions

The partition is a glass wall that separates two rooms. When there is no direct sun, the room temperature will drop. In winter, the windows of the building are closed, and the temperature inside can fall significantly. There is a negative correlation between the amount of solar radiation penetrating a window and the degree of thermal efficiency. The higher the number, the greater the heat loss. The relationship between the penetration rate of solar energy and the thermal performance of the glass (the higher the number, the lower the thermal coefficient).

There can be parts inside the room with a cooled room or a non-cooled room. How much intensity of heat is consumed by the partitions with non-cooled rooms is higher.

Partitions provide a way to store solar heat. As the sun heats the earth, the surrounding air gets cooler. This cooling effect increases the temperature difference between the inside and outside of the building.

Solar Heat Gained by the Roof

- In order for a building, roof or structure to have solar gain, the sun's rays (photons) need to enter the surface and pass through the material without being reflected out of the system.

- If sunlight passes through any material and is not absorbed by the material, then it is called transmittance. Transmitted photons travel through air and water and are not absorbed by those substances.

- Photon Absorption Absorbance x Transmittance

- Absorbance is the measure of how much light is being absorbed by a substance

- Absorbance rate 1/Transmittance rate

- When comparing two materials side by side, the material with higher absorbancy rate will always have greater solar gain.

- Absorbent Materials

- Glass

- Concrete

- Wood

- Steel

- Copper

- Aluminium

- Tarmac

Many people don't realize that they can gain solar heat by their roof. If you live somewhere warm, you may have noticed how hot the air gets on sunny days. This is because the sun causes all surfaces to radiate some amount of thermal energy back out into the atmosphere. In addition to being able to store solar power for later use, we can also use this energy to cool our house. Unlike traditional cooling systems, solar cooling does not require any moving parts. Cooling occurs naturally due to the transfer of thermal energy from the sun's rays. When sunlight hits a black surface, it is reflected away from the surface and absorbed by the surrounding air. Air absorbs the radiant energy and transfers it as heat. Radiant cooling works best in very dry climates where little moisture exists in the air - in general, these regions tend to have lower annual average temperatures. Additionally, a dark roof helps trap heat during the day and reduce heating costs at night.

Roof Heating Ventilation & Air Conditioning (RHVAC) systems use solar gain to raise the temperature inside the home. As sunlight passes through windows and skylights, it reaches the roof surface, heating the air and making it circulate throughout the attic space. If you live in a sunny area, RHVAC units may not be necessary at all. However, if you live in a climate where temperatures can drop below freezing in the winter months, you may need to have some type of heating system installed. The amount of solar gain depends largely on the quality of the insulation and shading material.

Heat Gained from Ceiling

A study was conducted to determine how much heat could be gained from ceiling fans in residential buildings. A typical room had a fan running at about 1,200 RPM (rotations per minute) and 15 inches of clearance from the floor. In this case, the clearance was less than half of the blades' diameter, meaning that some portion of the airflow would pass over the rotating blades instead of passing between them. Over a 5-hour period, researchers measured the temperature increase between 10 p.m. and 6 a.m. At 2 feet above the floor, they observed only a 0.9 degree rise in temperature, while at 4 feet, the temperature increased by 6 degrees. As expected, the higher the clearance, the greater the effect. Researchers concluded that the ceiling fan did not contribute significantly to heating the room, though its use was still recommended because of the air flow provided.

The amount of heat gain from ceiling systems varies widely depending on how much air conditioning is being used, where it's located, and what type of system is installed. If temperatures in the room rise too high, the evaporator coil may run hotter than its safe operating temperature. Heat gains can vary in increments of 10 degrees Fahrenheit (F) to 40 F. When the difference between the heating unit and room temperature gets larger than 20 F or 30 F, additional capacity may be necessary to keep the space comfortable.

It's recommended that ceiling fans should be set at least 15% lower than desired room temperature; however, if the fan speed isn't adjusted properly, it could actually raise the room temperature. Ceiling fan speeds shouldn't exceed 4,200 revolutions per minute (RPM), nor should they be set lower than 2,600 RPM. Fans should be installed close enough to the ceiling to provide good circulation, yet not too close. Fan blades should be smooth without sharp edges or points.

If your ceiling system doesn't have an automatic defrost cycle, the fan motor and blower should be inspected regularly. If the fan motor starts running before it's time, the system won't get cold enough to defrost. Also, if the air flow decreases noticeably after the heater turns off, the fan may need to be replaced.

When choosing a ceiling fan, consider these factors:

Ceiling height - The higher the ceiling, the more airflow required. Lighter ceiling fans might not work well in taller rooms.

Fan size - Smaller fans generally require less power and produce less noise. Look for models that use about 1/8 hp of electricity while producing approximately 50 cfm of air movement.

Shape - Round fans circulate air evenly throughout the entire area. Square and oval shapes move air vertically along the ceiling line. These types of fans tend to create localized hot spots by concentrating airflow along a single path.

Mounting position - A fan mounted near the center of the ceiling will draw air from opposite sides of the room and distribute it evenly across the floor. Mounted below the ceiling, the fan will pull air down from above and direct it toward the floor.

Power rating - The maximum voltage the fan operates at is determined by its electrical specifications. Most fans operate best at 120-volt AC. However, some DC-powered fans can run on either 110 volts or 220 volts.

Resistance - Low resistance produces better air movement. High resistance limits air movement and tends to increase fan noise.

Sound level - Noise levels range from 60 decibels (dB) to 90 dB.

Heat Absorbed by the Floor

Here I describe what air conditioners do if they are running and how much heat they take away from the floor below them. Air conditioners work by absorbing heat from the room being cooled (in the summer) or heated (in the winter). If we have an AC unit installed, we need to make sure the thermostat setting is set properly. This will keep the temperature of the house consistent throughout the day. The air conditioning units will release heat depending on the efficiency of the cooling/heating function. In the summer it would be releasing excess heat thus keeping the house cooler than outside temperatures. In the winter, it's the complete opposite; it absorbs heat from the home making it hotter than out side conditions. As always make sure to check and adjust the thermostat settings accordingly!

Heat absorbed by the floor is measured at room temperature, not at the ambient temperature of the room. If the ambient temperature is higher than the room temperature, then the amount of heat absorbed does increase. Because the surface area of the floor is larger than that of any other object in a room, it takes longer for heat to dissipate off of the floor. This means that the surface of the ground becomes warmer than the air surrounding it.

Heat Gained from Infiltrated Air

Heat gained from infiltration (or air-conditioning) is not just a cost savings measure, it's an environmental benefit too! Here are some reasons why:

Air conditioning uses electricity and fossil fuels. But when we use our air conditioning systems responsibly, we save money on utility bills and reduce our carbon footprint.

Heat gained from infiltration is free and even if you don't have AC units in your home, you still get the benefit.

When we turn off our air conditioners and open windows to let cool air flow into homes, we're using the heat built up inside our houses. If we didn't do that, it would be much hotter than it already is outside. By letting it out, we're reducing the heat lost from the building and cooling down our own environment.

If we had no air conditioning at all, we'd have to spend more time running fans or turning lights on and off throughout the day to stay comfortable. At least now we don't need to worry about having hot rooms in summer.

Heat Gained from People

Heat gained from the people in air conditioning systems is a loss of energy that isn't captured. When we talk about energy losses, we have three ways of looking at them. One way is by using the potential energy (PE) method. In PE it is explained that if you push something then it loses potential energy in the form of heat. After the air conditioner removes heat from the room, the remaining heat stays in the house until someone comes back inside. If the person doesn't return inside, the heat remains in the house and makes it warmer.

Building survey

Air conditioners require electricity and thus use a lot of power. In some countries, they are considered luxury items, while in others they are necessities. That being said, they don't always work well, if at all. A poorly-designed system may not provide enough cooling to make up for its high operating cost. Or it could cause problems such as making it hard to control humidity levels and even causing mold to develop inside the unit. To avoid these issues, it's important to have an AC contractor perform a building survey before installing any type of air conditioning equipment. Here are some things to consider about building surveys.

• The first thing to do is determine whether you need a new installation or an upgrade. If you're replacing an old or malfunctioning system, then you'll want to have an air conditioning contractor conduct a full inspection of the existing system. Otherwise, you should only have them look at certain components such as ductwork, vents, filter units, and blowers.

• Next, the contractor should check to see what kind of unit you'd like to install. While there are many different types of systems, you generally have two options: split units and packaged systems. Split units are similar to window units, except their compressor and condenser are separate pieces of equipment. Packaged systems consist of the compressor, evaporator, condenser, and fan coil all built into a single unit.

• Once you've determined the type of unit you want, you'll need to decide where to place it. You might think that putting it near a window would help keep the room cooler, but it also makes it harder to regulate the temperature. Placing the unit closer to the wall will help balance out the warmth from the sun coming in through the windows. And placing the unit in the center of the room will give you maximum airflow throughout the space.

• Finally, the contractor should ensure that the system meets local codes. Depending on how much space you have, the size of the area that needs to be cooled, and the amount of equipment you want to run, you may need to purchase permits or pay fees.

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