HVAC Load Calculations
What Does an HVAC System Do?
HVAC systems are designed to keep indoor spaces comfortable. They do two main things:
Make sure people inside are comfortable.
Keep the right conditions for products or processes inside a space.
To do this well, the HVAC system needs to be powerful enough and run all year. This means the system should be chosen based on the biggest needs of the space, called "peak load," and also be good for when it needs to work less, called "partial load." Since you can't really measure these loads exactly, experts estimate them using data.
How to Estimate the Work HVAC Needs to Do?
Before estimating how much work an HVAC has to do, a detailed survey is needed. This helps in choosing the right equipment for smooth operation. The survey should look at:
How much heat comes in or goes out of the space (heat gain or loss).
The actual work the air conditioning has to do to handle this heat.
Usually, heat gain and the work the system does aren't the same because the building's structure stores some heat.
What Goes Into a Good Building Survey?
To get a good estimate of cooling or heating needs, you need a complete survey. This includes checking things like:
Building’s Direction: Where the space faces (sun, wind, nearby buildings).
How the Space is Used: Is it an office, shop, hospital, factory, etc.?
Size of the Space: The length, width, and height of the room.
Ceiling Height: The height from the floor to the ceiling and any space above false ceilings.
Columns and Beams: Their size and position in the space.
Materials Used: What the walls, floors, and ceilings are made of and how thick they are.
Conditions Around the Space: The color of walls, if shaded, attic ventilation, and temperature of nearby rooms.
Windows: Size, type (single/double pane), and if there are shades or overhangs.
Doors: Where they are, type, size, and how often they're used.
Stairs, Elevators, and Escalators: Location and if they're open to non-cooled areas.
People: How many people are usually there and what they’re doing.
Lighting: The type and wattage of lights used.
Motors: Where they are, how powerful they are, and how they’re used.
Appliances: Location, power usage, and if they need ventilation.
Ventilation: How much air is brought in per person or per square foot.
Thermal Storage: How long the system runs and what the space can handle in temperature changes.
System Usage: If the system runs daily or only sometimes (like in churches).
Location of Equipment and Service:
When engineers design heating and cooling systems for a building, they need to know where to place the equipment and how to set up air and water systems. This guide helps them gather the right information:
Available Spaces: Find out where stairwells, elevators, old chimneys, pipes, and areas for air equipment, cooling machines, and pumps are located.
Possible Blockages: Check for electrical wires, pipes, and anything else that might get in the way of air ducts.
Fire Walls and Partitions: Note where walls that stop fire are, because these might need special fire safety parts.
Outdoor Air Intakes: See where the outdoor air comes in, making sure it isn’t too close to busy streets or places where dirty air might get in.
Power Supply: Know where the electricity comes from, how much power is available, and if it’s enough. Also, see if more power can be added if needed.
Water Supply: Find out where water pipes are, how big they are, how much water they can carry, and how strong the water pressure is.
Steam Supply: Check where steam pipes are, their size, and how much steam they can handle.
Cooling Systems: If the building has its own cooling system, know what type it is, how much it can cool, and how fast it can move water.
Design of Spaces: Choose air outlets that fit the look and style of the space.
Existing Ducts: See if there are any old air ducts that can be reused.
Drains: Find out where the drains are and if they can handle sewage.
Control Systems: Check if there is a source of compressed air and how strong it is, and look at the electrical controls.
Building Support: Make sure the building is strong enough to hold the equipment.
Noise and Vibration: Think about how the equipment's noise or shaking might affect quiet areas of the building.
Equipment Access: Ensure there are ways to move big equipment, like using elevators or wide doors.
Rules and Codes: Follow local and national rules about wiring, water supply, refrigeration, ducts, fire safety, and ventilation.
Estimating HVAC Load
When you plan to install an HVAC system, you first need to estimate how much cooling power it needs. This calculation considers the heat that comes into a building from outside and the heat generated inside. A "design day" is used as a standard day to make these calculations, which means:
Both dry and wet temperatures are at their peak.
The sun is shining brightly with no haze.
Normal activities inside the building are happening.
It’s rare for all these heat sources to peak at once, so adjustments called "diversity factors" are used to get a realistic estimate. The air coming in and out for ventilation is also considered.
Heat from Outside
The heat coming from outside includes:
Sunlight through windows: Sun rays bring in heat. To manage this, people use shades or blinds. There are tables and charts to help figure out how much sunlight comes in and how to reduce it.
Sun on walls and roof: This heats the building, especially when the outdoor temperature is high. There are tables to estimate how much heat comes through walls and roofs.
Hot air outside: This heat enters through windows and walls.
Humidity: When the air outside has more water vapor, it can pass through the building, but this is not always a concern.
Wind: Wind can push hot air through small openings like cracks around windows, adding heat.
Ventilation air: Bringing in outside air to freshen up the building adds to the cooling load, especially if that air is hot or humid.
Heat from Inside
what are Sources Heat inside the building comes from?
Followings are sources
People: Humans give off heat through body activities.
Lights: Lightbulbs create heat when they're on.
Appliances: Equipment like stoves or computers can add a lot of heat.
Machines: Devices like motors in factories or offices produce heat.
Hot pipes and tanks: Hot water pipes or tanks give off heat.
Other sources: Things like steam from cleaning machines or materials that absorb moisture can add heat.
Total Heat Load
In addition to indoor and outdoor heat, the air conditioning system itself can gain or lose heat. Fans and pumps that move air add heat, and air ducts can lose cool air or let warm air in.
Heating Load Estimation
Calculating the heating load (for winter) focuses on how much heat escapes the building and how much is needed to warm it up. This load doesn't consider heat from people or lights, as it’s for cold nights. Tables help to find out how much heat is lost through walls, windows, and ventilation.
High Altitude Adjustments
At high altitudes (like in the mountains), air is thinner, so more air needs to move to keep the space cool. Adjustments are made for this when estimating how much air conditioning is needed.
This simplified guide should help you understand how professionals estimate air conditioning and heating needs in a building.
Equipment Selection Simplified Explanation
Once you know how much heating or cooling your space needs (called the "load"), you need to choose the right equipment that can handle this amount. The equipment you pick should be strong enough to keep the space at the right temperature and moisture levels. The air coming out of the system must be just right to deal with both temperature (sensible load) and moisture (latent load).
Basic Terminologies
Here are some simple explanations for the basic terms we use in calculations and design:
Space: This can be any area, big or small, that doesn’t have walls or is divided by walls. It can be just one area or several connected areas.
Room: This is an area with walls around it, making it a separate space on its own. We usually think of a room as one complete area.
Zone: This is an area (or group of areas) in a building that has similar needs for heating or cooling. It’s treated as one unit so that people feel comfortable in the same way across that space.
Calculate Estimated HVAC Cooling Load
Conduction Through Exterior Structures
Explanation for finding the heat transfer through an exterior wall and Roof:
Q = U x A x CLTDc:
This is the main formula to find heat transfer through the wall. Let’s break it down:
U: This is called the “overall heat transfer coefficient.” It tells us how easily heat can pass through the wall. The value of U depends on the type of materials used in the wall and how well they resist (or block) heat.
A: This stands for the “area” of the wall, meaning the total size of the wall surface we’re looking at.
CLTDc: This is the “corrected cooling load temperature difference.” It’s a special value we need to find that depends on different things like the outside temperature, inside temperature, and location.
CLTDc Formula: To get CLTDc, we use this formula:
CLTDc = CLTD + LM + (78 - TR) + (TA - 85)
CLTD: This is the cooling load temperature difference, a value we can find from a chart.
LM: This is a correction for the month and latitude (location), also found in a chart.
TR: This is the desired room temperature (like 75°F, for example).
TA: This is the average outside temperature for a specific day.
By putting all these values (U, A, and CLTDC) into the main formula, we can find Q, which tells us how much heat is passing through the wall
STEP 1 Find Value of U For Wall
In the picture below, you see two boxes :
Vertical highlighted box: This box shows different values for U. U is the heat transfer coefficient.
Horizontal highlighted box: This box shows different types of materials. Each material has its own U value, so you can look along this box to see how different materials change the U value.
By using this chart, you can find the U value for the material you want to use. also remember group name.
For Roof
STEP 2 Find Value of CLTDc
CLTDc = CLTD + LM + (78 - TR) + (TA - 85)
1 - Find CLTD Value
For Wall
In the picture below, you can see different groups. Choose the group you remember when finding the U value. Then, you’ll see two boxes:
Vertical highlighted box: This box shows different time values to help find the peak time.
Horizontal highlighted box: This box shows the direction of the outside wall facing the sun, like the north wall in the example below.
Using this chart, you can find the CLTD (Cooling Load Temperature Difference) value for the material you want to use.
For Roof
2- Find LM Values
In the picture below, you will see two boxes:
Vertical highlighted box: This box shows the direction of the wall that faces the sun, like the north wall in the example.
Horizontal highlighted box: This box shows the latitude (how far north or south) and the month in the example.
Using this chart, you can find the LM (Latitude and Month) value.
3- Find average outside temperature Values
In the picture below, you will see two boxes:
Vertical highlighted box: This box shows the Daily Range of the specific country
Horizontal highlighted box: This box shows the country and Latitude
Using this chart, you can find the daily Range value and put this value in the formula average outside temperature value given below
Ta = To - (DR/2)
put all Above Values in the Formula we find Heat Transfer Through Exterior wall
CLTDc = CLTD + LM + (78 - TR) + (TA - 85)
ADD U , CLTDc and A Value in Given Equation and Get Q values
Q = U x A x CLTDc
How Heat Transfers Through Glass and Affects HVAC Systems
When it comes to heating and cooling (HVAC), understanding how heat moves through glass is important. There are three main ways that heat transfers through glass: conduction, convection, and radiation. Here’s a simple explanation of each:
Conduction
Conduction is the direct transfer of heat through the glass itself. Imagine you touch a hot window on a sunny day—the glass feels hot because the heat moves from the outside to the inside. The heat flows from the hot side of the glass (outside) to the cooler side (inside), which warms up the room.
2 Convection
Convection happens when air moves around the glass and carries heat with it. For example, on a hot day, warm air outside touches the glass, heats it up, and moves away, letting more warm air touch the glass. Inside, the glass then warms the air around it, creating a flow of warm air in the room. This is how heat circulates near the glass.
3 Radiation
Radiation is heat transferred in the form of energy waves, like sunlight. When sunlight hits a window, the glass absorbs some of this energy, which heats it up. The rest of the sunlight passes through and warms up the room directly. That’s why rooms with large windows feel much warmer on sunny days.
Why This Matters for HVAC
When heat comes through glass, it can make rooms hotter, increasing the need for cooling from the HVAC system. This can make it more challenging to keep the room comfortable. To help, many people use tinted glass, shades, or reflective coatings on windows. These block or reduce the amount of heat that enters, helping to keep rooms cooler and reducing the strain on HVAC systems.
By controlling heat transfer through glass, HVAC systems can work more efficiently, using less energy to cool spaces. This saves energy and keeps rooms more comfortable, even on hot, sunny days.
HOW to Find Heat Transfers Through Glass
WE USE THE FORMULA GIVEN BELOW TO FIND Heat Transfers Through Glass AND AFFECT HVAC SYSTEM
Q = SHGF × A × SC × CLF
This formula is used to calculate how much heat comes through a glass window due to sunlight, which affects the cooling needed in a room.
Here’s what each part means in simple terms:
Q: This is the total heat from sunlight that enters through the glass. It’s measured in BTU per hour (a unit to measure heat).
SHGF: This stands for "Solar Heat Gain Factor." It’s the maximum amount of heat the sunlight can add through each square foot of glass in an hour.
A: This is the area of the glass in square feet. So, it’s the size of the glass window.
SC: This is the "Shading Coefficient." It shows how much shading (like blinds or tinted glass) reduces the heat coming through the window.
CLF: This is the "Cooling Load Factor." It adjusts the heat value to match the glass type, so we know how much cooling is needed for the specific glass.
The formula Q = SHGF × A × SC × CLF helps in finding out how much cooling is required to keep a room comfortable by accounting for the heat from sunlight through the glass.
STEP 1 find Solar Heat Gain Factor Value
Find the latitude using the chart provided.
Check the direction the glass is facing on the wall (like north, south, etc.).
Identify the peak month with the highest solar radiation
With the help of all above info calculate the SHGF Value
STEP 2 Find Shading Coefficient Value
With the help of chart Given below find Value of Shading Coefficient
STEP 3 find Cooling Load Factor Value
with the help of chart given below we find Value of CLF
by using all above info and put value in given equation we find value of heat transfer through glass
Heat Produce and Transfer through interior factors
How To Find Heat Transfers Interior Partions
Explanation for finding the heat transfer through an exterior wall and Roof:
Q = U x A x TD:
This is the main formula to find heat transfer through the wall. Let’s break it down:
U: This is called the “overall heat transfer coefficient.” It tells us how easily heat can pass through the wall. The value of U depends on the type of materials used in the wall and how well they resist (or block) heat.
A: This stands for the “area” of the wall, meaning the total size of the wall surface we’re looking at.
TD: This is the “ temperature difference.” It’s a special value we need to find that depends on different between partions
How To Find Heat Produce by Lighting
This formula is used to calculate how much heat comes through a glass window due to sunlight, which affects the cooling needed in a room.
Here’s what each part means in simple terms:
Q: This is the total heat from sunlight that enters through the glass. It’s measured in BTU per hour (a unit to measure heat).
SHGF: This stands for "Solar Heat Gain Factor." It’s the maximum amount of heat the sunlight can add through each square foot of glass in an hour.
A: This is the area of the glass in square feet. So, it’s the size of the glass window.
SC: This is the "Shading Coefficient." It shows how much shading (like blinds or tinted glass) reduces the heat coming through the window.
CLF: This is the "Cooling Load Factor." It adjusts the heat value to match the glass type, so we know how much cooling is needed for the specific glass.
The formula Q = SHGF × A × SC × CLF helps in finding out how much cooling is required to keep a room comfortable by accounting for the heat from sunlight through the glass.
When heat transfers through glass, it usually happens in three main ways: conduction, convection, and radiation. Let’s break down each one in simple words:
Conduction:
This is the direct transfer of heat through the glass itself.
Imagine you touch a hot window on a sunny day. The glass feels hot because heat is moving through it from the outside to the inside.
The heat flows from the hotter side of the glass (outside) to the cooler side (inside), warming up the room.
Convection:
This happens when air moves around the glass, carrying heat with it.
For example, if the air outside is hot, it touches the glass, transfers some heat to it, and then moves away, allowing more hot air to touch the glass. This keeps warming the glass surface.
Inside, if the glass is warm, it heats up the air next to it, causing a flow of warm air inside the room. This is how heat circulates and spreads in the air near the glass.
Radiation:
Radiation is heat transferred in the form of energy waves, like sunlight.
When sunlight hits a glass window, the glass absorbs some of this energy, which then heats up the glass. The rest of the sunlight passes through and warms up the room directly.
Glass allows sunlight (radiant heat) to pass through, which is why rooms with large windows feel warmer when the sun is shining.
In a typical situation with sunlight, radiation is the biggest way heat enters through the glass, especially if the sun is shining directly on it. Then, conduction and convection add a bit more heat from the warm glass to the air inside, making the room even warmer.
Why It Matters:
This heat transfer through glass can make rooms hotter, which increases the need for cooling (like air conditioning) to keep a comfortable temperature.
Find the latitude using the chart provided.
Check the direction the glass is facing on the wall (like north, south, etc.).
Identify the peak month with the highest solar radiation
This formula calculates how much heat is added to a room from lighting, which affects how much cooling is needed.
Here’s what each part means in simple terms:
Q: This is the amount of heat coming from the lights, measured in BTU per hour.
W: This is the power of the lights, measured in watts (W). It’s basically the amount of energy the lights use.
BF: This stands for "ballast factor." It’s a number that adjusts for the type of light being used. value of BF is 1.
CLF: This is the "cooling load factor" for lights. It tells us how much of the light's heat needs to be cooled. value for CLF is also 1
So, Q = 3.4 × W × BF × CLF helps us figure out how much cooling is needed to handle the heat from the lights in the room.
How To Find Heat Generated by People
Heat Generated by People can be find by the formula given below
add all values together and find complete cooling load calculation