The designing of a ductwork is an important part before you install an HVAC system in your house or office building. The professionals shall obviously provide and install the system properly, but you need to supervise the whole process. This is of course if you’d like it because it can be a quite tedious process and may prove to be difficult for many. Supervising the whole process means that you need to know every little bit of the designing process.

When I mention design, it essentially means the way the ductwork needs to be put and fitted. There are certain calculations, and the science of physics is involved to make a ductwork function properly, like it was meant to be. If this particular process is followed, the system shall be efficient and can go a long way. It shall work effectively and give less pain.

HVAC duct systems are used to circulate hot or cool air throughout the house or inside an enclosed space. They need to be well-designed and afterward, well-maintained. Ducts that are not well designed do not work well and lead to increased energy bills and noise, poor air quality, and discomfort. Air duct cleaning equipment are also necessary to keep them clean after installing and using them.

Before buying or installing an HVAC system in your building or office area, make sure to take help from an expert or a professional. Everything depends on the equipment you choose to use. Make sure to choose the right one. 

This article shall take you through the HVAC and duct design process.

HEAT & COLD LOAD CALCULATION:

Before the duct’s design comes the critical step of figuring out the HVAC system’s heating and cooling load calculation. You first need to know; how much cooling or heating is required for your room. The unit is BTU/hr. Translate the BTU/hours into room-by-room airflow in cubic feet per minute. You can also use online software or applications to do the process. 

The ACCA’s manual S Protocol might help you select the right equipment. When you go to the shop, ask for the performance data table of the particular manufacturer. You need to find an equipment that meets the total heating and cooling of your home.

THE WEIGHT OF THE AIR:

After finding out your BTU/hour and the right equipment, you need to know that the air passing through the ducts carry weight. Air carries weight. One cubic foot of air at standard temperature and pressure weighs approximately 0.0807lbs. We are assuming that the composition is average.

The rule is to have 400 cfm per ton. So, if you get a 2.5-ton air conditioner, the normal airflow would be 1,000 cfm. This means that the blower pushes 81 pounds of air each minute. This weight needs to be balanced and moved around. Without resistance, air travels downward, but it takes energy to push it upwards in cold air and vice versa for warm air. 

More air pressure is needed to work inside reduced space. Hence, two factors reduce airflow – one is friction, and the second is turbulence. Rough surfaces slow down air. Hence, the smoother the inner surface, the better the airflow. 

When the air is pushed out of the blower, the air travels through the ducts, and it lessens as it reaches the rooms. Since the air gets diverted through each branch of the duct, the air reached inside of the rooms is very less. 

Everything fitted inside the system adds resistance and turbulence, and the air pressure drops to zero when it reaches the rooms. The fittings, grilles, registers, filters, balancing dampers, turning of the ducts reduces the air pressure. 

This is why the system has a blower that pushes the air at high pressure, and by the time it reaches the rooms, the pressure drops to zero, and you don’t feel a thing except for the hot or cold air coming in and giving you comfort. 

HOW DOES THE HVAC SYSTEM WORK?

The HVAC system contains an AHU (Air Handling Unit) which handles the air. It contains the blower. Air from inside the home or rooms gets pulled through the return ducts to the AHU. This air then gets conditioned accordingly and is returned to the home or rooms by the supply ducts.

On the return duct, the pressure is negative. As the air moves through the ducts, through the AHU, the pressure gets more negative. We are talking about relative pressure here and not absolute pressure. On the other hand, the pressure on the supply side is positive. As the air moves through the AHU and the ducts and finally into the rooms, the pressure becomes more positive. 

The pressure is maximum near the blower or AHU. The pressure drops to zero when it finally enters the room. 

BLOWER CAPACITY:

The blower needs a certain amount of capacity to push the air through the ducts. The blower works against pressure and also, has a speed setting. 

The blower is set by moving wires to different taps. The AHU or the Air Handling Unit has a Total External Static Pressure (TESP). This is the pressure that the AHU handles during the pushing and pulling of the air through the ducts.

Ask for an expert or professional help when deciding on the blower speed and pressure. Generally, a system is designed at medium speed. They have a TESP of 0.50 inches of the water column. This provides airflow of 899 cfm. 

You need to make sure at what inches of water column should your system function. The blower gives a pressure rise to make the air move into the ducts. The pressure falls due to the fittings and ducts inside the system. 

LENGTH OF THE SYSTEM:

The length of the duct is also an essential part of our duct design. For straight duct systems, the total length shall be the same as the length of the ducts. The effective length shall remain unaffected. This is the same case for any rigid metal or anything fitted tight.

This length will get a lot of fittings throughout. The ducts should go to each room and fulfil their goal of imparting air to all of them. 

Firstly, do a duct layout diagram, aka a blueprint of your building or office area. It shows all the fittings, vents, the blower system, the ducts getting into the rooms, sizes. Use the software RightSuite Universal to measure the length of the ducts. 

Now every extra fitting that has been added adds to the pressure drop and the total effective length. Now, we generally choose the fittings from what is available in the shops. Take control over the installation process and make sure to get it according to how you want it to be. This may take up a few days or some extra money because you ask for every part to be perfect and make no mistake. 

The total length will all the fittings and ducts are the total effective length of the system. The sum of the ducts and fittings is not the only length. The total length of the system includes all the pipelines stretching through the rooms plus the fitting and duct sizes. 

CALCULATE FRICTION RATE:

FR = ASP/TEL, goes the formula. 

Friction Rate = Available Static Pressure / Total Effective Length 

For the above mentioned 0.31 inches of the water column and with a total effective length of 424 feet, the FR would be:

0.31/424 = 0.00073 iwc/foot.

This number gives you the pressure drop. At every foot, there shall be a pressure drop of 0.00073. Therefore, all the turns or any fittings, shall help in pressure drop. 

Most people do not calculate the friction rate because of mathematics and so many decimal points. But it is one of the most vital things to keep in mind while setting up your HVAC system. If the decimals become difficult to understand, here is another way to show it around:

For 1 foot, there is a pressure drop of 0.00073 iwc.

For 100 feet, there will be a pressure drop of 0.00073 * 100 = 0.073 iwc.

Hence, for every 100 feet of length, there shall be a pressure drop of 0.073 iwc. Now, did it become easier? If not, let your professional knowledge, and make sure they keep this in mind. They shall obviously do so, but you need to supervise it personally! Also, keep in mind that the Total Effective Length of the duct is 424 feet taken by me. Your length will obviously matter, So, do accordingly.

The number achieved from the friction rate is important to tell you the amount of drop in pressure per foot. Remember, the pressure drops points I mentioned before? So, here you have to find that how much pressure is dropped for every 100 feet. The Total Effective Length needs to be calculated by you according to the required size of the length needed for your house or office.

If this number is higher, you need to use smaller, restrictive ducts, and if it is lower, you need to use larger ducts. Another way of seeing it can be that the available static pressure needs to be as high as possible, and the total effective length needs to be as less as possible. This is an ideal situation and hence unattainable in some cases. 

SIZING THE DUCTS:

There are two ways to size the ducts – sizing by friction rate and sizing by velocity. 

Sizing by friction rate is essentially another way of saying that the resistance of the duct system needs to be controlled. The more the effective length, the more will be the resistance. Friction is lost due to the length and the cross-sectional area. A knob is fixed, that shall be used to control the resistance. 

The duct area needs to be increased in case of more length. And if it is the other way round, i.e., the length is low, we can use smaller ducts. Now the important thing to know is that the friction rate and the airflow rate needs to be used to find the right size as per requirement. 

You can use duct calculators, and online software’s to do the same. As mentioned above earlier, the friction rate is 0.073 iwc per feet and let’s say that a ductwork needs to move 400 cfm.

The formula is friction loss/air quantity, lined up with 0.073 with 400 cfm. 

The more the value, more will be the resistance, more the static pressure, and the airflow shall provide below. 

Sizing by velocity means that the velocity of the air shouldn’t be too high. For 400 cfm at 0.073 iwc/100 corresponds to 725 feet per minute. This is fine for the supply duct, but bigger ducts are required for the return side. 

So, there it is! How to design your HVAC system! Call for a guide or professional to run you through the whole process and that shall be easier. Keep your research in place and do it properly!

After the tedious process of designing and installing is complete, you need to maintain it as well! Air duct cleaning is another essential part of the whole installing system. It generally comes after you install your system but do keep this in mind. If air ducts are not cleaned, you are prone to diseases and can lead to serious health troubles.