Hvac Duct Sizing

How to calculate the duct size?

Ducts are the conduits around a house through which conditioned air travels and gets into various rooms. The sizes of the ducts are an essential concern as they should be able to provide adequate airflow. This will be able to give you desired comfort without putting stress on the HVAC system. Appropriate duct size will also prevent you an arm and a leg on your energy bill. There are three methods of determining duct sizes. Let’s take a look at them:

Velocity Method- In this method the contractor considers the speed at which air flows through the ducts based on their general size. To determine this they have to know the cross-sectional area of the duct, the rate of airflow, and its speed. There are specificformulasin which they substitutethese values to determine the duct size.

ConstantPressure Loss Method-
This method uses more componentsthan the former to determinethe duct size. But it provides more accurate value of the actual pressure loss in the system. This is based on materials used to make the ducts and their layout.

Static Pressure Recovery-
It works towards ensuring the same pressure level at all vents and inlets for the system. This is acomplex method and is rarely used in residential situations.

Here is a quick look at the processes that the contractors usually undertake to determine the duct size.

Determine the CFM ofeach room: They will do theload calculation of the whole house as wellas room-by-room. This is done by using (Manual J*). Then they apply their findings in the following formulae-

Room CFM = (Room Load / Whole House Load) X Equipment CFM

*Manual J refers to the name of a particularprotocol which deals with “Heat Load Calculation” or “Cooling Load Calculation“. The protocol is used to determine the amount of heating and cooling a house needs to remain dry and cool in summer and warm in winter.

Calculate the friction loss rate: They compute the external static pressure of the blower using the specifications furnished by the manufacturer. From this they subtract the pressure drops generated by various complements that becomea part of the air distributionsystem. These are a/c coils, filters, grills, registers, dampers, etc. The resultant value is the available static pressure for the duct system. Next, they calculate the total effective length. The total effective length is distance between the farthest supply outlet through the equipment and the farthest return outlet. This will also include the equivalent lengths of all turns and fittings. Nowthey use the following formulae to calculate the friction rate.

Friction Rate = (Available Static Pressure X 100) / Total Effective Length

Note: A default friction rate of .05-.08 is usually used instead of making the elaborate calculation given above.

Duct Sizing: The next step is easy. The contractor uses a chart or software to select a ductwork based on the two calculations done before.

Busting the myths around vent installation

Installing HVACs and making ductworks in houses, offices and other commercial areas isa rampant project. There are many false notions that surround duct sizing and installation. It is always useful to know them so that you are not misled. Here are some of the myths, let’s bust them one by one.

Ducts are self-balancing.This is next to impossible. A self-balancing duct will be one that will ensure adequate return of cool air in every room through the supply outlet. This cannot be achieved automatically by getting a ductwork done in the house.They have to be adjusted individually after installation. There are balancing and testing methods to do the adjustments. Besides these, some instruments are also available.

There is a conventional method for effective grille sizing. Many people think that 1 sq.ft. per ton is the rule of thumb.But that is notthe case. If you go by this method of sizing grilles then you can get shockingresults. This couldinclude:

• Bad airflow
• Noisy grilles
• Poor filtration
• Duct leakage

According to Dave Fetters from Hart & Cooley, it is always better to plan for two cfm for each sq.in. of gross grille area. The conventional values that are widely accepted for good return is 144 sq.in. per ton. However, it can stuntthe system return by 30% or more.

A 4 ton unit will need a20-in. X 20-in. filter grille. Thisis a common practice. But it does not bring result.

Sealed ducts will increase efficiency and performance. This is not true. The size of the ductshould be optimum. It isonly after having arightlysized duct thatyou should considersealing it. Sealing ducts of improper size will lead to leaks. Leaky ducts would reduce the efficiency of HVAC. It can lead to zonal pressure imbalance. If there are leaks in supplyducts nearthe attic then it will result in unwanted infiltration. Having natural draft gas appliances in conditioned areacan cause back drafting. This is an alarmingconcern as this can result in carbon monoxide poisoning. If you have sealed undersized ducts then it can cause high static pressure. This can lead to compressor flood back and mechanical failure.

0.1-in. wc friction rate helps to determine the right size of the duct. This is not true. It results in poorly sized supply ducts that cause uncomfortable zones. It will not deliver the Btus and be the cause of noisy grilles. It has been found by measuringsupply duct airflow,
that0.06 is the best friction rate to ensure proper airflow.

An 8-in. flex duct delivers 200 cfm.This claim has been defied by the Air Diffusion Council and manufacturers of flex ducts themselves.

Just plugging in new air-handling unit will increase CFM.It is only true if you have ducts of right size. If the ductsare small the static pressure will be high and cavitation will occur.

Supply outlets can be located anywhere.Outlets in theceiling will allow the cold air to flow near the ceiling. Understanding the properties of air is important to maximize the performance of HVAC. You will need a psychometric chart.

A typical run of supply duct is less than 25 ft.For an adequate airflow, your duct-making expert should takeinto accountthe longest equivalent length of both supply and return side. It could up to 350ft and more.

Disclaimer: This document is intended to be used for reference purposes only, it is not a professional advice. Every job is unique in its way and the application may differ based on several variables at the jobsite.Always contact your local certified professional for assistance to your specific needs.



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