Duct sizing impact on flow

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We have a ranch with a wood fireplace in the living room. It's fantastic at heating the main living area, but the bedrooms down the hall get left in the cold.

I'm going to install insulated flexible ductwork in the attic, with a blower near the inlet (red), with outlets in each room. The initial duct will be 10", with 6" to each room.

This is roughly to scale, about 24' between the inlet and 2 aligned outlets. Fireplace is to the left:

I can either run 10" to a 3-way manifold, and take 6" runs from there. Or I can run 10" to an 8"/6" splitter, take one 6" off there, then run the 8" to a 2-way 6" splitter, and take the remaining 6" off there. The first is preferable, as I'll get more efficient use of the duct, which comes in 25' sections.

Will I see a noticeable difference in airflow one way or the other? If it matters, the lower-right bedroom is the master, and it's fine if airflow is a little less there (other 2 are kids' rooms, which we want a bit...

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Sizing of duct-works in ventilation systems can be done by the

Velocity Method Constant Pressure Loss Method (or Equal Friction Method) Static Pressure Recovery Method

The Velocity Method

Proper air flow velocities for the application considering the environment are selected. Sizes of ducts are then given by the continuity equation like:

A = q / v (1)

where

A = duct cross sectional area (m2)

q = air flow rate (m3/s)

v= air speed (m/s)

Alternatively in Imperial units

Ai = 144 qi / vi (1b)

where

A = duct cross sectional area (sq.in.)

q = air flow rate (cfm)

v= air speed (fpm)

A proper velocity depends on application and environment. The table below indicate commonly used velocities:

Be aware that high velocity close to outlets and inlets may generate unacceptable noise.

Related Mobile Apps from The Engineering ToolBox...

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Duct Sizing. Two Words and A Bit of Fun…

Shall we explore more?

Before that, a little introduction about ducts. Duct, is a dedicated passage, mainly for gases. The main function is to deliver the gas, from one point, to another, by means of forced, or natural flow.

Ducts in air conditioning systems are fitted with fans and/or blowers. Hence, air movement within the system is from the forced pressure difference at the fans and/or blowers.

This exercise is especially useful for central air conditioners.

Techniques for duct sizing:

There are two famous ways of duct sizing, the

equal friction technique, andthe static regain technique

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We’ll go deeper into the methods of duct sizing on this page.

Equal friction and static regain methods vary in the approach, but these two share some similarities.

Similarities between these methods are, we need initial data, iterations, and we need duct sizing charts,...

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DUCTWORK MYTH BUSTER

Before using your old Rule-of Thumb for sizing ducts, it might be good for you to read what Paul Wiebolt of Tradewinds Appropriate Technologies has to say about the subject.

It's often said that to do something right, you have to get back to the basics. Sometimes, the basics involve putting aside what you think to be true, and looking at it from a different perspective.

This is certainly true with duct design. There are many "myths" associated with duct design that result in common errors. Allow me to be that different perspective, as I debunk 10 common myths associated with duct design.

No. 1: Ducts designed per Manual D are self-balancing.
Duct systems are not self-balancing. The closest thing to a self-balancing duct system would be one with an adequate return in every room that has a supply outlet. This isn't very practical, and really can't be achieved in the real world. Installed...

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