3d Duct Design Software

[Merja Caryk]

Ifyou have a 2.5 ton air conditioner, the nominal air flow would be 1,000 cfm. (The rule here is 400 cfm per ton.) That means the blower has to push about 81 pounds of air through the system each minute. It takes work to move weight around.

When air comes out of the air handler, several things happen to it. It gets sent to the various rooms in the house. As it travels through a trunk-and-branch duct system, the quantity keeps diminishing because some of it gets diverted down each branch on the way to the end.

Each section of duct, each fitting, each turn of the air adds resistance to that air flow because of friction and turbulence. Grilles and registers, filters, and balancing dampers also add resistance. That resistance results in decreases in the static pressure, or pressure drops.

I am having ac work and new

I am having ac work and new flex duct system run off a main hard board plenum that comes up into the attic out of the ac unit closet. Florida install. The installer wants to run a flex duct directly off the end of the main plenum that comes off the blower/inside unit. I did install for a year in high school and we were taught to have the last run of the main hard board plenum no closer than 12 inches from the end. (to create back pressure) I told the ac guy he can use a prefab sheet metal elbow (12 inch) on a side of the main plenum instead of running it off the top/end. I thought that you never ever want to install anything on the end of the main plenum, even a flex duct system, because it will not allow or will decrease back pressure and therefore decrease airflow in the system?

I am noticing a few

I am noticing a few similarities between how water moves in a plumbing system, how current moves in an electrical system, mechanically pulling electrical cable through ducts (think a duct and manhole system, not hvac ductwork) and how air flow moves through an HVAC system.

Another issue in the electrical sector is the idea of skin effect. Where most of the current flowing through a conductor flows at the edges of the conductor, rather than the the center. Water pipes show the same phenomenon. Turn on your hose and notice that the water sprays from the sides and not the center. What we find in the electrical world, is that a 4/0 stranded conductor can carry a bit more current than a 4/0 solid conductor. So, I wonder if multiple, smaller pipes would lessen the pressure requirements at the blower motor? The idea is less turbulence in the center of the pipe and more consistent air flow across the entire pipe diameter.

Also, in the electrical world, if we have 2 separate, but equally sized conductors, starting and ending in the same spot, but tied together at both ends, the resistance drops by half. I have noticed similar behavior with water. And I wonder, since ducts all have the same starting pressure and ending pressure, if you were to run parallel groupings of smaller pipe (multiple outlets in the same room), if the amount of resistance from blower motor to room would drop? That would also lower the amount of pressure the blower would have to surpass to move the air.

I also wonder if other

I also wonder if other materials can be used for ductwork. In caustic environments, cpvc pipe is used. In the power industry, High Density Poly Ethelene (hdpe) pipe is used. They do make a low smoke version for use in confined spaces like electrical vaults and substation basements.

The reason why I am looking for something besides sheet metal pipe is 1) plastic is a much better insulator and therefore less heat is loss through convection of the duct itself. Sure, you can insulate a sheet metal duct, but why not buy a pipe where that is not necessary?

Another reason is the smoothness of a plastic pipe is better than even galvanized steel, which would then be better to reduce the turbulence mentioned in the article. And because it is a better insulator, there is less sound produced within the pipe than there is in typical sheet metal duct.

A fourth reason is that proper installation requires the pipe to be either fused together or joined by some sort of adhesive. This makes it air and water tight. No additional sealing required (think duct tape that is used with sheet metal duct), which would make for easier and quicker installation.

Fred, yes, it would absolutely be better for air flow to shorten that duct. A lot of poor air flow problems in ducts are on the return side so anything you can do there to fix it should result in noticeable improvement. Two possible reasons your return duct is so much longer than it should be: (1) The installer did it to reduce the transmission of noise from the blower into the living space; or (2) the installer was too lazy to cut the duct.

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You don't have to live with uncomfortable hot and cold spots anymore. With our 30 years of residential HVAC duct design and sizing expertise and your elbow grease, your entire home will be cool in the summer and warm in the winter.

We use the Professional Standards set by the ACCA Manual D HVAC Duct Design Criteria. ACCA is the Air Conditioning Contractors of America. They are responsible for setting and maintaining most of the quality and design standards established for HVAC contractors across the country.

First, your duct layout design expert will work from a drawing of your home that you provide. Then he/she will calculate the individual air requirements of every room in your home using custom industry formulations that ensure comfortable and balanced air delivery throughout your home.

For the low-static versions, shoot for duct velocities of no more than 400-450 feet per minute, which keeps the static pressures low even in imperfectly implemented ducts. It's fine to taper down for higher velocity at the registers. Mike MacFarland's nomographs of cfm to fps against duct sizing in this blog work pretty well, but his target velocities are more geared toward more powerful air handlers. You'd be working on the left third of the nomograph when using low-static mini-splits:

Consider using an oversized pleated filter on the return side to keep the filter's static pressure low even when filtering to MERV 10 or higher. The low air velocity through an oversized filter improves the filtering action, as well as keeping the static pressures down. (An Aprilaire 2400 isn't insane here, even though it's more commonly used with 1000 cfm+ air handlers.)

Stay away from return registers in the floor, these are garbage magnets. It is always best to have a filter right at the return register. Well sealed filter and return ducts means very little dust getting into your system, no need to ever clean it.

For supply registers, take a look at the room layout. You generally don't want to blow air across people sitting or sleeping. If you have no choice, try to reduce the velocity down to around 150 fpm.

No matter what, you need to install balancing dampers and set the air flows when you commission the system. Once you set the air flows, check your static pressure and configure the mini split accordingly (usually an install parameter on the thermostat).

I have been struggling to come up with a way to get returns up high in the bedrooms' zone. 1957 plaster walls so no desire to cut holes any holes other than for registers. I want the supplies up high so as not to impede furniture. My plan is closet chases but not much room for that in BR3, and even less in BR2. Maybe with some creativity I can at least get them on the wall at floor height so skittles don't fall in.

For the BR zone I really like the layout afforded by having the supply and return trunks parallel, but blowing directly into a T seems inherently bad for airflow. Is it more important to get smooth transitions on the supply side, at the expense of a less direct return layout?

Typically 50's houses have plaster over drywall lath, these are a bit easier to work with as they are smoother on the inside. From the basement, just cut through the bottom plate between the two studs and install the duct up to the register opening. Not too messy. I doubt you need more then a 10x3 1/4 duct for a return on a bedroom anyways and that easly fits inside the wall.

You can pretty much do what you want with the plenum as long as the pressure loss is fine. T shape is not the greatest but if you make it large enough, it doesn't matter. Adding in a V shaped flow splitter in the middle of the T would not hurt. I think two side elbows would be much cleaner and less restrictive though. Sharp bends are restrictive, doesn't matter if on supply and return side.

Yes you are correct, walls are plaster over blue drywall lath boards. Wall stacks sound like a good plan, I should be able to do that and well worth the effort to get the returns up off the floor. I actually have photos from the original house construction and don't see any blocking in the interior walls, so hopefully that's the case.

What is the purpose of the long red duct on the 12K unit (vs locating it quite close to the air handler)? My understanding is that with open doorways, return location doesn't matter - it's register throw that determines mixing and when that is correct, diffusion within a room happens without any assistance from return location.

There's an existing wall register there that opens directly into the basement. The previous owner had installed it to get hot air up from the wood stove that used to down there. I plan to enlarge it and then run the return ducting back to the 12k. Seems like the extra 10' of straight duct run has a negligible effect on Total Effective Length and friction/pressure/sizing calcs. Those are just openings (no doors) between the all the rooms in that 12k zone so I am hoping one appropriately sized central return will suffice.

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