Duct Sizing

robert griggs

Anyone know where I can get a good chart for sizing duct work for residential hvac systems?

Duncan_2

I'm guessing it's more complicated than that.

It was a real eye-opener for me when I took a course in forced air heating system design, there's a lot more involved than I had thought, and it took a lot of time to figure out.

The Manual D method involves calculating your heat loss, and the amount of cubic feet per minute of air to deliver that heat, After determining the longest run, with all the fittings involved, you can determine static pressure, which drives which blower you select. Air flow needed for other runs drives duct size for those areas.

Hope I'm not belaboring the obvious, I just didn't want anyone to think designing a duct system is as simple as using a chart.

I like ACCA's duct sizing wheel, but you need to do all that other stuff first.

Tom Anderson

Duct Sizing Calculators

Probably the most common duct sizing calulator everyone seems to use in my experience is the "Ductulator" from Trane. It is a manual rotary slide calculator. Similar in concept to the pipe sizing "wheel" the B&G Pumps offers.

Most designers size duct using equal friction method, sizing at .08 inches of water column per 100 equivelant feet.

SMACNA has a more sophisticated and pricey "HVAC Duct Design Calculator". This baby sports typical duct loss co-efficients, using velocity pressure, to nail down actual pressure losses for each fitting.

The Trane unit is fine 99% of the time.

For total pressure loss, once the required cfm is determined, I never use the duct software programs. Hand calcs work fine for most jobs.

A common quickie rule of thumb is to multiply total duct footage by 1.5 to account for elbows, etc. For a real twisty, convoluted duct layout, use a 2.0 multiplier. These are for general estimates, and usually, especially for small jobs, will work fine.

Hope this helps.....

Tom Anderson

don_9

duct sizing

Now duncan you know that not a guess,for a guess you sure hit the nail on the head.Duct calculator is only one tool to help get the job done.With most residential you have a low pressure system,velocities less than 2000 fpm and duct static pressure less than 2 inches wg.
Pressure rise/drop=airflow/cfm =air demand=rpm=velocity.
there are many variations of air distribution system and if you are willing to learn proper air distribution design and balancing you will be on top of your game and way ahead of most.Anybody can do ductwork, so they say, Good luck don

Earthfire

duct is a PIPE

for air , and needs to be sized for proper flow justlike steam or water pipe. Guestimates and rules of thumb are just that a guess. Take 4 identical houses on a culdesacand chances are the loads will be different and the ducts for any given room will be different size. Things like solar gain wind exposure and north south orriantation do make a difference

Steve Ebels

Here's a chart

That gives air flows up to 2,000 cfm for all different sizes of rectangular and round duct along with the "tons" of cooling each size will carry. I use this as a rule of thumb unless we get into something really wierd or gnarly.

The trick is to start with a good heat loss/gain so you know the BTU loads you are dealing with. From there you can get the temp rise of the equipment you're dealing with and then determine CFM required to do the job. (CFM X Temp rise X 1.08 = btu's) If I remember right.

Looked at an AC system that was just installed last month this afternoon. Goodman furnace with 5 ton AC connected to it. The owners complaint was lack of cooling and no air flow after it had run for a while. When it first started each day it was OK. The contractor (low bidder) that installed it refused to come out and check it, insisting it was running fine and the symptoms were normal. He had 8X24" for the return and an 8X10 + an 8X14 for the supply. Check out the chart and tell me why the evaporator coil was icing up.

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Steve Ebels

I don't

Have a clue what happened to the chart. The first time I posted it the 'puter defaulted to a text file format. The post was about 100 yards long all in a single line. I had to put the little picture on the page and post it as a jpeg file. It still isn't right and I'm still illiterate as far as computers go. Can someone enlighten me?

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