Furnace Air Velocity
I am in the process of designing ductwork for my home. I purchased the homeowner version of HVAC-Calc Residential 4.0. I am impressed with the software-it's very easy to use. However, I noticed that the software recommends an air velocity of 900 fpm for supply, and 700 fpm for returns. I did a comparison with a Ductulator wheel, which seems to be the standard of the industry for sizing ducts, and it seems to indicate a supply air velocity of only 500 fpm. So I entered my info with the lower speed, and resized all of my ducts. They came out significantly larger than they were before. The larger size seems more in keeping with what I would have expected, intuitively. Can anyone point me in the right direction here as far as recommended air velocity? I'd hate to undersize the ducts. Thanks.
We use to size supply at 450 fpm, I haven't did installs in quite some time but 900 fpm sounds high and noisy.
How big is the house?
900 FPm would blow your curtains around and probably create some comfort problems. It would also reduce the efficiency of the unit.
Does the house have AC?
I may be way off as I am a little rusty. Hope you worked it out.
If you have a ductulator, design using the air pressure drop, not the velocity. For standard low-pressure systems, you should be designing your supply duct to have a drop between 0.08" and 0.10" drop per 100 feet. The "air pressure drop per 100 feet" should show up on the ductulator wheel. Match it up with the airflow, and then size your duct.
What this means is that, as your airflow gets smaller, your allowable velocity gets smaller. A 5-ton unit, blowing nominal 2000 cfm of air, at 0.1", would be around 1150 fpm. But as you branch off, and have an airflow down around 300 cfm, you would only be at 725 fpm. Generally, even at minimal flow, you are not going to end up below about 450 fpm.
For return duct, you want to be between 0.06" and 0.08".
You may want to stay on the low side - 0.08"/0.06" - because residential units (usually) have direct drive fans (as opposed to belt-driven) and these generally have less static pressure drop capability.
While you should reduce the main duct as you go (and thus have less airflow in the duct after branches), most residential systems don't reduce the main much, if at all - this puts more reliance on individual branch flow dampers to control the flow to each space. If you are doing it yourself, you should try to reduce the main duct size at least once, maybe 2/3 of the way to the end of the run.
FYI, the airflow across a cooling coil is usually designed for no more than 500 fpm or so, because much more than that and you can risk blowing the condensation right off the coil into the airstream. But once you are out of the unit, you will be considerably faster than that.
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