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Limits of existing ductwork
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Posted by heretoplease (My Page) on Tue, Nov 10, 09 at 20:41
| The house I am in has ductwork that was sandwiched between a flat roof and a plaster ceiling. No attic. This ducting is impossible to replace without a major tear out of the existing ceilings.
Can I calculate cfm capability of this system using the dimensions and lengths of the ducts only?
After I know what the max cfm potential is, I can then have the proper system designed to match my limitations.
Thank you.
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Follow-Up Postings:
RE: Limits of existing ductwork
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I guess this was a tough question. I am new to the a/c, heating forum. I don't know who the experts are over here.
The pool forum was a bounty of help for my swimming pool and this forum has helped with a bad install.
If my question needs to be more detailed please let me know.
I did not include dimmensions because if the answer was "no" it would have had no purpose.
If I was to brief in my post please let me know. Always appreciated, Bryan. |
RE: Limits of existing ductwork
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Yes you can maximize the BTU/CFM capabilites of existing ductwork knowing it may not be sufficient to meet or exceed certain design temp requirements.
(Undersized unit for space using existing duct with it's limitations) The downfall is the extended run times at peak loads increasing energy consumption many times 2-3 fold. |
RE: Limits of existing ductwork
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| Thank you zl700, My problem is I don't know what the potential is of the existing system. My system may or may not be undersized. Is there a recommended way to calculate what I have? Should I just post the dimensions of the ductwork? I had bad luck with the company that installed my heat pump. I wish to be more informed with the whomever I hire now.
Thank you so much for all the information.
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RE: Limits of existing ductwork
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| If you have the actual dimensions of the ducts and access to the plenum (main trunk line from the furnace/fan-coil), they can be calculated from dimensions. When possible, I usually oversize ductwork to reduce air velocity and noise, BUT in cases where the ductwork's insufficient I've then modified systems to produce colder / warmer air than normal. There are a few caveats and limitations to this method, which a competent HVACR engineer can handle and work around. No matter what you do, you're not going to be able to move air below freezing through ducts without problems, similarly you're not going to be able to cool a 20x20 room with a single 4" duct without making substantial amounts of noise. I've had many arguments with various contractors about this over the years, but I've yet to have one of these modified systems have problems due to the modifications. I usually end up hiring a refrigeration contractor since most HVAC guys only sell boxes and aren't into tampering with things. The refer guys actually understand how this stuff works in a carnal nature and are understand how to modify equipment for the desired results. |
RE: Limits of existing ductwork
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Formula for finding CFM Airflow
If you can measure the air velocity coming from a duct, here is a rough ballpark formula to get the CFM:
CFM = (velocity in (FPM) Feet per Minute "times the 'square footage' of the duct area")
I.E.,Rd duct formula for sq.ins., 16" Rd duct 16X16=256-sq.ins. X's .7854= 201-sq.ins. / 144= 1.39494-sq.ft. X's Velocity of 800-fpm = 1117-CFM. If your meter just registers mph> X's 88 = FPM.
- udarrell |
Here is a link that might be useful: Sizing Duct Systems for Adequate Airflow
RE: Limits of existing ductwork
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| Thanks again for the responses. sean m, "If you have the actual dimensions of the ducts and access to the plenum (main trunk line from the furnace/fan-coil), they can be calculated from dimensions" Is there a calculating tool that is available? udarrell 2007, (hypothetical)
What do you recommend if you don't have an air handler installed? If I can't create velocity, what are my options? Bryan. |
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