Undersized Ducts


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Old 04-08-05, 09:50 AM
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Undersized Ducts

I posted earlier about concerns I had with my return ducts being undersized and Ed helped me with the problem. Well the further I looked at the system I could see that the returns being undersized was the least of my problems. I did a lot of research on duct sizing and what I have learned kind of scares me.

My supply trunk duct is only 8" X 16", it is 53 feet long and has 19 - 6" diameter supply ducts coming off of it. What really makes it bad is that the air handler is on one end and not in the middle and it is trying to shove 2150 cfm through this 8" X 16" trunk. In addition to this there are 3 90 degree turns in the 8" X 16" duct within 8 feet from the furnace, there are no turning vanes and one of the 90's is a square 90. From what I have learned this supply trunk is extremely undersized, I brought home a flow meter from work and tested my registers and a few of the registers on the furnace side of the trunk is actually acting like a return duct and is pulling air into the supply duct instead of blowing air out.

I can't make the present supply duct bigger so my plan is to add a second 8" X 20" supply duct that will run half the length of the house. I would then take the first 10 6" diameter supply ducts off of the 8" X 16" duct and connect them to the new larger duct. So the old smaller trunk would supply the end of the house furthest away from the air handler. The AH is in the garage, where it comes into the basement it makes a 90 degree turn, I would connect the new supply duct at this 90 and actually turn it into a "T". Since the new duct will be the path of least resistance, because it will be a straight shot down the new duct I want to add a trunk damper to limit the air into the new duct. I want to also add supply dampers for all 19 supply lines.

I know this will not be normal or an optimum set-up but I think it will be much better than what I have now. Does anyone see any major flaws in my plan?

Thank You
Frank
 
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Old 04-08-05, 10:55 AM
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Wink

Read this www. http://hem.dis.anl.gov/eehem/96/960109.html

I think you should rework it all from the furnace. Reduce the duct as you go is how it should go. You are right for sure a 8X16 cant supply 19 -6" runs. I look at it as the 3 90o L in it are like 15' more of drag in that 53' of duct.

ED
 
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Old 04-08-05, 11:44 AM
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Thanks for the article.

If I could rework the system I would, but I just don't have the room to enlarge the present duct, unless I remove the return. Looks like when they built the house they ran the trunks then all the other plumbers and electricians did their work. So there is plumbing, lighting, drains, vents, freon lines...... preventing me from doing so.

This is really starting to upset me, it's a 4.5 ton Lennox zoned system, I paid top dollar to an established licensed HVAC contractor for this system.

Frank
 
 

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