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Old 01-23-2014, 09:05 PM   #1
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Question about number of registers that can be on a trunk


My lower level does not get much pressure and it always takes a long time to get heat especially in the -40's. I have lot of drafts which is half the issue, and I'll have to figure those out, but I also notice my vents don't get much pressure.

I finally got around to removing the dampers (they were very old and the levers were loose so it was hard to tell their actual position) but it did not do that much of a difference. Then I started thinking perhaps the main trunk for that side of the house is just undersized.

I figured out that if I combine the square area of all the vents it is much higher than the square area of the main trunk, could this be an issue? It seems to me, you would not want to exceed that.

This is what I came up with:

Trunk: 21x31=651cm^2 (there are two, one for downstairs one for upstairs)

Single 5" duct: 181.4cm^2

5x ducts = 907cm^2 or 6x ducts = 1,088.4cm^2

(6x ducts is counting having one left on in the crawlspace, there are two in there but I closed one off)

Should I perhaps run a couple dedicated lines to the very end of the main trunk, and then just tie it in? Compared to upstairs when I put my hand in front of a vent I don't feel much air at all. If I close all the ones upstairs it also does not help much. so I know it's not the main plenum that's undersized.

Just want to make sure I'm on the right track here before I make any changes.

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Old 01-24-2014, 12:36 PM   #2
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Question about number of registers that can be on a trunk


Before doing any of that make sure you have proper return air. All the heat registers and the largest supply trunk in the world means nothing if you don't have adequate return air. Once return air is established then you can look at the temp. rise to make sure its within the manufacturers specs then look at properly balancing the system.

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Old 01-24-2014, 12:59 PM   #3
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Question about number of registers that can be on a trunk


Yeah there is plenty of return, though, now that you mention it, I wonder if perhaps the return trunk itself is too small. It is only slightly larger than one of the two main supply trunks. There's a vent in the basement (been wanting to add one in the crawlspace too) and two upstairs. Essentially the upstairs one is "shared" with the basement one, crawlspace one would be similar idea. But if the actual duct is too small then that may be an issue... The elbow that comes out of the furnace is larger but then it shrinks right away. So if I add another return I should maybe cut right into the elbow part.
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Old 01-24-2014, 02:25 PM   #4
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Question about number of registers that can be on a trunk


It is not uncommon for the area of the floor boots to exceed the area of the ductwork. Do you have a zone system? If so check the bypass damper. It my be partially open.
What would really help is to know how many branch lines run off your trunk to the boots and what size are they. What size is your trunk line? Is your ductwork sealed. Are the coils clean. What size is your unit.
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Old 01-24-2014, 02:26 PM   #5
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Question about number of registers that can be on a trunk


sorryi should have read your post completely
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Old 01-24-2014, 02:32 PM   #6
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Question about number of registers that can be on a trunk


how long is your duct system? are the branch runs metal or flex?
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Old 01-24-2014, 05:06 PM   #7
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Question about number of registers that can be on a trunk


All metal ducts (5") the duct in question is about 20 feet give or take (not at home to measure). Not sure of the BTUs, I'd have to check, but I think my issue is lack of air flow not so much lack of heat. There's no main damper on the trunk.

The 5" runs are also rather simple. The two on the end have a few curves but are no more than a few feet from the trunk to the floor. The further ones are straight runs, no bends.

Ducts are extremly dirty though, I should get those cleaned.
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Old 01-25-2014, 10:07 PM   #8
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Question about number of registers that can be on a trunk


Measured the return duct, it is 47cm x 20cm so 940cm^2

The trunk in question is 25 feet, the upstairs trunk is shorter, , about 15 feet or so. Oddly if I put my hand on the trunk it's not as hot as the shorter, upstairs one. Can the length really make that much of a difference as far as static pressure goes? It's almost as if the downstairs one is just not getting enough air flow, but it's all coming from the same place, and the returns are "shared". (the upstairs and downstairs is not enclosed in any way)

I guess I'll have to open up the trunk and look inside with a light, maybe there's some kind of obstruction. I'm almost scared what I could find lol.


Edit: Looked inside, I don't see anything wrong, there was a ton of dust bunnies and some dead bugs I am not sure what they were, but other than that it was not bad. Just occurred to me, I am forgetting that a lot of the upstairs ducts are dedicated. Bathroom and office. That leaves 4 running off the trunk, and two of those are in the basement and 2 in the far bedrooms, the return is in the hall.

Downstairs is 5 not counting the crawlspace, and any loss in the ductwork (minimal, but there's no tape). So I'm thinking that's why there is such a difference.

I closed off the crawlspace duct that I had left open, I figure I can leave the ductwork loss take care of the crawlspace, till I get around to taping it all, if I even bother, might just leave it this way.

Still thinking of maybe running a couple dedicated runs to the far end though. Also should I also look at making the return duct bigger?

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Last edited by Red Squirrel; 01-25-2014 at 10:38 PM.
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