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Furnace take off for supply

553 Views 5 Replies 2 Participants Last post by  Master Brian
I am trying to finish off my basement ceiling and thus need to change the supply trunk for 4 rooms. The current trunk is 8"x14" and runs on one side of a wood beam and is dangerous to my forehead. My research shows that's 490cfm. Two bedrooms off that are 7" round (125cfm each) and the other two rooms (kitchen and small pantry) are 6" round (85cfm each). So 420cfm is going to the 4 rooms... correct? Is all of that correct thinking?

What I'd like to do is get rid of the 8x14 duct and run half on one side of the beamand the other half on other side, so I can gain some head room. The beam is about 7" tall. I have a bunch of 7" round duct and even a fair amount of elbows. (For what is worth, I won't be adding any more bends than currently there.) The 8x14 trunk is only about 15' long before splitting off for the individual rooms.... furthest room is about 10' off that trunk.

My thought is come off furnace with a 9" round to a T and reduce to 7" for two of the runs. Each of those runs would then be 7" round for entire run. Then maybe an 8" round take off to a T then reduce to 7" and feed the two remaining up to their current 6" round supplies.

With that I wouldn't have to buy much of anything, except what I use to come off furnace. My other thought was 6x12 duct on each side of the beam, which seems to be 270cfm and then split each room off. **maybe I could use get by with two 6x10 and feed one 7" and one 6" duct with each of those.

I'll try to upload pic of the current furnace take off for the trunk in wanting to replace.

Any feedback on this? I want to get this done while weather is decent and furnace and air are off.
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I could do a separate takeoff for each run if that's better. Might actually be easier, just trying to find out if I need the flex boot in there. I can't recall the purpose, but am thinking it helps with noise?


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My research shows that's 490cfm. Two bedrooms off that are 7" round (125cfm each) and the other two rooms (kitchen and small pantry) are 6" round (85cfm each). So 420cfm is going to the 4 rooms... correct? Is all of that correct thinking?

What your research is showing is potential airflow. Whether you have that airflow or not would have to be measured. You may be moving a lot more or a lot less air than you think. If your looking to gain an inch or two of head room I think you'd be ok running 14x6 or 14x7 along the beam where the existing duct is now, your main duct is only 15' long so in that existing 14x8 section @ 500 cfm (assumed) you have a friction loss of .012" wc. 14x6 @ 500 cfm your loss is .026, a .014" wc difference. All based on .1" loss per 100' of duct. Very rough calculations but close enough for what your doing. Your velocity will increase in that duct section too.
So coming directly off the main stack with 4 individual runs might give needed air to rooms and might not.

I suppose I might be able to reduce the size like suggested by inch or two. I'm curious how the flow is actually measured.

I've tried to have several different, fairly large, hvac companies give me a quote to change this and none seem to want to. I've never had one do a calculation ever in any of my houses.
Running 4 separate runs would probably work too. Obviously, I don't have the big picture and don't know if you have existing performance problems. Because I have access to a sheet metal shop, my preference would be to change the main duct size instead of separate round pipe runs. You might find running "off the shelf" round pipe and fittings easier and cheaper. As far as measuring the actual airflow you would need tools/equipment that are expensive, Pretty sure it's not worth it in your case.
Well, I'll admit I still need to read up and learn more about the WC figures. I've looked before and to be honest, I gave up as it was a bit confusing to me. Maybe I should read up again....

That said, I decided to just go for it and eliminate the large trunk in this area and replaced it with two 7" round pipe and two 6" round pipe sections with Home Run takeoff's from the furnace. I installed dampers at each takeoff at the furnace. Just finished the last little bit up today and what a difference it seems to have made. Prior to this the lower access panel on lower section of furnace would vibrate, not noticeable unless next to it, but that is now gone. Air flow seems better in each room and I was able to come off one 6" pipe and one 7" pipe and I went a few feet over and added ceiling registers in the basement, so if I ever want, I can open them for a bit of air flow. I do plan to add a return air in the basement as well.

At some point, I think I'm going to do what I can to redo the other half of the house, if this is what I can expect....honestly seems sooo much better. I'm curious if it changes energy bills any. When I started it seemed the AC was running almost non-stop on a low 90* day we had. Today it's almost 100* and I've noticed it shut off several times! Does that really make since?
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