Air Flow Loss Help

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bowersox1
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Post by bowersox1 » Fri. Jul. 24, 2015 4:18 am

I'm not sure if this is the topic for this, if not i'm sorry. I am wondering if someone can give me a idea of the approx. CFM loss for my current hot air supply to a room. My house was built in 1956 and in 1962 or 63 a room was added. The added room only has a craw space the cellar has a solid support wall running down the center for thee floor joist. Heat was added and it is a somewhat mess. I am looking to see if it would be worth the added cost to redo the ducting for the gain of flow.

Here is the layout for the duct working supplying the added room. The main supply trunk for entire house is 20"x8", from this it taps off to a 8" round duct which runs for 4'6" then reduces to 6" round duct. On the reducer is a 6" elbow the runs straight for 3'5" then a elbow then runs straight for 2' then elbows up into the room.

Can you give me a idea of approx how many CFM this set up is losing and weather it would be worth the cost to knock out blocks and run new duct?

Thanks.

 
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Post by McGiever » Fri. Jul. 24, 2015 7:37 am

See 2 choices...educate yourself to what it takes to properly size duct work or hire a pro to do it.
There is too much to it to give a short answer on the internet.

 
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blrman07
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Post by blrman07 » Fri. Jul. 24, 2015 8:26 am

It all depends on how much air flow you want to that room. Is it currently a problem?

I have copied a link to a pretty good primer on duct sizing. There is a substantial difference in CFM's between a 6 inch and an 8 inch duct. It all boils down to is the extra work worth the effort and expense of taking out that block wall so you can run a bigger duct?

http://russellking.me/2012/08/19/duct-size-vs-air ... ow-part-1/


 
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Post by franco b » Fri. Jul. 24, 2015 10:06 am

The return capabilities of the room also greatly effect the supply.

 
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Post by Freddy » Fri. Jul. 24, 2015 11:29 am

franco b wrote:The return capabilities of the room also greatly effect the supply.
For sure! The return is as...if not more...important as the feed.

I'd either A: Give it a supply and return & if it's not enough make changes, or B: hire a pro for his thoughts.

 
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Post by Lightning » Fri. Jul. 24, 2015 6:08 pm

bowersox1 wrote:Here is the layout for the duct working supplying the added room. The main supply trunk for entire house is 20"x8", from this it taps off to a 8" round duct which runs for 4'6" then reduces to 6" round duct. On the reducer is a 6" elbow the runs straight for 3'5" then a elbow then runs straight for 2' then elbows up into the room.

Can you give me a idea of approx how many CFM this set up is losing and weather it would be worth the cost to knock out blocks and run new duct?
How big is the room?
How many of the wall are adjacent to the outside?

Hmmm.. can't really help with CFM numbers, but I have a similar situation. I converted my attached two and half car garage (about 500 sq ft.) into a living room. I dedicate about half of my furnaces output to it since three sides of the room are exposed to the weather, it also has some big windows and two entry doors. I have two 8 inch round ducts feeding it.

The less turns and bends the duct work makes, the better. The 8 inch getting cut to 6 inch in your situation drastically cuts air volume flow. If you could make it 8 inch all the way to the room and take some bends or elbows out of the path, I think it would be a big help. It's important that return air can get back to the furnace to be heated again, as mentioned. If return air currently can't get from the room back to the furnace, this should be the first thing that is fixed before attempting any rework of the warm air supply.


 
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Post by McGiever » Sat. Jul. 25, 2015 9:28 am

The rule on return air duct sizing is to have more/bigger size (sq. in.) return than the size of the supply (sq. in.) of the given space to be heated.

 
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Post by bern » Sun. Aug. 02, 2015 7:17 pm

They make a inline booster fan to help improve cfm runs about 30 bucks for a 6". They can work in some cases but I have to agree with almost everyone here who said get a pro. However I will add that a heat loss on the room in question is where I would start. many plumbing supply places will do one for free then simply add the but per run and see. Most cases with a cold room is its a question of thermostat placement and heat loss for the given area. I have run into duct issues being undersized and had to add one or more, if it looks like a mess it prob is.

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