How long does a section of ductwork need to be before you reduce it ? If I'm running an 8" round metal duct 25' or so, is there a point when I need to reduce it to 6" ? maybe 15' to 20' ?
Ductwork length and reduction
- Thread starter NUTTSGT
- Start date
Zeke
Well-known member
You reduce when you take off the duct as in a wye. In instances where the duct run is quite long, you increase the size. The friction and loss will take care of itself. Use smooth walled duct for maximum efficiency.
Not a real professional explanation, but that's the gist of it.
Not a real professional explanation, but that's the gist of it.
zmaxmotorsports
Well-known member
Where are you running the 8"?
OP
I'm planning on a new wood stove for the garage. It's a DAKA add-on stove. It has two 8" ducts coming out of the top. One will be used for one side of the garage. I'm going to run the second one to the far side of the garage.
The plan is to go up through the ceiling, across the attic and back down through the ceiling in the back of the garage, near the back room and air compressor.
Due to the fact the ceiling is low, like 7' and I have a block wall at the end, I'm going through the attic. My plan is to tape the joints of the duct, wrap with some Reflectix foil bubble insulation and tape those joints also. On top of that, I plan to wrap some fiberglass insulation to heat keep the heated air warm.
Strouty
Well-known member
Is there a blower involved?
OP
Yes, the stove has a blower.
Here it is.
http://www.menards.com/main/heating...28165847-c-12809.htm?tid=-4776738952900431056
Strouty
Well-known member
I will be curious to find out what is the best approach, I need to do something similar at my shop.
zmaxmotorsports
Well-known member
Those are normally meant to be added onto an existing furnace/ductwork.
Its been a long time since I messed with one,Does it have any provisions for a return air supply?
Its been a long time since I messed with one,Does it have any provisions for a return air supply?
OP
It's made to "connect to your existing duct system or use as a central furnace"... their words.
It also has a cold air box for a filter and states to run cold air return to both furnaces, if you're using it as an add on.
It also has a cold air box for a filter and states to run cold air return to both furnaces, if you're using it as an add on.
Ohmthis
Well-known member
The ones I have played with just connected to the existing system. It relies on the furnace/air handler blower. I like that unit, with 550 cfm you have more than enough flow for two 8" ducts. I have a quick question, how hot would the air be coming out of the plenum? Look and see what the bubble wrap temp rating is. You may have to wrap the first sections in fiberglass. I may look into one of these for my shop next year, I'll be looking forward to see how well it does for you.
Farmall Cub
Well-known member
As a general rule, reduction in duct sizing occurs as you add grilles or registers on the trunk.
So, if you have a 10" round duct and want a discharge 10' down the line, you reduce the duct sizing after that.
I do not know the calculations, you could probably find something on the web.
Say you have 1/2" wc in the duct, the reason to reduce it's size is to try and maintain the same pressure after relieving some of it through an outlet.
After a grill, that could drop to 1/4" wc but if you choke the duct down it would push the pressure back up (waaaay oversimplified).
This way you try to maintain the same cfm out of every outlet.
90 degree bends will also affect flow. Most people don't think about it but there is friction involved in moving air.
Next time you're in a big box store, look up and see where the duct size reduces and you'll see what I mean.
Duct dampers or adjustable grills can do the same thing but not as effiently.
So, if you have a 10" round duct and want a discharge 10' down the line, you reduce the duct sizing after that.
I do not know the calculations, you could probably find something on the web.
Say you have 1/2" wc in the duct, the reason to reduce it's size is to try and maintain the same pressure after relieving some of it through an outlet.
After a grill, that could drop to 1/4" wc but if you choke the duct down it would push the pressure back up (waaaay oversimplified).
This way you try to maintain the same cfm out of every outlet.
90 degree bends will also affect flow. Most people don't think about it but there is friction involved in moving air.
Next time you're in a big box store, look up and see where the duct size reduces and you'll see what I mean.
Duct dampers or adjustable grills can do the same thing but not as effiently.
OP
Good point, I will check into that. The bubble wrap won't be on the duct work until it goes into the attic. There will be about 4-5' of "bare" ductwork before the attic.
EDIT: It's rated to withstand temps up to 180°F.
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OP
I was looking yesterday and it appeared that the only straight boots that are stocked locally are for 6" duct work. Since I already have one left over from a previous project, I bought the rest of what I need last night at Menard's. We got into the store about 30 minutes before closing and glad that I had a list printed out as they were making the call for the store getting ready to close.
I will be running 8" duct to the other side of the garage and right before it turns down into the boot, I will be reducing it from 8" to 6". Since the DAKA unit has 2 8" take offs, the other one will be getting a Tee and elbow to direct it on that side of the garage. If I have any issues with the other side not getting enough flow, I'll do some trial and error with a damper in the elbow or tee.
Poor lighting makes for a crappy picture.
I will be running 8" duct to the other side of the garage and right before it turns down into the boot, I will be reducing it from 8" to 6". Since the DAKA unit has 2 8" take offs, the other one will be getting a Tee and elbow to direct it on that side of the garage. If I have any issues with the other side not getting enough flow, I'll do some trial and error with a damper in the elbow or tee.
Poor lighting makes for a crappy picture.
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Ohmthis
Well-known member
Eric, I have no real idea how you plan to run your duct. I'm just giving you options based on the numbers. The fan puts out 550 cfm, the two 8" ducts (hard round not flex) can support 230 cfm each. You could take two 8" up and tee them into two 6" (a 6" flows 120 cfm, again based on hard round). If the lengths of the 8" are similar and the 6" branches are similar, you will get the same (relative) flow. I know the costs go up a little, but the comfort I believe will too.
OP
Basically what I'll have is one duct running about 25, give or take a few feet to the other side. The second one will be nothing more than a Tee and elbow on top of that to give me two directions of heat for that side of the garage. The second will (well, should) have very little restriction.
I have a plan in my head but still open to ideas. I also have no issue with doing some trial/error work to see what works and what doesn't. I knind of like doing that.
All help and advice is well appreciated.
Highbeam
Well-known member
That furnace is 4.2 cubic feet so a pretty good sized firebox and with a decent blower should move the heat well. The automatic, non-electric, thermostat is pretty great too since it will allow you to leave the garage with relative certainty that the stove won't go nuclear.
There are rules about clearances to ductwork for a certain number of feet from the furnace. Be sure to read the manual since with a low ceiling and a rapid pass through to the attic you will likely hit combustibles really quickly.
Did you consider the drolet heatmax or heatpro? Those are more expensive but are EPA legal, have a window, and much bigger blowers.
There are rules about clearances to ductwork for a certain number of feet from the furnace. Be sure to read the manual since with a low ceiling and a rapid pass through to the attic you will likely hit combustibles really quickly.
Did you consider the drolet heatmax or heatpro? Those are more expensive but are EPA legal, have a window, and much bigger blowers.
OP
No I didn't check them out till you mentioned them but I think I stopped by them in the store and kept walking after seeing the price. Not saying they aren't worth it but a quick search shows a price tag of 2 1/2 times what I paid for the DAKA.
If I knew this was going to be the last place I lived, I'd probably consider a lot nicer stove but I believe this unit will work for me and the garage at this time. I went to Menard's today and picked it up.
EDITED for photo replacement.
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Ohmthis
Well-known member
Eric, any follow up on how this project is going? Did you get everything done and operational? Curious how you like it?
OP
I responded back to your PM but yes, the stove is up and operational. I got busy helping to clean up a house before it was tore down and then got sick with the crud that is going around.
I still need to finish wrapping the ductwork with insulation and securing that in the attic. I also replaced the barometric damper with a section of stove pipe. I kept smelling an odor, it really wasn't smoke but it didn't seem like a new stove/pipe smell. I decided to remove the damper and see if that resolved the issue. Unfortunately, it's gotten warm and I haven't need to use it that much so I might be waiting till next year to get it set up exactly were it needs to be.