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Old 01-13-2010, 08:09 PM   #1
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Air temp drop over length of supply


on the 1st floor on the registers closest (shortest duct length) to the furnance the air comes out at 138 degrees F... 50 ft down the line (other end of the supply duct) the farthest register only ha a temp of 110 degrees F when a heat call is running... is there anything to help prevent this 28 degree drop in temp? the other side of the house is always a lot cooler then the closest side to the furnance

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Old 01-13-2010, 08:18 PM   #2
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Air temp drop over length of supply


Thicker duct insulation?

That's my current project. My heat pump is doing good if I see 85 degrees at the first register so I'm upgrading the R4.2 insulation to R8.

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Old 01-13-2010, 09:17 PM   #3
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Air temp drop over length of supply


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Thicker duct insulation?

That's my current project. My heat pump is doing good if I see 85 degrees at the first register so I'm upgrading the R4.2 insulation to R8.
Right now none of it is insulated, just the sheet metal duct... the runs off the supply though are insulated 6" round duct, but the main line isn't...

not 100% sure how to do it with the way its installed... I bought some reflectix duct insulation to try out but I cant find anyone that sells the corner spacers (the plastic ones) or the rolls of the layer you put down first pre cut to 2" widths... wish someone sold that stuff around me so I could make that project easier... but will insulating it really help that much? or is it a waste of money?
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Old 01-13-2010, 09:27 PM   #4
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Air temp drop over length of supply


Helps a bunch blue. I have checked many systems that had less then 4 degrees drop from one end to the other with 1" duct wrap. Part of your duct runs in an unconditioned space doesn't it? My duct runs through the attic. It's wrapped with 2lb 2" duct insulation and has a half degree drop from furnace to far end.
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Old 01-14-2010, 06:30 AM   #5
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Air temp drop over length of supply


An 8"X20"X4' sheet metal duct has a surface area of 18.66 sq ft.
if the air temp inside the duct is 138F, and the room temp/air temp around it is 70F. It has a heat loss of 1269 BTUs an hour.

If that same duct under those same temp conditions is insulated with an R4.2 insulation. Its heat loss is reduced to 302 BTUs an hour.

302 divide by 1269 equals roughly a 76% reduction in heat loss.

Only draw back. Is if your basement ceiling isn't insulated. Your first floor floors will be cooler. And if you walk around barefoot. You will notice it.
In that case. Don't walk around barefoot.
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Old 01-14-2010, 09:30 AM   #6
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Air temp drop over length of supply


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An 8"X20"X4' sheet metal duct has a surface area of 18.66 sq ft.
if the air temp inside the duct is 138F, and the room temp/air temp around it is 70F. It has a heat loss of 1269 BTUs an hour.

If that same duct under those same temp conditions is insulated with an R4.2 insulation. Its heat loss is reduced to 302 BTUs an hour.

302 divide by 1269 equals roughly a 76% reduction in heat loss.

Only draw back. Is if your basement ceiling isn't insulated. Your first floor floors will be cooler. And if you walk around barefoot. You will notice it.
In that case. Don't walk around barefoot.

what, cooler then they already are? *LOL* I need to consider insulating the basement ceiling... the 1st floors are always cold... the second floor ones are warm...

Last edited by BlueBSH; 01-14-2010 at 10:40 AM.
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Old 01-14-2010, 12:12 PM   #7
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Air temp drop over length of supply


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Right now none of it is insulated, just the sheet metal duct... the runs off the supply though are insulated 6" round duct, but the main line isn't...

not 100% sure how to do it with the way its installed... I bought some reflectix duct insulation to try out but I cant find anyone that sells the corner spacers (the plastic ones) or the rolls of the layer you put down first pre cut to 2" widths... wish someone sold that stuff around me so I could make that project easier... but will insulating it really help that much? or is it a waste of money?
I insulated an (approx) 25 foot length of trunk, using Reflectix. I don't know if you can actually buy spacers and end strips, but I made my own.

How, you ask? By using 2" strips of reflectix and various bits and pieces, it worked out very well. Using the 4' wide roll, and using a board as a 'backer', I would unroll the material, mark the length, then using a straight edge, cut across the roll with a box cutter. You make the 2" strips the same way.

I find a double layer of the material does the job, used every 2 linear feet. You can use strips taped across square corners.

Before Reflectix, the heat would pour off of the trunk. Now, I can barely feel any heat.

V
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Old 01-14-2010, 12:47 PM   #8
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Air temp drop over length of supply


If the problem is that it is right next or close to a ceiling or wall, you could try loosining the hangers, get at least a crack between the duct and surface, then spray foam insulation in the crack, then retighten the hanger. Just a thought.
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Old 01-19-2010, 09:48 PM   #9
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Air temp drop over length of supply


V -- How did you apply the Reflectix around the duct hangers?

RST

P.S. To original poster, I measured a 45 degree drop with my system, I feel your pain. And my crawlspace was warmer than my house(!).
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Old 01-21-2010, 09:24 AM   #10
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Air temp drop over length of supply


In my house I only have a few runs that actually run for the first floor. 4 of the runs are about 7 feet, with one running the span of about 15 feet. Should there be much concern for the runs that are really short that end up feeding the second floor? Those that feed the second floor are 3ft and and two at 5ft. With the three that feed the second floor and going up through the wall, will I be wasting my time wrapping those lines? I will keep an eye on this post, all of your previous reply's are very informative, but I could not conclude an answer for my problem.
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Old 01-21-2010, 03:16 PM   #11
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Air temp drop over length of supply


I think the rule of thumb is that insulation only pays off for duct runs in unconditioned spaces (e.g., basement, attic) or those in exterior walls.

RST

P.S. Just reread your post... are asking about insulating the small portion of your second floor runs that is accessible in your basement? I would suggest sealing the ducts to prevent leakage but I can't imagine that insulating such short runs would make a difference. If you have extra insulation, sure, but probably not worth buying more.

Last edited by RST; 01-21-2010 at 03:19 PM.
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Old 01-21-2010, 03:19 PM   #12
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Air temp drop over length of supply


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I think the rule of thumb is that insulation only pays off for duct runs in unconditioned spaces (e.g., basement, attic) or those in exterior walls.

RST

yeah, wouldn't it being in a wall kind of make an insulation layer in itself? you have a duct enclosed in a ~14 inch wide cavity 4 inches deep sealed off by wood and drywall and maybe fiberglass if its on the exterior wall
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Old 01-21-2010, 03:39 PM   #13
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Air temp drop over length of supply


Supply runs in outside wall cavities have a higher heat loss and gain then runs in inside walls. Unfortunately. Most installers don't allow for this. And run the same size duct as though it was an inside wall. Then you end up with a warmer or cooler room then the rest of the house.
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Old 01-22-2010, 02:47 PM   #14
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Air temp drop over length of supply


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V -- How did you apply the Reflectix around the duct hangers?
You have to be a little imaginative. In my case they extended the clips and attached them to the ceiling. What I did, was when wrapping the trunk, I made a slit at the location of the hanger, from the closer edge and 'worked' the insulation around the hanger and continued the overlap. Just cut pieces of tape to seal the material around the hanger and the length of the slit. Do it similarly on the runs. You'll get the drift. You might need to insert some small pieces (with tape) to maintain your airspace.

V
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Old 01-22-2010, 04:26 PM   #15
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Air temp drop over length of supply


Well I finished wrapping the few long runs that I had. One roll of the reflectix 16" x 25' did three of my runs that I had, and the other roll did 3/4-7/8 of my longer run, I will probably spray foam the remaining length when I order my foam kit. I'm using foil tape that is sorta see-through to tape all the seams, same tape I used to seal all the joints/seams in all the runs and main duct prior to wrapping. It took a little long than I thought, and the elbows were a PITA. The air coming from the ducts is much hotter now, I wish I would have taken temp readings before I started for a comparison.

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