Same run/dia different temp

[GOOSE2]

I have checked temps comming from the registers in two locations (two different bedrooms). The difference is 8 degrees!
Checked for leaks either end - no luck there.
Changed both ducts with new 6.0 r value runs. same diameter same lengths, no bends or separate attic areas.
I do not know what is causing one to be 8 degrees lower than the other!
Any help would help.

[Gary]

Tell us about the main duct these branch ducts are connected to. How far apart are the connections? How far from the end of the main duct? Did you use the same thermometer to measure each?

[Andy]

Hi, GOOSE2,
what way is the flow balanced in this system? Have the flow dampers in the grills been set up for the design air volume?
Regards. Andy

[Dan]

Let's assume the air is at one temperature before it branches off the main duct.

If this is a cooling question, the colder air must be the air delivered most swiftly and the warmer air must be moving much slower permitting greater heat transfer within the branch duct.

If this is a heating question, the colder air must be the slower air for the same reasoning.

Gary's and Andy's questions are addressing this consideration.

Try this: restrict the air flow from the grill that has the desired temperature and see if the grill that is 8 degrees from the desired temperature achieves the desired temperature.

If it does, you have an extremely imbalanced air flow. If it doesn't, then we need to think further.

[GOOSE2]

Hey Guys,
First off thanks for the replies. I guess I did not provide enough info.
Both runs come dirrectly out of the plenum at opposite ends. There are no obstructions in the plenum (ie dampers of any shape or form).
I have no real technical way of determining the airflow through these, except that of the sheet of paper being deflected. I did try that (again not very scientific) yet both registers seem to deflect the paper the same amount.
If I have missed any other info please let me know.
I could understand a couple of degrees difference, but not 8!
Oh, and I did use the same thermometer on both registers.
Thanks again for all your help.
Goose2

[frank]

it could be that the air entering the plenum is at different temperatures as it comes off the coil/heat element (due to bypass factors) and then it is distributed down opposing ducts without having the chance to combine/eqalise.

If this is so then an equalising grid on the inlet to the plenum would help. What is the distance from the coil/heat element to the plenum inlet?

Frank

[GOOSE2]

Frank,
The coils are located just prior to the plenum. The distance from the edge of the fins to the plenum mouth is 2.5 inches. the plenum mouth measures 22 x 22 x 48 and has a wedge shape with runs comming off 3 sides. Unit sits horizontally in attic.
Thanks,
Goose2

[frank]

Try taking temp measurements across the face of the fins and see if you get the 8F temp diff at this point

[Gary]

I'm thinking a picture would be worth a thousand words.

The plenum should be capped at the opposite end from the mouth. All branch connections should be off the sides at least a foot from the end. There should never be a connection in that end cap, as the air is being tossed directly into it, robbing the others.

[GOOSE2]

Success!
I tapped the plenum in a different area (same face) where I had checked temp and the difference is now only 1 degree!
Thanks for the tips to all who responded!
GOOSE2

[herefishy]

I am no A/C-duct man, but could we have some other problem that we are bypassing by moving the location of the tap?

I am thinking a long the line of an unevenly fed "A" coil, perhaps a restricted distributor tube or two. Or could we just have a poorly designed plenum?

[frank]

Good point - but if the coil is fed from the side with a restricted distributor etc then the coil would surely be even temp from left to right but not from south to north. The air temp leaving the coil would also be even from left to right etc.

I think this problem is really down to a lack of space for the air temp to eqaulise as is bourne out by the statement that the plenum is only 2.5 inches from the coil and the take off ducts are at opposing ends.

Frank