register temps

[mrb8828]

My master bedroom runs about 3-4 degrees warmer than other rooms upstairs. I'm only about 1-2 degrees difference in upstairs/downstairs register temps (which is pretty good for a two story in texas with 1 a/c unit). I noticed that I have very low airflow from one of the two registers in the master bedroom. Air temp is also 6 degrees higher than other register at other end of the room (68 deg vs. 62 deg). Went up into the attic and noticed that the duct run for that particular vent is probably 25-30 ft long! I have no idea why it is that long, but could that be the culprit? Can a duct run that long pick up that much heat and lose that much velocity (CFM)? It is regular 8" flex duct, r-30 isulated attic. I haven't checked for kinks or collapsed areas in duct yet. The run from the main trunk to the register in question is probably 15 ft max. If it is just too long is that something that I could easily fix myself?

Thanks for any suggestions.

[amickracing]

Your right with your guess's. Flex kills air flow (it can be designed to flow as much as hard pipe, but rarely is). Also the longer the run, the more chance for it to pick up heat in the attic.

1st I'd crawl back up there, check for any kinks or sharp bends and fix them. Also be sure the flex is pulled tight and in as straight of a line as possible.

If that doesn't work you might be better off to throw 10' of pipe up there to replace some of the flex.. leaving 5' on the end won't really hurt anything and that should get the air moving a bit faster and it won't transfer the heat it did before. But this hard pipe also needs to be insulated too.

Could you do it yourself? Most likely... but to keep some of the people around here at bay and not breathing fire... pretend you didn't hear that from me. But, on the other hand... if you pay someone to do it, then they get to crawl through the hot attic, get all full of insulation, and it's thier problem if they step through the sheetrock celing, they fix it, not you.

[BaldLoonie]

Bury that sucker into that R30 as deep as you can. Also can lay batts of R11 or R13 on top of the duct. Flex easily picks up attic heat.

[t527ed]

if you are saying you have 25' of duct for 15' run then shorten duct before you insulate.

[tinner73]

flex is garbage. unfortunatly, now you know why. bad for airflow and usually installed poorly.

[docholiday]

The flex isnt garbage the installation is.

Flex most often comes in 25' lengths. It sounds like they didnt want to take any back and it was the last run. As suggested, make sure the run is straight and tight. Odds are, the designer specified the right size flex and the installers either shortened the trunk for time and materials sake or simply used the whole length of flex.

See if that doesnt help your problem. Also make sure the flex is connected to an elbow at the register. If it is made to make that last turn 90 degrees down that also cuts the flow down considerably.

Your difference in upstairs downstairs can usually be cured by a pair of balancing damers in the two supply ducts. In summer the upstaires duct should be wide open and the downstairs adjusted slightly closed. This will take a couple of days to find the point. once your happy, mark the damper positions and tag them summer. Come winter, you will need to readjust them, slightly closing the upstairs and opening the downstairs. Get it even and mark those positions as winter and make sure to adjust them seasonally.

[tinner73]

sorry but i have to disagree with the good Dr. on this one.

flex is installed for one reason and one reason only, it's cheap.

it's bad for airflow.
it's cheap labor wise.
anyone with a knife and a strap gun can install it. poorly paid/fairly skilled installers and this is what you get. that is why you get 25' runs snaked through the attic. this is the result. how many times have we seen it on here?


again.... i probably go through more boxes of flex in a couple months than most here do in a year. we buy flex by the pallet per job. i know why it's used. because they don't want to pay me to hard pipe everything. COST IS THE ONLY REASON. we put no more than 5' at a time. these systems would never pass code around here.


if metal was cheaper to install to you think the flex systems would still be put in?


just wait until it deteriorates.


sorry.....but true.

[robnjr]

How long does it take to "deteriorate"???? I have a 15 year old home with flex (it was built that way, not my choice, I admit), and I see no signs of deterioration yet. And yes, I have inspected **inside** the ducts (OK, only really the first few feet at each end) and see no problems. No "deterioration" seen yet with the duct vapor barriers either (or radiant covering, whatever you call it) ....

BTW, I have one run of 10 inch flex that is 35 feet long, and temps are only 2 deg higher (with slightly less airflow) at that duct than some 8 inch ones that are less than 10 feet, .....but then again, most of my ducts are now buried in loose-fill cellulose, which helped ***a lot*** (as well as sealing all the leaks I found when I first moved in!!)....

[dash]

Ductboard and flex ,in homes with Hot Florida attics,installed in the 1970's,still in great condition.

Metal pipe,installed in the 1960's and 1970's,most didn't last 20 years,duct wrap falling off ,duct sweating,mold growing in ciling and walls from leakage,what a mess.


Air flow for flex versus metal is identical,if sized properly,and installed correctly.

[tinner73]

what took you so long?

[dash]

It hot and busier then usual,thanks for being cocerned!!LOL

[docholiday]

Tinner, as you and I both pointed out, its the installation of the flex but not the flex itself, I haven't yet found eteriorated flex when it is installed properly but for some that was not, yes you're right