Takeoffs at Plenum are Different Temps
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  1. #1
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    I just had my ancient HVAC unit replaced with a new Trane system (92% gas upflow/13 seer XR series AC). The new system is WAY better than the old (of course), BUT there does seem to be a problem.

    One branch is not giving out air that's as warm as the other branches in the house. At first I thought it was due to running through an unheated crawlspace (about 17' length), yet another, shorter branch (about 8')in the crawl was blowing air 10 higher at the register. Even this warmer of the two crawlspace branches isn't as warm as the branches that run through the basement, but the difference there is only about 3-4. That seems ok to me, but the one being 10-14 less than the rest of the house doesn't.

    The crawlspace branches are the only ones using flex ducting (6" R4.2), with the rest of the branches using the original sheet metal ducting in the basement except for the new takeoffs and such to connect new system to old ducts.

    Upon further investigation I noticed that the takeoff for the coolest, problem branch is substantially cooler to the touch than the other takeoffs right at the furnace plenum. The other two takeoffs are rectangular, probably 8x12, but the cooler one is 8" round.

    I did a temp measurement at the point that this same branch changes from the sheet metal duct to the flex, and it substantiates the fact that the air in that branch simply is cooler to begin with. (I stuck a thermometer up into the takeoff for the flex duct, into the main part of the branch.)

    It's obvious that if the air starts out cooler at the point it leaves the furnace, there's no way it can be the same temp (same being within 2-3) when it leaves the register vent. But how can the air leaving the plenum into the takeoffs be different if they are all coming out of the same plenum?

    I'm going to call the contractor back about this, but I wondered if someone could give me some idea why this may be happening? I don't want to have paid all this money and end up with one room of the house that is always colder than the rest.

  2. #2
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    Hard to imagine different temperatures at the plenum,after the air has passed thru the furnace and A-coil.

    Anyone have an answer?

    cakins,can you post a picture of the equipment and plenum??

  3. #3
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    I can take some digitals tonight after workd and post.

  4. #4
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    cakins

    I had replied yesterday with:

    2 possible reasons for colder air on the one duct.

    1. If the cooling coil base is leaking air around the coil where it attaches at the furnace, the colder unheated bypass air will go out the duct on that side. A thermometer reading just above the base on the downstream side will show that.

    2. Depending on coil configuration, if the system is not properly charges and the downstream refrigerant flow is on the side where the colder duct is, the air will not be as hot when leaving on said side.

    Hope this helps, makes it to the thread, and please post back as to what you find.
    tom

  5. #5
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    Request for reposting of answers to this question.

    Could those of you who gave me some great leads on this problem repost your answers that you had written yesterday? It seems the forum had a meltdown last night and lost all the replies - doh!

    The links to the photos of this install, once again, are:

    http://www.extraordinaryimage.com/images/plenum1.jpg
    http://www.extraordinaryimage.com/images/plenum2.jpg

    Regarding your reply, twcpipes, can you clarify about the reason for the downstream refrigerant and such. If my problem is with the heating (not cooling) how does the refrigerant or the systems charge affect it at all?

    Also, are you saying to take a thermometer reading on the outside of the a-coil box on the side that's cold? Not quite sure where to put my thermometer.

  6. #6
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    All of the following dimensions are guesses from looking at the pictures.
    That 8" goes to an 8" X 12" rect. duct, which then goes for how long, to how many 6" flex runs? Does the rect. duct go through the crawl? A cooler temp at a branch can be caused by low air flow(restriction from the flex or the 8"?) through that branch. If accessible, remove 1 or 2 flex ducts at takeoffs on trunk and see if it warms up. Also, remember that warmer trunk is what, 8X16? That trunk will have a lot more air flow and will definitely be warmer. But you're using 8" X 14" long and 1 8" X 90 to feed an 8X12-
    thats 50 sq. in. opening into 96 sq. in. rect.- there will be loss in pressure there that may be carried to the restriction at the takeoffs for the flex- maybe they could keep closer to the trunk(8X12) dimension for their plenum takeoff, which should raise the pressure in that branch.

    [Edited by rimek on 02-10-2005 at 07:55 PM]

  7. #7
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    cakins

    >>can you clarify about the reason for the downstream refrigerant and such. If my problem is with the heating (not cooling) how does the refrigerant or the systems charge affect it at all?

    Also, are you saying to take a thermometer reading on the outside of the a-coil box on the side that's cold<<<

    1. I don't know your coil configuration but I am saying it is possible the refrigerant is cooling off at the end of the coil if the coil configuration is made so the refrigerant is warmer from left to right as, say, a slant coil or coil that flows from left to right (entering/leaving refrigerant)and thus the refrigerant temperate is colder because the charge is not enough.

    2.Take a thermometer reading just downstream of the coil where the air just leaves the coil. If the temperature is lower than the rest of the supply air it is possible the air is going around the coil instead of through it and mixing with the warmer air. If it is bypassing air because the coil is not sealed, the discharge temp will be colder where it leaks. Did you seal the coil at the furnace when you set it so it could not bypass air or just "plunk" it down on top the furnace?

  8. #8
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    The velocity pressure at the top of the plenum will have a greater effect on the 8X12 takeoffs with the 12" side parallel to the plenum top than on the 8" round takeoff. Try an 8X14 for the long run now fed by the 8" round.

  9. #9
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    Well after 2 failed tries to edit my post- here's another.

    Has this part of the house always been cooler? I wonder if that 8" plenum takeoff isn't reducing cfm too much for the 8X12 rect. The reason I suggested removing 1 or more of the flex at the duct takeoffs was to determine if the greater restriction was the flex or the 8". How long are these flex runs and how many?

  10. #10
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    Update on Ducting Sizes

    My first post on this question included what I thought were the correct sizes of the various duct runs, but they weren't totally accurate. Tonight I measured them.

    Three branches come off the plenum.

    #1 is 7x12 and goes to feed the one vent in the LR and one of two bedrooms. It splits just before going through the basement wall into the crawl so that it ends up as a 4x12 branch on the crawlspace side. The old ducting design included a conversion to a 7" round, which now has a 7 to 6 reducer on it, with 6" flex to the new vent location.

    #2 is 7x16 and is completely within the basement. It splits into various runs, all the orig. hard ducting that feeds DR, LR, and kitchen

    #3 (the problem child) was originally also a 7x12 which goes through the basement wall, into the crawl, and feeds the other BR and bath. It's still a 7x12 for a few feet, but doesn't come off the plenum that way. For some reason when they put the new system in the installers changed that one into a 8" round (seems more like about 7 1/2 actually) as it comes out of the plenum, but connected it back into the bottom of the 7x12 ducting. This branch originally had two 7" takeoffs just after entering the crawl, which went to 7" hard round pipes into the vents. The BR vent wasn't in the proper place, though, and the bath is totally being re-done (to the studs), so my contractor suggested re-routing the BR run with 6" flex for ease of working in the crawl due to extreme space limitations.

    So, #3 run, the cool one, comes into the crawl as a 7x12, but has 2 6" takeoffs, each with 6" flex runs. One run goes about 17' to BR, and the other will go maybe 12-14' to the bath vent when the bath is put back together. Right now the flex just comes up through the hole in the bath floor.

    Not being an HVAC person, I had the system replaced by a contractor, but also needing to save some money, he let me do the actual labor in re-running the flex duct. I put it what he told me, though. Didn't do this on my own lest anyone thinks, "Well, that's the problem. A homeowner trying to do it himself."

    Anyway, as suggested in one of the replies, I already took a temp reading in the 7x12 duct prior to putting on the flex ducting and the temp in the 7x12 portion itself is cooler by a good 10 than the other branches. That's what led me back to the furnace to begin with. That temp reading told me that the flex had nothing to do with the lower temp at the vent. The air was not even getting to the flex at a similar temp to the rest of the system. And we're only talking about probably 4' from where the new 8" round connects to the 7x12 ducting to the takeoffs for the 6" flex. It's not far at all, with 2/3 of that being in the basement. So it's certainly not losing temp just getting to that point. It's definitely starting out cooler at the point of departure.

    Thanks for the leads so far. Any reason why the installers wouldn't have used rectangular connections on all the new transitions? They did on two of the three branches. But this one, they used the round to turn up into the 7x12 run.

    I've called my contractor back, and he's supposed to meet with me next week. His first reaction was that he didn't really know what could be causing such a thing. That worries me a bit, though he's one of two HVAC people on our Board of Trades here which certifies mechanical trades people of all types.

    Right now his only suggestion is to put a "scoop" on the side that's too cool to get more air into it. I'm not sure what that is, but does that sound reasonable?

    One of the posters from the other day whose answers were lost in the server failure said he had fixed problems like this a number of times. If you're reading this, could you re-post?

    Also, and this is it as I know this is long, what would be the best way to show all your tips to my contractor? I don't want to sound like I'm telling him his business, but since his first reaction was to be unsure, I'd like to at least give him everyone's suggestions. Would any of you take offense if a customer suggested how you might fix a problem?


  11. #11
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    Originally posted by rimek
    Well after 2 failed tries to edit my post- here's another.

    Has this part of the house always been cooler?
    I actually haven't lived in this house yet. I'm totally redoing it. As it's a one bath house and the bath is gutted it's hard to live there!

    The old system barely worked until it died completely just as it was turning colder (of course), so I can't really say.

    Also, one of the original posters to this question (pre-server failure) had posted a link to a great web page with info on where and how different types of ducting should be sealed. Any chance of getting that link again?


  12. #12
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    Are the 17' and 14' flex runs nice and straight? Could you have run 6" sheet metal instead? The flex has greater friction, especially if it is installed improperly, with sags, droops, using less than 3" wide strap, etc. Then you have that restricted start setup, Have you taken a temp in the 8", about 12" from furnace?
    Do you have a temp at the end of the "7X12"? Is that really 7 X 12, not 8X12? It looks like the 8" just fits the narrow dimension.
    Before, with the "7X12" dimension all the way to the supply would be good for 400 cfm@ .1" sp, (the 8", at .1"sp can move about 250 cfm) then depending on length of "7X12", the two 7" could move about 160 cfm @ .1" sp. If the 7X12 is really 8X12, flow goes up to about 500cfm @.1"sp.

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