One room not cooling...suspect plenum/duct design

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Old 01-11-14, 07:15 PM
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One room not cooling...suspect plenum/duct design

I just had this system installed recently. I have one room that is not heating well. I have attached pictures. Since i have talked to multiple techs at the company and am now completely certain they are totally incompetent, I need help. HEre is the setup:

The duct that is coming off of the END of the plenum feeds a room that is problematic. Its an 8' duct, about a 30 foot run. It was just moved here the other day by the tech to get air that is hotter. I now understand this is wrong. Two other techs also recommended this.

On the pic where you can see the Carrier Logo, the plenum closest to you is a 6" duct. This duct is 20 degrees colder than the hottest duct. Its a 30 foot run or so.

I have AC register temps that vary 20 degrees. Im just trying to get it balanced out and functioning as it should, but I get a LOT of different answers and I don't know what is RIGHT and what is pure opinion. Im reading somewhere that the plenum should have no ducts off the end and the end should be 18" from the last duct. All ducts should be off the top and sides. Please help me...im about to lose my mind over this system. IM dealing with techs that have been in the business for 30+ years, so Im more than disheartened about it.
 
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Old 01-11-14, 08:42 PM
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Are all supply lines coming off the plenum without a trunk line. For a 30 ft. run or for that matter almost any run they should come off a trunk line. Then the supply pipes with dampers installed to balance the system should come off the trunk.
With flex pipe any bends or sagging is going to restrict air flow.
I have about a 12 ft run on mine because of the system layout and I was even nervous about that but because of the dampers luckily it worked, but I can't even fathom a 30 ft run.

A word of caution. If your return line is too small it will also restrict you airflow.
You will only get as much heat out as you have return air coming back.
 
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Old 01-11-14, 08:43 PM
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Duct work is trash I rip it all out and start over. Flex duct should never be longer than about 12 ft. There should never be a branch line coming out the end of the main duct or branch lines coming out less than two feet from the end this is basic 101 duct installation. Good luck in getting your contractor to fix it FYI there should not be more than a degree difference for any room on same floor
 
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Old 01-12-14, 12:17 AM
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With all due respect.....I can't really get bogged down in a 'flex duct sucks' argument. Its what 90% of the companies in Houston use, most exclusively. Its used in the ghetto, its used in multi million dollar homes. I don't know what to say about it other than most companies know how to utilize and calculate frictional loss for it properly. Is it wrong? I dunno...who cares.

There are no trunk lines. This unit is behind a kneewall and serves 1500 square feet or so. One large room like a living area and a bedroom. The bedroom is the problem.

Im more concerned with the plenum and what needs to be done with the current flex setup. I can tell you right now that getting them to fix that as it is will be a HUGE battle. There is no 'rip it out and start over'. I know that. You know that. There might not be a 'redo the plenum' either, but I need to know where to start. "rip it out and start over" is not a viable option. It is almost certain that I will need to tell them what to do though.

Also, unit is behind a knee wall and all supplies must go through a short cathedral ceiling with 2x12 joists spaced 12" OC into the actual attic before going to their registers.

The Returns are good because I caught them at install and forced an additional return to be installed. Didnt get to this though.
 

Last edited by agdodge4x4; 01-12-14 at 12:44 AM.
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Old 01-12-14, 10:33 AM
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Sorry with what you got there and 30ft flex lines you can't make it right
 
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Old 01-12-14, 11:20 AM
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OK, yikes. I personally believe flex is fine as long as a contractor is willing to put forth some effort to size it properly and calculate, many pros feel the same. My 5 ton system is 100% flex aside from zone plenums, but it was sized properly and I don't have these issues with it. Most homes I have seen are like this. Hard duct is rare to find in homes around here unless they are old.

So, I have a fix of 'rip it out and start over'. Wish me luck with my contractor.
 

Last edited by agdodge4x4; 01-12-14 at 12:46 PM.
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Old 01-12-14, 02:14 PM
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I don't have a problem with flex as that's what I put in my house. As you said though, it must be sized right. The problem I see with yours is everything being supplied off the plenum and the long runs with no dampers to control the air flow. Is that plenum large enough to supply all the lines.

With a trunk line you use air at various outlets along the line and then you decrease the size a you go for that funneling effect. You've got everything coming out at the same time and at full volume. You're probably getting some lines that really blow and others not so much. This is where the dampers would help.
 
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Old 01-12-14, 03:00 PM
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They are going to install dampers on every branch duct. To answer your question, the plenum supports all of the branch ducts. However, I do NOT think its technically 'big enough'.

If I understand correctly, the end of the plenum should have no ducts coming OUT of it, and it should be 18" from the last duct to allow for pressure to build from the end up to the discharge. If that is true, then yes the plenum is big enough for ducts...but not big enough to be technically correct.

I do have some registers that put out a LITTLE more air than others, but its honestly not noticeable. Dampers will fix that, but I don't know that it will fix temperature variance. Maybe.

The duct off the end of the plenum has a damper....its the only one with a damper...lol.
 
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Old 01-12-14, 09:09 PM
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When you say temp difference are we talking room temp or air temp right from the duct.
If its air temp from the duct is there a way you can increase the outlet air temp before the blower comes on.

Back in the day as they say they used a mechanical control called a fan/limit relay that was temp adjustable. Today they use timers and your at their mercy. The problem is that in the minute or so it takes the blower to come on the air might not be hot enough to be comfortable and that is a common problem.

Your body temp is around 98 so the air has to be warmer than that or it feels cool to the body. Once the blower comes on there has to be a balance between the supply and return to give the return air a chance to pass by the heat exchanger to get reheated. If it goes by to fast it will not heat up and will be uncomfortable.

Also is your blower speed is too high it will move the too fast to get heated.
Did he turn up the blower speed to get the air to travel that 30 ft.

Slow speed for heat. Higher speed for AC. is the general rule. That's why they have multi speed blowers.

About the duck coming off the end of the plenum, that is a definite NO NO for the reason you described. The plenum has to pressurize and needs a closed box to do that.

Air like anything else will take the path of least resistance. With the top open there's less pressure to force the air out through the other ducts.
 
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Old 01-13-14, 09:24 AM
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Was a heat load ever done?
 
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Old 01-13-14, 02:31 PM
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When you say temp difference are we talking room temp or air temp right from the duct.
Both. But generally, im referring to the fact that when the heater is running, there is a 20 degree difference between certain ducts and others.

As for the end of the plenum, what is the difference if you put a duct off the END of teh plenum, as in my case, or out of the TOP of the plenum? I don't understand. For whatever reason, when he put it on the END of teh plenum, the air flow stayed about the same as before for all ducts but that one duct had a warmer temperature, which is good. So, Im curious why its bad, when in practice, it had more than one benefit.

Was a heat load done? I have no idea. There was a guy out before install that inspected the area, the insulation, and I was told he takes all that back and designs the system. If it wasnt done, then they got really lucky because the system runs in the summer time without any issue, and it maintains 40-50% humidity without any problem. Either way, it does not short cycle or run absolutely continuously.
 
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Old 01-13-14, 08:17 PM
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There is nothing wrong with a takeoff at the end.
I misunderstood, I thought you meant the top.
 
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Old 01-14-14, 10:24 AM
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Wait....ive read and been told by a LOT of people ducts that are taken off of the 'end' of the plenum are a no-no. The pictures below show what was done. That pipe is the emergency drain so you can see the orientation. Those pics represent the same plenum. One looking at it from the air handler, and the other is looking at the end of the plenum back toward the air handler. I have ducts off the sides, top, and 'end' of that plenum. Are you saying this is all OK? Im really really confused now about the supply plenum on my unit and where ducts are OK and NOT OK to be.

Can you elaborate? On this setup there ALL are 8" ducts except for 2.

Why can't you put a duct off the 'end' if all branch ducts have a damper for proper balancing?

Also, to better balance this system, couldnt I simply ra-arrange the ducts? Short ducts near the air handler where teh air is cooler and not as strong, longer ducts near the end where the pressure is greater and air is hotter?
 

Last edited by agdodge4x4; 01-14-14 at 11:05 AM.
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Old 01-15-14, 03:35 AM
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Running a branch duct from the end of a trunk being wrong is a persistent myth. It comes about from thinking that the velocity of the air traveling the length of the trunk will somehow translate to a higher CFM flow out the end than it will out of side-connected branches. The truth is that installing a balancing damper on ALL branches totally eliminates any velocity issues.

Each section of ductwork will operated at a lower pressure than the section preceding it. Velocity is a function of the volume of air, the pressure and the cross-sectional area of the duct and this is why trunks are often tapered as they run from the furnace plenum to the outer end. As air is removed (through the branch ducts) the pressure and velocity of the remaining air is reduced. You MUST have a pressure differential to create air movement. By decreasing the duct size with lesser amounts of air you retain the pressure needed to force the air through the branches. Using balancing dampers can serve the same purpose and is far easier than trying to calculate all the different duct changes that would be necessary with a decreasing volume system.

You need a certain velocity in the ducts to move the heated or cooled air along without losing or gaining heat along the length of the duct. Too high a velocity leads to noise and too low a velocity leads to heat loss/gain along the duct AND poor mixing with the room air once it exits the register into the room. It is ALL a big balancing act and one of the purposes of a heat loss / heat gain calculation is determining just what total flow of conditioned air is necessary to properly heat and/or cool each individual room AND the entire house. Once it is known how much air (at a specified temperature) is needed then the number of duct runs and the sizing of the ducts takes place. These same types of calculations are also necessary when designing the return air ducts.

As you can see it CAN get quite involved when done correctly. Unfortunately residential systems are often NOT designed correctly and even when they are they are often not installed correctly. The result is a heating/cooling system that sort of works but has "issues" that can be anything from a slight nuisance to quite irritating.
 
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Old 01-15-14, 12:45 PM
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The issue I am going to have, even if I get the air flow balanced, which I can do, is that the temperature differential between the ducts will be pretty big.

FV4CN5002T00

It seems that there is a potential for the plenum I have to be bigger than the duct flange on teh air handler. Can anyone verify the dimensions of the duct flange? I don't see it in the manual so I guess I need a service manual of some sort. The plenum dimensions are16"x19"x35".
 
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Old 01-15-14, 10:05 PM
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The issue I am going to have, even if I get the air flow balanced, which I can do, is that the temperature differential between the ducts will be pretty big.
With properly sized ducts the temperature difference between ducts should be minimal. Of course if you have an eight inch duct where it should be only a four inch would mean that the velocity is so low (from closing down the balancing damper severely) that the air will cool (or heat) in the duct run before getting to the room. My example is, of course, extreme as that represents a four-fold reduction in cross sectional area.

The plenum is sized to fit the required ductwork, there is no fixed relationship between the size of the plenum and the outlet of the furnace.
 
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Old 01-16-14, 07:19 AM
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Yes, the plenum is sized to fit the ductwork but the size of the plenum in relation to the size of the duct flange makes all teh difference in air temperature for a duct when its set behind the main flow of air. Itll never be as hot.



My setup is like C.

See the duct on teh left, closest to the air handler in C? Thats a COLD duct. It will ALWAYS be a cold duct and there i no amount of ducting that will correct that so far as I can see unless its the only duct on teh plenum.

I think we can all agree this has almost nothing to do with ducts and almost everything to do with plenum design.

Ill probably end up fixing this myself. I have had 4 techs out over the course of 2 months, and now they won't even return calls.

Teh 30' 6" duct IS a problem though. That should have been an 8", and it happened to be one that was thrown in after the fact. However, its over an air return and I don't even think it needs to be there, so Ill probably close it off completely with the damper. It doesnt produce much air (because its small) and it doesnt have hot air (because its small and in the wrong spot on the plenum). Its useless.
 
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Old 02-04-14, 11:11 AM
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Just have all the ducts moved to the side of the plenum. Having them run off the end of the line screws up the static pressure created by the unit. Ducts run off the top won't get as much air flow as if they were run on the side aswell. I not only installed and fabricated duct systems for 12 years but I did it in Houston. And I'ves installed both flex and hard duct systems in attics.
 
 

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