HVAC length of pipe from return

I have a situation on an inspection I did today I need some help with. The homeowner stated that the air flow from the registers in some rooms is stronger than others. When I got to the house and went in the attic I discovered multiple problems with the ductwork; crimped piping, improperly installed support straps, branch distribution laying over the return pipe, etc. Overall the system is a mess and could have been arranged better. There are 2 returns going into the attic unit.

My question is in the ductwork from the main ceiling return going into the unit is about 35 ft long. I’ve never seen one this long before. What generally is the appropriate distance for this? I have attached a photo of the attic.

I have no idea. I’m not a system designer. Refer it out if you believe there is a problem.

You have all kinds of imporper bends in those ducts. Go to the Air Difussion council - they give you the standards for duct installation.

Like Larry said, get a local HVAC contractor out there to sort it out.

Guys there stands the problem local contractor was the one that made the mess to begin with do you really think he is going to say its wrong:roll:

No 35 feet is not an excessive run for a return air see them that long all of the time matter of fact I installed a longer return air run than that in my own house this last summer works just great.

Hubert correct me if wrong but are you saying you think the weaker outflow from some registers might be due to the long return?

Just wondering as I do not think it would effect some and not others.

It would just be weaker overall from my understanding.

Did you trace the weaker register ducting from the attic to see if it was bent?

Bob the problem lyes with the supply trunk not the return just look at it some of the suppy lines are not on a 45 degree angle from the trunk line thay are actually trying to reverse the direction of air flow with the take off branch. The size of the trunk appears to be maybe 14 inch (SWAG) to many take offs on one trunk it will over feed some branches and under feed others

Thanks Charlie as I noticed the bends ,but was wondering if Hubert was trying to factor the return as part of the problem.

I do not calculate either, but seem to recall (little off subject ) that it is 10 feet added for each bend, correct?

How much of a factor is the blower on the unit.Yes I see what you are saying though.

Ok then reccomend your client to send pictures to Charley for evaluation over internet with remedy as necessary. :mrgreen::mrgreen::mrgreen:

I just luv it when you get cute I am feeling generous today one of my compitors just started work at Lowes :Dsend me all of your pics

Yes I think he was trying to contribute lack of air flow to the return distance wrong approach on this one. I have made the comment many times that a system can and often is killed by the return air but don’t think it is the case here

The size of the blower is very important must move the proper CFM to maintain Duct static pressure don’t you mean 10 feet taken away for each bend not added

BED time for this old man hard day tomorrow

No I meant added , because if you deducted 10 feet for each bend , then bends would be more efficient

Yeah I am reminded of the return above the supply vent in that redneck DIY presentation.
Always gets a laugh when I do the homebuyer seminar…

Thanks for the responses everyone. I used an infrared camera to get a visual picture of the air flow from the registers in each room and then traced each room from the register back to the source. The lines toward the front of the picture had good flow, but the lines closest to the unit with had the bends had significantly less air flow. Some had next to none, as you barely see the register in the infrared photo. I concluded from this that the air was attempting to branch out but the obstructions (bends, crimps, etc) were forcing the air back down the supply line to the other rooms. Generally, the farther away you go with the supply, the lower the air flow, but this was just the opposite. And yes you are correct in that I wanted to know if length of the duct from the return to the unit was part of the problem and if I should cite it as well as a contributing factor to the problem. She had a HVAC tech look at it and he said it should be shorter, but she wanted an independent opinion. And I really did not know, but I had never seen one that long in an attic before. Most HVAC techs install the unit in the middle of the attic close to the return but this one was not done this way. I am siting the location of the unit as undesirable due to the fact you can’t get to the backside of the unit.


There is a second return also feeding into the unit from the garage bonus room. It is about 20 to 25 feet long. Is there a maximum length for flex duct here? Is this considered improper?

Each 90 degree elbow or bend adds as much resistance to air flow as 10 feet of straight pipe. The term used is “equivalent length”. Each boot/transition piece, hood, grille has an “equivalent length” number. e.g. some exhaust hoods with a simple swinging backdraft damper have the resistance or “equivalent length” of 60 feet of smooth straight pipe.

Using spiral round or flex duct adds about double the resistance to airflow as the same size of smooth galvanized pipe. e.g. If you need 6" pipe to supply/exhuast an area properly and you’re using a flex duct, increase the diameter by 1" to ensure adequate airflow. In other words, the 7" flex is like using 6" smooth.

Residential duct systems in general are some of the worst designed (non-designed!!) parts of houses. Studies have shown that from 20-40% of the air moving through the air handler/fan, does not get to or from the rooms it was designed for due to duck leaks.

Can you tell me the diameter of the main trunk line I can see at least 4 take offs on it in the pic how many taps on the supply plenum or was the whole home being fed from one trunk line

That is why, whenever I refer something to another professional, I recommend “qualified” professionals. :smiley:

I’ll try to lay out the diagram here. There are 2 return lines coming in. Sorry, but I did not measure them. Then from the unit there are 2 main trunk lines: 1 you see in the above photo and another that runs to the attic on the other side of the vaulted area. The first main trunk line (above) has 9 taps. The second trunk has 8 taps. Here are more photos.



Also, the house is 1816 sq ft and the unit is a 3 ton unit. I feel the system should have been installed with 2 zones, one for this attic and one for the other side.

The complaint is about differential airflow.
Is there a comfort issue here? Different amounts of air are required in different parts of the house due to load requirements.

Basically, you probably have a duct system that is not balanced with the appropriate dampers installed.

From what I’m gathering in your post, it appears that there are too many takeoffs to have sufficient air “throw”. The more takeoffs you have the lower the volume of air because you have more holes in the duct.

The length of the return should not be an issue if it’s properly sized. The static pressure of a return duct must be lower than on the supply side or adverse conditions will exist.

Properly sizing the supply air duct, using the friction loss method will require the Main supply trunk line to be reduced in size as air volume is removed at takeoffs, maintaining static pressure along the entire length of the supply trunk. This design ensures an even static pressure throughout the supply. They can’t stop there.

Balancing dampers must be installed in the shorter high volume takeoffs to restrict airflow to increase static pressure for the longer/crooked takeoffs.

These calculations are performed using a ductulator. As you remove CFM at takeoffs, you adjust the design friction pressure per 100 feet of duct to the remaining CFM scale. This indicates the new duct dimensions that the line should be reduced to.

[size=2]Remember, the HVAC unit is a sucking machine not a blowing machine! It’s about removing the air from the house, reconditioning the air to the proper temperature/humidity and putting it back in the right proportion at the right location.
It’s not about blowing hot/cold air into a cold/hot room.

Both the supply and return systems must be balanced and properly located.

Also remember that one air duct system cannot be efficient in both heating and cooling modes. It must be designed for one or the other.

The location of your supply and return grills are critical for comfort. If you have the return registers and the supply registers in the ceiling, air stratification will occur and short cycling of heat air will occur at the ceiling resulting in a cold floor (in the heating mode).

What is the actual complaint?