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Help! Duct work--flexible vs. rigid?

abelan's picture

I'm a newbie in need of quick help re: duct materials.  We are in the process of replacing our boiler with a gas-fired furnace in our 1100 sq. ft. bungalow.


We were hoping that the duct work that was installed for our A/C several years ago (by previous owners) would suffice for the furnace. All of the duct work coming off the air handler is the flexible duct material (flexiduct?). We have one contractor saying that we should really replace it with a rigid trunk and only use the flexiduct to get from the main trunk to the registers, and another contractor who says all houses now use flexiduct and that we needn't worry about the lack of a rigid trunk for a house this size.


<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />Contractor 1 wants $2200 to replace all the duct work. Contractor 2 wants $150 per length to replace any compromised flexiduct runs (we think there are 2 that need replacing).


Who is right? It's getting chilly and we're without heat, so we want to move fast but not stupidly.

(post #109699, reply #1 of 17)

A properly sized and configured air handling setup seem to combine rigid trunk lines and flex duct branches to each registers.

And there can be different size trunk and flex lines in one system.

(There is also the issue of metal vs. pressed fiber trunk lines....)

I would consider getting a second opinion if someone is telling you flex lines alone can be used. It is possible but probably less then ideal.

I do not speak as an installer but as a very satisfied homeowner who dealt with a reputable contractor.

(post #109699, reply #2 of 17)

It's becoming much more common to see flex lines but they normally take off from a rigid trunk. Your house is quite small so thare may be a way around that, depending on the location of the furnace plenum etc.

Flex do tend to sag and compress, inhibiting air flow after a time but initial cost is much less so when you are doingthis on bid basis, you are only going to see flex mentioned unless you spec rigid.

.

Excellence is its own reward!

 

 

Oh Well,

We did the best we could...

(post #109699, reply #3 of 17)

The HVAC contractor I have contacts with has said that flex duct is cheap and effective but not long lived or efficient. It can collapse. Is more subject to damage by anyone who goes into the attic and can necessitate the use of a larger blower motor to compensate for the extra turbulence it creates.


Duct board is a step up and insulated metal ducts a few steps better than that. As quality goes up so do costs. Some saving can be had by using better ducts only on central lines and a cheaper product on lines going to individual registers or even just the ends of runs. 


If mold or contamination is a concern be aware that neither the flex ducts or duct board units, according to my HVAC buddies, can be effectively cleaned. Insulated metal ducts can be cleaned. The flex ducts might be cheap enough to replace.

(post #109699, reply #4 of 17)

So far we are very pleased with the workmanship and results from the rigid ductboard trunk line setup (for A/C only).

Unbelieveably quiet, even with the air handler in the attic.

And there is no noticeable draft either, which is very nice.

We have wind socks the kids bring home from school taped under many of the ceiling registers for visible clues.

However, if the trunk lines ever have to be replaced, we are going to insist on insulted sheetmetal, which we asked for in the first place but was talked out of because we were told they would not be as quiet.

Can someone clarify this ?

Thanks !

Alan

(post #109699, reply #7 of 17)

I have heard, literally, metal ducts making noise. If not properly designed and built it can rumble and boom. The sound of the sheet steel vibrating like a drum or oil-canning can be quite loud. If properly built metal ducts should make no more noise than any other kind. A word of advice: before receiving a quote for metal ducts. Sit down. The difference can be substantial. Some of this can be retrieved in time by allowing the use of a smaller blower and lowering the total cost of ownership.

(post #109699, reply #8 of 17)

Yes, the installer also mentioned the much higher cost of metal duct.

Since his reputation is impeccable in our area I had no reason to resist his overall recommendation.

With my other thread on mouse traps, their fondness for flex duct is disconcerting....Fortunately the thread concerns the empty house next door, not the one we live in.

Alan

(post #109699, reply #10 of 17)

Alan,

"However, if the trunk lines ever have to be replaced, we are going to insist on insulted sheetmetal, which we asked for in the first place but was talked out of because we were told they would not be as quiet. Can someone clarify this?"

Sheet metal ducts that are sommonly used in residential installations is 8" deep, sometimes 10". At 8" deep. trunk lines closest to the air handler have to be pretty wide to move the air. Since they are closest to the unit, they are also usually sized at a greater pressure loss/foot than duct work farther down the line. A wide surface area, higher pressures make the duct "tink" when pressure is applied and relieved. The higher velocities can make it "roar". LINED sheet metal will absorb and/or dampen these noises. Lined sheet metal is very UNcommon in a residence. Duct board, is basically a rigid duct liner, with foil instead of galvanized sheet metal for an exterior.

(post #109699, reply #12 of 17)

Thank-you, Tim, for explaining the difference.

It appears noise, even when sized properly, will be louder then ductboard.

I do know our installer will size the trunk lines accordingly depending on the location and capacity of the system, which he has done so with the ductboard trunk lines.

It is possible new technology could emerge that is superior to both metal and ductboard.

I hope not to be dealing with a replacement/repair for at least five more years.

The system was installed (in a vented attic) for A/C only in 2000.

Alan

(post #109699, reply #13 of 17)

I recently had the opportunity to design a system that is "newer", at least to me. It is called "DuctSox" and is a fabric supply duct system. It was done in a "sound sensitive" area, and, not only does it produce little internal noise, it absorbs sound from the space, as well. It has its drawbacks, though. Cannot be used for return ducts and it is much more susceptable to damage and other soft material maladies, like mold growth potential. This was used in a pool room dehumidification system, so I expect no mold will grow.

(post #109699, reply #14 of 17)

There aren't many absolutes in this world...but I think Tim's post(24063.10) is a thing of absolute beauty. Nice one... Regards, Mongo


There are 10 kinds of people in this world; those who understand binary and those who do not.


(post #109699, reply #15 of 17)

Mongo,

Thanks for the words of encouragement. It is always nice to have positive feedback from knowledgeable folks.

Tim

(post #109699, reply #16 of 17)

"However, if the trunk lines ever have to be replaced, we are going to insist on insulted sheetmetal...."

How is "insulted" sheet metal different from a regular sheet metal?

Do they talk mean to it at the factory? Tell the metal it's stupid, and no better than flex duct ???



(Sorry - Couldn't resist)



What luck for rulers that men do not think. [Adolf Hitler]

(post #109699, reply #17 of 17)

Ok, Boss, you got me...

I do need to crawl around in the attic to see how the flex supply ducts are holding up.

(post #109699, reply #5 of 17)

Flexduct reduces air velocity some and can sag (making the situation worse) if not installed properly. 


If you have even the SLIGHTEST opportunity for rats or mice to get into your dwelling, they love to chew into and make nests in the stuff.


 

 

(post #109699, reply #6 of 17)

we're all saying that flex duct CAN SAG.

I move that we drop the pretenses and tact.

It WILL SAG!

.

Excellence is its own reward!

 

 

Oh Well,

We did the best we could...

(post #109699, reply #9 of 17)

AC,

"We were hoping that the duct work that was installed for our A/C several years ago (by previous owners) would suffice for the furnace."

IF, the ductwork was sized properly for cooling air flows, it will ceratinly be fine for heating. Most furnaces have four speeds, that are set by which "taps" are used at the furncae blower motor control. The highest speed is used for cooling and one of the other three are used for heating.

If you were starting from scratch, I would recommend against any flex duct in the returns and no more than 2 or 3 feet at connections to supply registers, and then only when necessary.

"...another contractor who says all houses now use flexiduct and that we needn't worry about the lack of a rigid trunk for a house this size."

This is true, but it is still a poor practice. The size of the house is a non-factor (Did the roofer do a poor job because it only 1100 sf?, I doubt it.) I would consider using that contractor only if you want more examples of poor practice in your house.

"We have one contractor saying that we should really replace it with a rigid trunk and only use the flexiduct to get from the main trunk to the registers.."

This one is only partially correct. You SHOULD replace as much of the flex as possible, but if you do this I would not use flex for ALL of the branch ducts, this is only a slightly better practice than using all flex ducts. If you are going to spend the money to gid rid of the flex, get rid of all of it.

If it were my house, I would give the existing duct work a try with the new system, unless you only have access to it now and you will not have access in the future. If the duct work is exposed and is not going to be enclosed immediately, it can't hurt to see how it works. And although it is not ideal (you didn't mention how well the AC worked, air distribution-wise), it should suffice.

Something else to consider, as is common with contractors who do poor work-like the one that installed this AC system, if they installed all in flex, they probably didn't treat the returns properly. An otherwise well done system can rendered ineffective by limited or inadequate returns.


Edited 10/18/2002 9:00:16 AM ET by Tim

(post #109699, reply #11 of 17)

Tim,


Thank you very much. Your answer addressed exactly what I needed to know. It helped me clarify my thinking and has me inclined to go with your advice to give the current duct system a try, while knowing that it is less than ideal and will need some attention in the future. But since we're looking at having to do asbestos abatement in conjunction with this job, it's an expense we can avoid right now.


Thanks to everyone for your thoughts and help!