Cost of duct work

[Texas Proud]

To answer the latest questions...

Furnace is in the attic above the second floor..

Return is a 20X25 and a 12X24... or 788 sq. in.

We have 21 vents... most look about 8X12... or 2016 sq. in.. let's round that down to 1900 with the smaller bathrooms...

The fan is sucking in the filter big time... both people have said it is starving for air...

The run in question is going all the way across the attic... say 20 plus feet... you can easily stand in my attic, so there is no space issue.. but, all 3 ducts are exposed to the attic air... so in my thinking I am losing some heat just because of this... I think he said a 16" line... (I think that I might have been wrong... they might be 8"... will have to check)...

Are vents are in the ceilings here... not the floor or walls.. the one guy even said that some older installers say that how they are installed are correct... the one guy said that people are surprised how much better a system works when they turn them around... the cost of doing this is minimal.....




Ducts are not metal.... they are flex...


There are 3 that I know are kinked... the return air and two that are going where the return air is located... they were just laid down and bent down a chase... no support or solid elbow...

 

[ERD50]

Good comments from samclem (as always). Yes, the ratio of return to supply doesn't tell the whole story, I was just trying to get some scale of comparison to our house.

The fan is sucking in the filter big time... both people have said it is starving for air...

Hmmm, there should be some more objective measurement of this. I think the pros use a water-column measurement device (manometer).

It could also be that the fan CFM is larger than what you really need, so it is high relative to the ducts, rather than the ducts being too small. I know I've seem some calculation for this on the web somewhere.

The run in question is going all the way across the attic... say 20 plus feet... you can easily stand in my attic, so there is no space issue.. but, all 3 ducts are exposed to the attic air... so in my thinking I am losing some heat just because of this...

From your earlier post, I take it these are the three lines that supply the downstairs? By 'exposed to the attic', do you mean they are not insulated? That could sure help explain at least pert of the reason the downstairs is colder than upstairs.

Are vents are in the ceilings here... not the floor or walls..

OK, so all the supply vents are in the ceiling? And returns are as described previously - 20X25 at the top of the stairs and a 12X20 downstairs in the ceiling.

I guess I could see how downstairs, the air is flowing from ceiling to mid-wall height, and not really getting the room warmed up that much.

samclem makes a good point about the doors and airflow back to the returns. Often, carpeting gets installed and there isn't good airflow under the door. But having the doors closed, with a good gap at the bottom, would help force the hot air down to that gap, and then up to the return. With the doors open, the air will tend to travel along the ceiling, not really warming the whole room.

In general, you want the heat to enter at the floor, blow across the floor (we use deflectors in our house t help with that), and then it flows up to warm the room as it flows. You are probably optimized for cooling, which would be opposite. I've seen installs where there are two louvered vents in the wall in the same supply duct, one at the top, one at the bottom. Open the top in summer, open the bottom in winter.

There are 3 that I know are kinked... the return air and two that are going where the return air is located... they were just laid down and bent down a chase... no support or solid elbow...

Well, if the return ducts are kinked, that sure is a part of your apparent 'starved for air' condition. Can you tell if these are the upstairs or downstairs returns? It sure sounds like this needs to fixed/bypassed. I think I'd get this fixed first and then see how everything acts to avoid unnecessary work.

-ERD50

 

[Hermit]

... the furnace is sucking the air filters strongly.... the guy even said he had not seen one being pulled like this... that is why they are suggesting either one larger return in our game room or three smaller ones in the bedrooms... This only helps on the upstairs... we will have to see what they can do for downstairs...

Why don't you try running without the filter? When my filters get dirty, I start getting the temp diffs you are talking about. If that makes the problem go away, then you could possibly work on putting a larger filter in the vent you have at an angle or find a less restrictive filter.

 

[Texas Proud]

Thanks for all the help....

With them I have been able to make some observations...

The ducts are getting old and the foil is starting to peel on some of them... the last guy said I did not 'need' to replace them all, but it will only get worse..


I have a quote with R-6... but think that it would be better with R-8... comments

As to the comments about return air... all 3 guys have looked at the unit I have (5 ton AC with the right sized blower).... and all have looked at how big the duct work is from the two returns.... all have said that it is not enough... seems to me to be an easy fix to add a return and fix this problem... not much cost...


ERD50... yes, three lines that supply most of the downstairs (are master bedroom is supplied from a separate line and gets plenty of air)... they are 10 or 12 inch lines... they are insulated, but I do not know what R value they have... they run across the attic, one for the kitchen and dining room...another for the living room and breakfast room and a third for the wash room, small toilet and two more in the large living room...

 

[samclem]

Comments:

The ducts are getting old and the foil is starting to peel on some of them... the last guy said I did not 'need' to replace them all, but it will only get worse..

I have a quote with R-6... but think that it would be better with R-8... comments

I'm just not a fan of the slinky duct. If you can get real metal ducts or at least "ductboard" ones at a reasonable price (i.e. in places with easy access, esp the bigger ducts) I think it would be a smart move. It has less resistance to air flow, it can be cleaned on the inside (if the ductboard is the right kind), and it doesn't sag, kink, collapse.

Insulation: It seldom makes sense to insulate any ducts that are in conditioned spaces. A >great< way to do the ducts in the attic, if it is practical in your case, is to lay them close to the floor and cover them with loose-fill or batt insulation. Just 6 inches heaped on top of the duct = R 18+ at very low cost in materias. And, if there's little insulation between the ducts and the room below, but lots on top, then the air in the ducts will not be nearly as hot in the summer/cold in the winter when the system turns on, which improves comfort. The installers may not like this idea--they want to get the ducts in and scoot as quickly as possible without raking insulation around or shooting more in/hauling batts up there, etc.

I'd probably go with at least R-8 on the ducts just to assure there's no condensation/dripping on the outside during your hot, damp Houston summers. But I'd want to have quite a bit more.

Which ever way you go, the installers should be confident that you know what to look for and will be going up in the attic to see that all the connections are tight, that there's mastic where there should be, etc.

As to the comments about return air... all 3 guys have looked at the unit I have (5 ton AC with the right sized blower).... and all have looked at how big the duct work is from the two returns.... all have said that it is not enough... seems to me to be an easy fix to add a return and fix this problem... not much cost...

Sounds reasonable. But take a look at the actual size of the last section of return duct that goes into your air handler, and the areas "upstream" of that. There's no point in paying for a bunch of return ductwork/grills if they exceed the size of that final plenum. Obviously there should be some allowances for losses/constrictions through the filters, etc, but you don't want to pay for a new superhighway that really leads to a one-lane road right before your air handler.

 

[Texas Proud]

Comments:

I'm just not a fan of the slinky duct. If you can get real metal ducts or at least "ductboard" ones at a reasonable price (i.e. in places with easy access, esp the bigger ducts) I think it would be a smart move. It has less resistance to air flow, it can be cleaned on the inside (if the ductboard is the right kind), and it doesn't sag, kink, collapse.

Insulation: It seldom makes sense to insulate any ducts that are in conditioned spaces. A >great< way to do the ducts in the attic, if it is practical in your case, is to lay them close to the floor and cover them with loose-fill or batt insulation. Just 6 inches heaped on top of the duct = R 18+ at very low cost in materias. And, if there's little insulation between the ducts and the room below, but lots on top, then the air in the ducts will not be nearly as hot in the summer/cold in the winter when the system turns on, which improves comfort. The installers may not like this idea--they want to get the ducts in and scoot as quickly as possible without raking insulation around or shooting more in/hauling batts up there, etc.

I'd probably go with at least R-8 on the ducts just to assure there's no condensation/dripping on the outside during your hot, damp Houston summers. But I'd want to have quite a bit more.

Which ever way you go, the installers should be confident that you know what to look for and will be going up in the attic to see that all the connections are tight, that there's mastic where there should be, etc.



Sounds reasonable. But take a look at the actual size of the last section of return duct that goes into your air handler, and the areas "upstream" of that. There's no point in paying for a bunch of return ductwork/grills if they exceed the size of that final plenum. Obviously there should be some allowances for losses/constrictions through the filters, etc, but you don't want to pay for a new superhighway that really leads to a one-lane road right before your air handler.

Thanks for all the comments... very good indeed....


As for the plenum... I am getting a new one... they will be adding a whole house filter... it is supposed to allow better air flow...

 

[meierlde]

Thanks for all the comments... very good indeed....


As for the plenum... I am getting a new one... they will be adding a whole house filter... it is supposed to allow better air flow...

In particular if the area of the whole house filter is larger than the sum of the areas of the return grates it should help since you do take the filters out of the return grates as part of the change.

 

[samclem]

As for the plenum... I am getting a new one... they will be adding a whole house filter... it is supposed to allow better air flow...

Sounds good. The thick "media filters" do a good job and only need replacing every 6 months or so. And this will give you a chance to ask the installers how big this filter element should/will be. Their answer will be interesting. The largest commonly available filter element is 20" x 25" = 500 sq inches. You already have 740 sq inches of return duct area--if the ducts are the same size as the grills (they may not be). You can see where this is going: if they install this 500 sq inch filter element and you already have 740 sq inches of duct leading to that bottleneck (with its own filter element, adding to the restriction), then adding even more return air ducts before that filter won't improve your system's overall performance. Now, maybe adding a separate new return duct before the filter solves a local problem (distribution, etc), but that would only be the case if you've already got enough supply air to the "trouble spot" and insufficient return air (under doors, through halls, etc). That can happen, but it's not very common if any thought was put into the design of the system in the first place.

Sorry for the long reply.

 

[Texas Proud]

Sounds good. The thick "media filters" do a good job and only need replacing every 6 months or so. And this will give you a chance to ask the installers how big this filter element should/will be. Their answer will be interesting. The largest commonly available filter element is 20" x 25" = 500 sq inches. You already have 740 sq inches of return duct area--if the ducts are the same size as the grills (they may not be). You can see where this is going: if they install this 500 sq inch filter element and you already have 740 sq inches of duct leading to that bottleneck (with its own filter element, adding to the restriction), then adding even more return air ducts before that filter won't improve your system's overall performance. Now, maybe adding a separate new return duct before the filter solves a local problem (distribution, etc), but that would only be the case if you've already got enough supply air to the "trouble spot" and insufficient return air (under doors, through halls, etc). That can happen, but it's not very common if any thought was put into the design of the system in the first place.

Sorry for the long reply.

Not a problem with long replies...


From what I was told, and am also reading, the whole house filter is less restrictive...

But, thinking about it.... my return air is NOT what I said... I have a 16" round duct and a 10" round duct.... or about 280 return area.... the filter size should not be the determining factor....

This also means that my supply side is calculated wrong as I would have to figure the duct size instead of the register size.... a 4" line can only supply so much no matter what size vent you have....


So, if they add a 25x25 whole house filter (just guessing at this point, I will ask what it is), I can add another 16" return without a restriction....

 

[eytonxav]

We do not have any dampers in the ducts... we have adjusted the outlets in the rooms to try and help as much as we can... still have airflow problems...


At this time, we are only looking at replacing and upgrading what can be done without tearing up walls... I do not want to go to that level....

Its unusual not to have dampers just off the distribution box for balancing purposes. You might want to try installing dampers first, as it is far better than trying to balance with the room dampers.

That said, if you have flexible ducts, the insulation they use for ducting today is far superior to what was used many years ago, so this might be a good upgrade, as would having a return in each room.

 

[samclem]

From what I was told, and am also reading, the whole house filter is less restrictive...

The thick, pleated media filters (usually 4" or more thick) last a long time and filter the air well and can be less restrictive than a "flat" filter, but they still restrict airflow appreciably compared to having nothing there. So, there's not much to be gained by having return ducts feeding it that are any bigger than the filter size itself (unless the ducts are >very< long or have a lot of losses for other reason--elbows, etc).

So, if they add a 25x25 whole house filter (just guessing at this point, I will ask what it is), I can add another 16" return without a restriction....

The largest "stock" 4" media filter I know of is 24" x 24", but the biggest one I can easily get at HD or Lowe's near me is 20" x 25". But, as you say, whether the filter is 24" x 24" or 20" x 25", you still might benefit from additional return air ducting size since you haven't reached the limit of the filter size yet.

 

[Live And Learn]

Sorry - didn't read the entire thread in detail, but I have to have my ductwork replaced. 1500 sq ft house with 9 vents. Two quotes - one for $2,000 one for $3,500. Saw your post about $200 - $300 per vent. I knew the second quote was nuts but you just proved it !

 

[Texas Proud]

OK, the day is here when I am doing the change...


I was surprised at the 3 quotes... two were very expensive... one over $8K and one over $7K.... the one that seemed the most knowledgeable and the one that did more work to see what I had already came in just under $4K...


I took the low bid... they are certified by Trane and have other certifications that the two high bids did not... they also were telling me some of the positives and negatives of the various decisions which the others were not...

We are getting a zoned system, a whole house filter and new ducts...

I opted out of the wi-fi thermostats as they were going to cost me an additional $175.... not worth it at that price...

 

[Bestwifeever]

Thanks for posting this discussion. We have an old house and never know when we might find threads like this helpful!

The low-range costs seem reasonable, TP, and you have certainly done your research!

 

[samclem]

.... the one that seemed the most knowledgeable and the one that did more work to see what I had already came in just under $4K...


I took the low bid... they are certified by Trane and have other certifications that the two high bids did not... they also were telling me some of the positives and negatives of the various decisions which the others were not...

We are getting a zoned system, a whole house filter and new ducts...

Sounds like a good choice of contractor. Hopefully the extensive work (and $$) will fix the problem. "Zoned" = multiple air handling units? Sounds like you are getting a good price. Probably not a big deal where you are, but where the weather gets really cold having another source of heat in the house can be a very nice thing.

 

[H2ODude]

Comments:

I'm just not a fan of the slinky duct. If you can get real metal ducts or at least "ductboard" ones at a reasonable price (i.e. in places with easy access, esp the bigger ducts) I think it would be a smart move. It has less resistance to air flow, it can be cleaned on the inside (if the ductboard is the right kind), and it doesn't sag, kink, collapse.

Ooops. That was samclem's paragraph above. Not sure how I fouled that up.


Agree entirely. Built a house in FL in 87, thought I'd be there a long time and used sheet metal, compared to the flex plastic a real PIA. Now in a house 20 YO and hate the way the flex ducts look, especially under the house in crawl space. In FL I was convinced that attic temps would make the flex plastic crumble in 10-20 years.

 

[Texas Proud]

Sounds like a good choice of contractor. Hopefully the extensive work (and $$) will fix the problem. "Zoned" = multiple air handling units? Sounds like you are getting a good price. Probably not a big deal where you are, but where the weather gets really cold having another source of heat in the house can be a very nice thing.

Nope, zoned is electronic dampers that close off either the upper or lower floor... the cost of putting in two complete systems was not worth it to me...

 

[Texas Proud]

Comments:

I'm just not a fan of the slinky duct. If you can get real metal ducts or at least "ductboard" ones at a reasonable price (i.e. in places with easy access, esp the bigger ducts) I think it would be a smart move. It has less resistance to air flow, it can be cleaned on the inside (if the ductboard is the right kind), and it doesn't sag, kink, collapse.

Ooops. That was samclem's paragraph above. Not sure how I fouled that up.


Agree entirely. Built a house in FL in 87, thought I'd be there a long time and used sheet metal, compared to the flex plastic a real PIA. Now in a house 20 YO and hate the way the flex ducts look, especially under the house in crawl space. In FL I was convinced that attic temps would make the flex plastic crumble in 10-20 years.

I didn't get into the cost of sheet metal ducts.... I could be wrong, but from what I have read they cost a lot more money than flex...

Also, my attic is not set up for sheet metal ducts.... not a lot of straight runs... in the house where I grew up, the main duct ran down the center of the house and the side ducks came off of them at a 90 degree... it was a one story....

 

[meierlde]

Nope, zoned is electronic dampers that close off either the upper or lower floor... the cost of putting in two complete systems was not worth it to me...

Will the fan run at different speeds when 1 or both dampers are open. I.E. if only one is open will it be a gale in the house>

 

[Texas Proud]

Will the fan run at different speeds when 1 or both dampers are open. I.E. if only one is open will it be a gale in the house>

Not at different speeds.... I think I will go for one of those when I have to replace the AC system...

It is a bit more breezy, but they have to run a loop from the supply side to the return side... or do some other thing to get the air out...

From what the guy said, I have 1200 CFM of ducts for the downstairs and 800 for the upstairs... my system runs 2000 CFM (not sure of this number though)... so, he said it needs someplace to take that extra CFM... sometimes they just have an open duct that works when either of them are on...


The good thing I can tell with one hour of running is that the temp is the exact same upstairs and downstairs...

 

[Texas Proud]

Decided to give an update since we have had the system in a few days...

So far, it is doing exactly what I was hoping.... the temp upstairs and downstairs have been the exact same if I want... today it was nice outside so I did not run our heater in the morning... the upstairs got to 75 and the downstairs was 72... but I could easily make them the same if I wanted...


My thermostats have a fan mode that will run at random times to make things the same... I am now trying to see if that is something I want to do...


But, so far, it is good.....