Replacing wide ducts with narrow ovals

don_185

Hey now,that bule filter become very efficent once it gets
loaded.LOL.

D107

To reclaim some attic floor space

contractor plans to replace all 6 supply ducts --see attached photos--with ovals placed under attic floorboards in between (parallel) to the 5.5" deep stud bays. Says that re-making the manifold in photo #4434 with a 'rounder shape' will increase static pressure, so air flow will not be diminished.

As you can see from photos there will probably be at least one or two ducts that at least in part have to be above the floor since they have to cross at right angles to get to the supply register. The current ducts vary in size from 6-8" --inside the visible insulation--but they will all be same size oval.

Unit is 15yr old Bryant 3-ton FB4ANA036 air handler. Square footage: Main Floor: 743, 2nd floor: 535 = 1278 Total. + 100 for closets = 1400. But not counting two bathrooms or porch which have no registers and are usually kept closed = 1100 sq ft. 1924 colonial has blown in cellulose, and a very hot unvented attic until we can spray foam. Last year we had no trouble keeping house 76 deg on hottest days.

Contractor also wants to widen existing return register. Currently 12 x 18, should be 24 x 18 but with pre-filter maybe 24x24. We hope to adapt an HRV or ERV to this system as well.

Does this sound right?

Thanks,

David

Dave_4

Let's See....

"The current ducts vary in size from 6-8" --inside the visible insulation--but they will all be same size oval."

If the ducts are carrying different airflows why would the ducts be the same size? He asked rhetorically. What is the actual oval duct size (and round equivalent) that will be used, David? If all are equal to the larger size- an 8 inch- then may not be so bad.

The way I see it and absent any measurements, the 3-ton unit should be pushing out about 1,200 cfm which seems more than reasonable for that size house and condition.

Each duct should be, if equally distributed, 8" round at 200 CFM. If the oval ducts will be hard ducted, they will work better at a given size than the flexible duct will, at least. So maybe the ductwork change is a wash in terms of performance. Oval duct tends to be more expensive in my experience; why not rectantular and get the benefit of greater area? If the oval is stock material and the rectangular is custom, that may be the answer right there.

I am a big fan of return filter grilles and the larger the better within reason. Promotes ready service (makes neglect more difficult).

Random Drive By Thinking

Brad

D107

Thanks, Brad. Well.....

The maximum space between attic floor and bedroom ceiling lathing below is about 5.5", so I'd assume max duct width would be 5" or maybe less since it should be insulated.

Yes, makes perfect sense to go rectangular for the extra space. If the depth is 5.5" then width must be at least 14" (assuming 16" oc beams). Don't know if that width rectangular duct could be found, but re: price, I'm not going to use that much. But if 8" round equiv, I guess dampers at the manifolds could be used to specify the flow to each room (and tweaking at the room registers as well.)

The return grill currently has no filter. I believe they'll put in an electrostatic one --in addition to the inline folding hepa filter that I haven't touched in 7 years. Does going from 18 x 24 return grill size for three ton to 24 x 24 make sense to compensate for the new return grill filter?

Hey if I get a new modcon and abandon the chimney as a flue perhaps I can run another duct down to the basement (500 sq ft) to cool that. Could 3 tons cover that extra load?

The other thought is to rent out the attic area for the cyberworld remake of 'The Crawling Eye.'

Thanks as always,

David

Brad White

David, I would figure

ducts at 4" sheetmetal dimension may be your best bet. That will allow at least nominal insulation but sealing must be scrupulous to prevent condensation.

For 8" round equivalent, a duct at 4" depth would be about 16 inches. I would not go flatter (4:1 aspect ratio) and that would not likely fit in between the joists anyway. If the biggest duct you can fit is a 4x12 allowing for insulation, that duct would perform roughly the same as a 7" round and will handle about 150 cfm comfortably.

Most "off the shelf" rectangular ductwork comes in 8" depth. Beyond that and nominal 3-1/4" ducting, is custom anyway.

FYI: The stock "3-1/4" x 10" duct is the equivalent of a 6 inch round by the way. Point of reference.

Without calculating your cooling load, I cannot say if the extra load will be absorbed on the worst day but I suppose an 8" round duct or whatever you could fit, will not kill the system. You could always damper it off if it is a problem. If you do that, the load on the upstairs ducts will be that much less. Say you take about 150 CFM down to the basement (low cooling load probably but will help dehumidify), that will allow the remaining six ducts at say 12x4 size to handle about 175 cfm each. Pushing it but not bad.

My overall thought is, to get the best distribution and your duct sizes are getting pinched, add more of them if you can.

don_185

Noise

Lets not forget when you increase the pressure you also increase the velocity..fpm thru your duct as well.

And please whatever you do not let them installed that electrostatic filter.I've found more doa compressor because of them.

D107

Would noise be heard in registers in

bedroom ceilings below? I forgot to mention that for the first floor, once the 3 attic ducts do 90 deg turns downstairs, the air goes through a circular metal duct (currently uninsulated) through closets of corresponding size to the current above floor ducts i.e. 6, 7 and 8". 8" being for the living room. I guess the living room only needs that larger duct. I would guess that the amount of CFM flow is determined by the narrowest part of the run, not how wide it eventually gets.

Seems from what Brad said there could be a problem with the living room duct if the max I can get at attic is 150 cfm (4 x12), regardless of the wider vertical duct it goes into.

Thanks,

David

Brad White

Don makes excellent points

The flexible ductwork you have has a side benefit of attenuating noise somewhat. Make sure you have volume dampers kept as close to the main as possible.

Also, I recommend you use pleated filters, 2" if you can, MERV-7 or MERV-8 (30% type).

The balance point or critical limit of airflow and velocity tends to be noise.

Adding more branches if you can will give you more options.

don_185

Well said

Well said as always Brad.

It never cease to amaze me when guys make improvements on the supply side of the distribution system only to mess it
all up on the return side of the system.

I hear from customer all the time when I tell them that we need to removed the electrostatic filter because, the pressure drop is to great and we're not moving enough airflow.

They reply I paid alot of money for that filter...and I always reply more then you will ever know.

D107

point of clarification

Thanks Brad. re: the 2" pleated filters, you mean for the inline I assume, not for the pre-filter in the return register? The existing inline 902B Bryant (3"+?)filter is pleated in a metal frame in a box. I assume the pre-filter would be flat and fairly thin, otherwise there would be CFM loss.

As for Don's points about jobs done wrong, easy to see from this job that it wasn't top notch work whoever did it. The pvc drain line had no unions to facilitate cleanout, no water trip switch, and the overflow line--suppposed to be fit into side of gutter--somehow ended before the gutter. More specifically right above a bedroom ceiling.

Thanks,

David

Brad White

With your 3\" Unit Filter, David

that is excellent. Those deep pleated filters are so much better than the blue "read a newspaper through them" cheap filters.

My concept of using a 2" pre-filter is for the return grille as you suggest. It does two things:

1) Prolongs the life of the more expensive deep pleated unit filter and

2) Keeps the return ductwork cleaner.

There would be some pressure drop (airflow loss in a fixed speed system) but by over-sizing the filter to 24x24, you keep this to a minimum.

A 2" pleated filter like I am thinking about has a pressure drop of about 0.33 inches at 500 FPM (2,000 cfm at 24x24 size). At 1,200 cfm as in your case, the PD drops to about 0.12 inches (reduces by the square of flow roughly). Therefore the loss is reasonable and can be dealt with. You can also use thinner filters too, just as a pre-filter which is what you want.

Good news that you have that deeper unit filter... too often I see a one-inch nominal see-through filter at best.

D107

correction

Brad I measured the width of the inline accordion-like hepa filter in metal frame and it turns out to be 6" not 3". (Metal box the frame's in is 8"). So does that change your recommendation for a 2" pre-filter given potential cfm loss.

Also given that register width (upper hallway) is fixed at 12" to fit between joists, it would have to be 48 x 12 to get the 4 sq ft. I guess it would be better if we could somehow get another register on the main level that would be preferable to divide the intake load. And if we ever did snake a supply duct down the abandoned chimney to the basement, a register there too would be best, yes?

Thanks,

David

Brad White

The pre-filter is just a bonus

to keep the return ductwork clean and to prolong the life of the deeper filter. Any pre-filter, even a see-through type, will help in that regard, just not as well. By installing a filter return grille, no one says you HAVE to put the filter in there. It just means that you can. You can use any available size but why not 24x24? A little carpentry, (not to minimize it but cutting the ceiling is the messy part you are going to do anyway) and you can make any size you want. I stick to filter return grilles in the standard sizes (16x16, 20x20, 20x25, 24x24 etc.)

Yes, the termination of any duct assumes a register, even the basement, for distribution and control. Were you thinking just an open-ended duct otherwise? If not a finished space it does not matter, but then why supply air to it? Maybe I do not understand your question.

Brad

D107

i'll try to clarify:

Regret I couldn't make my question clearer--probably because I know so little about AC.

First, I have no problem with the 48 x 12 size register or pre-filter in the register. I assumed that size would be equal in desirability to 24 x24, and since it saved a joist cutting I preferred it.

It was the thickness of the pre-filter that I was asking about. Since the inline turned out to be twice the thickness I'd thought, I thought perhaps you might say put a 1" max instead of 2". So now that I carefully re-read your prior posts I see that with proper measurement of pressure drop, the best pre-filter thickness and register sizes can be arrived at.

The third issue revolved around how to distribute return registers in an AC system. I assume having the registers more evenly distributed on the air conditioned floors would be optimum--instead of having only one on the upper floor as it is now. So, again re-reading your posts-- a good AC contractor should be able to carefully calculate pressure drop and flow requirements, the right register sizing with pre-filters can be arrived at.

If only my wife and I could agree on what house temperature should activate the AC--she's perfectly comfortable at 82F with a blanket.

Thanks, and happy holidays to you and yours,

David

Brad White

I think I see, David

You will be hard pressed to find a 48x12 filter unless you cut your own from roll (Facet) material, which may be just fine for you.

Air distribution is key and as with heating is proportioned according to sensible heat gain an a given space. The quality of the air in terms of humidity is created at the air handler of course. From there it is all about delivered and returned quantity. Hope that helps!

Thanks for the good holiday wishes. I wish for you and yours the same. (I get both this year, we are hiding the eggs this year in the Mmmmmmmmfffp@

Sorry. I was sworn to secrecy

D107

AC guy came out today

He pointed out potential problems with burying the ducts:

1-Being that ceiling is plaster, the excess will stick up into the joist bay, taking away some of the 5.5" depth.

2-Some bays are likely to have BX cables and light fixture junctions going through them. Wasn't looking for more electrical work.

3-He suspects that with 16 OC horizontal ceiling beams (covering house width of 27ft, balloon construction) each resting on a center support halfway over, so in effect some of the beams become sistered part of the way, and so they narrow the space down a bit. I guess I can verify this by inspecting the beams.

Oh well, always more things to think about. Good part is the company has its own sheet metal shop and can custom size ducts.

Thanks,

David

clammy

changing suppies

Not for nothing but if you change your flex to sheetmetal ducts espically 7 or 8 oval you will in all probality have to change your 8 x 8 cieling boxes to 10 x 10 to fit 7 and 8 oval into them and maybe instead of increasing your central hall return why not put some returns into your bed rooms that will really even out your second floor room temps .I myself do not like to use more then 1 filter example if your have 1 hepa filter at your unit there is no need for a pre filter then you are only increasing the static required to pull enough return to the a/h .Also go for a good open bar retrun grill instead of stamp face and have your return boxes and supply cans wrapped ,always opt for externally lined duct work also with putting all your duct work in the bays you will also be removing insulation from eaach bay lower the r value of your cieling and increasing the heat lose and gain properties of the structer .if you can have your a/c guy chaange you flex to r -8 instead of r4 peace and good luck clammy

D107

thanks clammy--always good to be reminded on insulation.

especially in this unvented hot attic. I do intend to Icynene foam the eaves and gable walls, which should make any loss of bay insulation less crucial. It may be that in this old house, short of a major renovation, we'll have to choose between a nice usable attic floor and a well-running AC system that on a 95o+ day can get inside temps down to 72 degrees and remove humidity. Right now it can do about 75, which is liveable. System is also not zoned, but that's another big job. An easier job, says the AC guy, would be to add a simple ERV that connects to the existing system. Once we foam the attic and basement band joists a little makeup air won't hurt.

Thanks,

David

lee_9

what pre filter? Air systems should only have 1 filter between register and unit. More then 1 will greatly reduce air flow. If you have more then 1 return grill then you can have a filter at each grill and only at the grill.