Flexible Duct sizing help

[duckman06]

Thanks Duckman06,,, am looking for someone (pro) , to do me a load cal,,I want to have that to show them.

someone came out yesterday from a/c company to check,, said they are coming this week to change plenum,,and if that does not fix problems,,they well see what we do from their,,

one more thing,,should the plenum open into a bigger box in attic,,,or as long as it is full size of air handler opening is good,,,or is their a size it should be???thanks again for the help,,,

so far the info i have been getting from the site (i put a couple of posts)has helped my argument with this installer,,,

thanks again all!!!!


if anybody does load calc let me know,,,miami Fl

I don't quite understand what you are saying. So I will just post what I think should have been done but remember I can't see the whole install so doing so excated thinking.

The plentum should be the same size as the opening of the air handler. If it was me I would have installed a plentum that was at least 4 foot long and ran the drops off of it and size manual j. This way I could ensure that static would stay in check along with supply enough cfm to cool the home. What about your return? What size is it?

[Gib's Son]

What's going to kill that system (from the limited photos provided) is the stupid supply plenum.....huge system effect.....which adds allot of external static pressure. The fan cares not, nor knows not, what the duct sizes are. It only cares about the total static pressure it needs to overcome to deliver the desired airflow.

My personal opinion is that plenums like that should be against code as most people do not know how to properly design a system with a supply plenum. A properly designed supply plenum, in your case, should run horizontally across your attic and extend a few feet past your last (furthest) take off to allow the air flow to straighten out and become non-turbulent.

Size of the flex, or any duct, is really going only affect two areas of a system....static pressure (resulting in CFM the fan can provide) and noise due to velocity. A fan will push 400 cfm out of a 2 foot 8 inch duct but will only push a fraction of that air if the duct is 100 feet because of the added friction. Duct sizing for a desired air flow is all about the fan laws.

If the size of the duct was the primary deciding factor for duct size, I.E., a 6 inch duct is only good for x amount of air flow, then all hand held duct calculators and sizing charts would size ducts that way...not provide values of pressure loss/ 100 feet of duct.

Over the years I developed a few spreadsheets for duct sizing that work pretty well but you need either the SMACNA book with fitting coefficients or an ASHRAE book with theirs. If your a pro and would like a copy PM me and I can share a copy. Warning though....if you do not already understand the proper SMACNA or ASHRAE methodology for sizing ducts by doing the hand calculations to determine total system static, don't ask, as the results will be bogus....garbage in garbage out you know. (Just be patient as I am pretty busy.)

Have not read this yet but a cursory view looks interesting.
http://www.ashraebistate.org/sites/a...uct_Design.pdf

[bullmoose]

We gave you advice on your last post. Guess you chose not to follow it.....

Sent from my SPH-D710 using Tapatalk 2

[duckman06]

What's going to kill that system (from the limited photos provided) is the stupid supply plenum.....huge system effect.....which adds allot of external static pressure. The fan cares not, nor knows not, what the duct sizes are. It only cares about the total static pressure it needs to overcome to deliver the desired airflow.

My personal opinion is that plenums like that should be against code as most people do not know how to properly design a system with a supply plenum. A properly designed supply plenum, in your case, should run horizontally across your attic and extend a few feet past your last (furthest) take off to allow the air flow to straighten out and become non-turbulent.

Size of the flex, or any duct, is really going only affect two areas of a system....static pressure (resulting in CFM the fan can provide) and noise due to velocity. A fan will push 400 cfm out of a 2 foot 8 inch duct but will only push a fraction of that air if the duct is 100 feet because of the added friction. Duct sizing for a desired air flow is all about the fan laws.

If the size of the duct was the primary deciding factor for duct size, I.E., a 6 inch duct is only good for x amount of air flow, then all hand held duct calculators and sizing charts would size ducts that way...not provide values of pressure loss/ 100 feet of duct.

Over the years I developed a few spreadsheets for duct sizing that work pretty well but you need either the SMACNA book with fitting coefficients or an ASHRAE book with theirs. If your a pro and would like a copy PM me and I can share a copy. Warning though....if you do not already understand the proper SMACNA or ASHRAE methodology for sizing ducts by doing the hand calculations to determine total system static, don't ask, as the results will be bogus....garbage in garbage out you know. (Just be patient as I am pretty busy.)

Have not read this yet but a cursory view looks interesting.
http://www.ashraebistate.org/sites/a...uct_Design.pdf

This is what I was trying to say but you did a much better job with the plentum design details. The ideal thing would be for it to extend out not up in this case due to pics. I would think run it horz. 4-8 foot both ways depending on what limits are in place with construction design. Either way the way it is now will allow staic to be high and not deliver air flow as designed.

So the variable speed that knows one thing and that's to deliver the proper amount of cfm to the ductwork. With current set up I'm welling to beat variable speed motor will ramp up after just a few mins of running to try to overcome this set up. Also what about the return what size is it? This also will effect the way a system will run and provide air. If undersized (beat it is) will do the same thing and make the variable speed ramp up to overcome.

[david280]

wow,,,thank you very much guys for this info,,

The handler is in a closet in my house 26 inches off the floor,, with a vented door,, return is facing down toward floor,,

thanks again guys,, this is really helping me resolve this problem

Bullmoose the other post just let me know that it was a very bad install,,over-sized ,,and small plenum...

after this I hope the company that installed this in my house does not do this to anyone else

they are coming back this week to change plenum

thanks to you guys I was able to talk like I new what I was saying,,and was able to put them in their place
because at first they were giving me the run around,,,

[udarrell]

Miami FL 2.5% SUMMER DESIGN is 90dry bulb 77 WB or around a high 77% RH.

That means the system needs to be downsized so it has long runtimes to remove the latent heat of humidity.

Pay attention to what DAN SW FL said...we are betting the condensing unit is oversized.

Pay attention to the return grille sizes; make them plenty big enough. The Return-Air filter area needs to be sized for 300-fpm velocity or less.

If a cheap throw-away media type filter is used, the free-air-area will only be around 64% of the physical size of the filter (s).
A 3.5-Ton system requires 1400-CFM / 300-fpm vel. = 4.6-sf of open-air-area (Ak). 4.6 * 1.36 is 6.25-sf * 144 is 900-sq.ins. or a 30X30 physical filter size. You would need two separate filtering areas which would then be 700-CFM threw each filter, with each filter sized accordingly.

Reducing the size of the system will help reduce the required Return-Air filtering area.
A 2-Ton system at 800-CFM / 300-fpm = 2.6-sf (sq.ft.) of open-filter-area or use a 24X24 media type filter.

A lot of things affect the total static pressure of a system; I would not use 1" pleated filters as they are too restrictive & load debris fast only on their surface area.

[duckman06]

wow,,,thank you very much guys for this info,,

The handler is in a closet in my house 26 inches off the floor,, with a vented door,, return is facing down toward floor,,

thanks again guys,, this is really helping me resolve this problem

Bullmoose the other post just let me know that it was a very bad install,,over-sized ,,and small plenum...

after this I hope the company that installed this in my house does not do this to anyone else

they are coming back this week to change plenum

thanks to you guys I was able to talk like I new what I was saying,,and was able to put them in their place
because at first they were giving me the run around,,,

While changing the plentum design and lay out might help it still want correct oversizing. I would talk with them about doing a load cal for proper sizing!!! Although if this was what they do for customers they would already have done so good luck with that.

I just hopes it all works out for you. As this system you will live with for the next 15-20 years or until the ductwork supply and or return kills the compressor. Just saying!!!

[beenthere]

Miami FL 2.5% SUMMER DESIGN is 90dry bulb 77 WB or around a high 77% RH.

That means the system needs to be downsized so it has long runtimes to remove the latent heat of humidity.

Pay attention to what DAN SW FL said...we are betting the condensing unit is oversized.

Pay attention to the return grille sizes; make them plenty big enough. The Return-Air filter area needs to be sized for 300-fpm velocity or less.

If a cheap throw-away media type filter is used, the free-air-area will only be around 64% of the physical size of the filter (s).
A 3.5-Ton system requires 1400-CFM / 300-fpm vel. = 4.6-sf of open-air-area (Ak). 4.6 * 1.36 is 6.25-sf * 144 is 900-sq.ins. or a 30X30 physical filter size. You would need two separate filtering areas which would then be 700-CFM threw each filter, with each filter sized accordingly.

Reducing the size of the system will help reduce the required Return-Air filtering area.
A 2-Ton system at 800-CFM / 300-fpm = 2.6-sf (sq.ft.) of open-filter-area or use a 24X24 media type filter.

A lot of things affect the total static pressure of a system; I would not use 1" pleated filters as they are too restrictive & load debris fast only on their surface area.

90°F DB with a WB of 77°F is only a 56%RH.
90°F DB with a 77%RH is a WB of 83.7°F.

[udarrell]

90°F DB with a WB of 77°F is only a 56%RH.
90°F DB with a 77%RH is a WB of 83.7°F.

You're absolutely right, beenthere!

I had shut down my PC for the night, then I was wondering if I had typed 77 instead of the 56% RH, - that I had looked up; I didn't want to boot my PC back-up again just to check that, so took a chance & left the dumb error sitting their; thanks beenthere for bringing it to my attention this morning.

I was trying to find the online grains of moisture calculator; search didn't turn it up; I don't have that easy online calculator, (does anyone have that link?). At 90-DB & 77-WB that would be a lot of grains of infiltration moisture. Pays to get that infiltration down to the minimum in high humidity hot climates.

[udarrell]

Well, after a ton of different searches I finally found the online grains of moisture calculator - I used with my old PC.

When I do searches; this new windows 8 OS system seems to get all the pay web pages, oh hum; could no longer find a free audio/video producer program...that would function after downloading it.

Here is the link: http://www.uigi.com/WebPsycH.html

At 100' above sea level 90/77 is 120.17 grs moisture per lb of air.

Indoors, 76-F/ 63-F WB is 48.81 %RH; only 65.74 gr of moisture per lb of air; therefore the A/C has to remove a lot of grains of moisture due to infiltration which ought to be kept as low as possible.

These new evaporator coils have a lot of fins per inch therefore, with an oversized tonnage coil & low airflow velocity through it, a cold coil ought to perform a lot of condensation...

[udarrell]

A Clarification: Concerning the best coil & condenser combination for high humidity conditions; these new smaller compressors to the tonnage rating of the condenser makes it more difficult to get the evaporator coil cold enough - using an oversized coil.

The old 6 to 8-SEER condensers had a bigger Btuh compressor to condenser rating therefore, they could get the temp/pres down lower & get the oversized coil colder than these high SEER units, therefore, with these high-SEER units, with a heavy heatload on the E-Coil, your usually better-off using the same tonnage Evap-Coil with minable airflow through it.

To keep them from automatically parsing links in your post content, best done before typing content: under Miscellaneous Options; UNCHECK Automatically parse links in text!

[beenthere]

You can download Carriers free psychrometric software.

Will tell you all you want to know bout the air properties.