Return Air Confusion

[AustinScoobee]

Hi everyone. Despite reading a couple hours of posts I am still confused. We have had 3 different HVAC service crews out here, each with a different answer.

The Question: Is our Return Air sized correctly? We run a 12Seer, 4 ton AC/Heat Pump. We have ONE 20X20 Filter which fits directly to the bottom of the unit. We have one vent opening feeding the unit which measures 13X 29 (377sq in)

We were led to believe that you need roughly 800sq in return air opening to feed 4 tons, so we were told to add another return air opening of about 450 sq in. Which we can easilly do BUT. How on earth would this change the air getting through the 20X20 filter?

My thinking is to configure EACH of the return air openings with it's own filter and remove the 20X20. Am I wrong here? None of the HVAC guys would comment.

Any help much appreciated, G

[Noon-Air]

Leave the 20x20 filter, thatway *ALL* of the return air is forced to go through the filter. As to the returns, for a short return(less than 10 ft, I use 24x24 (minimum) to the filter base, if its more than 10 ft, then I use multiple returns into the 24x24 RA plenum, sized appropriately.

[AustinScoobee]

They have return grills which have integral filters just behind them, this way all the air would be filtered. The base of this unit (Trane TWE048C140B2) is pretty much exactly the same size as the 20X20 filter mounted under it. Its mounted on a box which opens into a hall, less than 2 feet away which is where the

13X29 grill is. It is also in a corner, so I "could" add another opening/return air from the other hall. But still I dont see how more air would get through the 20X20. G

[Noon-Air]

Its not a matter of getting more air through the 20x20 filter, its IS a matter of reducing the restriction of airflow to get to that filter at the bottom of the unit. BTW, with that 20x20 filter base (with a filter in it), thats the *ONLY* filter you need in the system.

[gofish]

What you, and a lot of other people dont understand is that total airflow (lets call it cfm) is dependent upon other factors than just "size".

none of the hvac guys knew, because they too were/are clueless.

I suggest that you educate yourself, so that you can educate the hvac techs who show up at your doorstep.

google External Static Pressure, and Total External Static Pressure.

here's a nice place to start:

[daytona°]

I'll put away the Ductalator....you pretty well cover it

[AustinScoobee]

Thanks for all the info here guys. I would like to visit one last thing before I go ahead. So. This unit sits atop a framed in "box" or Plenium which measures roughly 22X22 by 40" deep. Its in the corner of a utility room. A hole was cut for the return air grill (13X29) in one of th ends ( a 22X22 one?) drawing from a central hall . I can cut another return air grill entering a side from another hall into one of the 22X40 sides of the same box.This appears to be our plan since, if we have problems we can just close is off and we are back to starting point. The whole issue came about when I dared to read a bit, and pointed out to a hvac "pro" that I thought a 4 ton needed a bigger return grill than 13X29. The 13X29 , I should add was fitted so that it enters one of the 22X22 ends of the "box" ? Therefore the top 6 or 7 In of the grill enters stud space. It is sealed but still the last 6in air at the top would have to turn to get to the box. What a mess huh? Baring some outstanding wisdom we plan to add a 24X14 return into the

22X40 side of the box so the air goes straight in, and will leave the existing 13x29 in place as well . This I conclude gives us about 700 sq in. return air. We could go to a bigger for the one we plan to add to get to the " rule of thumb" 800 sq in if nessecary.

Waiting for the "outstanding wisdon" (another supposed hvac pro to come today) we will post back what he recommends and again , any and all input appreciated, thanks again, G

[Noon-Air]

You never stated any of this previously.... your best bet is to call a local pro who can lay hands on what you have, and do it right. Your DIY stuff is going to do nothing more than create a bigger mess.

22X40 side of the box so the air goes straight in, and will leave the existing 13x29 in place as well . This I conclude gives us about 700 sq in. return air. We could go to a bigger for the one we plan to add to get to the " rule of thumb" 800 sq in if nessecary.

Waiting for the "outstanding wisdon" (another supposed hvac pro to come today) we will post back what he recommends and again , any and all input appreciated, thanks again, G

[udarrell]

Depending on the Return air grille design, you need the recommended amount of free air area. They recommend 200-sq-ins per ton on a filter grille. The "Return air duct" should be around 22" Rd duct or 280-sq" for a

4-Ton system. Below is a paste from my HVAC Web.

Air Filter Rack Sizing For efficient operation - Gross Return Air filter grille area: 200-sq. ins. or more per ton, is required. Keep air velocities through the filter(s) as low as possible. Filter Grille(s) Velocity 350-FPM. All filter mfg'ers should print the free air area of the clean filter on the edge of the filter along with the pressure drop data.

It would be good if they listed on their filters the recommended air velocity in FPM.

- udarrell

[New Directions In Building Ser]

22X40 side of the box so the air goes straight in, and will leave the existing 13x29 in place as well . This I conclude gives us about 700 sq in. return air. We could go to a bigger for the one we plan to add to get to the " rule of thumb" 800 sq in if nessecary.

Waiting for the "outstanding wisdon" (another supposed hvac pro to come today) we will post back what he recommends and again , any and all input appreciated, thanks again, G

In all of this - is there any OA being induced into the RA stream? All I'm hearing is that there's a closed loop with OA make up only from leakage if there's a toilet exhaust or some other means of having negative pressure in the dwelling (are your codes so slack that this is allowed?? or have you not heard of ASHRE55 etc). Remember that if you do have an OA supply, without fan assist, to the Return Plenum - and you start reducing the static pressure there. You reduce the OA quantity being introduced.

[AustinScoobee]

No there's no Outside air intake, but the house isnt airtight by any stretch of the imagination, lots of doors, etc.

I dont want to persue this any further now but I will say, I think that the "HVAC" profession as a whole needs some serious looking into! What other profession can get away with answering the same question so many different ways? ( The last "pro" to come out Sat had even another idea) I had 5 companies come out, 3 from national companies and 2 from local ones and none came even close to agreeing. If you ask an electrician what size wire you need he cites NEC code. Where's the code for these guys?

Again, the system is "basically" functioning so we will put this to rest till I have the time to research this myself.

Thanks again for all the answers and resouses G

[Noon-Air]

I dont want to persue this any further now but I will say, I think that the "HVAC" profession as a whole needs some serious looking into! What other profession can get away with answering the same question so many different ways?

Doctors, Lawyers, Politicians, etc........

( The last "pro" to come out Sat had even another idea) I had 5 companies come out, 3 from national companies and 2 from local ones and none came even close to agreeing. If you ask an electrician what size wire you need he cites NEC code. Where's the code for these guys?

IMC

Again, the system is "basically" functioning so we will put this to rest till I have the time to research this myself.

Instead of calling the ones with the biggest ads, or the ones with 50 trucks where you don't have a clue what your getting, maybe you should call a small company where your dealing directly with the owner. Check with family, friends, neighbors, in-laws, outlaws, and see who they use, and are they happy with them??

Thanks again for all the answers and resouses G

[<kjpro]

Electricians will disagree too. Don&#39;t believe me, just ask an electrical question.

[gerry]

Try and get any two periodontists to agree ;(

gerry

[Geoman]

I dont want to persue this any further now but I will say, I think that the "HVAC" profession as a whole needs some serious looking into! What other profession can get away with answering the same question so many different ways? ( The last "pro" to come out Sat had even another idea) I had 5 companies come out, 3 from national companies and 2 from local ones and none came even close to agreeing. If you ask an electrician what size wire you need he cites NEC code. Where&#39;s the code for these guys?

Again, the system is "basically" functioning so we will put this to rest till I have the time to research this myself.

BASICALLY FUNCTIONING AND YOU WAISTE FIVE COMPANIES TO COME OUT ABOUT A SIMPLE FILTER/RETURN AIR QUESTION?

YOU HAVE TO BE KIDDING ME!!! WHAT THE HELL IS WRONG WITH YOU!!!! YOU THINK THE HVAC COMPANIES HAVE TIME FOR YOUR BS?

IF YOU CALLED ME OUT ON A SATURDAY FOR THIS CRAP I&#39;D BEAT THE CRAP OUT OF YOU!!

Man, some people are born stupid AND YOU ARE THE WINNER!! Your question doesn&#39;t even deserve answering and you waist five companies time and this newsgroup as well.

Thanks again for all the answers and resouses G

[gofish]

the bigger question Rich is of the 5 companies who responded to this persons call, how many of them charged & collected a service call fee, whether they called it a trip charge, truck charge, diagnostics or whatever.

[<kjpro]

More than likely, ZERO.

If they had, he&#39;d have bitched about it.

[Noon-Air]

Especially considering that Saturday is OVERTIME

[AustinScoobee]

You must know I expected all the above rantings..

Anyhow, no I didnt pay all of them but I paid: The original svs call, who SUGESSTED THE ADDED VENT , and did a few tests. Paid the second a svs call $95 just to talk... The 3rd lives down the road, freind of a freind. The fourth OFFERED to come NO-CHARGE and paid with the last , who could ONLY come on sat.

The last was the best to me and he will be paid to do any work we need...

""BASICALLY FUNCTIONING AND YOU WAISTE FIVE COMPANIES TO COME OUT ABOUT A SIMPLE FILTER/RETURN AIR QUESTION?""

If it was such a simple question, why couldnt anyone here answer it..?

Cheers, G

[<kjpro]

Do you have a digital camera?

Do you have the required tools to take the readings we need?

BTW, what&#39;s wrong with my truck... it won&#39;t run?