Correct return size?

Hey everyone,

I finally scheduled a date for the installation (A/S ycx036gm). They will install only one return, since there was no previous returns as the house has a flat roof, and the original equipment was a swamp cooler. The house layout is pretty open, and it's only 1000 square feet (two bedrooms only).
In any event, this is a 3 ton package unit, so I was wondering what would be the correct size of a single return. As some of you might know, many houses in Arizona, specially older ones like mine (1972) are built with brick, adobe, or block, which means that they are harder to modify. However, I do see that a lot of houses with these package units and only one return duct (or so it seems to my untrained eye, hehehehe).
Oh, and last but not least, I ordered a VisionPro 8000 (it was only a few bucks more): was that overkill, since this is not a variable speed or two-stage system?

I thank you for your help and look forward to your response.

The VisionPro is not overkill, they make a few different models. One just happens to be the TH8110, which is a single stage heating/cooling stat.

The VisionPro is an AWESOME stat!

“They will install only one return” mlbussert, I assume that “they” is a qualified HVAC Contractor, if so “they” would have designed the air distribution system to meet the required manufacture’s tolerances for pressure drop and velocity limits. Both of these factors depend upon the duct geometry and the type duct of material. Selecting the correct blower for the system weighs into the mix, and there are many factors to be considered there also. Driving the equation is the fact that your residence is located in Arizona thus it’s ACTUAL capacity needs to be carefully analyzed. Cooling is made of two components, sensible and latent. The former is what you see on a thermometer (delta in degrees) and later is what you see on a humidity gauge (delta in relative humidity). Arizona is a decidedly “sensible” only environment therefore all latent cooling capacity should by subtracted from the aggregate total capacity stated (3-Tons).

In your region the qualified Contractors realize all the above and hopefully use sound professional judgment to design a system that will meet the cooling loads as calculated by ACCA Manual J procedure (or similar methodologies).

I have to smile at your focus on the thermostat, which while obviously a necessary component in the mix, albeit such a minor one in the total scheme of things. So many times I’ve ask the caller “Who is the manufacture of your unit?” only to be told WHITE-RODGERS. Thus the highly visible thermostat gets top billings and the remainder of the components remains a mystery. I agree that the user interface is omnipresent in the owners mind, however if you are installing a completely new system please try to come up the curve on all the elements in the equations giving the most credence to the most critical of the aforementioned.

How much thought have you placed on the length of warranty being offered, and does it cover compressor only, parts and compressor (if so are they both covered for the same delta time) and labor. I would be far more concerned about getting a 10-year compressor, 10-year coil; 5-year other parts and 1-year labor (minimum). For a very reasonable price you should be able to get 10-years on everything. Mlbussert, these are extremely important considerations that YOU control, does this make sense?

I would also want to know if the unit size was determined by a cooling load calculation and if so I would want to review the input data used and see if room-by-room loadings were generated. If the contractor is able to accommodate these request, then assuming you’ve checked references you can relax a bit knowing that you’re indeed in competent hands.

The other factors in the equation would require a substantial time commitment on your part to study the necessary methodologies to become competent enough to make meaningful decisions on this design issue. For your immediate questions start with ACCA Manual D, ISBN 1-892765-00-4 (2002 with 2004 reprint available also) available free of charge through your local library via inter-library loan. If your not in a hurry and have lots of free time then camp out on this HVAC Forum and drink in the knowledge made available by a wonderful multitude of talent, enough said.

God Bless;

Quote:

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Originally posted by faith
“They will install only one return” mlbussert, I assume that “they” is a qualified HVAC Contractor, if so “they” would have designed the air distribution system to meet the required manufacture’s tolerances for pressure drop and velocity limits. Both of these factors depend upon the duct geometry and the type duct of material. Selecting the correct blower for the system weighs into the mix, and there are many factors to be considered there also. Driving the equation is the fact that your residence is located in Arizona thus it’s ACTUAL capacity needs to be carefully analyzed. Cooling is made of two components, sensible and latent. The former is what you see on a thermometer (delta in degrees) and later is what you see on a humidity gauge (delta in relative humidity). Arizona is a decidedly “sensible” only environment therefore all latent cooling capacity should by subtracted from the aggregate total capacity stated (3-Tons).

In your region the qualified Contractors realize all the above and hopefully use sound professional judgment to design a system that will meet the cooling loads as calculated by ACCA Manual J procedure (or similar methodologies).

I have to smile at your focus on the thermostat, which while obviously a necessary component in the mix, albeit such a minor one in the total scheme of things. So many times I’ve ask the caller “Who is the manufacture of your unit?” only to be told WHITE-RODGERS. Thus the highly visible thermostat gets top billings and the remainder of the components remains a mystery. I agree that the user interface is omnipresent in the owners mind, however if you are installing a completely new system please try to come up the curve on all the elements in the equations giving the most credence to the most critical of the aforementioned.

How much thought have you placed on the length of warranty being offered, and does it cover compressor only, parts and compressor (if so are they both covered for the same delta time) and labor. I would be far more concerned about getting a 10-year compressor, 10-year coil; 5-year other parts and 1-year labor (minimum). For a very reasonable price you should be able to get 10-years on everything. Mlbussert, these are extremely important considerations that YOU control, does this make sense?

I would also want to know if the unit size was determined by a cooling load calculation and if so I would want to review the input data used and see if room-by-room loadings were generated. If the contractor is able to accommodate these request, then assuming you’ve checked references you can relax a bit knowing that you’re indeed in competent hands.

The other factors in the equation would require a substantial time commitment on your part to study the necessary methodologies to become competent enough to make meaningful decisions on this design issue. For your immediate questions start with ACCA Manual D, ISBN 1-892765-00-4 (2002 with 2004 reprint available also) available free of charge through your local library via inter-library loan. If your not in a hurry and have lots of free time then camp out on this HVAC Forum and drink in the knowledge made available by a wonderful multitude of talent, enough said.

God Bless;

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Faith and Jultzya,

I thank you for your thoughtful responsenses.

I have actually been reading many sources of information for several weeks, including this forum, several publications by numerous associations, institutes, and governtment agencies. Thanks to the helpful input of memebers of this forum, like yourselves, I stopped looking at the brand and looked for a qualified contractor. I picked this contractor because I checked out the company with BBB, and the City, for lawsuits, because it has been in business for 20 yrs, has some NATE certified technicians, and a friend of mine who is an independent contractor recommended them to me (he said they were a little more expensive than other companies, but worth it); oh and the contractor offered references.

I believe the system was sized carefully (although I could be wrong), as the contractor performed a Manual J load calculation (he took careful measurements, and showed it to me days later, along with a scaled plan of the house).
From what I can surmise by looking at other posts and/or brands, the warranty is pretty decent:10 yrs compressor and coil, 5 yrs. all other parts, including the thermostat, 20 yrs. heat exchanger, and 2 yrs labor. The contractor also includes a 100% satisfaction warranty or your money back. So this was also a factor in my decision.

So, that only left me with the ductwork. The house has limitted space to do ductwork, as this is a flat roof house. This means there is no attic or any other space to put additional ductwork or equipment, other than rebuilding the whole roof (same reason for the package unit). There is only one main trunk in the house. The contractor measured the ducts, but I don't know if he used any calculation method, he only said that they were appropriate.The current system is a swamp cooler, so there is no return.

This brings me to my original post. The contractor explained to me that the installers would be putting in a new return, which would have to be trhough the ceiling and over the roof. What he did not tell me and I did not ask is the size of the return, hence my posing the question in the forum. I just wanted to confirm that the contractor would install an appropriately-sized return. Obviously I trust him, but I would like to confirm this.

I hope I was not too long-winded. Like I said before, it's always easier bo be diffuse than to be concise an clear hehehehe. I thak everyone for taking the time and patience to read this and maybe post a repply.

I don't understand why you are so concerned about the size of the return? Is the contractor not the professional you called to do the job.

You said:

"I picked this contractor because I checked out the company with BBB, and the City, for lawsuits, because it has been in business for 20 yrs, has some NATE certified technicians, and a friend of mine who is an independent contractor recommended them to me (he said they were a little more expensive than other companies, but worth it); oh and the contractor offered references."

AND

"I believe the system was sized carefully (although I could be wrong), as the contractor performed a Manual J load calculation (he took careful measurements, and showed it to me days later, along with a scaled plan of the house)."

It seems to me that the contractor has been very diligent and is doing should be done. He is even showing you his work. Now, what if someone here says that the return isn't right? How do you know that the person saying this is correct. Have I, or anyone here been at your house? An educated homeowner is a good thing because they understand that value has a high cost, but for a HO to get ACCA manual J and D to verify the contractor can only cause confusion. He has the training and experience to use those manuals correctly.

If you really trust the contractor: ask questions-it's good to be informed so that there are no misunderstandings, offer suggestions where appropriate, but don't try to also design the system and then compare notes.

Quote:

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Originally posted by on call


It seems to me that the contractor has been very diligent and is doing should be done. He is even showing you his work. Now, what if someone here says that the return isn't right? How do you know that the person saying this is correct. Have I, or anyone here been at your house? An educated homeowner is a good thing because they understand that value has a high cost, but for a HO to get ACCA manual J and D to verify the contractor can only cause confusion. He has the training and experience to use those manuals correctly.

If you really trust the contractor: ask questions-it's good to be informed so that there are no misunderstandings, offer suggestions where appropriate, but don't try to also design the system and then compare notes. [/B]

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I think I gave the wrong impression, or misrepresented my original intent in asking questions. The contractor has been indeed very diligent, and I do trust him; that is why I explained how I arrived at the decision to pick that particular company. However, I do believe that doing independent research is not inappropriate nor excesive; in other words, I trust but I want to verify. It's that simple. Or, to put it bluntly, there are very few people whom I trust blindly, hence my questions. Furthermore, I did not in any way "designed" the system nor did I tell the contractor how to do his job. I am not trying to compare notes or anything close to it. I just wanted to be informed so that I can make a judgment and sing off on the job once it is comleted, which is something I have to do (it's in the contract) and it is a responsibility I take seriously; I would rather that everything is okay now and not have to call the contractor back later. That being said, I did not mean to offend anybody's sense of what is and what is not an appropriate way of dealing with an HVAC contractor.

So... after the scolding, I understand that there are many variables that will influence the design of a system and that it is impossible to get a precise response w/o having done the work yourselves, so I will rephrase my question:

With a 3 ton gas electric package system, in a pretty open, but small house (about 1000 square feet), where a single return will be installed, is there a recommendable minimum size for said return?

Again I thank everyone, and please do not take my comments in a negative way, I am just trying to explain myself (which I obviously do very poorly hehehehehe). Best regards to all.

As others have said, there's no single answer we can tell you because we don't know how much static your system can tolerate based on the blower power, filter restriction, and pressure duct restriction. But here's a ball park number. I'll assume your air handler could flow up to 1200 CFM. You'll need an approximate minimum of 240 square inches of return DUCT (10x24) if metal. Larger is usually better. Definitely even larger if flex duct or something more restrictive than smooth metal. Your return GRILL needs to be even larger, about 600 square inches (30x20).

Suemarkp,

Thanks. I think that kind of minimum (ballpark) parameter will help me understand better what I should expect. I think that with all the comments and help from people here I should be okay. Like I said before, I will post pictures. Thanks to all.

By the way Suemarkp, I lived in Seattle before (my wife is from there), and I must say that I sure hope to have the chance to live there again. It is, in my opinion, among the top 5 cities in the US (obviously, to continue with the nuances, that will vary depending on the criteria that you use to judge cities). I envy you hehehehe. Best regards.

What the heck. I am probably less qualified than anyone else on this thread, but I do understand your question and have worried about return capacity myself.

First answer, indeed your HVAC contractor should be designing a correct sized return in the first place. Hopefully his proposal will agree with what you learn.

Manual D is rather intimidating, but is the "Gold Standard" for designing an entire duct system. You cannot go wrong if you follow Manual D correctly (in the event something I say might contradict Manual D, assume the latter is right and I am wrong).

There is a rule of thumb on return sizing, which sometimes produces "good enough" results. One of my HVAC technicians gave me a table which evidently has the goal of air speed to stay below 600 feet/min (we use flex a lot here). It tells me 3 tons will be 1200 CFM and needs a 20-inch return (or two 14-inch). In my hot-humid climate, we often assume a standard 400 cfm/ton but in your drier climate you may well be running a higher airspeed. In which case you might need larger.

Returns are one area where it's safe to say "bigger is better", with no real drawbacks. You can go larger and the result is to be gentler on your equipment, draw slightly less electricity, be better all-around. I used to have one 18-inch return for a 3.5 ton system flowing 1400 cfm, which is undersized. In my case I went up to double that size and that lets the whole system work better.

Filter area is another thing you did not mention directly, but is related and is also important. You would like to get 200 square inches per ton, or more. I originally had 171 sqin/ton, and after adding returns am now around 300-400. I estimate filter air speed is around 140-180 feet/min. Because of this I have no fear of those notorious high MERV 1-inch pleated filters. If you have less filter area, you might get too much restriction from such filters, in that case the safer answer is to use the see-thru kind.

Your HVAC contractor should be able to measure ESP (External Static Pressure) after he installs your system. Ideally you want a number good and low, perhaps under .50 inch water column. If the good man gives you enough duct size that will be the case. In any event you want that ESP number to be compatible with your air handler. My original system ran about .70 inch w.c. and its air handler manual stated never go above .50 -- get yours designed and installed *better* than mine was!

There. I have the sense you were looking for some tangible numbers, I gave you a few, and I believe none of it will contradict what a real pro would say. You should know I am a homeowner, very interested in getting my house right but very much an amateur at these things.

Hope this helps -- P.Student

P. Student,

I think your answer, and the others', have helped me understand a lot better what I should ask the contractor before and/or during installation. I believe what happened is that I focused most of my energy in finding the right contractor and making sure that the system was sized adequately, and in the process I neglected to get more information about duct size; I did read a very informative website though, from one of the forum members, which talked about the myths surrounding duct cleaning, mold, etc. I forget his name, but I thought his website was a very valuable resource.
In any event, I think now I have a better idea of what to look for. I didn't want to be like the lady whose mechanic told her that she needed to change the radiator in her VW Bug, hehehehehehe. Best regards to everyone.

While discussing what your HVAC contractor is going to do, there might be an opportunity to nudge him toward larger return choices. In my case the HVAC guy offered an 8-inch (flex) return to the master bedroom, I replied "why don't we make that a 10-inch?" and he readily agreed. It will depend on the personality and background of the man, if he has designed by-the-book Manual D then I might expect him to be more firm (perhaps even stubborn) in his opinions. My HVAC contractor articulated the philosophy "if you're happy then I'm happy" and I just suggested what would make me more happy. Since we were still in the stage of negotiating, that went down well with both parties.

My own personal opinion is strongly "bigger is *not* better" when it comes to A/C equipment, just that filters and return ducts are another matter.

BTW I bought the ACCA books on Manual D and Manual S, they have given me increased respect for the people who actually understand and use them.

Good luck -- P.Student