Page 1 of 2 12 LastLast
Results 1 to 13 of 14
  1. #1
    Join Date
    Aug 2010
    Location
    Dallas / Ft Worth Metroplex
    Posts
    7

    Effect of reduced return air on performance ?

    I've read a number of posts regarding return air that emphasize the need to correctly size the amount of return air. A contractor recently stated: "a system can only put out what it takes in" . He was pointing out that each of our units had less than adequate return air and if we expanded the return air we would likely see a significant improvement in performance. (This was a side note and not a selling point).

    Each of our 4 systems have only one return air that is typically an opening in an adjacent wall and in one case the wall goes to an adjacent closet with a louvered door. The largest return air opening we have is 8" x 20" on a 3 ton system with a 16" x 25" filter. The rest of the return air openings are smaller and that includes a 5 ton unit. The house was built 35 years ago and is located in northern Texas.

    My questions: If we increase the vent openings to match the size of the filters, what performance gains might we see. ie: overall percent increase in system performance, increase in EER or SEER, shorter runs times ...etc. I am interested in some kind of qualified measurement.

    Also, the three ton unit I mentioned has the loudest return air noise of all the systems even though it is identical to one other system. So if we increase the return air openings will that increase our interior noise levels?

    Thanks

  2. #2
    Join Date
    Jul 2010
    Location
    Maryland
    Posts
    35
    your noise level will decrease with the proper return openings. The excess system noise you are hearing is being caused by the blowers having to increase the static pressure to compensate for the lack of return. Also the return opening will need to be larger than the filter size if the return grills are louvered due to the louvers affect the free are of the grill. the noise is also much louder if the returns are not ducted with at least one elbow or 8' of duct before the return grill. In most cases the return sizes you are listing would not work at all leading to the evaporators freezing. so as far as increased performance you are lucky they are currently working at all

  3. #3
    Join Date
    Jul 2004
    Location
    Massachusetts
    Posts
    6,837
    The problem is there's no science in either your systems design or your question. What the tech said is that if you have an under sized return, then the supply air volume is being controlled by the return volume. However, increasing the size of the return may have no effect overall if the supply side is also restricted, which is most probably the case. I say most probably because if the system(s) had been properly designed at the beginning, then the returns wouldn't be undersized and neither would the supplies. So before you make any moves or spend any money, please make certain that the company you're dealing with is properly qualified. Proper qualification is easily observed when a company states that they can't determine any remedies or problems with airflow in your system without for doing a static pressure test and then a full, room-by-room load analysis on the home. The purpose of the static test is to determine if your system suffers from "high blood pressure", a sure sign of duct problems. If the static is high, then the room-by-room load analysis is the only way to determine if the equipment is properly sized for the house (or each zone in the house) and based on equipment size and airflow needs, the duct system can be analized and recommendations to relieve the "high blood pressure" can be made. Any other procedure is just a technician guessing at what you need and you gambling with the money you give him that he guessed correctly.
    If YOU want change, YOU have to first change.

    If you are waiting for the 'other guy' to change first, just remember, you're the 'other guy's' other guy. To continue to expect real change when you keep acting the same way as always, is folly. Won't happen. Real change will only happen when a majority of the people change the way they vote!

  4. #4
    Join Date
    Aug 2010
    Location
    Dallas / Ft Worth Metroplex
    Posts
    7
    The contractor's observation / comment came up while preparing a quote for one new system. He was stating that he would significantly increase the return air on the new system and after explaining why, I asked him to look at the other three systems return air flows.

    This is my in-laws 35 yr old house with original duct work. I believe they have been repairing and replacing components over the years without taking an overall system approach that included ductwork assesment.

    The oldest condenser unit, a Janitrol, began tripping the breaker on these hot Texas afternoons. As it is an R22 unit, it was recommended that we replace both the condenser and evaporator with R410 units. We were told the yard men damaged the cooling fins on this over 10 yr unit.

    As my in-laws were complaining about their last month's electricity bill doubling from $600 to $1200, I began to suggest that they might want to consider a complete system replacement with a high SEER rating and taking advantage of the Federal Tax Credit.

    After reading some comments on this board that suggest changing an 8 to 10 SEER system for a 16 SEER system will not generate the claimed savings in electrical cost nor pay for itself in 7 years, I began to look for other cost saving measures that might generate a higher return on investment such as increasing the return air volume. I didn't know how to quantify an increase in return airflow and was looking to add that to an ROI list of options.

    I have two system quotes and will be getting two more soon. I have also scheduled two energy audits, one of which includes a blower door test.

    Any comments on the overall situation are appreciated.

  5. #5
    Join Date
    Jan 2008
    Posts
    2,004
    Utility bill from 600 to 1200?
    That's the best reason I can think of to have the ductwork fixed first, then go from there.
    Just having a "complete" system replacement,(condenser and evap), will not solve the whole problem.
    "Hey Lama, hey, how about a little something, you know, for the effort." And he says, "there won't be any money, but when you die, on your deathbed, you will receive total consciousness." So I got that goin' for me, which is nice. - Carl Spackler

  6. #6
    Join Date
    Jul 2010
    Location
    Maryland
    Posts
    35
    Although this would probably go against a lot of peoples beliefs I like single stage compressors which limits you to in most cases 15.5 SEER. In my opinion there is less to go wrong over the life of the system. When I quote system I look at the AHRI rating not the manufacture ratings. I have seen many unit called 16 to 18 SEER by the manufacture only produce 15.5 SEER according to the government testing any way. To be fair this is because the test are run at the high speed not there more efficient low speed. Really in my experience the smaller size unit hit the higher SEER ratings easier than the larger units. In 5 ton models it is difficult to reach the extreme high Seer ratings no matter what. I think mostly due to the size of the evaporator (you can only get an indoor unit so big before it won't fit in most applications). I would have any duct work issues addressed when changing the equipment. Also remember the FTC is capped at 1500 per year no matter how much you spend and it doesn’t apply to the ductwork.

  7. #7
    Join Date
    Aug 2010
    Location
    Dallas / Ft Worth Metroplex
    Posts
    7
    Thanks Turner Mech. I particularly appreciated your comments on the noise levels. Two systems do not have elbows nor 8 ft of duct in their returns. Good info. Problem is, you would have to move the entire systems to the far side of their closets in order to put in an elbow.

    Skippedover; None of the A/C contractors have offered duct work assessments. Three are highly rated and well established and one was recommended to me. I'm hoping the energy audits might help with the big picture.

    2old2rock; During the month that the electricity bill doubled, my sister in law was running a summer camp. Lots of indoor / outdoor traffic.

    Have I read correctly that a 16 SEER system is unlikely to pay off in 7 years?
    Electricity rate is ~14 cents during summer months.

  8. #8
    Join Date
    Aug 2010
    Location
    Dallas / Ft Worth Metroplex
    Posts
    7
    With no response to my electricity savings question I have to conclude a new system can't pay for itself.

    Just FYI, I'm the one who thought is was possible to get a 10 to 15 percent return on investment. I gleaned this concept after reading energy articles on the benefits of upgrading to a high SEER rated system.

    None of the four contractors have suggested this was possible, only that a new updated system is a good idea and that we would enjoy the benefits of the new warranties and greater comfort. To their credit, the ROI argument has not been a selling point from any of the contractors.

    I'm still waiting on the last two bids and a blower door energy audit. I expect they will all be complete this week.

  9. #9
    Join Date
    Nov 2007
    Location
    Texas
    Posts
    425
    Here is a link to a energy cost calculator. http://www.hvacopcost.com/ Choose your area, and energy cost/kw. Then enter your existing seer in the standard, and the proposed replacement seer (be sure to change to variable speed if that is being quoted), and this will calculate the expected energy savings over a 20 year span. I have not followed up on this, or endorse the accuracy. There are many calculators like this that can be compared. Good luck.

    edit: I checked this against a Lennox site calculator, and the 5 year projections were close ($90 difference). That being said, the efficiency of the system will not be attained without the proper installation. A top seer rated system with poor duct work, improper line set, improper charge etc. can still cool the house but at a significant loss of efficiency.

  10. #10
    Join Date
    Sep 2006
    Posts
    1,253
    Roadszx,

    I have traveled the path you are on with my own home. If I may, let me share with you what I learned in the process.

    When I purchased my home five years ago I had high energy costs and was not comfortable. On the second floor of my home there was a 10+ degree difference between the front and back of the house and the hot side (summer) was where the master bedroom was. Air flow in that area was almost nonexistent.

    Of course, I did not know squat and turned to local contractors for help. Everyone was trying to sell me something - typically new systems. One even went so far as to tell me the high SEER would resolve my comfort issues. Another guaranteed (verbally) that I would be comfortable and save money but could not tell me what he would do to make sure that both happened. Lots of BS.

    Started to read and research and found things like load calc, blower door test, flow hood air flow measuring, and such. Conclusion from all this is you have to be able to measure performance of each of the systems and compare it to the needs of the home. Measure, evaluate, and then develop a plan of remediation. Only with good data can you make good decisions.

    The absolute beginning of the process is the development of a comprehensive load calculation. Without it everyone is totally guessing on whether any measured performance meets the needs of the home. A properly prepared load calc will be done on a room by room basis and it will provide you with both the heat and cool loads and also how much air flow is required for each room.

    With that data you can then start assessing what the existing system are doing with regards to how many BTUs are being delivered and how much air flow each room actually has. This would also include whether return ducting is sufficient. Static pressure testing should also be part of this process.

    You are likely to find that the systems are oversized and the duct systems are undersized, especially the returns. But now you will know what each room needs and the remediation plan can address each issue.

    In my case the returns were way too small and I was not getting anywhere close to the amount of air through the system. The supply ducting was not as bad, but on the smallish side. But once the equipment was sized right the reduced air flow requirements were handled by the supply ducts reasonable well. Balancing dampers added to each branch run allowed room by room air flow balancing.

    Look for contractors that up front will tell you they will do a load calc and design/remediate duct work using Manual D. Do this by phone so you don't waste your time or the contractors. If they do both then they are likely to also do the right thing on helping you come up with a plan to deal with the whole house.

    Good luck.

  11. #11
    Join Date
    Jun 2010
    Location
    Houston
    Posts
    36

    Let me recap what mchild says

    Another homeowner here.

    The summary of a 6 month story: if you don't have the return supply sized adequately, you can pretty much give up. Period. My old system had returns that (to put this in terms a mere mortal understands) would hold a filter in place securely, but not try to "eat it", were not noisy, and seemed to be OK. When we got through I had twice the return. That one change was night and day. THEN, we started fixing problems - and that is a long story. The end result is a system that is running right now and aside from the slight, slight breeze of cool air, I can't tell it is on. Keeps the house at 73 with < 50% humidity in 95 degree Houston weather. This is with run times of 12 - 15 minutes typically.

    I don't think this piece can be stressed enough. After this, then duct work for the supply. As my contractor says, "It's all about air flow."

  12. #12
    Join Date
    Oct 2009
    Location
    Phoenix
    Posts
    155
    Quote Originally Posted by Roadszx View Post
    With no response to my electricity savings question I have to conclude a new system can't pay for itself.

    Just FYI, I'm the one who thought is was possible to get a 10 to 15 percent return on investment. I gleaned this concept after reading energy articles on the benefits of upgrading to a high SEER rated system.

    None of the four contractors have suggested this was possible, only that a new updated system is a good idea and that we would enjoy the benefits of the new warranties and greater comfort. To their credit, the ROI argument has not been a selling point from any of the contractors.


    How about my real-world example:

    I doubled my attic insulation (to R50) , replaced half of my windows, installed an 18 seer two-stage heat pump - (doubled the return/filter size as well) and installed a heat-pump water heater, to replace my standard electric tank, changed over to CFL's and LED lighting, etc.

    This is on a 1600 sq/ft house in Phoenix. 22 cents/kw "on peak" pricing.

    According to the power company, similar-sized homes in my neighborhood payed $323 for electricity in July - I payed $223. <--- July is typically our "worst" electric bill of the year.

    No funny business, the thermostat has been set at 74 degrees 24/7 all month...

    It's a decent bill for Phoenix, and I can certainly find somewhere to spend that $100 I saved, but we have several months where we don't really have to heat or cool the house, so best case, we might be saving $600 a year - with all of those changes.

  13. #13
    Join Date
    Jun 2010
    Location
    Philadelphia PA
    Posts
    2,190
    Your 20x8 return openings (with a grill?) are a little more than 1 Sq. Ft. the rule of thumb is 1 SF/400 to 500 CFM face velocity ( mostly for noise) SO you should look for an 18x18 Grill as a starting point.
    Your description about return openings going into closets with louvered doors( is that right) is not close to Good Practice.
    Thing is your unit has a finite amount of pressure available to deliver the air. What you describe just wastes that air pressure that it take a fan motor and $$$ to produce that pressure diff
    You have got to learn from other people's mistakes! Because God knows you don't live long enough to make them all yourself !!!!!!!!

Page 1 of 2 12 LastLast

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Comfortech Show Promo Image

Related Forums

Plumbing Talks | Contractor Magazine
Forums | Electrical Construction & Maintenance (EC&M) Magazine
Comfortech365 Virtual Event