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  1. #1
    Join Date
    Jan 2005
    Posts
    84
    I recently had a 2.5 ton upstairs unit replaced with a 5 ton. The installer added a 14" return to two existing 8" returns. The 14" only has an inlet opening of 12X14 due to existing electrical obstructions. No supply ducts were added. The new AHU has three oulets. One is a 16" supplying two 12" ducts. One of these 12" ducts feeds two 8" and one 6" ducts. The other 12" feeds one 8" and one 7". Another 8" outlet supplies one 8" duct. The last 8" outlet supplies a 6" and 5" duct. The 8" and 7" ducts have 6X14"" registers. The 6" ducts have 6X10" registers. The 5" duct has a 4X8" register. This unit is cooling approx. 1500 ft2 plus a foyer (1700 ft3) that gets very hot in the DFW heat. He set the VS blower to run at 450 cfm/ton to help cool this area and augment my 21yr old downstairs units. The unit is also set up in comfort-r setting. Humidity removal is decent (60%). However, the wind noise at the registers, esp the undersized return is quite noticeable when blower goes on high speed. He knew this might be a problem at the current blower speed setting. Backpressure? My main question here is how can I determing if I need to add either return/supply ducts and if so, how many/size? The unit currently cycles on the small compressor with the 5 ton running only occasionally. Run time is 12hrs/day. It seems to me a unit capable of moving up to 1800-2300cfm needs more ducting. I'm running a disposable fiberglass filter upstream of a Honeywell F300E EAC. After approx. 300 hrs of runtime both are still fairly clean.
    I'd like to get some educated opinions before confronting my HVAC guy. Is this system being optimized?

  2. #2
    Join Date
    Apr 2005
    Posts
    36
    OK, it seems to be airflow class day on this site.

    First of all, your HVAC contractor should have done a Manual J to properly size the A/C equipment and subsequently done a Manual D to design and layout the ductwork. If he can't immediately produce these printouts, then unfortunately you hired a hack. No contractor should ever double the size of a unit in a house without a load calculation to justify why he's doing that. You didn't say if you remodled the house and doubled the upstairs floor space, but that would be about the only reason that I can think of to do this, unless the 2.5 ton unit was way too small to begin with. However, one wouldn't know that without a proper Manual J.

    With that said, how do you move forward?

    It sounds like you are way undersized on return and probably undersized on supply for a 5 ton. You mention that they supply registers make noise, you can change the stamped metal grills that you probably have for higher airflow, more expensive fully adjustable curved fin grills which will reduce air noise.

    You should have a minimum of 360 sq. inches of ductwork for your supply and return respectively. According to your statements you have: 254 sq inches return and 301 sq. inches supply (one 16" and two 8" directly off the plenum)

    Your 12x14 return grill probably has a "net free" area of 50% of the actual grill Sq. Inch measurement so it is probably about 84 sq. inches net free area if it's a Hart & Cooly grill. This means your 154 sq. inches of 14" duct is restricted at the grill opening to only 84 sq. inches. So in actuallity you're worse off than 254 sq. inches of return duct. You didn't mention what the 8" ducts for the return use for grills, but assuming they are 12 x 12 grills, you're not restricting the 8" duct like you are the 14" duct. As a rule of thumb, the minimum duct sizing for a 5 ton unit on return would be two 16" duct runs. Remember that 16" duct does not equal two 8" ducts in net free area. To determine sq. inch area of a circle you do: Radius squared x PI

    I would expect that your static pressures are way high, your Variable Speed motor has a higher than rated Amp draw to move the air against such a high Static pressure. You condensor head pressure is probably high due to inadequate airflow across the evap coil, and you are basically killing this new unit slowly but surely.

    It's a safe bet your genuius installer never heard of Manual J or D and probably doesn't have the equipment to check your static pressures or do an accurate Manual J or D.

    You can read my response to the previous post on high static pressure for more details of Static pressures. I'm sorry you've been screwed. Anyone who sold you on the idea of augmenting an 21 yr old downstairs unit by dumping more cold air upstairs should be Stooge Slapped in public.

    Good luck.

    Mike








  3. #3
    Join Date
    Mar 2002
    Location
    Concord, CA
    Posts
    2,633
    The problem with trying to ascertain if your ducts are too small is that we don't know the total equivalent length of the ducts.

    But even with short runs those returns are small. A 14 inch duct has about three times as much area as an 8 inch duct. So the 14 would need to handle about 1350 CFM and the 8's would need to handle 450 CFM each. That would mean a velocity of over 1200 feet per minute! That's twice the recommended velocity. Except for the 14 being replaced with a 16, your supplies mirror the returns. So you're going to have high velocities there as well.

    To add to your problems is the fact that your VS blower is probably not capable of delivering 2250 CFM (450 CFM per ton times 5) with such small ducts. So it's most likely running flat out (as fast as it can) which will could theoretically shorten the motor controller's life. Though at the same time I'd disagree with Mike about it being beyond its amp rating. VS blowers don't just speed up until they break. They have limiters and will stop at a point that they're rated to run for - or at least they should.

    And I might disagree with Mike about being screwed. If your guy warned you about this possibility AND if he offered solutions (duct upgrades) that you declined because you didn't want to spend the dough, then you may very well be getting what you deserve. But if he sugar coated it and told you not to worry, then he's an idget.

    Here's my biggest question: Why 450 CFM per ton? If you're running about 60% indoor RH then that needs to be lower. A setting of 350 CFM per ton sounds right for your humid area. At 350 CFM per ton your blower would push 1750 CFM. Most 5 ton VS blowers CAN push that much air against some pretty darn restrictive ducts, probably including yours. In other words the blower would not be running flat out. You'd get better humidity control. The unit would probably be running within the blower performance data chart's allowed limits. And it would be quieter. Do that and upgrade to some high quality diffusers and while it wouldn't be an optimum situation it might be workable.

  4. #4
    Join Date
    Apr 2005
    Posts
    36
    Irascible,

    Good point on the TEL, and my 72 sq. in per ton rule of thumb is just that. Without SP readings and more info it's just a quick way to see how much trouble we're in when looking at a customer's ductwork.

    That's why I said a Manual D was in order.

    Also, the VS motor should stop before it breaks on Amp load, but perhaps I wasn't clear in my statement. What I should've said is that ECM motors do draw more amps as the Static Pressure rises. The more resistance you put on them, the more amps they draw trying to push their programmed CFMs. The older AC motors don't do that, they actually draw fewer amps the more resistance in SP you put on them, and the fewer CFMs they move.

    I know this because we rigged up a test system that we could put on a VS motor airhandler and a standard AH and we checked the CFMs produced and the Amp draw as we opened and closed a damper to increase SP on the motor. I know it's geeky, but it was enlightening.

    The last point you made good, 60% RH is lousy and symptomatic of an oversized unit. Slowing the fan speed down should slow the air down across the coil and draw out more moisture, but it is also going to need a service guy who can set the charge properly. Still my opinion is that this guy should ask for a Manual J & D printout from a recognized ACCA certified load calc tool, and if he doesn't get it, he should demand restitution from this "idget".

    BTW: what does Idget stand for?

    Mike




  5. #5
    Join Date
    Mar 2002
    Location
    Concord, CA
    Posts
    2,633
    I'm not sure that idget stands for anything. I believe Bugs Bunny used the term along with other esoteric insults like "maroon".

    Your expanded explanation on VS blowers hits it on the head. For such an obviously smart guy I figured the "beyond its rating" remark was more a slip of the tongue then anything else. Either that or I'm being pedantic. He'll still be using unfortunately high amps at 350 CFM per ton. But at least there his system isn't as likely to be "off the chart" anymore.

    I'm not so sure about this "recognized ACCA certified load calc tool" business. I use the professional version of HVAC-Calc. I'm not sure that it's a recognized ACCA certified load calc tool. But it does the job. I think it might be better said that he needs (or needed, it's too late now) a real load calc (as opposed to rules of thumb and short forms). Of course, that's basically what you were saying. I'm just being pedantic again.

  6. #6
    Join Date
    Apr 2005
    Posts
    36
    Good word use of the word "Pedantic"! LOL

    I can't vouch for HVAC Calc, I use Wrightsoft and love it. I took Hank Rutkowski's EPIC instructor course a few years ago and got convicted of the importance of Manual J & D, etc. as a tool in my bag as important as a gauge set. However with all due credit to Hank and ACCA, Manual J manually is a pain in the youknowhat. So, we invested in Wrightsoft and I've worked the calculations manually and with Wrightsoft a few times and found Wrightsoft to be pretty darn close. Of course it has ACCA's seal of approval which according to Hank is like making a deal with the devil. Wrightsoft is a donor to ACCA and therefore gets the "seal of approval". Hank seems to think that if you don't do it by hand, you are gonna screw it up. The problem with doing it by hand is there are too many inputs you can screw up, tools like Wrightsoft are easier to use, and therefore less prone to math and input errors.

    Responding to posts on this website acurately enough to not draw critisim of pros like you but still be clear enough to a layman is a dificult line to walk. Especially when the laymen obviously doesn't know the lingo and jargon, much less understand the principles and relationships that the various components have. That's why I don't respond more often, but I have time to kill today! For a change.

    The 72 Sq. Inch thing per ton actually comes from the Mechanical Code. And although I haven't actually done the legwork on this, I believe it's based on a 900 FPM velocity rating from the Ductalator. One of the biggest problems I encounter is undersized returns and sometimes undersized supplies. The number two problem I see is oversized systems. And of course crappy quality installs are ever present. That's why I responded to this thread in the first place.

  7. #7
    Join Date
    Mar 2002
    Location
    Concord, CA
    Posts
    2,633
    I ran both Wrightsoft and HVAC-Calc on a job once and they came out pretty close. But I don't think HVAC-Calc's author paid ACCA off. And of course that was based on J7. HVAC-Calc never upgraded to J8. So I guess I'll never know if I have adequate exposure diversity. From what I've seen that's just code for being oversized by that much more. J7 has been documented to oversize as it is. Given the fact that I cooled a supposed 5 ton load with 4 I tend to believe the study. J8 appears to oversize even more - or at least that's what those in the know tell me.

    People that do Manual J by hand, wow - God bless 'em. Our word of the day seems to apply towards those types. But I don't want to overuse it. :^)

  8. #8
    Join Date
    Jul 2002
    Location
    Zelienople, Pa
    Posts
    2,965
    tmoss,

    Good luck with your future compressor replacements...
    How tall are you Private???!!!!

  9. #9
    Join Date
    Mar 2002
    Location
    Concord, CA
    Posts
    2,633
    While the idea is not without merit, I think you're being alarmist Yellow. The worst performing 5 ton Trane VS blower can push 1750 CFM at .9 inches water column. I'm sure most OEMs aren't far off. If the coil PD was .3, the filter .2 and the grilles and other fittings .1 that would still leave .3 inches ASP for the ducts. Even those very small ducts can most likely (unless they're reeeal long) accommodate 1750 CFM with that much available static.

    Now if there's a fixed metering device and tmoss likes it 68 degrees in the house then maybe. But that's more a problem of usage than ducts.

  10. #10
    Join Date
    Jun 2005
    Location
    Clearwater,FL.
    Posts
    34
    another example of improper sizing & application of very expensive high end equipment at the cost of the customer & reputation of hvac companies that would have never done this garbage.probably the low bid

  11. #11
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
    Posts
    3,304

    Duct noise?

    Tmoss, I would expect some major whooshing sound coming from ducts which were designed for 2.5 tons, and now flowing enough air to serve 5.0 tons. If you hear excessive duct noise, that is more evidence the ductwork is inadequate (if you don't maybe things are better than we think). I think the Manual D air speed recommendation of 900 fpm for hard duct (600 fpm for flex) are in line with what you prefer for quietness.

    It's sad and amusing how some AC guys take their inspiration from Tim Allen of "Tooltime". Whatever the situation they try to give you "MORE POWER!!!". You would be more comfortable and pay lower utility bills going with someone with more brains than that.

    Best of luck -- P.Student

  12. #12
    Join Date
    Jan 2005
    Posts
    84
    OK OK OK! Enough of the personal attacks on my HVAC guy. I'm soliciting PROFESSIONAL opinions and possible solutions. Calling him a hack etc. without knowing all the details is immature and makes me think some of you just want to sound off without positive comments. I pursued this site for help.

    Here are the facts. Last year this same guy ran a Manual J and determined I needed between 3.5 and 4 tons. Since we were only staying in this home for 5 years, instead of replacing the existing undersized unit he added an additional return and a larger blower to help make the upstairs tolerable. He gave me a quote for a 14 seer unit if I elected to change out the 20 year old existing unit. Well, we now have 10 week old twins and are staying in this home for 10+ years. I contacted him two months ago to install a more capable unit. I asked if him a XL19i would be more cost effective since we're not moving for awhile. After reviewing my home layout etc. he thought that the 5 ton XL19i would probably only run on the 3 ton compressor most of the time. This is, in fact true at this time. My electric usage is down 25-30% and I'm keeping my upstairs 3 degrees cooler AND keeping my foyer area cooler as well compared to the old inadequate unit. The run time on one of my downstairs units has decreased with no loss of cooling comfort.

    The 16" return he added has a 12X30 inlet grill to a closet. (remember this is retrofit; this was the best option for a large return.) The ceiling has a 12X14 return because an existing light fixture prevented the proper size from being installed. He pointed out that the light needs to be removed for him to make it right. He also commented that another 8" return would help the unit when running on high speed. BUT, unless I keep the T-stat on 70 it seldom runs on high. In fact, it's usually set on 76. 73 get me 50-53% RH but that's too cold for my babies.

    Did he run a Manual D? I doubt it. He knows my duct work and has made comments about adding or upsizing a couple of them as well as changing out registers, etc. for better airflow. Do I have it all in writing? No. I've known this guy for 6 years. His service and advise to me has been just fine. He kept me from wasting money in my last home. He's a co-owner working 60+ hours during the hot DFW summers. Does he want to make my system perfect? Well, he's pragmatic. He discourages people from wasting money. (I now have 5 kids) His primary goal was to get my new unit installed and running. He told me to let him know when it would be convenient to upsize the return duct and add/upsize any supply ducts. My wife appreciates him not wanting to inconveniece her any more than necessary. Right now our home has never been more comfortable. So, it hasn't been our number 1 priority especially with twin babies.

    I titled this post air balance because I wanted to know how much ducting this unit needs to run the way it is designed to run. I trust my hvac man. I know he won't encourage me to spend money doing something he doesn't think is beneficial. If I want to add or modify my ducting differently than what he suggests I need to have some intelligent and valid data to support it.

    Positive comments, please.

  13. #13
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Your HVAC guy though honest and well intentioned,likely doesn't have the tools or training/knowledge to do what's needed.It's certainly not uncommon,is this industry.

    Yes ,calling him a hack ,is out of line,IMHO.

    They did a Man. J ,but installed a five ton ,instead of 3.5?My guess is this was to help cool a foyer area that may be two stories,which is fine if you have a supply and return in that area.


    He needs to use Man. D to redesign the duct,likely needs a Man. J,room by room,to know the required air flow to each room.

    High humidity, ,go to 350 cfms ,per ton,if comfort R is properly set up,your humidity is likely to be reduced closer to 50%.


    Without the Man. J and D,it would hust be a guess to advise on the changes to be made,increase some or all dcuts and grilles,etc,etc..

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