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Return undersized?

Hello, thanks for providing such a forum. Wish I'd found it sooner. :~)

Can anyone clear up what is the ballpark minimum return size required for a 1400CFM air handler, assuming adequate supply side, sealed ducts, etc.?

Specifically, at issue is a 2/3 reduction in return duct capacity after a new heat pump/air handler installation. The installed unit is a Trane XR15 3.5-ton heat pump (4TWR5042G1000A) and XR single-speed ECM air handler (GAM5B0C42M31SB). The air handler is mounted in a garage HVAC closet of our 40-year-old, 1.5 story, partially underground, 2400 sqft home (the home being partially below grade was the rationale for undersizing by 0.5-ton). Ducts were sealed 10 years ago.

The house was built with 24x24 and 12x12 returns which meet at the base of the air handler, for a total of 720 sq.in. Trane specs the AH at ~1400 CFM, which requires 700 sq.in. by the "2CFM per sq.in" rule-of-thumb for AH sizing. Therefore duct size should have been adequate.

But due to the greater height of the new air handler, the installer downsized the returns into a 9" high x 24" wide box under the air handler. That's 216 sq.in., or 432CFM per the 2x rule. The HVAC company owner says this is more than enough by a significant margin.

To confuse me further, Trane's recommends a 20x22 filter for this AH model, which is only about 430 sq.in. internally, or 860CFM.

So the rule-of-thumb's 700 sq.in, the contractor's 216 sq.in., or Trane's own 430 sq.in. . . . what is adequate for 1400CFM? ?

Many thanks.

Comments

  • Don_197Don_197 Posts: 184Member
    When in doubt............the manufacturer wins. So you have to ask yourself "self?............should the installer have followed the manufacturers instructions??" (I think you know the answer)
  • Don_197Don_197 Posts: 184Member
    Also............pay attention to the installation manual in regards to what it says about a possible difference in sizing for side return vs bottom return..................there might not be any difference........but read carefully.......and insist that the installer follow the manufacturers installation instructions MINIMALLY.
  • EBEBRATT-EdEBEBRATT-Ed Posts: 6,026Member
    The 9 X 24 box under the air handler will handle 1400 cfm with a pressure drop of .10 inches of wc/100 ft of duct. This should be ok but has no extra capacity. Some would choose a lower pressure drop (thus a larger duct). the 20 X 22 air filter comes out with an air velocity of 500 feet/ min @1400 cfm which in my opinion is a little high on velocity but you are probably limited by the size of the air handler. (I am assuming the filter is in the AHU)

    The 24 X 24 & 12 x 12 you mention I am assuming are return grill sizes, not duct sizes. 3.5 tons should be 1400 cfm so that matches. All units require 400-450 cfm/tom
  • unclejohnunclejohn Posts: 1,420Member
    Send a picture of how the return ducts tie into the box. As mentioned above the 9x 24 box is ok for 1400 cfm @.10 static which is fine.
  • lost_n_austinlost_n_austin Posts: 3Member
    unclejohn, EBEBRATT-Ed, Don,

    Thanks very much for your enormously helpful replies. Good news, I hope? :~)

    The .10 value... is that a baseline value of sorts? What about filters, grills, dirt in ducts, etc?

    An interesting observation after the first filter change: the filter was definitely dirtier on the side adjacent to the return connection. It looks to me like more air is entering the AH on that side, and less on the side farthest from the return connection. Any significance to this?

    Would you recommend I have the pressure tested? I'm concerned that you think we are on the edge of adequate capacity. I listened carefully to suggestions that I choose the smallest tonnage practical. If airflow isn't adequate, we may be in trouble on blazing south Texas summer afternoons.

    An EZ-Filter base was installed under the AH. I'm not sure why as the AH has its own slot. The base takes up 4", which could have been used to increase the box height a by ~45%. What are your thoughts on this?

    I chose a Glasfloss MERV 8 20x25x2 filter. I found research data that suggested a .18 pressure drop for this filter. I'd like to use minimal filters at the registers to catch hair, etc. but haven't yet due to pressure concerns. How restricted am I as to filter choice?

    The duct dimensions are filter size, but the 24x24 duct is 25x25 externally at the AH, though it could be smaller elsewhere along its bizarrely circuitous, mostly hidden path. I'll take a closer look at the 12x12 tomorrow - it's as straight shot to the closet, only about 4-feet.

    I can't see the duct junction with the AH in place.

    Lastly, I have no desire to be "one of those customers" (which is why I'm asking here :~)) I just want to make sure I haven't unnecessarily lost capacity that I had and still need. The installer could have raised the AH, but that would've meant more work and materials rebuilding the supply connection. I believe cutting down the return size was the quick and easy path. But was it best choice for the efficiency of my system?

    Thanks,
  • SpenceSpence Posts: 316Member
    "What is adequate for 1,400 CFM?" This question you asked is an interesting one, because your question should start with another one: "What CFM is adequate to neutralize my cooling loads?" Your sensible load (total minus latent) and your delta T (from your sensible heat ratio) determine a target CFM, not some magical duct number.

    Trane uses the 2-per as a guideline for keeping you out of trouble, while hoping that installers do proper sizing. Your comfort and economy should mean more than depending on rules of thumb.
  • lost_n_austinlost_n_austin Posts: 3Member
    Spence,

    Thanks very much, Spence. At this point, my return capacity, AHU capacity, condenser tonnage are all fixed values which I must work within.

    The variables are the changes made by the installer, including the rather substantial loss of return connection size.

    Within that context. I need to figure out my next step, even if that step is to stop being concerned. :~)
  • Empire_2Empire_2 Posts: 2,343Member
    Looking at you detailed explanation, Don (hello Don) and Ebebratt-ed are on the money. I too have come up with 1400cfm on your current return set up. The MERV 8 should not be a problem in fact it is a standard pleat style filter and should be OK. Since you stated that the return extends to some remote location, we have to look at the inter connecting duct or cavity in may go through. That would determine your totals.
    In principal, 400cfm/ton is a designed factor, but I can and will use lower cfm in both design and application to effectively increase Latent heat removal as most mfg. do, especially with variable speed blowers. (Nice to have)
    Q; Do you know if your system utilizing an expansion valve (TXV) or fixed restrictior? I would assume, (I hate to do that) the contractor chose an evaporator with TXV.
    Your concerns are valid, but looks at least on the surface to be OK.



    Peace;
    Mike T.
  • SpenceSpence Posts: 316Member
    I understand you have fixed values, so let's see what we are facing. Your GAM5 is a constant air flow machine, so it will work to provide the set point regardless of the distribution system. However, the cost can be noise, poor performance, and elevated energy use.

    No one can tell that your system is right or wrong without a pressure test to compare to the blower performance chart in your IOM. Ask your installer to do one for you if not done at start-up.
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