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  1. #14
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    Thread Starter
    My house is 2725 sqft with 3 & 3.5T AC. 419 sqft/T. As you say, maybe I could go smaller with AC. From energy audit, windows U value estimated at 0.51. i will have to look further into costs of windows, insulation, etc. Thank you for advice.

  2. #15
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    Thread Starter
    And if I could go *smaller* in my AC, wow, that upsets me that contractors are pricing high SEER units with larger tonnage than my current systems....

    I appreciate all the advice. I want to spend my money wisely on new efficient air cooling, and I am learning a lot here.

  3. #16
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    Jan 2004
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    Quote Originally Posted by clmssun View Post
    Thank you. This is something I am trying to clarify and better understand.

    I do note the higher SEER and greater rebates from the local utility company for "multistage", two-stage, and "variable capacity" units, but I am not sure that I would really all the benefits. (But why would a Phoenix electric company offer a larger rebate if the climate here is not conducive to benefits from multistage, two-stage, or variable capacity?)

    What is the difference between "two-stage" and "multi-stage"?

    2 stage is just 2 stages, as in low and high. Multistage is 3 or more stages, as in 20%, 40% 60%, 80%, 100% of capacity.

    The "two-stage" and "variable capacity" units require a "communicating" thermostat, but the "multi-stage" does not.

    Most 2 stage can use a standard 2 stage thermostat.

    Are "two-stage" and "variable capacity" already accounted for in SEER? Or do "variable capacity" and "two-stage" pave the way for better efficiency regardless of SEER?

    Already account for.

    I see that the four Lennox units have a different sequence of "SEER" than "EER". I am pretty sure that the hot dry climate of Phoenix is well outside the "standard" SEER climate; I also understand that EER is not variable with climate. In my hot dry climate, should I pay more attention to SEER or EER?

    Thank you again for all the advice.
    In very hot climates, EER. Unless its over sized.
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    How many times must one fix something before it is fixed?

  4. #17
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    Apr 2012
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    Millsboro, DE
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    Like I said, need to do other stuff today, I'm rushing! Your existing number is 419 SF/Ton and the people at the Alpha Hotel are trying to up you to 341. You only need about half the existing capacity, and a proper MJ calc should verify that.

    To follow up on the leakage:
    Existing: 124 CFM, 0.33 ACH (!!!)
    MJ "Tight": 0.05 ACH, 4+ CFM.
    The home's about as bad as it gets: ELA4 (equivalent leakage area) is 67.3 sq. in. (think 9" hole in the wall).
    2010 ASHRAE ventilation requirement: 60 CFM for 2 or 3 BR, 75 CFM for 4 or 5.
    I usually neglect the infiltration because the ventilation exceeds it - when you provide an outdoor air intake.

    Yes, with all of conscientious people we see in the Forum, there are still smooth talkers out there actively giving the Industry a bad rep. Sad but true. In this day and age there are people who just look at the existing nameplates and match the existing capacities, going larger when they have the opportunity. Two stage being a prime example: "Wouldn't you like a little more, to be safe?".

    The other thing is ductwork workmanship, which ACCA tells us "can make a 100 to 400 SqFt per ton difference" - Manual J is a book, not software, check page 94. That's why MJ and MRS require a duct survey as well as an MJ calculation for every replacement.

    I always look at SHGC (Solar Heat Gain Coefficient) first with windows, thus the 0.35 & 0.20 mentioned earlier.
    Last edited by ferd1942; 02-23-2019 at 06:08 PM. Reason: Corrected for 2,725 SF

  5. #18
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    Apr 2012
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    Millsboro, DE
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    Rule of Thumb: "NEVER GO BIGGER".

  6. #19
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    Feb 2019
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    Thread Starter
    So I have quotes from two contractors each with four different models of AC from their respective preferred manufacturer (Trane and Lennox). For the "most efficient" models (from *any* manufacturer), it seems 3.5T (the original size of my upstairs unit) is not offered, so I am being quoted for 4T upstairs. Given that all of these "high efficiency" units have multi-stage compressors and variable capacity, would that mean that the "oversized" unit would be running most frequently at "reduced capacity"? Does that reduce/eliminate the "oversized" concern? I understand how an oversized single stage unit is terrible, but I am wondering if multi stage variable capacity brings things back in line?

    Also, I hear it rationalized that multi-stage variable capacity units "run all the time...at varying speeds/capacities"... And I further hear it rationalized that that is better (for reliability and efficiency and temperature control) than a single stage "constantly cycling on full power then off then on full power". Is there merit to this train of thought? Or is this just slick marketing to encourage people to buy expensive multi-stage variable capacity units?

    https://www.airconditioningarizona.c...r-for-arizona/

    (I have not spoken with the folks who wrote the above blog article.)

    Thank you for your help.

  7. #20
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    Apr 2012
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    You are so like me: Beating a Dead Horse Until It's mush; my daughter talks about chalk and blackboards.

    Forget about two stage, you don't need it. If you absolutely must have it, buy two two's (700 SF/Ton high speed, 1,000 low speed). You'll save what, $15K? AND if you're lucky enough to have metal ducts to start with, sizing becomes a non-event, and you're only into UL tape or mastic (mostly), rather than having to enlarge 0.10" stuff to accommodate more air than it was "designed" for. Fibrous glass board installed in 2004's gotta be replaced, to include D Boxes on radial systems (MD Appendix 3 shows how to design the ACCA-required rectangular boxes, forget about the optional diffuser).

    Do put some of the 15 away for when you have to spend $1,000 to replace the indoor blower motor (that's the part, labor and incidentals are extra).

    With all due respect to the Skilled, Dedicated people who post here: There are contractors out there who deliberately oversize as part of their business plan. Do not let them scam you!

    Another option: My guy set up a meeting with a rep from a German manufacturer of fixed indoor CFM, variable speed compressor heat pumps he was raving about; I've been specing them lately and the clients and Builder are also raving. Their three ton unit can be set for 1-1/2 to 3 tons, in half ton increments (I'm done with sizing on this thread, so please don't ask about their 5 ton).

    Good luck!

    PS: I read the article; you gotta wonder about a contractor installing 5 ton units (5,000+ SF from one system).

  8. #21
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    Feb 2019
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    Thread Starter
    Thank you all for all the help and advice so far.

    I am now requesting two more (different) contractors to come to my house for estimates. I can now speak much more educatedly with them thanks to all the help you folks have offered me. I will seek a Manual J Load computation before final estimates, and I will inquire extensively about their recommendations for my home.

    Thank you for all the help. It has been very confounding and disappointing for the first two contractors to configure estimates for equipment which seems to be oversized, overspeced. I hope when I meet with the two additional contractors this week that I come to an understanding and "agreement" about what my needs truly are. (And I think I will be better served by improving insulation, etc.)

    Thank you again for all your help. I appreciate your advice, and I will certainly be back on hvac-talk if the next two contractors prompts more questions from me.

  9. #22
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    Apr 2012
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    Millsboro, DE
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    I work with a couple of local builders, and one called me a few years ago. It was time for him to replace equipment and upgrade his ducts, he'd received bids from two HVAC contractors who do his work and wanted my opinion. Would you believe they tried the exact same thing? Exact replacement on one and up to the next even number on the other. I'd have loved to be a fly on the wall at those meetings.

    So don't feel bad, it's what some contractors do, and it's a Code violation.

  10. #23
    Join Date
    Nov 2004
    Location
    SW FL
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    How much DUCT SURFACE AREA ( A ) in the 135'F - 150'F afternoon Attic ?

    Q. Sensible = A * DT/ R.duct

    R.duct = 6
    ….. Attic T - Supply Air T
    DT = 136'F - 58'F = 78'F

    Q = 13 * A ( SQ FT)

    100 feet of 10" diameter =~ 250 SQ FT
    100 feet of 8" diameter = ~ 200 SQ FT
    100 feet of 6" diameter = ~ 150 SQ FT

    _ _ _ _ _300 lineal Feet = 600 SQ FEET Surface Area

    Q = ~ 8,000 BTU/ HR ___ a.k.a approaching 1 ton
    ________ ____ ____ ________ … when one factors in the S.H.R. of ~ 0.8

    5 Tons is deemed reasonable for 2,725 Sq Ft 2 story =
    ______ 2,100 CFM in the AZ climate 108'F .:. ~ 20 Diffusers
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  11. #24
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    Apr 2012
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    Millsboro, DE
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    Thanks Dan!

    OMG...The carefully fabricated Snark Containment Box DW gifted me at our 1971 wedding as Engineers' wives do, and which we've carefully maintained for decades since, and hoped to reinforce on my return here has exploded!!!

    I don't the cause, but do beware when quoting "Science" , SF/Ton, etc.

  12. #25
    Join Date
    Nov 2004
    Location
    SW FL
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    With diurnal swings of 22'F - 32'F, I would definitely be pursuing 2-Stage equipment.
    https://www.usclimatedata.com/climat...saz0166/2018/8

    Attic temperatures are definitely a SIGNFICANT issue Given the location of ducts and the two A/C systems.

    … Post # 23
    1. Radiant barrier - 126'F max.
    2. R-8 ducts
    Q = A * DT/ R =
    ____ A * ( 122' - 58' ) /8
    Q = ___ A * 8 = 600 * 8 = 4,800
    _ _ _ Lots of $$$ to save ~ 3,200 BTU/HR
    _ _ _ _ _ _

    I would expect window replacement pay-back period
    to be > 70 years.

    Your A/C Systems definitely have a finite Remaining-Life knowing
    that operating hours per year actually are > ~ 3,300.
    _ I presume going on ~ 50,000 hours to-date.
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  13. #26
    Join Date
    Apr 2012
    Location
    Millsboro, DE
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    Oh Shit it bit me! Moderators forgive me, please as it's not my fault.

    I always liked D's back in the day Dan, didn't have a problem finding them when granted consent...they were humongous. On the other hand, some who granted consent had two points but nothing else...
    Last edited by ferd1942; 02-26-2019 at 07:41 AM. Reason: remove a "D"

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