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  1. #1

    Did 'Manual J' calculations...

    thoroughly as possible using HVAC-Calc Residential 4.0. At the default setting of indoor summer temperature of 75 the BTUH is 25,059. Dropping the indoor summer design temp to 70 increased the BTUH to 28,637. [And regardless of the humidity my wife will set it that low].

    Seeing how we'll eventually sell the house [we don't plan on dying here] and there is only the two of us would a 3 ton two stage unit be a wiser choice seeing as how whoever buys it will probably have children?

    I've been thinking about Rheem and I've been doing more homework. To get the tax credit I'd have to use an aftermarket coil because the none of the furnaces on Rheems page of qualifying matched systems http://216.122.22.11/FetchDocument.a...5-f72b5eccd482 are narrow enough.

    Any advice?

    Searching the CEE site http://www.ceedirectory.org/ceedirec...ultSearch.aspx gave me some matches that earn the tax credit and that would fit [the furnaces are RGPR-07A models, the narrower ones}

  2. #2
    Join Date
    Jun 2004
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    4H: Hot, Humid Houston H.O.
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    Quote Originally Posted by Thebil Illpay View Post
    thoroughly as possible using HVAC-Calc Residential 4.0. At the default setting of indoor summer temperature of 75 the BTUH is 25,059. Dropping the indoor summer design temp to 70 increased the BTUH to 28,637. [And regardless of the humidity my wife will set it that low].

    Seeing how we'll eventually sell the house [we don't plan on dying here] and there is only the two of us would a 3 ton two stage unit be a wiser choice seeing as how whoever buys it will probably have children?

    I've been thinking about Rheem and I've been doing more homework. To get the tax credit I'd have to use an aftermarket coil because the none of the furnaces on Rheems page of qualifying matched systems http://216.122.22.11/FetchDocument.a...5-f72b5eccd482 are narrow enough.

    Any advice?

    Searching the CEE site http://www.ceedirectory.org/ceedirec...ultSearch.aspx gave me some matches that earn the tax credit and that would fit [the furnaces are RGPR-07A models, the narrower ones}
    What number is the sensible (thermometer) component, and what is the latent (humidity removal)? You need your equipment to meet requirements on both, as a homeowner I went through Manual J on Hvac-Calc and was told on this board that usually sensible capacity is the limit reached sooner (among other things). If you show the report to a pro who does your install, he should be able to make the choice of equipment which will satisfy requirements.

    Just curious what are the design conditions for outdoor summer temperature and humidity? I live in a hot-humid climate and worry a lot about humidity, you may not face those challenges. Maybe it's just making conversation rather than necessary info, but I am curious about where you are.

    Best of luck -- Pstu

  3. #3
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    HVAC-Calc is not "an approved" software and thereby shouldn't be used. You should use Elite or wrightsoft along with manual S and manual D. It would be wise to let someone with experience do this for you.

  4. #4
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    Quote Originally Posted by air2spare View Post
    HVAC-Calc is not "an approved" software and thereby shouldn't be used. You should use Elite or wrightsoft along with manual S and manual D. It would be wise to let someone with experience do this for you.
    Interesting point of view to be posted on this board...
    http://hvaccomputer.com/talkref.asp

    While I have no doubt that Air2Spare would do a fine job, it's not always wise to "let the perfect be the enemy of the good". There are pros and cons of every approach, even the latest version.

  5. #5
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    "Interesting point of view to be posted on this board..."

    Not really a point of view, but a fact.

    Could be an issue if local authorities or contracts require use of an ACCA approved load calculation program.

    Why isn't HVAC-Calc approved? Got me. Could be nothing more than not wanting to pay licensing fees to the organization granting the approvals.

    HVAC-Calc seems to work just fine to me, though.

    OP said: To get the tax credit I'd have to use an aftermarket coil because the none of the furnaces on Rheems page of qualifying matched systems are narrow enough.

    Ah, but then will you have a "matched and approved" system?

  6. #6
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    Quote Originally Posted by neophytes serendipity View Post
    Ah, but then will you have a "matched and approved" system?
    Absolutely - who will certify this combination of condenser, aftermarket coil and furnace? Certainly not Rheem! You may be better getting the RASL condenser - a little bit more money but a wider range of qualified combinations. Unfortunately, the 3 ton/70,000 BTU Rheem combination has the fewest number of qualified matches for some reason (?!)

  7. #7
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    Quote Originally Posted by Thebil Illpay View Post

    Any advice?
    Yes.

    Choose a manufacturer that offers a certified combination right out of the box.

    If the combination you choose isn't on a list of certified combinations, no tax credit for you.

    If you have narrow or other specific installation requirements, then you may need to do some more work to make the equipment fit if you want that tax credit.

  8. #8
    Well, the ARI number for the combination I prefer is 3385978. This uses an ADP coil. There are two other combo's that get me the tax credit, but one uses an Aspen and the other uses a benchmark. If I recall, I didn't like the Aspen because it uses a galv. sheet metal condensate tray with stainless as an option. Benchmark doesn't even have a website extolling the virtues of their units. 3385978 is CEE Tier 3 (Advanced) 13 EER, 16.5 SEER - bingo - tax credit.

    Yes, Rheem has right out of the box units that the earn tax credit. But none of the nine or so options fit my situation. I actually called Rheem this morning to find out whether the standard width furnace cabinet -17.5" [A]- would work but I was told no, it had to be the 21" [B]. His explanation had something to do with restriction of airflow into the coil by using the standard width furnace going into the larger coil case. If he'd of said no problem it would have been a simple matter to order the adapter.

    I like the phrase "don't let the perfect become an enemy of the good" -excuse me if I'm slightly misquoting. I just ignored the comment about HVAC-calc not being good enough because using HVAC-clac the results are close enough to size an A/C. The standard jumps in BTUH capacity of compressors seem to be around 6000. If a tech exceeded the BTU of a 2 ton unit by only 500 BTU when using a real Manual J calculation and performing it perfectly would he recommend a 2 ton or a 2-1/2 ton? I already know the answer.

  9. #9
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    Quote Originally Posted by neophytes serendipity View Post
    [B
    OP said: To get the tax credit I'd have to use an aftermarket coil because the none of the furnaces on Rheems page of qualifying matched systems are narrow enough.

    Ah, but then will you have a "matched and approved" system?

    Third party coil manufacturers rate them. it is a match system as far as ARHI is concerned.
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  10. #10
    Quote Originally Posted by pstu View Post
    What number is the sensible (thermometer) component, and what is the latent (humidity removal)? You need your equipment to meet requirements on both, as a homeowner I went through Manual J on Hvac-Calc and was told on this board that usually sensible capacity is the limit reached sooner (among other things). If you show the report to a pro who does your install, he should be able to make the choice of equipment which will satisfy requirements.

    Just curious what are the design conditions for outdoor summer temperature and humidity? I live in a hot-humid climate and worry a lot about humidity, you may not face those challenges. Maybe it's just making conversation rather than necessary info, but I am curious about where you are.

    Best of luck -- Pstu
    Sorry I didn't get back to you sooner pstu. My sensible gain @ 70 design temp was 25,417 and latent 3,220 for a total of 28,637. At 75 design temp the sensible gain was 22,391 and the latent 2,668 for a total of 25,059. And I am reasonably certain these numbers are low because HVAC-calc doesn't allow me to enter the actual R value of the insulation in my walls which is around R-9, I think (3" of faced fiberglass, circa 1967). It is possible that a pro could come up with a better match than I have, but not by much, I think. A variable speed blower is supposed to slow the air down enough to remove more humidity.

    As both totals were close to a 2-1/2 ton requirement, and the 3 ton two stage unit removes about 20,000 sensible BTUH and from 5000 to 9000 BTUH (depending on specific combination) latent in stage 1, I think I'm pretty well covered.

    What I do not comprehend at all is how the unit knows to kick into stage two if there isn't an outdoor temperature sensor that tells the unit the house is receiving more gain.

    The relative humidity here around New Orleans is 91% and up in the summer. As you know, HVAC-calc doesn't allow direct input of this. I just had to trust that the RH was automatically assigned after I entered the location.

  11. #11
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    >>What I do not comprehend at all is how the unit knows to kick into stage two
    >>if there isn't an outdoor temperature sensor that tells the unit the house is receiving more gain.

    My two stage AC kicks into stage two when the indoor temperature reads 2 degrees more than the setpoint (if stage one is sufficient, it will never reach that gap). Once stage two is engaged, it will stay on until setpoint is reached.

    Hvac-Calc may have some shortcomings, but as homeowners it is an invaluable defense against the service technician who wants to size by square footage, seat of the pants, same nameplate tonnage as before (plus an oversize to be "safe"), etc. In my experience a technician has told me "Manual J gives a different answer every time depending on who inputs the data". I could have said "garbage in, garbage out" and that would have been true but it probably would have offended him and I did not pursue that conversation. Still, it is very reassuring to have my own number from Hvac-Calc, even though it is the older Version 7 of Manual J.

    >>The relative humidity here around New Orleans is 91% and up in the summer.
    >>As you know, HVAC-calc doesn't allow direct input of this.
    >>I just had to trust that the RH was automatically assigned after I entered the location.

    Hvac-Calc did input a humidity number and it was just in unfamiliar units. When it says design conditions, look for "grains of humidity". That is a measure of absolute humidity, adding in the temperature defines the relative humidity number. The only way to understand this (that I know of) is to use either a psychrometric chart (not easy), or a psychrometric calculator such as this one:
    http://www.linric.com/webpsy.htm

    My Houston area outdoor design conditions I remember as 94F, 114 grains humidity. Plug those numbers into that calculator and you can see relative humidity. It's an eye opener to see what it tells you. I find that if you translate absolute humidity into dewpoint, that is a familiar number in the HVAC industry and communicates well.

    Hope this helps -- Pstu

  12. #12
    Thanks for the education. I plugged in 91%RH and 96 db [numbers that I understand] and came up with 240 grains of moisture. Plugged that into HVAC-calc and the numbers jumped. The system still works, but it will probably be running continuously in the second stage through most of the hottest months. I may go back, now that I understand this, and do a month by month assessment.

  13. #13
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    Quote Originally Posted by Thebil Illpay View Post
    Thanks for the education. I plugged in 91%RH and 96 db [numbers that I understand] and came up with 240 grains of moisture. Plugged that into HVAC-calc and the numbers jumped. The system still works, but it will probably be running continuously in the second stage through most of the hottest months. I may go back, now that I understand this, and do a month by month assessment.
    Manual J summer design conditions for N.O. is 92 degrees dry bulb/78 degrees wet bulb. That gives you a relative humidity % of 54, a dew point of 73, and grains of moisture per pound of dry air of 127.

    Your 96 dry bulb/91% relative humidity renders a dew point of 93 and 247 grains of moisture. If such a day were to ever occur, you could walk outside with a glass of indoor room temperature tap water (even if that tap water was at 85 degrees or higher!), and the glass would sweat. In short, it would be unwise to plug in these numbers; you'll be horribly oversized for most every other day your system would need to operate under.

    In your climate, being smart about humidity control is the best strategy. This involves a cooperation between the structure of your house and your HVAC system. If your house is leaky you'll always struggle to keep humidity levels in check. Build it tight and ventilate it right, as they say.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

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