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  1. #1
    Join Date
    Nov 2009
    Location
    Fort Worth, TX
    Posts
    6

    Which unit for 900 SF

    I'm a new guy here, an occasional lurker for months, just registered, and I even read the rules, so here goes. Hopefully this isn't too long.

    We live on a lake in the Ft.Worth, TX area and have a 40-year old detached guesthouse/apartment that is being increased from 600 SF to 900SF, all under the same roof. My 80-year old parents in-law are moving into this guesthouse and paying for all remodeling, about 37K in all. We have hired a General Contractor who we have known for 24 years, and we know his work (built our previous house) and we trust him. We told him up front that cost was very important on all items of this remodel.

    The guesthouse is a wood frame structure with 8-foot ceilings and 6-inch joists, insulated with fiberglass, and still has the original flat roof, covered in asphalt and gravel. A sloped/pitched roof was added later. Our lot slopes about 20-25 degrees, I think, and a 4-foot retaining wall surrounds 2 sides of this guesthouse and is about 30-inches from the house walls. We plan to have the exterior re-sided with Hardie and Tyvek and we currently have fairly new Simonton vinyl double-pane windows. There are large native oaks on the east and south sides, extending some to the west.

    The current electric A/C / heater is a 2-ton unit, and cools and heats the 600SF pretty good considering there is only one intake and one outlet air vent in the main room which is over 500SF. The outside compressor unit is a 1980s Janitrol and the air handler is Airtemp from who knows when.

    Included in the A/C bids for this remodel job is running six new R-6 supply ducts to new vents in various areas. The air handler will be on an outer wall of the guesthouse, and the compressor unit will be right outside that wall, probably about six feet away. The parents-in-law didn't want a heat pump, but that may not be out of the question.

    The GC gave us a good bid on a 2.5 ton 14 SEER Lennox, which seems a little on the high side as far as capacity, IMO. The Lennox condensing unit is the 14ACX using R410, and the air handler the CBX26UH. 5-year warranty on compressor. I didn't specify a warranty requirement to GC in the beginning so I just found out the warranty. He's trying to keep costs low.

    The local A/C guy we want to use gave us bids on 2-ton Rheems, one convential A/C and the second a heat pump. While we were talking before the bids came back, he said he thought a 1.5 ton should be sufficient, but his bids came back with 2 tons. His models on the conventional A/C are the RANL024JAZ condensing unit and the RHLLHM2417JA air handler, overall 15 SEER. On the heat pump, he offers a RPNL024JAZ and same RHLLHM2417JA air handler, 14 SEER . Both units have a 10-year compressor warranty, per my request before bidding. Both have 10KW strip heat.

    I realize these units are pretty much basic units, but under the circumstances, they are what we are looking for. Since I can't quote prices or comparisons, I hope I can get away with saying that the Lennox bid is about 76% the cost of the conventional Rheem A/C unit, even with the Lennox at .5 ton more. The Rheem heatpump is a few hundered dollars more than the conventional Rheem A/C.

    Ok, here are my concerns/questions if you haven't gone to the next thread by now. Our GC keeps mentioning trying to get to a 30 degree difference factor, I guess between outside temp and inside. But I gotta tell ya, the old unit we have kept the guesthouse comfortable in the Texas summers and in the winter. Also, my 80-year old parents in law are cold natured these days. I'm a little worried about the 2.5 Lennox unit not running enough to remove the humidity, especially here by the lake, given the energy efficiency (maybe not) of the guesthouse, and surrounding ground on two sides of the building, and closeby large trees.

    I guess my main questions are should I go with the 2.5 ton Lennox bid with 5-year compresssor warranty or maybe get a bid on a 2.0 Lennox with a longer compressor warranty to see if costs come out about the same. Or, take the GC's current 2.5 bid and check into a Lennox extended warranty if in-laws agree. I think I prefer the Rheem unit, but its not my money being spent. Not sure if I should ask for another Rheem bid for something with a 5-year compressor warranty to better compare to the Lennox bid. I guess it all boils down to what tonnage would work best in this situation, Lennox brand versus Rheem brand, and the warranties.

    Sorry this was so long. I really would appreciate any thoughts, opinions, questions, or recommendations.
    Last edited by lakerat; 11-15-2009 at 01:37 AM. Reason: added one more piece of info.

  2. #2
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    66,833
    If you oversize. Your in laws will have to turn the stat down too low to get the humidity down during milder days(high 80's, low 90's).
    2 tons is 1 ton per 450 sq ft. 2.5 tons would be 360 sq ft per ton.

    Do your inlaws really like it to be 70 inside when its 100 outside?
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  3. #3
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,007
    OPTION for the open minded who wish to have Flexibility and
    High Efficiency (low operating costs)

    SANYO or equivalent manufacturer to meet your special situation.
    Review the features and see if this just might be Best suited for your guest house.

    http://us.sanyo.com/HVAC-By-System-T...Duct-Heat-Pump

    http://www.mehvac.com/Products/itemD...ProductID=1492

    Size to your guest house and personal needs.
    These VRV units have infinitely varying (inverter driven) capacity.
    Last edited by dan sw fl; 11-15-2009 at 05:59 PM. Reason: sp
    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

  4. #4
    Join Date
    Nov 2009
    Location
    Fort Worth, TX
    Posts
    6
    Thanks beenthere. My GC kept saying 400SF per ton, but not sure if that is his belief or his A/C guy's. Then he rounded up instead of down. Sounds overpowered to me. The in-laws would probably be comfy with a 78-80 temp on 100-degree day. In your opinion, would the 2-ton be a better fit, or would a 1.5 ton be enough? Thanks again.

  5. #5
    Join Date
    Mar 2006
    Location
    The South
    Posts
    1,976
    lakerat

    for best comfort/operating cost, I would insulate to the max you can afford including windows. Then have quoting dealers perform a load calculation for both heating and cooling. Next would be equipment selection and certainly you want to take advantage of tax credit if available for guesthouse.

    IMO

  6. #6
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,007

    Hmm

    Quote Originally Posted by lakerat View Post
    In your opinion, would the 2-ton be a better fit, or would a 1.5 ton be enough? Thanks again.
    Yes.
    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

  7. #7
    Join Date
    Nov 2009
    Location
    Fort Worth, TX
    Posts
    6

    Smile

    Thanks Dan and tigerdunes,

    At this point, I think if I add any more complexity with bids and other analysis to this decision, my wife would divorce me and her parents couldn't move in. Wait a minute, maybe I do need to re-think this and drag it out a little longer.

  8. #8
    Join Date
    Jan 2008
    Posts
    1,863
    I say 1.5 ton, but that's just a guess. Sounds like your contractor is guessing too.
    "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

  9. #9
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    66,833
    Quote Originally Posted by lakerat View Post
    Thanks Dan and tigerdunes,

    At this point, I think if I add any more complexity with bids and other analysis to this decision, my wife would divorce me and her parents couldn't move in. Wait a minute, maybe I do need to re-think this and drag it out a little longer.


    ROFL
    2 ton would be a better fit then a 2. ton.

    How well a 1.5 would do, on 100 degree days, I can't say.
    A load calc should be done, to determine that.

    You can do your own with HVAC Calc, pay the fee, and remove some of the complexity.
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  10. #10
    Join Date
    Jun 2008
    Location
    Fort Worth,TX
    Posts
    56

    Fort Worth Area

    Lakerat

    Being that I live in the same area I was reading your post and thought I would jump in. I am not a HVAC pro , but have gone through the same senerio that you are.

    Be sure of the size of the unit you need. It is important if you want the humidity to stay at the right level. ie: if you oversize the humidity will stay on the low side and if you undersize both cooling and humidity will not be correct.

    78 to 80 is where I keep my thermostat in the summer and it keeps the humidity about 38 to 40 %.

    As far as a heat pump is concerned. I think they work well except for the blast of cool air when first starting up. A variable speed air handler helps , but is reallly not the answer. The savings over heat strips are a 100% better, but the operational upkeep I believe are going to be higher in the long run. If you have natural gas in that area you would most probably be better off, considering the cost of elect. and heat strips. If you use heat strips they will be going on if your parents turn the thermostat up more than two degrees and the heat pump takes over.

    Sizing and duct insulation is what you should be really looking at before you get price.

  11. #11
    Join Date
    Jan 2008
    Posts
    1,863
    If you oversize, the humidity will be on the high side.
    "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

  12. #12
    Join Date
    Nov 2009
    Location
    Fort Worth, TX
    Posts
    6
    Thanks for the feedback guys.

    In order to compare apples to apples and dollars to donuts, I emailed both the GC and the Rheem guy with new requests. I think I'm just going to go with a conventional 2.0 ton unit and live with it, or let the in-laws live with it.

    Request to GC / Lennox unit:
    1. Request bid on 2.0 ton Merit series instead of 2.5 ton. 5-year warranty.
    2. Request for cost of Lennox extended warranty on above unit.
    3. Request bid on 2.0 ton Elite series with 10-year warranty.

    Request to Rheem A/C guy:
    1. Request for bid on 2.0 ton Rheem with 5-year warranty.
    2. Request for cost of Rheem extended warranty on above unit.

    Once I have these, I can compare the units based on warranty and features and let in-laws decide.

    Thanks for the suggestions. I'm still open at this point.

  13. #13
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    4,485

    Arrow Do everything in the proper sequence

    Everything needs to be done in the proper sequence.

    Evaluate everything you can do to reduce the heat-gain/heat-loss.

    Then have the load calc performed for each room.
    Then a manual D for the duct system & air handler selection.

    I am betting in Fort Worth TX if you do what is possible to your home, a 1.5-Ton with plenty of airflow through the evaporator would do the job.

    When everything is done right, you'd be amazed at what a 1.5-Ton A/C system can accomplish.

    If possible oversize the return air filter area as much 1.5-CFM per sq.in of filter area. Or, 675-cfm / 1.5 = or 450-sq.ins., a 20X25 or 500-sq.ins., or more is better.

    Always use Manual D to plan & design the duct system for optimal airflow.

    Dallas/Fort Worth summer design 100 dry bulb 75 wet bulb a 25-F drop, or 33% RH! In your climate, have plenty of airflow through the E-Coil > to get closer to the units BTUH Rating.

    If you are near water, humidity could be higher than the summer design figures above. - Darrell
    Last edited by udarrell; 11-15-2009 at 11:10 PM. Reason: Clarification...

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