HVAC-Calc help and opinions - Page 3
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  1. #27
    Join Date
    Dec 2006
    Location
    Arkansas
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    292
    Quote Originally Posted by Doral View Post
    All the upstairs ducts are being replaced.....so they should be "tight"
    Could be a bad assumption. Just because they are new does not mean tight. I test ducts in new houses all the time with leakage as high as 300 to 400 cfm. You need to be specific with what you want and you may still need to go back and do some caulking/foaming/sealing unless you put it in the agreement with the installer. A lot of people say they install tight ducts - I usually can prove them wrong with a pressure test.

  2. #28
    Join Date
    Sep 2007
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    79
    Quote Originally Posted by dcmcm5 View Post
    Jimj is correct - the assignment of excess latent comes from Manual S. This can be significant and I have seen units that were just a little below on sensible that they went to the next larger size, which unfortunately was a 1 ton jump, and resulted in a unit that was way too large.

    A couple of comments/questions:

    - You show walls with R-2.5 1/2 in. Extruded poly board. This is not common. Do you really have a continuous insulation board on the outside of each wall? That's great if you do.

    Yes, we have that silver foam board all around the outside of the house, except for the wall that is inside the attached garage.

    - If the walls are insulated with fiberglass, unless you know for sure, they are probably not giving you a full R-11. May want to do a couple of runs with something a little less to see how it comes out if you have little margin on capacity.

    [B]Yup...the walls are insulated with fiberglass and I believe that my husband was able to open some areas and read the label. Also, thanks to Katrina --we got a look inside exterior bathroom walls and they had R-11. Might be old and have lost some of its zip but that was how it started out life.B] - You list several windows as Double pane; Metal frame; Low emittance. Did you have a film put on the windows? Otherwise, I don't think you would have a low-e metal frame window.

    Yes indeed.........we have double pane, e-glass METAL frame windows. All new...thank you Katrina. Also, they were not just plopped into the hole. They were installed with insulation and finished with that sealing tape stuff. We know they aren't as efficient as vinyl but.....my husband wanted to go with them anyway as he is still dubious about the long term effects on vinyl in the direct intense sun.

    - If the attic insulation is R-19, you should look at adding additional insulation (you can blow cellulose on top of the batts) to get you to at least R-38. This is usually not that expensive and will really help where you are located.

    Yes, we already are thinking that we will do that sometime in the future when things are better under control. - You are not following Manual J, and I believe you are making a mistake, by using design temperature values beyond the design. I show for New Orleans the 1% values are 92 deg F for cooling and 34 deg F for heating. The ASHRAE mean extremes are 96 and 23. You are outside of the extremes. You load calculation is too high assuming all the other parameters are correct.
    Thank you...I will pass that along

  3. #29
    Join Date
    Sep 2007
    Posts
    79
    [QUOTE=dcmcm5;1662643]Could be a bad assumption. Just because they are new does not mean tight.
    They haven't been put in yet. That is the plan. My husband is going to watch the installation CAREFULLY

  4. #30
    Join Date
    Feb 2007
    Posts
    999
    Quote Originally Posted by jimj View Post
    As per Manual S guidance if more sensible capacity and less latent capacity
    is needed you add one half of excess latent to the sensible capacity.
    No dip switch or other adjustment.
    Using HO data she has target CFM of 1559 CFM at a 19 degree delta T.
    Re: your comment about Arizona, is the above applicable to dry climates only?

    AM

  5. #31
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Quote Originally Posted by Doral View Post
    We are using and indoor temp of 75F and outdoor temp of 98

    -Sensible: 32586
    -Latent: 6444
    Sq. feet 2324

    Please note that most Mfrs. specs aren't at 98 and 75 indoors.

    The outdoor difference can usually be interpolated,from their chart.

    The indoor used is usually 80F ,and for every degree below that you lose 835 btus of sensible capacity ,per 1000 cfms of air flow.


    This loss of sensible ,is converted to latent and you can then use half of the excess latent as sensible.
    It's all in Manual S from www.acca.org

  6. #32
    Join Date
    Dec 2006
    Location
    Arkansas
    Posts
    292
    Quote Originally Posted by dash View Post
    Please note that most Mfrs. specs aren't at 98 and 75 indoors.

    The outdoor difference can usually be interpolated,from their chart.

    The indoor used is usually 80F ,and for every degree below that you lose 835 btus of sensible capacity ,per 1000 cfms of air flow.


    This loss of sensible ,is converted to latent and you can then use half of the excess latent as sensible.
    It's all in Manual S from www.acca.org
    The information provided by Dash is why you don't take the load calc, divide by 12000, and use that as the tonnage required. I no longer tell people that have me do a load calc an estimated tonnage. I give them the sensible and latent loads and the design parameters they were calculated from and caution them to match the equipment to those loads. But as Dash stated, most of the equipment is rated at 80 deg. I don't know why the equipment manufacturers don't change that to 75 deg, which would take the majority of corrections from having to be done.

  7. #33
    Join Date
    Sep 2007
    Posts
    79
    Thank you for the help. My husband is busily modifiying numbers based on your suggestions. He has been out of town for a long time and has been working on this, trying to understand all, since he got home Friday.

    Any further advice is always appreciated.

  8. #34
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    If you get specs on capacity from Carrier,Trane,Lennox,you'll see the deduct for lower indoor temperature in the footnotes.Might help the hubby understand it's not BS.

  9. #35
    Join Date
    Sep 2007
    Posts
    79
    Yuppers...we found the specs and he is reviewing those also.

    After spending so much time learning, we realize that it is important not to oversize a bunch. Our current old AC system is a meat locker and frankly we have a little electric bill cuz I hate the cold clammy feel. Now I know why.

    My husband has already contacted the contractor and they are going to compare numbers tomorrow night. I doubt that the contractor put all the detail into his calculation that my husband has...which would account for their BIG differences in tonnage.

    6 months ago I would have said..."oh throw in an extra ton above what we already have...it's getting hotter". That's the way everybody does it around here.

  10. #36
    Join Date
    Sep 2007
    Posts
    79
    We followed advice and did revise all the data as suggested in the Manual J. Our contractor is reviewing all and agrees that he had missed some of the detailed information such as the e-glass, foam board, etc. Anyway, on review they agree that the 4T. Infinity (one unit with zoning) would be the correct size.

    To add another twist ...we can go back to the 2T. up and 3T. down IF we reuse the old lineset. This puts two compressors right by our bedroom wall and the head of our bed. Also...2 T. is too big for upstairs (700 sq. ft) but that is the smallest heat pump that they make. Currently the upstairs is heated with heat strips. Original compressors are ~16" apart from each other as space is so limited. We would go 24" but it will be tight.

    We have read much here warning us off using one unit for upstairs/downstairs - too much wear and tear on one unit, reliability, inability to distribute heat/cooling evenly, etc.. We are getting cold feet on the one unit concept due to comments on the complexity of zoning and reliability of all the parts.

    So...would you stick with 2 units and old linesets and noise by your bed ?(Moving compressors elsewhere isn't an option..can just barely squeeze the 2 back in. Or take a chance on zoning ---which given re-doing about 70% of the ducting for change to one unit is committment. ) Going back to 2 + 3, we are oversized.

    Would you buy a house with one ac unit for two floors even if it was zoned with the top of the line Infinity?

  11. #37
    Join Date
    Apr 2007
    Posts
    6,285
    Is the upstairs an open area? Have you thought any about a mini split for upstairs? They make heat pump version from probably way less than you would need to way more.
    Last edited by BigJon3475; 11-01-2007 at 09:17 AM.

  12. #38
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
    Posts
    3,304
    Quote Originally Posted by Doral View Post
    We followed advice and did revise all the data as suggested in the Manual J. Our contractor is reviewing all and agrees that he had missed some of the detailed information such as the e-glass, foam board, etc. Anyway, on review they agree that the 4T. Infinity (one unit with zoning) would be the correct size.

    To add another twist ...we can go back to the 2T. up and 3T. down IF we reuse the old lineset. This puts two compressors right by our bedroom wall and the head of our bed. Also...2 T. is too big for upstairs (700 sq. ft) but that is the smallest heat pump that they make. Currently the upstairs is heated with heat strips. Original compressors are ~16" apart from each other as space is so limited. We would go 24" but it will be tight.

    We have read much here warning us off using one unit for upstairs/downstairs - too much wear and tear on one unit, reliability, inability to distribute heat/cooling evenly, etc.. We are getting cold feet on the one unit concept due to comments on the complexity of zoning and reliability of all the parts.

    So...would you stick with 2 units and old linesets and noise by your bed ?(Moving compressors elsewhere isn't an option..can just barely squeeze the 2 back in. Or take a chance on zoning ---which given re-doing about 70% of the ducting for change to one unit is committment. ) Going back to 2 + 3, we are oversized.

    Would you buy a house with one ac unit for two floors even if it was zoned with the top of the line Infinity?
    As a homeowner, I would not shy away from buying a house with zoning and a Carrier Infinity system, however there are other options. For my master bedroom suite I have been shopping 9000 btuh mini-split heat pumps -- one ton is 12,000 btu. I *know* they make mini-split models in several sizes between, one of those might be just right for you. Myself, I am thinking about getting something 17-20 SEER and just letting it run all it can, that will take load off the larger and less efficient system. The natural downward flow of cool air might aid you doing that in a 2-story if you want to.

    The mini-split technology is very widespread in Europe, Asia, everywhere outside the USA. They appreciate quality and efficiency as much as we do, and these products are pretty advanced. You could Google "Mitsubishi Mr. Slim" to get a feel for what they offer.

    My best opinion is the "wear and tear" argument is bogus -- if that were so then oversized ACs would last longer and they don't. An AC is a machine designed to run at peak output indefinitely. The "reliability due to more parts" argument has *some* validity, but these parts aren't really expected to break every couple years, are they? Whatever the added expense of zoning is, that is a solid argument. There is a lot of conventional wisdom, including redundancy and simplicity, in having a dedicated system for each floor. Less demanding of the AC pros also, that may be important if skill is in short supply in your area.

    It's good not to oversize, but with two systems or a 2-stage system it's not the same level of urgency as with one single-stage system. As long as one of your systems has long runtimes, it ought to remove humidity well from your whole house. If one of your systems necessarily has short runtimes, you might consider optimizing by making that one cheaper and lower SEER. Most of the professional thinking about oversizing, is based on the assumption that one single stage system will do everything -- with multi systems and multi stages you are getting outside those assumptions. This is not to advocate oversizing, just to recognize when and how much it matters.

    I am a homeowner in S. Texas and so have a taste of what your heat and humidity is like. And sympathy for the ever-present tension between a homeowner and... all those other people, most of them well meaning and a few whose interests are not aligned.

    Hope this helps -- Pstu

  13. #39
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,271
    To address the upstairs/downstairs/zoning/two vs. one unit discussion, let me put it this way...if I owned a two story house, the only way I'd consider using one unit to heat and cool two floors would be to put a chiller/heat pump on the ground that chills water in summer, heats it in winter, and then put an air handler upstairs with a hydronic coil, and one downstairs with a hydronic coil. The outdoor unit would chill water that is circulated to each coil for cooling, and heat the water for heating in winter. My other consideration would be to use a Daikin-type multi-split system, where one outdoor unit handles multiple indoor air handlers. Such systems are available as heat pumps.

    For direct expansion (DX) systems, one unit per floor is optimal, and should be sized correctly. Especially in a humid climate like New Orleans. It sounds like you may have already taken some steps in this direction, but for a humid climate I would place emphasis on controlling air infiltration into the house from outdoors. You don't want to live in an airtight coffin, but you don't want the house to leak like a sieve, either. Ideally, you're after controlled air changes, which is important for healthy indoor air quality. 75 degrees/50% relative humidity is the standard Manual J indoor design condition, and for New Orleans is probably a good target to shoot for.

    Oversizing an a/c system in a humid climate is counterproductive. Your cooling coils will run too warm and not extract enough moisture from the air to feel comfortable. When that happens, the temptation is to turn the thermostat down lower to get comfortable, which only yields a cold and clammy feel, which you report you've experienced in your home so far. If you oversize your new systems, you'll be no better off in this issue...cold and clammy...and indoor humidity levels running too high.

    What you and your husband seem to be learning, and what I'd wish more of us in the trade would take seriously, is the importance of good system design for a residence. IMO, true success is found between a combined effort of better design of the structure's thermal envelope (walls, windows, ceilings, etc), along with better HVAC system design, from equipment sizing and selection to duct sizing and installation to placement of supply and return registers for proper air distribution and removal. All too often the common thought is to throw more equipment (and too much to boot) at what is perceived to be inadequate thermal envelopes, vs. improving thermal envelope efficiency, thereby reducing the intensity of energy required to run the climate control system.

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