Manual J software.

Discussion in 'General Discussions' started by Kogashuko, Mar 18, 2013.

  1. Kogashuko

    Kogashuko Member

    Here is my current situation described here with number crunching in detail.

    I do not trust the software I have really tried that estimates 1.5 tons for my first floor and 1.5 ton for the second floor because of the poor cold weather performace of my current 4 tons of cooling. The software does not take into acount that the second zone is over the first one. There is also not the option for R30 walls that are in my addition.

    I will have 3000sqft house located in central va with 1000sqft with R30 walls and R38+ attic. By the estimates in the software a 4 ton geo unit would be plenty.

    However, I currently have 2 2ton air source units and they could bairly keep up at below 20 outside with the current 1880 sft (actually closer to 2100sft since the builders finished the additional space for two walk in closets in the master bedroom which wasnt there before) without HVAC in the addition. I dont know if this was because of the cold air efficency loss described in the thread above or that I actually need 4 ton cooling for the current 1880 sft house.

    So I want to check my numbers before replacing my two air source pumps with one geo. I need to determine if replacing the second floor 2 ton with a geo 3 ton would be a better option and hopefully be able to come up with the cash for a geo 2 ton before 2016.

    IF anyone knows of any good free software or spreadsheets that would be appreciated. Thanks.

    I dont need anything too crazy because all the ceilings are 8 foot ceilings with standard insulation with exception of what I will do to the addition.

    Thanks as always.
  2. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader


    Try Slant Fin. They had free software way back when.

  3. Kogashuko

    Kogashuko Member

    Thx will start digging some more.

    All in all I would be more comfortable with my margin of error going 5 tons instead of screwing w 4 since they don't make a 4.5. Downside is the flow center I picked up is a qt with only one pump. Is there anyway to boost the flow on those? At $200 lightly used I couldn't pass it up.
  4. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    Yes we can boost or add a pump.
  5. Kogashuko

    Kogashuko Member

    I think I like the idea of just going with a 5 ton unit and being done with it. Even using the down and dirty 500 sft (standard house) to 600 sft (energystar) or my current 1.5 ton per zone with RHVAC v9 it still puts in in the arena of 5 ton instead of 4. I really dont mind oversize but I dont want to freeze like this winter or cook like one day in august last year.

    I might have spoke too soon about the pump. lists a head height. If that is what I am thinking and I am placing the flow center only about 6-8 feet above the loop then this thing says the flow is in the 20 gpm area... should be plenty.

    Thanks again for the help. I will look for the software and run a manual j again with second software before the actual install.
  6. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    The fact that your current systems don't meet load is NOT a valid reason to condemn the ACCA Manual J (as well as Manuals D, S, and T) process.

    A nominal 2 ton air source system won't produce anywhere near "2 tons" (24,000 Btuh) of heat when it is 20*F outside. Air source heat pumps are typically rated at an outdoor temperature of 47*F, a far cry from 20*F.

    Geo systems operate independently of outdoor air temperatures.

    There is zero justification for using ANY "x square feet per ton" rule you may have come across.

    Feed proper data into an accredited Man J package, accept the results, (but also apply Manual S - actual system capacity at design operating conditions)

    You may have one or both of an excessively air leaky house (resulting in garbage data going into Man J) or your present systems are connected to woefully crappy ductwork (violating Manual D and guaranteeing that systems can NEVER perform anywhere near rated output)

    Your stance amounts to shooting the messenger rather than properly reading and reacting to the message.
  7. Kogashuko

    Kogashuko Member

    Yes read my long thread on the actual estimated btu of my current system. That being 1.5 tons per zone equals 4.5. Betting that the geosystem will be able to cope w being undersized half a ton and then get a fluke uncalculated issue isn't worth it to me. I would rather be oversized.
  8. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    Any program is subject to GIGO. However you also said your current systems could "barely keep up under 20 degrees" depending on where you are and the heat pumps they may be undersized by 100%......while a like-sized geo could be right-sized or oversized.
    Last edited: Mar 19, 2013
  9. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    4 ton sounds more than adequate in your scenario. However, even a 5 ton HP can be served by a single 26-99 pump on the QT flowcenter. It depends on the pressure you design you loopfield with, we do it all the time to save the customer the second pump operational costs.
  10. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    We can lead a horse to water...

    (y'all know the rest of it...)
  11. Kogashuko

    Kogashuko Member

    Ok I ran the manual J software from slantpro and this is what I got. I was not able to calculate R30 wall insulation in the 1200sft addition and R19 was the best it would do. Which lowered the below numbers by about 1000btu.

    Designing the system for 3 different outside temps with a 70deg inside gave me the following.

    0deg = 63106 btu
    10 deg = 54093
    25 deg = 40568

    So the 4 ton heatpump I am looking at with 32 deg loop temp gives 39000 btus which would be plenty for anything above 25 deg. So strip heat would be needed for anything lower.

    A 5 ton gives 4500 btu with a 32 deg loop temp.

    My thing is, what type of loop would actually get to a 32 deg temp???

    At 50 deg loop temps and the lowest flow I am looking at almost 45000 btu.

    I am thinking the 4 ton with maybe money better spent going adding insulation to the outer walls. I have to redo floors in a couple of rooms. Nailing frames with R15 insulation up and building the wall up might help a bit...
  12. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    A quick scan of central VA design temps suggests that 15*F is reasonable, so your Man J load is likely no more than 50kBtuh

    A 4 ton with envelope improvements is a good bet, possibly 3 ton if you get serious about the envelope.
  13. If you over-size it you will be seriously cold but not dehumidified due to short run times on the cooling side. You really seem to need a pro to do a heat/cool loss/gain. Did your program allow for cold partitions? Do you know what they are?

    This seems like a palm to forehead moment.
  14. Kogashuko

    Kogashuko Member

    Yes it did. I need to go through and take exact measurements because what is on the plans for the existing house might not be as accurate. But yes I have a very large closet space that is also a giant cold partition. I am thinking 4 tons with 15kw strip heat backup will be fine. I am not too worried about dehumidification since the unit I am looking at has that option as almost another stage.

    The program did not have a heat gain figure, at least not that I have found yet, but we had no problems in August last year, other than one day, which leads me to believe the heat gain is less but of course not sure until I do the calculations. Also, the cooling seems to be much better in terms of total BTUs for the cooling stage of this unit. Where my rheem units only have a 2000btu difference on paper. If it is big enough for heat my bet it that it is big enough to cool. I would rather dehumidify in the summer than freeze in the winter any day.
  15. TowPro

    TowPro Member

    Im just a follower, here for the learning experience, but I paid for HVAC-Calc for 2 months. I just checked, it has R30 8.5" walls as well as R33 9-10" walls.
    I have tried a couple other free ones, and HVAC-Calc was always right in there with similar results.
  16. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader


    Keep playing before you reach for that check book.

    I use two older things that keep me out of jail. I use Slant-fin Heat loss explorer for manual J heat loss calcs. It was free if you asked nice. I know the not get sued load, built into the version I have and use it. It lowers the heat loss to met the real world. I know it and can do very fast heat loss computations and know they will keep people warm. I do go through each and every room in the home.

    I get to do this over and over to know I am correct. You get to pray what you chose works for the rest of your life.

    Pick a number and stick to it.

    Let me know if I can help.

  17. TowPro

    TowPro Member

    thanks Mark, Slant/Fin Hydronic Explorer is still available, found the link right on this site. It shows my house right in the 24,000 BTU heat loss area ball park that every other application I have tried shows. And as Engineer says, "Don't be afraid of a few dozen hours of aux operation". (or turn my Pellet stove on to supplement the Geo. for those couple days it drops below my designed 11 deg nights)
    the more I research, the better I feel about my 2 ton size selection.
  18. BTW, a cold partition is a wall, floor or ceiling that is next to a (possibly insulated) but unheated space.
  19. Kogashuko

    Kogashuko Member

    Luckly my big one will be gone when I finish the addition.
  20. TowPro

    TowPro Member

    I also went back and looked at this software after you pointed out the cold partition. I noticed I made a mistake and had exterior wall and cold partition mixed up.
    Now my load is down to 22,000 :). I will certainly be happy with a 2ton unit for my little 24' x 50' home, and if there is overhead built into this software I might never see the Aux heat turn on.
    Last edited: Mar 22, 2013

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