Round up or round down pn Manual J Calc?
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  1. #1
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    This forum is very, very informative!!! My thanks to all the pros who participate.

    Question to the pros--when doing a Manual J calculation, is it customary to round up or down when the final number of tons is calculated?

    My HVAC contractor did a Manual J load calc on my home (two story colonial, about 3000 square feet--70 feet long x 20 wide, central new jersey, 23 year old home) and came up with a value of 4.2 tons, which then was rounded up to five tons. The rationale was that the two speed unit (Lennox XC21) would run at low speed most of the time anyway. Does this sound reasonable?

    Thanks again all!

    LJM

  2. #2
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    I would much prefere to round down to a 4 ton system than up to a 4 ton. The 1st stage latent capacity is not very good at all on those systems, so oversizing will likely result in humidity issues.

    If it isn't a heat pump system with an electric air handler, the contractor may be able to match the 5 ton outdoor unit with a smaller indoor coil to get the sensable capacity down closer to what you need, wich would also give the system more latent capacity. This would lower the SEER of the system, but that is ok, since you can run the thermostat a little higher when you have good humidity control.

    The key is to have a contractor that knows how to properly match up a comfort system to the requirements of the home. To many of them just plop in the boxes that match up to the highest SEER, then don't understand why the customer complains about things like high humidity.

    If it is a heat pump system with air handler, the equipment matchup options are likely limited. I'm not sure how many matchup options Lennox has on those systems.

    Generally you would be better off going with the 4 ton system and adding some insulation, or something, to reduce the load on the house.
    If more government is the answer, then it's a really stupid question.

  3. #3
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    what do your $$$ want?
    if RH% control not important, then having larger size is ok.
    the estimate is only as good as the assumptions --
    do you plan to upgrade your house to have less load?
    what is the efficiency of each at projected load?
    what is the estimated energy use for each choice?
    how many hours per year would the smaller unit be undersized? what would be the result to your comfort? what will you pay for more | less comfort?

    remember, fuel costs will not go down.
    machinery run at full load is more efficient than with partial load; starting & stopping are hard -- listen to the news about the Shuttle!

  4. #4
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    I am a homeowner with a two stage Trane XL-19i in Northern Virginia with summer climate much like yours.

    With your load calc at 4.2 tons, I would look at what other changes you may make in the house in the next few years that will drive you to round either up or down. If you were to change nothing in the house, the answer depends, it depends.

    You have two questions here. If you are in a high humidity area, and if you buy a single stage unit, you should round down, so that the machine will work more to deal with the humidity. But if you end up with the two stage unit, your contractor is playing safe. And you are covered with the first stage running most of the time.

    But running the load calc is not an exact science. Much of it is an art. Did the contractor explain all the numbers he entered into the process? And do you agree with them? Did he already "round up" some of the significant factors to play it safe? Did he consider the type of windows as well as the size? Color of roof? Insulation R-values? By the way, did he enter a summer temperature inside the house of 70 or 75 degrees? That makes a big difference. If you used to run down the thermostat with the old system to 70 or 72 to make the humidity go down, you may need to keep the thermostat higher with the new system because the air should be less humid.

    Depending on what a 4 ton system really means, it may be better, even if two stage.

    But to look at the 4 ton system, you need to know whether it really gives a 48,000 BTUH capacity, or maybe if the condenser is rated at 47,000, and the best evaporator coil for humidity performance brings the combination down to 46,000, you'd have to think again.

    Whichever way you go, ask your contractor to pick the evaporator coil that will give the highest BTUH for latent heat to deal with humidity.

    Al

  5. #5
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    As far as the basic question, I agree with Mark. Round up if over halfway, and down if less, i.e. 4.2 would be 4 ton, and 4.7 would be 5 ton. Keep in mind, that if you want to be 'exact' then you can usually come up with indoor and outdoor combinations that come pretty darn close to what the load calls for. In my geographical area, humidity is a major concern, and for that reason alone, we would always trend down as opposed to up. Also, as we have indicated here before, the seer rating is based on some pretty exact criteria from design temperatures, to length of line set.
    Everyone has a purpose in life..........even if it's to be a bad example.

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  6. #6
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    You need to look at te detailed cooling capacity for actual sensible and cooling btus,there are no 4 ,4.2 or 5 ton systems.

    Then if your load is correct,reduce the load to get to the smaller size,or raise the indoor design,if you'll be happy with that.

    Otherwise I'd go to the larger size,so you get the indoor temp. you wanted ,at the outdoor desidn for your area.Any two stage,will or should have dehumidifing features ,at least close to the Infinity,so you'll be fine.



    If your indoor design is less then 80,don't look at ARI or some mfrs. ratings,because they are at 80.


    What are your design conditions??

    [Edited by dash on 08-07-2005 at 11:04 AM]

  7. #7
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    Originally posted by mark beiser
    I would much prefere to round down to a 4 ton system than up to a 4 ton. The 1st stage latent capacity is not very good at all on those systems, so oversizing will likely result in humidity issues.
    Does this apply equally to the two stage scroll systems and to the dual compressor units? I am very happy with the latent heat capacity with the Trane XL-19i which has a 1.2 ton first stage compressor and a 2.8 ton second stage. It is set up with the optional humidistat. However, this units comes only with R-22 refrigerant.

    Also, does the Lennox scroll compressor split stages at 66% or 80%? The web site info is mute on this point. Why would any design favor the 80% split that was mentioned in this forum about a month ago?

    Al

  8. #8
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    I just dug out the paperwork from the HVAC contractor, which includes the printed Manual J report. The assumptions were an indoor temperature of 75 in the summer with indoor relative humidity of 50%. The calculation yielded to values, which I don't understand, as follows:

    3.62 cooling tons, with a sensible/latent split of 88%/12%.

    4.16 cooling tons, with a sensible/latent split of 77%/23%.

    Not sure what sensible/latest means! In any event, given the length of the house, we decided on a 5 ton Lennox XC21 unit, which was installed last week. It is hard for me to judge whether this confuration will keep the relative humidity below 50%, because problems with the Arzel zoning and bypass damper are causing the system to continually cycle on and off.

    The HVAC company has a 100% satisfaction guarantee (they are part of Service Experts), and thet claim they will remove any equipment I am not satisifed with. I am considering having them remove the Arzel zoning (question--will it be a problem for them to close the holes they put into my 25 supply trunks to insert the dampers) because, now that I have done the research, there is no way I am going to allow a bypass damper to dump 4 tons of cooling back into the coil. I am also considering that I might want the condenser downsized from 5 to 4, but--would it be worth it if the five ton condenser keeps the house at 50% humidity???

    thanks all!

    LJM

  9. #9
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    You have a zoned system and went up in size?

    That was a bad desicion.

    You should have looked for other improvements you could have made to the house to reduce the load.

    Shouldn't up size on a zoned system (exspecially with a bypass).


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  10. #10
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    Originally posted by machese
    I just dug out the paperwork from the HVAC contractor, which includes the printed Manual J report. The assumptions were an indoor temperature of 75 in the summer with indoor relative humidity of 50%. The calculation yielded to values, which I don't understand, as follows:

    3.62 cooling tons, with a sensible/latent split of 88%/12%.

    4.16 cooling tons, with a sensible/latent split of 77%/23%.

    Not sure what sensible/latest means! In any event, given the length of the house, we decided on a 5 ton Lennox XC21 unit, which was installed last week.
    Homeowner here again. To me, the numbers mean the 5 ton is the wrong way to go. The 3.62 ton line tells me to go for a 3 1/2 ton system. The 4.16 line tells me to go for a 4 ton system. Both lines together tell me to take the 4 ton system which would do nicely to take care of latent heat, but maybe the experts here will correct me on this.

    Sensible heat is the type of heat that you can feel. Out in Nevada, a house with a load calc calling for 42,000 BTUH capacity will do nicely with a 3 1/2 ton system. In the mid-Atlantic region with humidity, a house with a load calc calling for the same sensible heat capacity of 42,000 will do poorly because you typically have to handle maybe another 4,000 or 5,000 BTUH capacity of latent heat. That's the energy required to convert water vapor (gas) to liquid.

    Originally posted by machese
    It is hard for me to judge whether this confuration will keep the relative humidity below 50%, because problems with the Arzel zoning and bypass damper are causing the system to continually cycle on and off.

    The HVAC company has a 100% satisfaction guarantee (they are part of Service Experts), and thet claim they will remove any equipment I am not satisifed with. I am considering having them remove the Arzel zoning (question--will it be a problem for them to close the holes they put into my 25 supply trunks to insert the dampers) because, now that I have done the research, there is no way I am going to allow a bypass damper to dump 4 tons of cooling back into the coil. I am also considering that I might want the condenser downsized from 5 to 4, but--would it be worth it if the five ton condenser keeps the house at 50% humidity???
    I suggest turning off the Arzel with all dampers open until you can get a handle on what the basic system is really doing. After settling on the final condenser/evaporator combination, then get the contractor to make the Arzel work right.

    Some techs in this forum seem to dislike Arzel. But did you consider using one of the zoning systems provided by Lennox that would give you a fully integrated capability? Why not?

    By the way, what is the rest of the configuration? All Lennox? SignatureStat?

    Al

  11. #11
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    Aug 2005
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    Thanks for the replies!

    The rest of the configuration is as follows:

    Three zones--

    Zone1: first floor (app 1500 sq ft)

    Zone2: second floor minus master bedroom (app 1100 sq ft)

    Zone3: second floor master bedroom (app 400 sq ft)

    equipment as follows:

    Lennox XC21 condensing unit (five tones
    Lennox CX3462C6F coil
    Lennox furnace G61MPV (104,000BTU, 94% efficiency)
    3 Dual Stage Lennox Signature thermostats
    Aprilaire 600 humidifier
    Aprilaire 2200 Media air cleaner
    3-Zone Arzel system (air boss panel, 26 dampers, gravity damper bypass)

    They worked five days to install all this, totl cost app. 15K (parts & labor).

    I was going for the "cadillac" solution, because I plan to live in this house for over 20 years (perfect size & location), and I consider proper climate control to be a huge factor in home enjoyment.

    The dealer (who is a "premier lennox dealer") said he doesn't use the lennox harmony III zoning because "the motors might break, & pneumatic will work better". I made the erroneous assumption that the arzel system worked like the lennox system (i.e, integrated with the VS fan to modulate airflow--bad assumption on my part). subsequent research shows that the lennox zoning system would have been better, or even a second system cooling system upstairs instead of zoning.

    Having run the system for about four days, here are my observations:

    1) the bypass damper (which is located directly on top of the furnace) is causing continual short cycling by essentially freezing out the unit (think about it: 2-4 tons of cooling directed right back into the blower).

    2) because of this, I am unable to reach moderate setpoint of 74 degrees when it is 87 outside, and the humidity isn't able to go less than 60%--pretty unacceptable.

    My belief (and I wish that I had researched this further before having this stuff installed) is that any system relying on a bypass damper right on the furnace will always cause the furnace to short cycle, and since there is no other way to reduce the airflow w/ the arzel system, it will have to be eliminated, either in favor of a better zoning solution that can control airflow w/ the VS fan, or in favor of a second unit. I am very disappointed in the dealer for not understanding this in advance, but I guess caveat emptor is the rule... Given that there is this 100% guarantee (the salesman said that if I am not satisfied with the system, they will remove it--and I have this in writing), I am not reluctant to tell the vendor the remove pieces of the kit and swap out others, and if he balks, to remove everything & I will start out with a different contractor (we have two in town that use Carrier & Trane equipment). It will be an interesting conversation tomorrow!!!

    Thanks to all for your insights on the matter!!!


  12. #12
    Join Date
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    East Grand Forks, MN
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    Question to the pros--when doing a Manual J calculation, is it customary to round up or down when the final number of tons is calculated?

    Depends on equipment capacity for a given sensible heat ratio.

    My HVAC contractor did a Manual J load calc on my home (two story colonial, about 3000 square feet--70 feet long x 20 wide, central new jersey, 23 year old home) and came up with a value of 4.2 tons, which then was rounded up to five tons. The rationale was that the two speed unit (Lennox XC21) would run at low speed most of the time anyway. Does this sound reasonable?

    NO!

    You need to look at te detailed cooling capacity for actual sensible and cooling btus,there are no 4 ,4.2 or 5 ton systems. ....

    If your indoor design is less then 80,don't look at ARI or some mfrs. ratings,because they are at 80.


    If the contractor is a pro, they should be able to determine which equipment is right for your house.
    Yes ARI ratings are at 80 degrees, but you can interpolate to any magic number. Each manufacturer has certain numbers to use to figure out other indoor design temps!

    I just dug out the paperwork from the HVAC contractor, which includes the printed Manual J report. The assumptions were an indoor temperature of 75 in the summer with indoor relative humidity of 50%. The calculation yielded to values, which I don't understand, as follows:

    3.62 cooling tons, with a sensible/latent split of 88%/12%.

    4.16 cooling tons, with a sensible/latent split of 77%/23%.

    Not sure what sensible/latest means! In any event, given the length of the house, we decided on a 5 ton Lennox XC21 unit, which was installed last week. It is hard for me to judge whether this confuration will keep the relative humidity below 50%, because problems with the Arzel zoning and bypass damper are causing the system to continually cycle on and off.


    According to their report, you should have had a 4 Ton installed! and especially with a zoned system.

    The HVAC company has a 100% satisfaction guarantee (they are part of Service Experts), and thet claim they will remove any equipment I am not satisifed with. I am considering having them remove the Arzel zoning (question--will it be a problem for them to close the holes they put into my 25 supply trunks to insert the dampers) because, now that I have done the research, there is no way I am going to allow a bypass damper to dump 4 tons of cooling back into the coil. I am also considering that I might want the condenser downsized from 5 to 4, but--would it be worth it if the five ton condenser keeps the house at 50% humidity???

    Really, what does 100% satisfaction guarantee, one season only, then too bad!
    Anyway, nothing with having a zoned-system. It's probably the contractor that doesn't understand how to design a zoned-system.


    The dealer (who is a "premier lennox dealer") said he doesn't use the lennox harmony III zoning because "the motors might break, & pneumatic will work better". I made the erroneous assumption that the arzel system worked like the lennox system (i.e, integrated with the VS fan to modulate airflow--bad assumption on my part). subsequent research shows that the lennox zoning system would have been better, or even a second system cooling system upstairs instead of zoning.

    not necessarily true.

    My belief (and I wish that I had researched this further before having this stuff installed) is that any system relying on a bypass damper right on the furnace will always cause the furnace to short cycle, and since there is no other way to reduce the airflow w/ the arzel system, it will have to be eliminated, either in favor of a better zoning solution that can control airflow w/ the VS fan, or in favor of a second unit. I am very disappointed in the dealer for not understanding this in advance, but I guess caveat emptor is the rule... Given that there is this 100% guarantee (the salesman said that if I am not satisfied with the system, they will remove it--and I have this in writing), I am not reluctant to tell the vendor the remove pieces of the kit and swap out others, and if he balks, to remove everything & I will start out with a different contractor (we have two in town that use Carrier & Trane equipment). It will be an interesting conversation tomorrow!!!

    If i were you, i would call Arzel zoning and see how to make it work, otherwise you can call me if i'm around your area.




  13. #13
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    Aug 2005
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    32
    thanks arc8! I am i central new jersey. where are you?

    to add insult to injury, now the condenser (again, a brand new unit purchased last week, top of the line Lennox) won't turn on even though all three stats & the arzel panel are calling for cold. the fan in the furnace is running, but the condenser is stopped in its tracks. probably because it has been short cycling all weekend... if the dealer / installer doesn't give me a good answer tomorrow, I am telling him to pull his equipment out of my house and I will find another HVAC firm to work with. my only concern is that his guys put a hole in all my supply ducts to slip the arzel dampers in, but i am sure that can be repaired. I am more than a little upset with how this turned out!!! alas...

    LJM

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