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  1. #1
    Join Date
    Jul 2008
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    20

    Why different size recommendations with load calcs?

    NOTE - sorry that I double posted my question, it was an accident and I don't see an option to delete the duplicate.

    I have received several estimates, all with Manual J load calcs being done by the contractors. If proper load calcs were done then what would account for having 2 different sizing estimates? Shouldn't all load calcs yield the same result or are there certain Manual J variables that would lead to different results? I have estimates for a 2.5 ton system and a 3 ton system. I don't know if this is too general a question but as I make my contractor selection are there certain Manual J variables that depending on what they used would cause the sizing estimates to yield different results?

    Currently I have a 3.5 ton system. I never felt uncomfortable inside but it could be slightly oversized based on short off/on cycles so 3 tons makes sense to me but with 2.5 being a full ton less than what I have I am not sure what to do now. I live in Massachusetts which has a fairly short cooling season.

    Home is a split level raised ranch with around 1400 square feet of living space. Home is about 50 feet in length, 26 feet in width. House is on the sun side of the street so there is sun all day long. Upstairs is fully air conditioned, the downstairs has supply registers in only 2 of 3 rooms that are about 16 feet long and 13 feet wide. I know this is extremely general and windows, walls, house orientation are all a factor but just looking for a guideline as I talk to the contractors - If I have been comfortable with my current 3.5 tons does 2.5 make sense or is 3 tons more likely the way to go.

    Also one more question. I like to have the inside temp at 72 degrees. Does this factor into the sizing so that if the contractors used 74 or 75 degrees do I need to tell them to use 72 degrees and would that yield a different sizing result?

  2. #2
    Join Date
    Dec 2002
    Location
    Arizona
    Posts
    4,421
    Yes using 72 in lieu of 75 will make a difference.There is a set of does and don'ts that many people don't follow. They will do things like use 72 degrees and not 75 as an indoor design temp for cooling..

    You do not want to add to the outside design temps, or fail to give full credit for wall and attic R values, true window values with overhangs and blinds and orientation of the home. Not worst case direction..

    Make sure that manual S is used for sizing of the equipment and that up sizing is not taking place.

    Ask for a copy of the J load and look at it. Is it your home or have the values been incorrectly set?

  3. #3
    Join Date
    Jul 2008
    Posts
    20
    Quote Originally Posted by jimj View Post
    Yes using 72 in lieu of 75 will make a difference.There is a set of does and don'ts that many people don't follow. They will do things like use 72 degrees and not 75 as an indoor design temp for cooling..

    You do not want to add to the outside design temps, or fail to give full credit for wall and attic R values, true window values with overhangs and blinds and orientation of the home. Not worst case direction..

    Make sure that manual S is used for sizing of the equipment and that up sizing is not taking place.

    Ask for a copy of the J load and look at it. Is it your home or have the values been incorrectly set?
    Appreciate the reply, this process is more confusing than I thought it would be. This is my home. Don't know what values were used. Neither contractor is anxious to share the Manual J report unless I go with them for the job - I was a little put off by that but I have seen comments here that it is not unreasonable for them to do that so that I don't use their work to go with another contractor. But whatever contractor I choose I will tell them that I need to be comfortable with their selection and will want them to go over their values with me.

    I understand that I do not want to add to the outside temp but is it OK to use 72 vs whatever the Manual J standard temp is or would that lead to incorrect sizing? If I would be able to keep the home at 72 except for those rare extreme outside temp days then should I go with the Manual J standard inside temp.

  4. #4
    Join Date
    Dec 2002
    Location
    Arizona
    Posts
    4,421
    I also will not give a client my load calc unless they go with our company, but I will always sit down with the customer and go over it with them. Many contractors in my area claim to do loads but NEVER show there work, I wonder why?

    Is there a reason you want 72 degrees? Is that the temp you want to maintain? If it is that's fine, if it's not that don't do it. Know that anytime your system is not operating at peak load its over sized. With multilevel on one system long on cycles are very desirable.

  5. #5
    Join Date
    Jul 2008
    Posts
    20
    Quote Originally Posted by jimj View Post
    I also will not give a client my load calc unless they go with our company, but I will always sit down with the customer and go over it with them. Many contractors in my area claim to do loads but NEVER show there work, I wonder why?
    I plan to tell the contractor that I choose that I will want to see their work. If they say that they need a deposit first is it acceptable that I ask them to give me something in writing that the deposit is refundable if they cannot show me the load calc work that was done.

    Quote Originally Posted by jimj View Post
    Is there a reason you want 72 degrees? Is that the temp you want to maintain? If it is that's fine, if it's not that don't do it. Know that anytime your system is not operating at peak load its over sized. With multilevel on one system long on cycles are very desirable.
    Why 72 degrees? I guess because I am a sweaty boy in the summer and when I have my thermostat at 74 I am not comfortable but at 72 I am comfortable. Come to think of it since I think that my current 3.5 tons is oversized could that be an oversize indication that I have to have my thermostat at a lower setting to feel comfortable?

  6. #6
    Join Date
    Dec 2002
    Location
    Arizona
    Posts
    4,421
    [
    QUOTE=bostonguy;1956380]I plan to tell the contractor that I choose that I will want to see their work. If they say that they need a deposit first is it acceptable that I ask them to give me something in writing that the deposit is refundable if they cannot show me the load calc work that was done.
    For sure. I would love to have a customer like you!


    Why 72 degrees? I guess because I am a sweaty boy in the summer and when I have my thermostat at 74 I am not comfortable but at 72 I am comfortable. Come to think of it since I think that my current 3.5 tons is oversiazed could that be an oversize indication that I have to have my thermostat at a lower setting to feel comfortable?
    [/QUOTE]
    Could be, but if 72 is what you really want go for it. Did either contractor ask you about your desired temps? If not I would be concerned.

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