Thread: Why different size recommendations with load calcs?

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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. 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?

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Originally Posted by jimj
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. 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.

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Originally Posted by jimj
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.

Originally Posted by jimj
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. [
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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Part of the reason you're not comfortable unless you set the stat at 72* is because your humidity may be too high due to over sizing of your present system. The difference between 2.5 and 3 tons may be because of different manufacturers' equipt. specs. Load calcs are only as accurate as the info they receive. Make sure the windows, shading, location, direction, and insulation valves are accurately entered.

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Originally Posted by jimj
[

For sure. I would love to have a customer like you!
Come to Boston and the job is yours

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.
Negative, I was not asked. Really didn't think about it until I started to educate myself here and on the net in general. Now if you are saying that they should have asked me what my desired temp is then I have more reason to be nervous. I am a bit concerned which is why at this point I have had about 6 contractors come. I know that is probably too many but I have not been comfortable with anybody but the two that I am considering, but still confused because one has quoted me for a 3 ton and the other a 2.5 ton. Again I know that "ballparking" is incorrect but based on my home description does 2.5 or 3 tons make more sense?

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Originally Posted by Gunslinger
Part of the reason you're not comfortable unless you set the stat at 72* is because your humidity may be too high due to over sizing of your present system.
I have suspected that my current system is oversized so thanks for your comment.

The difference between 2.5 and 3 tons may be because of different manufacturers' equipt. specs. Load calcs are only as accurate as the info they receive. Make sure the windows, shading, location, direction, and insulation valves are accurately entered.
I will try to verify this info, thanks.

10. As Gunslinger indicated the nominal tonnage differences could be because of manufacture specs. Questions that you could expect to be asked would include things like what temp you keep your thermostat at, cold or warm rooms, indoor air quality concerns, noise concerns, any brand preference, energy concerns and a number more. We would also check your duct system and system statics.

Also Thanks, for the job offer but its a little out of my work area.

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Originally Posted by jimj
Also Thanks, for the job offer but its a little out of my work area.
Yes, I was afraid that you would say that. Based on what I read in this forum it seems like the professionals that respond to us clueless home owners are much better than any contractor that I have seen so far. If there is a professional contributor here that is in the Boston area I have a job for you if you want it.

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Manual J recommends an inside design temp of 75 degrees with 50% RH. Without you specifically requesting otherwise then this should be what your system is designed around.

The discrepancy in sizing is most likely due to several factors. You should understand that very rarely will two people come up with the exact same numbers when performing a load calc. There is usually at least one or two cases of "grey areas" that can be interpreted differently. Also depending on the manufacturer, system capacity and blower performance will vary, causing slightly different sizes.

(rant on) As a contractor that is also apparently cold natured I just have to say D@mn man 72 degrees is fackin cold!!! I keep my t-stat at 79. Why don't you try getting up and moving around a bit, get the blood sloshing around some eh? Maybe go outside once in a while and make friends with that giant hydrogen explosion in the sky. Its all about acclimation. It irritates me to no end dealing with the endless stream of morlocks that think their house should be a friggin icebox. (/rant off)

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Originally Posted by JohnnyValvecore
(rant on) As a contractor that is also apparently cold natured I just have to say D@mn man 72 degrees is fackin cold!!! I keep my t-stat at 79. Why don't you try getting up and moving around a bit, get the blood sloshing around some eh? Maybe go outside once in a while and make friends with that giant hydrogen explosion in the sky. Its all about acclimation. It irritates me to no end dealing with the endless stream of morlocks that think their house should be a friggin icebox. (/rant off)
I laughed at this. My older brother likes to have his thermostat at 69 in the summer, guess we are just one hot, sweaty family.

At 72 it does not feel that cold to me. Again could be that my current unit is oversized so that 72 is not actually cold but that is the setting I need to get the humidity out? Just wondering.

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