# Thread: Manual J question

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Should the outside or inside of a house be used to figure load calc.

1 contractor measured inside (sd 2.5 ton a/c)
the other measured outside & sd (3 ton a/c)

Thanks

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Originally posted by lit'l cat
Should the outside or inside of a house be used to figure load calc.
To do it professionally requires a room by room calculation. You can't do that from outside. Sounds like the 'outside' guy is just calculating the square footage of the house - and that won't yield reliable results.

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A whole house load calculation CANNOT be performed accurately by measuring square footage for the outside of a house.

The type of construction entails both inside and outside factors as well as design temps for your area.
I usually do a room by room load instead of a whole house load so that I can be sure of getting the proper amount of air to each room.

Hope this helps.

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The difference may be in "block load" and "sum of peaks load". I'm not familiar with manual J, I use the Trane 700 load program.

The block load is just that-the length and height of the walls are mesured (can be done on the outside of the house) along with the area of glass. Taking into account the orientation of the walls (N, E, W, S) and the load is run.

Sum of peaks is a room by room calculation.

The sum of peaks load will be higher than the block load since the peak load of each room is added together. This is used to calculate how much air needs to be delived to the room.

The block load is used to select equipment.

Both loads should be performed to select equipment and determine the airflow for each room.

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Originally posted by sean88
The block load is just that-the length and height of the walls are mesured (can be done on the outside of the house) along with the area of glass. Taking into account the orientation of the walls (N, E, W, S) and the load is run.
Interesting. So a 2500 square foot home with eight foot ceilings shows the same load as a 2500 square foot home with 11 foot ceilings?

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Yes it would if you had a 3 ft plenum dropping the ceiling height from 11 ft to 8 ft. All wall mesurements should be floor to floor with the plenum taken into consideration.

It would not if you were lowering the total building height 3 ft.

I came from a commercial background so like I said I don't know what Manual J states how loads should be run. I use Carrier or Trane programs.

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Sorry, I overlooked that you said "height and length" of the outside walls. I must have read that as "width and length". You must have meant "height, width, and length".

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I guess the 1st Guy (outside measurements)did a block load. He sat down & calculated everything in front of me. Oddly he did say I should add 2 supply ducts on either end of the house to make enough airflow. (brick rancher 1954 currently 7 rtns on outside walls & 11 supply ducts on inside walls but 2 are sm ducts in the bathrooms)

Other contractor (2.5ton ac) sd my current furnace possible was so oversized to make sure enough airflow. But he didnt think I needed any additional supplies w/a 2.5 Ton a/c

(as if I wasnt already confused enough)

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If you have a sketch in pdf format with dimensions of walls and windows. I can run a load for you in 20 min.

I also need to know what the walls are made of brick, insulation, gypboard, is there is any insulation in the roof and what kind of windows do you have--single pane or double pane.

e-mail is seancantrell@comcast.net

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Originally posted by sean88
If you have a sketch in pdf format with dimensions of walls and windows. I can run a load for you in 20 min.
Don't you also need to know where the OP lives, what design temperatures he desires, and the orientation of the various windows and doors? How about the number of fireplaces and the number of people living in the house?

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Don't forget infiltration - that 51 year old house may leak like a sieve!

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PS you are right about this house being leaky but oddly 2 1/2 years ago we got bids to replace AC only, one contractor mentioned in a few years we wd have to replace furnace so why not wait & replace both. Sounded good BUT since then weve replaced single pane windows which were VERY drafty (all needed to be reglazed) with double pane low E gas windows & the 2 new bids we have received are the same as 2 years ago (back then 1 sd 2.5 ton & 2 sd 3 ton)

Anyway, I'm not sure how much difference it makes but I would think it would count for something

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Originally posted by lit'l cat
I'm not sure how much difference it makes but I would think it would count for something
When you get a room-by-room Manual J done, you'll know.

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