# Thread: Manual J Calc question:

1. Professional Member*
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Originally Posted by ACFIXR
According to appendix 5 it is the "outside dimensions" for exterior walls.
Mr FIXR, I did not know that.... I had to look it up.... I didn't believe you....

2. Originally Posted by rickboggs
Mr FIXR, I did not know that.... I had to look it up.... I didn't believe you....
Please don't call me Mr., it makes me feel ancient.........................

3. I don't have a copy of the manual j, so I can't comment on whatever the appendix said regarding "exterior walls", but I still say that common sense dictates that it does not apply to spaces with knee walls. Why not ignore the insulated ceiling and enter the measurements of the roof? Same logic applies to knee walls. All we care about is the heat load on the "conditioned space". Build a 5' x 5' dog house in an attic and imagine using the attic exterior walls dimensions to size the cooling load; same principal would apply to a space surrounded by knee walls. This is the kind of crap that results in oversized a/c's.

4. Originally Posted by tipsrfine
When doing a load calc on a home, or room by room calc, where is the perimeter of the home or room measured, on the interior of the room, or the exterior?
Take for example a room surrounded by 14" thick brick walls, the area measurements of the room are going to be completely different if you include the whole wall as being conditioned space.
I'm not trained & experience in Manual J's at all, but it seems logical to measure the inside perimeter, because we are not trying to condition the 14" thick brick walls.
Any help is appreciated.
No it's not logical at all.............................It's the 14" wall that causes the room to need conditioning.
It's the outside surface area that is used in the loss/gain calculations. Then the volume is used to calculate infiltration values. You should invest in yourself and buy a MJ then you could answer some of your own questions. How did you come up with this thought in the first place?

5. Originally Posted by ACFIXR
No it's not logical at all.............................It's the 14" wall that causes the room to need conditioning.
It's the outside surface area that is used in the loss/gain calculations. Then the volume is used to calculate infiltration values. You should invest in yourself and buy a MJ then you could answer some of your own questions. How did you come up with this thought in the first place?
When I get the money to buy I will. At this point I am just trying to learn on the cheap. Common sense tells me that the only thing we are concerned with is conditioning the living space. We need to know what the load is coming through the walls into that living space. We don't care about what is going on IN the walls, only what is coming out and into those walls. Explain to me why it is that the calcs want the ceiling height, and not how high the roof is above the ceiling?
And especially explain to me how you would do a manual J on a room in a finished attic where the outer walls of the attic are 25' x 40', but the conditioned area is defined by knee walls of 15' x 25'? All we care about is the heat load coming into the conditioned space INTO the conditioned space. Logic dictates that the load CAN ONLY come into the conditioned space through the envelope of the conditioned space. (not counting internal loads of course). Do a manual j calc on a small insulated & conditioned dog house in the middle of a large warehouse; by your logic, you would size the equipment needed to cool the dog house based on the exterior wall measurements of the warehouse, and that is just obviously the wrong way to do it.

6. When doing a room by room load, measure and round up each interior wall.

When doing a block load, measure and round up each exterior wall.

7. Another thing to consider:
http://www.energyvanguard.com/blog-b...ce=Blog_Email_[Big%20News%3A%20The%20R-Valu]

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Originally Posted by tipsrfine
Common sense tells me that the only thing we are concerned with is conditioning the living space. We need to know what the load is coming through the walls into that living space. We don't care about what is going on IN the walls, only what is coming out and into those walls.
Manual T is a really good read. In it you'll find that we are concerned about the tempature of the walls and ceilings. When the wall is colder than me I radiate... no, I'm not radiant... but my wife is, wait, what was I saying, oh yeah... when I radiate because an object is cooler than me I feel cold... the comfort or discomfort depends on the tempature difference. If the object is warmer than me it radiates and I feel warm. That's why my load is the ceiling and walls....

9. Originally Posted by tipsrfine
When I get the money to buy I will. At this point I am just trying to learn on the cheap. Common sense tells me that the only thing we are concerned with is conditioning the living space. We need to know what the load is coming through the walls into that living space. We don't care about what is going on IN the walls, only what is coming out and into those walls. Explain to me why it is that the calcs want the ceiling height, and not how high the roof is above the ceiling?
And especially explain to me how you would do a manual J on a room in a finished attic where the outer walls of the attic are 25' x 40', but the conditioned area is defined by knee walls of 15' x 25'? All we care about is the heat load coming into the conditioned space INTO the conditioned space. Logic dictates that the load CAN ONLY come into the conditioned space through the envelope of the conditioned space. (not counting internal loads of course). Do a manual j calc on a small insulated & conditioned dog house in the middle of a large warehouse; by your logic, you would size the equipment needed to cool the dog house based on the exterior wall measurements of the warehouse, and that is just obviously the wrong way to do it.
No, the load on the dog house would be calculated on the temperature differential between the two based on the HTM's and the surface area of the doghouse walls. The warehouse size,location etc does not matter.

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