Reply to Thread

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.

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....

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

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.

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.

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?

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.

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.........................

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....

Originally Posted by second opinion

you are correct, it is the interior dimensions used for the calculation.

According to appendix 5 it is the "outside dimensions" for exterior walls.

Wall thickness may not have a big difference normally, but consider a finished attic with knee walls. The actual conditioned space area can be much less than if you measured at the exterior walls. Take ceilings and how they are entered into Manual J. You don't ignore the ceiling and enter the roof dimensions. Same goes for knee walls. Common sense says that all we care about is the load on the conditioned space. If you built a small dog house in an attic and conditioned it, you wouldn't throw 2 tons of a/c into it just because the attic it is in is bigger.

Isn't that why we position the registers to sweep the perimeter walls?

Tips here is some good info for you. Attachment 375331

Fudging input data on indoor/outdoor temps will seriously throw things off.

Agreed, splitting hairs, but since he asked...

Tips says hes not trying to condition the wall, but in reality you really are. Heat enters the wall structure and is transfered out at whatever rate. The surface area of both the interior and exterior matters, both conduct, and at different rates depending on which side is warmer. I cant comment on which dimension is the assumed dimension by the manJ folks.

Me? Id measure the easiest way.

The OPs post is the kind of smart questions that need to be asked and answered. Splitting hairs? yes but a small miscalculation here and a mismeasurement there along with a few incorrect assumptions can send the end result far from acceptable accuracy. Be as precise as you can with measurements, and make as few as possible assumptions to get as close as you can to the actual load. Ideally: measure everything (twice) and assume nothing (if you can)

Just don't follow the unofficial standard of measure sqft/ton and nothing else, or replace "Like with Like" and assume everything will be great.

Originally Posted by alamo1718

I am currently enrolled in a Residential design class, so my real world experience is very limited but I have the book with me.. We are using Manual Jae(Abridged Edition). It is not as many pages and much easier to teach from. It says in Appendix 5 that "Exposed above grade wall is recorded to the nearest foot(use the outside dimensions for exposed walls and the centerline dimension for interior partitions)". I have class on Tuesdays and I also have access to a Manual J. I'll ask my instructor and look at my Manual J and let you know what else I find.

You are correct! And yes the full Manual J8 reads the same as Manual J Abridged.

Your splitting hairs! Your wall load is a small fraction of you total load..........so if your off by a few sq' feet its NOTHING! Worry about your windows, there direction,there placement in the structure, there type, overhangs, and interior and exterior shading and blinds. And as you know Tips your inflitration is very important, don't forget about that blower door!

I am currently enrolled in a Residential design class, so my real world experience is very limited but I have the book with me.. We are using Manual Jae(Abridged Edition). It is not as many pages and much easier to teach from. It says in Appendix 5 that "Exposed above grade wall is recorded to the nearest foot(use the outside dimensions for exposed walls and the centerline dimension for interior partitions)". I have class on Tuesdays and I also have access to a Manual J. I'll ask my instructor and look at my Manual J and let you know what else I find.

Thanks. I have a contractor I'm working with who came in with a bid that appears to be oversized; he included the 16" thick walls in the rooms conditioned area.