PDA

View Full Version : How to handle this manual J calculation

gander
02-19-2007, 04:09 PM
I will be getting a new ac this spring and in the process I will be getting a manual J load calculation. I need to make sure that the individual performing the manual J calculation correctly handles my non-typical house construction situation. Let me describe the layout of the house and all will become clear. The house is a 2700 sq. ft., two-story home, dual heating and cooling, located in the Houston area. Think of the house as being divided approximately into thirds with the center third having a sloped ceiling rising from the first floor and becoming the ceiling of the 2nd floor. The low end of the slope is at the standard 8 ft. height and at the apex about 18 ft. There is a bridge for traversal from the left and right sides. My question is what is the correct way to calculate for this sloped ceiling? The problems are listed in the next paragraph.

I can see and hear you grinning now. You say, "of course you cannot easily heat nor cool this house"! In the winter the heat fly’s up the sloped ceiling where it encounters the upstairs t-stat. The upstairs t-stat rarely calls for heat as the downstairs heater is satisfying its conditions, leaving the upstairs bedrooms cold and all of the downstairs cold. In the summer the opposite happens, the upstairs’ ac unit runs flat out because part of the cold air falls to the first floor. The downstairs is nice and cool while the upstairs is hot. We have been in the house for two years now and in that time additional soffit vents have been added, all light fixtures and outlets have been sealed for air leaks and additional attic insulation was added to bring the upstairs attic to an R-38. Nothing can be done about the sloped center section. There is no way to add insulation to it and it will always be a R-20 roof. Because cooling is my dominant concern, I really need to get the upstairs’ ac size correct. Currently the upstairs’ ac is 2-ton unit and the downstairs’ is a 3-ton unit. All insights are welcome. Thanks.

seatonheating
02-19-2007, 04:25 PM
Easy design.....2 zones!!

Manual J "averages" ceiling height, won't be a problem. Programs these days can factor all that in.

BTW, I'll be in the Houston area from March 25th thru 27th. Would be more than happy to help out(for a small fee of course :). If not, find one of the guys on here locally, sure they will help you out.

gander
02-19-2007, 05:42 PM
Thanks for the offer. If things do not go well with the hvac contractor, I might need to take you up on the offer. Help me to understand what you mean to have two zones. I thought that I already have two zones, one for upstairs and the other for downstairs. Thanks.

seatonheating
02-19-2007, 06:47 PM
One furnace/AC or 2? Not sure you clarified that.

Special Ed
02-19-2007, 07:15 PM
Easy design.....2 zones!!

Are you kidding me?

Whether it's zoned or it has two systems he's still gonna have the same problem. Besides, he has 2 systems which is sorta like zoning only far better. And, zoning is for comfort & doesn't make the system more efficient - in fact, it probably takes away from the efficiency.

If I were to recommend anything it would be to re-locate the t-stat to a more "protected" spot like a bedroom.

cem-bsee
02-19-2007, 08:16 PM
put a return in ea rm upstairs back to that unit --
keep doors closed --
t-stat in M bdrm

gander
02-19-2007, 08:20 PM
Yes the house has two heating/cooling systems. One for upstairs(2 ton) and the other for downstairs(3ton). I have never seen an application where the t-stat is not within a few feet of the return air. I can easily see how this would provide a more even temperature than at its present position. Thanks for the idea.

seatonheating
02-19-2007, 08:46 PM
Are you kidding me?

Whether it's zoned or it has two systems he's still gonna have the same problem. Besides, he has 2 systems which is sorta like zoning only far better. And, zoning is for comfort & doesn't make the system more efficient - in fact, it probably takes away from the efficiency.

If I were to recommend anything it would be to re-locate the t-stat to a more "protected" spot like a bedroom.

Don't bash until you know where I'm coming from. I didn't really interpret his post as meaning 2 systems. He said Dual, but being a homeowner he could've just been talking about the fact it has heating and cooling.

Now to his problem. It is a good idea to relocate the stats but only if your supply/return pathways are in ideal locations first. Being that you have both heating and cooling, there will be more than 1 solution. May have to try a couple of things.

rlj1117
02-19-2007, 11:31 PM
i have member title , but answer to your question is , most load calc programs will average ceiling heigth and take that into consideration, but being nebb certified a room or space does not see anything as long as you have correct cfm for that certain space . questions seaton?

seatonheating
02-19-2007, 11:34 PM
i have member title , but answer to your question is , most load calc programs will average ceiling heigth and take that into consideration, but being nebb certified a room or space does not see anything as long as you have correct cfm for that certain space . questions seaton?

you mean Gander

pipe
02-20-2007, 03:31 AM
You have got air being stratified and the cheapest way is to install a ceiling fan with low rpm in the region where you have a high ceiling. The underlying assumption in all load calculations is that you have a good mixing of air. It might look a bit odd but you will have air movement in/out of the pockets where stagnant air gets trapped. Try this first before moving the stats.