That brings up the questions: Do we have the correct imput on our calcs and do we design the house for the few days of abnormal conditions? And is there a better way to solve the problem? For example to control humidity on high demand occasions and in mild temps--use a whole house dehumidifyer.
Originally posted by Carnak The latent load seems too low for Houston.
I don't use manual j but when they say 47 grains for 50% does that mean that the ambient air holds 47 grains more than the room air?
******That's my understanding. I think it is converted from the coincident WB at the design temperature.******
If so that would imply that Houston has a design ambient of 112 grains, I would guess it would be more like 130 or higher.
*****The magnificent one may guess, but Manual J has looked it up in the ASHRAE books. PNM has been to Houston a couple times and tends to think it might be closer to 200.*****
All I have is worst case humidity data for Houston and it ranges from 137 to 147 grains.
*****When there is a hurricane you lose power and your AC won't work anyway.*****
I would guess the latent load would be more than double what was calculated considering 0.3 air changes, plus six occupants and mama simmering some chili.
*****Manual J assumes mama turns on the vent fan over the range when she cooks. But Manual J ignores the humid air the fan sucks in from all corners of the house. So moma needs to be trained to open a window in the kitchen when she cooks.******
Originally posted by uktra Do we have the correct imput on our calcs and do we design the house for the few days of abnormal conditions?
If I have bought a 5200 sq. ft. home my guess would be, either a lot of occupants or a lot of parties call it abnormal conditions may be perfectly normal to some and you brought up a good point as to why you can't totally rely on old J & D.
By the way did old J&D put common sense out of business? if so were all in deep po-po.
Mrbillpro--well if huge parties are given all the time then the people can afford two stage equipment, can't they. Of course you do explain to all your customers this would be the best way to go instead of slamming in 2 extra tons--right?
3.5 tons on 5200 square feet. I'd say run the Manual J one more time. Might be a good idea to take into consideration internal load...lights...computers, TV sets, people, pets, refrigerators and the like. Going to be interesting on a 4th of July party with 25 people over too.
Manual J (7th Edition) states that one of the assumptions is, "That the sensible loads associated with people, lights and minor applicances are included in the 300 Btuh allowance that is applied to each occupant."
It also says, "Usually the number of people is estimated to be twice the number of bedrooms." This house has 10 people in it.
Manual J says to "Include 1,200 Btuh as a sensible load in the kitchen. This is considerably less that the rated output of the appliances, but intermittent operation and exhaust hoods make this a reaonable value."
I should note that today's refrigerators are far more efficient than they were when J7 was written.
Mrbillpro, I have been in this argument with others here before and some don't understand that when its hotter than design or if they have several people over and it won't cool they are not happy. They don't want to hear it was sized according to manual J, they want it cool because they paid good money for a system to cool their house when they want it cool. Oversized most of the time, Yes but then again there is no perfect size for any house due to changing outside conditions. If I installed all my systems according to manual J I would have been out of business along time ago.
The first commercial job I ever got the distributor came and sized both the furnace and A/C (I honestly can't say what he used to come up with the loads) and both were to small. He paid for the equipment and I furnished the labor and learned a valuable lesson.
Manual J (7th Edition) states that one of the assumptions is:
Does man figure up all this? is so there is room for imperfection! is that included? unless the perfect man of course is figuring all this up and I don't know if God is into Manual J design or not, but I bet you got an answer to that also.
Originally posted by trane Mrbillpro, I have been in this argument with others here before and some don't understand that when its hotter than design or if they have several people over and it won't cool they are not happy. They don't want to hear it was sized according to manual J, they want it cool because they paid good money for a system to cool their house when they want it cool. Oversized most of the time,
I totally understand they have a product to sell and like any good old politician they will take there arguement to the grave, I am just having some fun with Mr. Know it all I thought Rocky and Bullwinkle kicked him out of the club.
Man what happened? guess the server room a/c was undersized and the server overheated the a/c company probably depended on old J&D a little to much and forgot about Mr. Common Sense.
So here is the $64,000 dollar question, do we install a border line a/c and hope we never have to entertain or cook on a 100 degree day because if it's borderline you have no more you ate all your cookies, or do we have a tad to much to were if we want it 70 degrees after a long hot day at work we can crank it down! and be comfortable? this argument could go on forever, but I will tell you one thing you come to Houston with your idea you better bring a good attorney with you because I don't think the Judge will let J&D take the stand in your defense you will be spending a lot of time in the cool, oversized courtroom.