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-   -   Credibility and reality of a manual J (http://www.diychatroom.com/f17/credibility-reality-manual-j-147079/)

Missouri Bound 06-14-2012 11:30 PM

Credibility and reality of a manual J
 
I have been curious as to how many have had there HVAC equipment replaced without a manual J done. And I wonder how many swear by the need of it being done to find out that house size and window size calcultions produce the same results with insulation and climate considerations factored in. I would think that with the "newer" two stage compressors that dialing in the size with a manual J would be of lesser value since the HVAC equipment can actually adapt to the needs more appropriately. I'm sure there are a lot of older generation HVAC techs here that have never thought a manual J was necessary, and a lot of "now" generation techs that think it's an absolute must. I myself being of the "older generation" think that it's most likely a good idea in new construction, when all the factors are documented, including duct length, size and type and construction materials are clearly noted....but in existing construction.....not sure of it's relevance. Seems to me if it worked well as it was sized originally, then replacing it with like equipment would be the way to go.:whistling2:

REP 06-15-2012 04:11 AM

You know when I first got started all heating equipment had pilot lights and a/c units were all about 8 seer.
In todays world energy is just too expensive.I remember the first time I did a manual J for a small house that had a bad 90,000 bryant furnace.
In the course of owning that house the older couple replaced most of the original windows ,had blown in insulation and did a few other things to tighen up the house.I got a 28,000 heat loss with the manual J.I also lost the sale to another "contractor who installed a 100,000 btu furnace.
As I said this was an older couple so they paid for that beast to short cycle for the rest of their lives because the other guy just put in a simuliar unit.
So if you figure the extra cost to put in the same size unit for 20 years ,it makes the $50 or $200 that gets charged for doing an honest quote look like lunch money.(when I started doing the manual it was long form and took a while and I did charge for it.Nowadays its on a laptop and you have to do it for free in order to be compeditive)

beenthere 06-15-2012 05:59 AM

Quote:

Seems to me if it worked well as it was sized originally, then replacing it with like equipment would be the way to go.:whistling2:
Problem is that many people think worked well means it cooled the house to 68 for them to be comfortable. When a properly sized one would only need to cool the house to 72 or 74 for them to be comfortable. And would ave them money on their cooling bill.

Quote:

I would think that with the "newer" two stage compressors that dialing in the size with a manual J would be of lesser value since the HVAC equipment can actually adapt to the needs more appropriately.
A 3 ton 2 stage in a house that only needs a 2.5 ton, often works ok. But if the house only needs 2.2 tons, it doesn't work real well, since first stage capacity is often anywhere from 2.1 to 2.4 tons. And the first stage SHR is usually high(excluding the Trane 20i which does run at 50% capacity in first stage).

Quote:

And I wonder how many swear by the need of it being done to find out that house size and window size calcultions produce the same results with insulation and climate considerations factored in.
Less then 30% of the time is the old unit the right size. usually find 1/2 to 1 ton of over sized.


Often find a 2000 to 2200 sq ft house with a 3.5 or 4 ton A/C. When a 3 ton is what it needs. And it is cooling the house ok. Usually because the duct system is under sized enough that they are only getting 3 tons of cooling from it.


After 37 years in the trade I still learn something on a regular basis.

Missouri Bound 06-15-2012 12:55 PM

REP, the efficiency ratng is a good point. But like furnace effeciency, can the seer rating be applied to cooling equipment sizing? Such as, if a 2 ton unit has a 8 seer rating, and a new unit has a 15 seer rating, would this calculate to a smaller unit? Does a higher rating mean more efficiency with the same output, or better performance which would result in a smaller unit?

beenthere 06-15-2012 03:15 PM

SEER, is how many BTUs you get per watt over the entire cooling season. But a 2 ton 8 SEER, and a 2 ton 15 SEER, are both 24,000 BTUs an hour. So no, a higher SEER doesn't mean you can use a smaller unit to get the same BTUs.

beenthere 06-15-2012 03:19 PM

On another site. Someone has posted about replacing his 2 A/Cs. he has 2 floors, both 1750 sq ft. And has a 4 ton A/C for each floor. His house is a good example for doing load calcs to find out what size is really needed. maybe only a 2.5, and 2 ton can't cool his house.

chp 06-15-2012 05:48 PM

I just did pretty much what MB is suggesting. My 3 ton has never cooled the house on the hottest days. All the local contractors wanted $400 to do a manual J, so I found one that would just give me what I wanted at a great price. I put in a 2 stage 4 ton with VS air handler. He also added 6" returns in all bedrooms except the Master. The house is 2400 sqft, with no shade, and the sun beats down on a lot of glass in the afternoon. We really tested the system last week with cool wet weather. The house kept 40-50% RH @ 75 degrees with our 70 degree 95% humid nights. I'm anxious to see how it does when we have a pool party. The old system would let the house get up to 80-82, and not cool back down until 1am with all the of the kids running in and out. I'm hoping the 4 ton will be able to keep up on those hot days.

beenthere 06-15-2012 05:52 PM

Quote:

Originally Posted by chp (Post 944150)
I just did pretty much what MB is suggesting. My 3 ton has never cooled the house on the hottest days. All the local contractors wanted $400 to do a manual J, so I found one that would just give me what I wanted at a great price. I put in a 2 stage 4 ton with VS air handler. He also added 6" returns in all bedrooms except the Master. The house is 2400 sqft, with no shade, and the sun beats down on a lot of glass in the afternoon. We really tested the system last week with cool wet weather. The house kept 40-50% RH @ 75 degrees with our 70 degree 95% humid nights. I'm anxious to see how it does when we have a pool party. The old system would let the house get up to 80-82, and not cool back down until 1am with all the of the kids running in and out. I'm hoping the 4 ton will be able to keep up on those hot days.

MB was asking if the opposite of what you did could be done.

Marty S. 06-15-2012 06:34 PM

I'm just a tech so manual J is above my pay grade but I do have some questions. If I understand correctly the indoor temp default is 75 for cooling. If that's correct and the average home owner changes places every 7-8 years how do you account for personal preference? For example seniors on oxygen seem to like 80 for a room temp and big boneded folks seem to want 66-68. Old folks buy the system then move on to assisted living in 7 years when biggie buys the place. Neither can handle humidity well so without selling a whole house dehumidifier how do you satisfy the wants of several home owners over the life of a system?

How about weather extremes? Our design outdoor temps in this area are -2 and 95 which is supposed to cover 98% of the time. In my life there's been cold low as -28 and highs up to 108. Granted those are out of the ordinary but every year there will be a week of over 100 and a week or more of 10 below zero. Tell the customer well there's going to be one week in the summer and one week in the winter where you wont be comfortable with this new $10,000+ system but that's what the manual J calls for?

Not knocking a load calculation at all,just some questions.

beenthere 06-15-2012 07:01 PM

I never size an A/c to only do 75 at summers outdoor design temp. I do most for 72 indoor. if someone moves in that sets teh stat to a higher temp and cals that its humid in the house. the blower can be slowed down, since it was set for 400 CFM per ton to do 72 inside temp.

An A/C sized by Manual J(how many BTUs at design) and S(selection of equipment to meet that BTU capacity at design temps) to do 72 indoor at 95 outdoor, will generally have a few BTUs to spare.

We have a 92 outdoor design, but often hit 95 and have several days of it in a row of it, and usually have 3 days of 98 or better. And yes, some people call up and say its 76 in our house and we have it set for 70. Most people will accept a few degrees warmer for a few days, as long as its keeping the humidity in check.

If you do capacity checks. You'll find that many 4 ton units are on duct work small enough, that the system is only delivering 3 to 3.3 tons of cooling. So a 3 or 3.5 ton will actually cool the house better, and to a lower temp.

REP 06-15-2012 08:28 PM

Quote:

Originally Posted by beenthere (Post 944152)
MB was asking if the opposite of what you did could be done.

I could be wrong,but if I'm right then just adding the returns might have solved your problem.
If I am right you will spend that $400 in the next 2-3 years in extra cost.Now multiply that by 15-16 years and tell me how much money you saved and tell me how much you enjoyed a cold clamy interior because the unit you have now is dropping the temp,but not be on long enough to get the humidity.
Penny wise,hundreds of dollars foolish.

ionized 06-15-2012 08:35 PM

Calculations on a lot of old construction are not going to be all that accurate. For new construction they should be. The calculations are the hypothesis and the installation is the experiment that confirms or disproves it.

In replacing a system, if a existing capacity is serving well, the same size will serve well. You can not possibly do better with calculations based on guesses about the magnitude of air leaks and quantity, condition of insulation, etc. Of course, that assumes that it is a well-designed system and the duct work is in good shape and will not have to be modified or improved significantly.

Of course, how does the installer know if the old. broken, system is/was operating well? Some homeowners can judge that well, but others can not.

beenthere 06-15-2012 09:32 PM

A few simple questions to the home owner will tell you if the old system was maintain temp. And if it was controlling humidity.

Taking static pressure readings of the duct work will tell you how close the air flow is to proper.

Learning the construction practices of the olde homes in your area will tell you how well the houe is insulated, and give you a good idea of its infiltration rate. A blower door test will tell you its infiltration rate.


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