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Thread: Fed up with Manual J!
03-21-2012, 10:04 PM #1New Guest
- Join Date
- Mar 2012
Fed up with Manual J!
I have paid for and received a Manual J (loads by room), Manual S (equipment selection) and Manual D (duct design) design for a heating-only residential project in the San Francisco area. The calculations were done using the ACCA approved WrightSoft software package by a consulting firm in the Midwest who appears to be credible.
So far, I have talked in person or by phone to 9 residential HVAC contractors in the San Francisco area, and each has proposed a system that is 3-4 times larger than the system indicated by the Manual J. Believe it or not, non of them have ever heard of Manual J. Some have refused the job or refused to guarantee it if I insist on the sizing indicated in the Manual J report.
A couple more data points:
My house is 33% the size of the project building and has a furnace with twice the capacity of that proposed by Manual J for the project building.
I have an engineer doing an electrical load calculation for the project. I spoke with his partner, also a licensed PE in California, who does the mechanical engineering work for their firm. He also had never heard of Manual J, but gave me some "rough" rules of thumb for sizing systems in my area (ie: typical BTU per foot, etc).
I am totally confused. If I go with the Manual J calculations and they are wrong, then the duct system will be significantly undersized and it will be very expensive to fix.
Is there any way to validate whether my Manual J calculation is correct (without hiring five engineers and averaging the results)?
Can anyone tell me why I shouldn't just go with the bigger system and play it safe? The additional cost is peanuts compared to the size of the project, we don't have any LEEDS, Energy Star, or regulatory compliance requirements, and the hassle that this is causing doesn't seem to be worth the effort to try and do it the 'technically correct' way.
Thanks in advance for your guidance.
03-21-2012, 10:27 PM #2
03-21-2012, 10:41 PM #3Professional Member*
- Join Date
- Aug 2007
- Northern VA 38 degrees N by 76 degrees W
At the top of the page when you log in is a load calculation that you can do that will give you an idea what your home needs. Post a copy of the load calculation that was done
03-21-2012, 10:42 PM #4
Ask your California contractors if they have ever heard of title 24? And what title 24 bases it's loads on?
If your PE has never heard of manual J.Make your expertise uniquely valuable.
Make your influence uniquely far-reaching.
03-22-2012, 12:32 AM #5New Guest
- Join Date
- Mar 2012
Thanks for your comments and help. Yes, this is a residential project.
Attached is the Manual J report done for the project building. Please let me know if you see any errors/problems or have any questions about the design assumptions.
Despite all the attention/press given to Manual J, I have come to understand that there are two other acceptable load calculation systems in addition to ACCA's Manual J. These are California's Title 24 and ASHRAE, and I assume both have their own software programs to do the calculations. To be fair to the PE I referenced in my earlier post, he implied in our conversation that he used Title 24 for load calculations. But I was still shocked that he didn't know what Manual J was.
For this project we also had a Title 24 report (a requirement in California) done early in the project in order to get the building permit. My impression is that in California, the State mandated Title 24 reports for residential structures, as long as you "pass" and don't require a HERS inspection, etc., are pretty much done for permit sign off purposes and aren't used or are accurate enough for HVAC system design. My Title 24 report for this building was considerably less detailed (for instance, no duct design) and informative as my Manual J and D reports that I had done. If you'd like to see the Title 24 for this building I can post it. None of the California HVAC contractors I've talked to have asked to see the Title 24 report.
In our case the Title 24 report showed a total building load a little over double what the Manual J showed. I asked the Manual J consultant how this could be, and they 'reconciled' the two and said 1) the actual ratings of the installed windows/doors is much better than the assumptions made in the Title 24 calcs, and 2) the Title 24 calcs had significant heat loss from fireplaces.
The fact that the Manual J consultant could account for the differences was reassuring and suggested to me that the Manual J consultant knows what he is doing. However, there were other things said/done by this consultant that didn't really pass the 'common sense' test to me, which is why I'm nervous about throwing caution to the wind and buying the Manual J calculation without further investigation.
The issue is that if the numbers/design of the Manual J consultant are wrong on the down side, the fix will be extremely costly. If I go with the larger system that everyone else proposes, I really have no risk.
How do I figure out who is right? Who do I believe when there is a lot of money at stake?
Please let me have your thoughts and advice.
03-22-2012, 04:44 AM #6
Manual j is based on ASHRAE.
He is already recommending furnace with a 50% larger output then what the manual J load requires. What size are the other contractors recommending?Contractor locator map
How many times must one fix something before it is fixed?
03-22-2012, 06:11 AM #7
My own experience with manual J has worked. I don't do any residential or sizing for that matter but did in school so for my home I pulled out my 25 year old notes and followed the forms and calcs. I own an older home, 1958 and had a 125,000 btuh furnace 80%. After calcs I put in a 75.000. I was apprehesive but I checked my numbers and remembered how my furnace cycled on zere degree days, on for maybe 15 min off for 5. Back on for 15 min etc. After install unit is on, on zero degree days for almost a full hour off for a couple of minutes. My gas consumpsion dropped by a third consistantly. I do wish I put in the 2 stage system but they were pricey at the time. I would follow manual J but if a PE is involved they are putting their name on the final result. Also as was mentioned as far as multi dwelling goes is there ventilation air (OA) being supplied. Do we have to consider walls adlacent to other units as conditioned spaces or exterior in case a unit is empty and the equipment shut off? Just askin.ckartson
I didn't write the book I just read it!
03-22-2012, 06:37 AM #8Professional Member*
- Join Date
- Feb 2006
- Float'N Vally, MS
You are listed as a new guest....
My experience with WrightSoft is that someone using it for the first time will screw up big time.
Any experienced contractor using this type of software will hit it on the nose every time.
Key work is experience.Life is too short, Behappy!
03-22-2012, 07:21 AM #9
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03-22-2012, 07:34 AM #10
behappy, I'm not a new guest, and I said that manual J did work for me. Are you replying to confused hvac?ckartson
I didn't write the book I just read it!
03-22-2012, 09:00 AM #11
Confused, I use Manual J on every job I do for many years and it still shocks me at the size of equipment recommended. Contractors have used rule of thumb for so long (which is always grossly oversized) that thy are afraid to install properly sized systems. They have the same fears you do.
I routinely install three ton systems where five ton systems were before and I have had no complaints.
You need to find a local contractor that will perform their own load calculation and stand behind their work. No credible contractor should or would go on the strength of someone else's load calculation and be responsible for the results.
You asked what the consequences of over sizing will be, you will have higher utility bills, short cycling poor comfort and shortened equipment life. If you were having A/C installed over sizing would be a bigger problem regardless proper sizing is always preferable.
03-22-2012, 11:02 AM #12
Manual J is only as good as the entered data.
Wrong data- wrong results.
Every contractor should know what Man J is, so those who don't- I'd eliminate from my list. As for refusing the job- I wouldn't take a job based on someone elses calcs either. I'd do my own & guarantee the outcome.Always drink upstream from the herd
03-22-2012, 11:31 AM #13
This is a perfect example of why I will always support a "separation" of energy auditors and people who perform manual J, from the day to day A/C & Heating business. If the A/C company was not involved in the manual J there would be no confusion,as we see here, "and" it would let the responsibility fall upon one company. Maybe if I preach this sermon long enough, or enough A/C businesses end up in court because of a bad manual J, they will eventually get the message. How on earth a day to day small A/C business can afford to send out a $25.00 hr. tech to perform a manual J, and possibly "NOT" get the job is beyond me.