+ 1 precision hvac
+ 1 precision hvac
I run manual 'J' calcs on a daily basis, sometimes multiple times in a day. So for giggles, I checked a house on which I did a load calculation yesterday for my area, which is south of Boston, MA. The heating need at 0°F for that 3300 sq. ft. home was 73,233. However, when adjusted up to 38°F, as would be anticipated, the load dropped to 34,582 or to less than 1/2 of what was needed at 0°F.
The problem you are encountering is resistance to science. If none of the HVAC contractors has heard of Manual 'J', then they likewise are probably ignorant of the Air Conditioning Contractors Association, ACCA, NFPA and the many other alphabet groups that strive to keep people safe and comfortable.
Above is a website you may want to visit. You can copy it and show it to your nay saying contractors who should be ashamed that they don't even know about Manual 'J' when it's cited in Department of Energy educational literature. Perhaps your contractors can't read???
Stick to your guns. My only concern about the whole issue is using one furnace to do an entire 13,000+ square foot home. Personally, I'd shop very, very, very (did I say very?) carefully because the potential for gross discomfort in a home of this size is great and if these guys and gals don't know Man 'J', then they certainly don't know Manual 'D', which is where the other half of the knowledge issue really can rear it's ugly head.
As an additional thought, it is entirely likely that on a job of this scope, the GC has been consulting with commercial companies rather than residential companies. In our state, where sheet metal licensing (any air system that moves mechanically is now required to be installed by a licensed sheet metal mechanic) was recently enacted and where a home of greater than 35,000 CUBIC feet requires a ratio of 1 to 1, journey man to apprentice on the job, is basically pushing large homes to toward the commercial market. The commercial market uses Manual 'N' and most frequently the calculation is done by a mechanical engineer so that the HVAC companies only have to install, install, install. Could you be suffering such a fate?
I'm a little leery to respond to a poster building a 14000 s/f luxury house with no a/c(?), but will anyway.
The problems as I see them:
7 btu's per s/f is a big red flag, triple it and I'd still be a skeptic that needed proof. Did I read the report wrong? 100k furnace serving 14K s/f?
1 furnace/3.5 mil to build? ? Give me a break. "oh, were gonna zone it"...please. break it down to unit per floor, or unit per exposure. Short sighted design with no logic or creativity. Define the outcome and work back to the best solution
Call Mechanical Air Service and see what they have to say about the project. I don't know them or seen their work, but they have many awards and promote themselves as "the best ever". Who knows, they may live up to their opinions of themselves.
Your house must be big for a 60k btus heat loss.
Is cooling not needed there?
I do manual J calcs with heating & cooling to determine the greater of airflows. With what you have so far; an 80kbtu 3-ton furnace might be adequate, perhaps a 4-ton instead.
Manual J's are one part, but if manual D and manual S are ignored why do HVAC companies even talk about "manual J"!
All architects, PE's, and ME's should know about MANUAL J! There is something wrong when they don't.
Would they guarantee and warranty everything they put in? Would they guarantee comfort, high efficiency, durability at at least 5 years (parts and labor) for their design and install; I think not!! They would probably only cover the equipment, like most do.Quote:
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.
I say keep searching.
Good points... I have been wondering how long it would be until this "load calc" issue became an irritation in the industry... here are my thoughts:
I know I would NEVER install a system and guarantee it based on someone else's load calc... that is just spitting into the wind IMO.
On the other hand (OTOH), I do not do free load calcs either.
The reason for this view is simple: There are some subjective things in a load calc (things that cannot be accurately & scientifically measured); and as such ultimately the opinion of the one doing the calc affects the end result. Having said this; a THOROUGH energy audit will catch the better part of these issues... however maybe not so well on new construction.
I think as soon as the economy firms up a little... load calcs will no longer be free... which IMO will be good for the industry.
So Mr. Bill, you would have the jobs reduced to the lowest bid then, correct? If all the HVAC companies are forced to rely on a 3rd party load calculation, duct analysis, Manual 'T' outlet sizing and Manual 'S' equipment selection, what's left except the install? And how does a company separate itself from the rest under those circumstances? By claiming to be better?
One pays for the cost of load calcs by having it done by a professional sales person who works on commission. Make the sale, get paid. Don't make the sale, no pay. And I for one am not about to stake my reputation on someone elses determination of what is a proper load calculation. My company has removed thousands of furnaces, boilers, AC and HPs, by far the vast majority improperly sized. So while you may have a hard time understanding how run a business, some of us have it figured out pretty well so it works for everyone. :.02:
OK, I have read the second page of the thread and looked at the load calc attached.
MeThinks a 100K furnace for a home of this size is waaay out of line; regardless of the numbers. For one thing; the ductwork to distribute this small volume of air over a home of this size would be a real mess. Personally, I would not even touch this job... because of the potential problems.
As noted in previous posts: I would keep shopping until you find someone that understands Manuals J, D, and S... then have them look over your plans. If they are interested in your project, only then tell them you have a load calc already done.
My guess is the contractor will want to do another one... as they will be wary of this one... you probably will have to pay for that.
The bottom line: While all us contractors want to do work; we also do not want to get involved in a project that will not function properly... and this project has that issue written all over it.
I am not trying to be negative... just prepping you to understand the local contractor is probably going to want more control over the load calc, duct plan, and equipment selection. There is more going on here than just numbers and specs and prices... Just the way things work.
As noted in my previous post... there is a lot more going on here than numbers and details and prices.
Looking at the load calc provided I can easily believe that the load is only 60K especially with a 38* outdoor design temp. My house in Houston was built in 1971 has minimal wall insulation and old single pane windows, the indoor temp does not drop below 50* on days in the mid thirties if we do not run the heater. I also agree that one furnace of any size will be inadequate for the airflow requirements.
I also wonder at the lack of A/C for a large tight home, in the long run this is going to come back to haunt you.
I would not use a manual J I would run a manual N commercial load calc and have a commercial company design and install the system. From the drawings I would look at hydronic hating and going with a Unico SDHV (small diameter high velocity) system using their unchiller for A/C and heat.
If you contact UNICO directly they will connect you with a trained and certified UNICO installer and help with the design and sizing of the system.
My approach is to explain load calcs to the HO... and give them a hard $$$ Proposal that states we will do the load calc AFTER the sale is made. Most of the folks I talk to are agreeable to that... unless they are trying to get a free load calc... in which case it is likely a Craig's list guy (hack) will be doing the install. Either way... no sale for me; so no load calc for them.