A Manual J calculation is calculation of how much heating and cooling capacity a home will need under design conditions. The book used for the calculations is the ACCA (Air Conditioning Contractors of America) Manual J. It is used exclusively for load calculations in low-rise residences. It's calculations are based on many factors, where you live, what biulding materials you use, how much insulation, what kind of roofing, etc....

This calculation is the first step in designing a residential HVAC system and must be completed correctly before equipment may be selected and ducts designed. If your sub is about to select equipment and hasn't done a Manual J yet, you need a new subcontractor. Talk to your general contractor and tell them because this is a BIG problem and if not taken care of now will cause you trouble down the road.

Hello,
In the 1970's NASA developed computers to help design rockets. Prior to this all engineering was done using a sliderule and by drafting on paper. In the early 1970's, I helped design a 200ft ship. Computers were not used. We almost built a sinking ship. Today that would never happen. At best we had a few calculators. When I graduated from high school in 1971, there were no hand calculators. Imagine not having computers or hand calculators. Homes were designed on the back of a napkin, like the first Compacq computer was. Today we have computer programs to help do taxes, accounting, and HVAC load calculations. They guarantee that a mistake is not made in simple math. One might argue this is a simple process. IT AIN'T. Tell him to get with the program, move into the 22th century. Use a Manual J to verify the size of equipment needed. Then use a manual D to help design the ducts to utilize the new higher SEER equipment. It aint the same old HVAC equipment.

This is like asking a doctor to set a broken bone without an xray. My sister has an 8 month old baby, that almost died from a brain tumor. 2 days ago the doctor thought there was a problem, did a catscan then a MRI. The next day the baby had brain surgury. 9 hours of waiting, and the baby is healthy today. Without a few modern tools, the baby would have died today(Jan 18, 2007). It was that close. Tell your GC to wake up and get with the program.

Ummm... Let me take a slighly different tact.

Are you being asked to choose the model or the exact unit? i.e. he can as you to choose between an 80% and a 90% without the calc. He can't tell which size you need, though.

Bentruler - I've told HVAC Guy I want 14 Seer and 92% efficiency, Goodman furnance and a/c, no heat pump. But, I need him to do a Manual J Load Calc, then give me a price.

He's come back (after installing the first floor duct work) and says the price will be $13,500 for system and all duct work (without specifying the tonnage of either the upstairs or downstairs units) and, by the way, I'll have that load calc done before I start the install.

My thought, which prompted my posting the original question, was: Doesn't he need to have the load calc performed before he orders the equipment and before he can give me a price quote?

Thanks for all the information.

Is a Manual J necessary when replacing just the furnace itself?
No duct work will be changed.

I am planning on replacing a 66,000 BTU Carrier furnace with leaky heat exchanger tubes.....
All of the contractors that have come and given me quotes stated that they plan on using an equivialent sized furnace as a replacement.

Is this alright?

Soonermagic... YES, he does have to have the man J done before he can order the equipment. If he does not have the loads, he cannot know what BTUs to order for the furnace and what tonnage to order for the A/C. And, without a manual D, the ductwork sizing is simply their best guess.

As a homeowner, I would seriously put an immediate stop to it all -- sounds like they are trying to put one over on you.

How unique is your house?

If you built like 99% of everybody else in your neighborhood, chances are that the HVAC sub already knows pretty much how big of a furnace and compressor you'll need. Same with ductwork.

I highly doubt that the HVAC sub performs a manual J for every house. It isn't as if AC compressors for example come in an infinite tonnage variations. If you have a fairly standard house for your area, the sub will know, if you need a 4 ton or a 5 ton unit simply because of experience.

Of course, if you have an exotic house, e.g. ICF with an unusual amount/size of windows and a strange room arrangement, such a calculation becomes much more important.

In other words, if 99% of all houses in the area use the same tonnage AC because they are so similar, why go through the expense and trouble of a manual J ???

soonermagic,

If you want to see a few pages from a manual J calculation, I'll email it to you. Just contact me with your email address.

Don't do anything without the calculations in your hand.

Hello,
The thing that really blows my gourd, is someone on this forum claiming to know something, with the intelligence of a rock. The same house orientated n, s, e, w will require different btu's. This can vary as much as 2 tons depending on trees. This is what I have found in N. Texas over the last 20+ years. The number of members and age of the family can change the requirements by a ton also. As I recently explained, the difference in a system correctly filled, and a refill can affect a 5 ton system by a ton also. So those that are just spouting off, shut up.

Furthermore, I have seen identical homes in a subdivision, where one house is comfortable, and another is hot. Across the street from one another.

jrldh...I need LASIK... when are you available? I have 2 eyes, just like my neighbor, and they are both blue, too.

What about tomorrow. Conveniently, I am in Dallas, too. Let me know what time you want your eyes done. I'll give you a really good deal. Blue eyes are a little more expensive, though.

My thought, which prompted my posting the original question, was: Doesn't he need to have the load calc performed before he orders the equipment and before he can give me a price quote?

Yes, and No. He needs the calc to order the equipment, but not to quote you a price. He can guess the size and quote you a price based on that. Given a full install the differece in his cost between uints is small enough (Most of the cost is likely labor) that the time to do a full ManJ / ManD might not be justified until he's got a signed contract. My heating contractor did it that way, saying the number of times he's wrong is far less than the investment of hours to do the full engineering work on each bid given his bid award rate.

But in all honesty, that's a best case scenario. YMMV

""The number of members and age (sic) of the family can change the requirements by a ton also"

Bwhahaha, bob_brown.
Is that's why people take their AC with them when they move. And you talk about "intelligence" LOL!
Maybe it makes a difference if there is 1 person or 10 people in the house but for a typical family it is irrelevant.

To make it clear to you: I was referring to a typical house in a neighborhood. Not about one single story house in heavily treed neighborhood A vs. a two story duplex on a totally cleared lot in neighborhood B. Most of the neighborhoods in the US that I have seen are fairly uniform. The huge variations that you talk about don't usually exist within a neigborhood. Even the orientation of houses is the same for the vast majority and I am sure that a HVAC person doing business in an area already has a good idea what the required tonnage is.

If you think that a builder in a new subdivision does a manual J for every house of the same floorplan, I've got a bridge to sell to you...

BTW, no need to become all nasty and offensive.

I just looked at this discussion and I know the feeling of bob brown......I was at a large wedding in 1976 and my father-in-law and I drew up the blueprints for our soon-to-be house on the back of a napkin. We built that house a few months later.

I agree with jrldh. Who knows, this contactor may have done so many installations he does know from past experience. He may be a great contractor but works with a low profit margin to keep a crew working. Maybe when he does do a load calc it comes out the same as when he wings it so why should he waste his time? Time is money! You guys are condemning a contractor you don't know anything about. I got some better advice to give soonermagic. "Get references for this contractor and make as many calls as you can". See if past customers are satisfied. I've got over 30 years in this trade and worked for some very reputable companies. Not every job was done by the book and I don't recall any major screw ups in an installation. And bob brown, they size septic systems according to the number of people in a household but not air conditioning

Hello,
Experience is a good teacher. But only when you learn from mistakes. Most people do not learn enough from mistakes, when they make a profit. Unless you have the knowledge in Manual J calcs, you don't know squat. If you think you understand manual J methods, you are wrong. If you are truly an HVAC contractor in Dallas, you will be caught sooner or later. You are wrong. You are wrong. You are wrong.

The problem with this industry, is the lack of training. Texas requires continuing training for HVAC contractors. Duct sizing is very important when designing for the newly designed variable sized equipment. I predict that this summer we will be hearing a lot of problems from variable speed gear owners. We already have seen a couple of complaints. If you use this site to judge America, this problem will become pandemic.

Most installers do not have a clue. They throw some duct at the house, and think they are an expert. When the system does not work, they quickly disappear from the job. Many contractors leave money on the table, when the systems do not perform as specified. They plan to leave money on the table.

Anyone can build a doghouse without plans. Cut an unlevel leg on a chair. Fix a faucet leak. HVAC is the most complex part of building. Even the structural design is simple in comparison to the HVAC. The jerks that ignore this are hurting the rest of the industry.

This isnt about money. This is about doing the job correctly. 30 years don't mean squat in new construction. It means you are knowledgable of the industry. It does not mean you are doing the best job.

Maytag sold America on quality. To have a higher P&L they then made a lot of junk. The once proud company was sold as scrap, when the public learned how pathetic thier product line was.

C'mon Bob, as the old saying goes "POOP OR GET OFF THE POT!" In the same conversation here you state Texas demands continuing training for their air conditioning people but then in the same paragraph, as well as countless times before, you say how useless and stupid the contractors are in your state. So I have to wonder, just what is the problem in Texas when it comes to air conditioning? Are those contractor training meetings BYOB? Are the people who teach the seminars totally stupid or are the people who attend those meetings just plain uneducable? The way you talk, as far as air conditioning goes in the state of Texas the only systems that work are the ones you've had participation in. You have a hard time convincing me every tech in Texas is a dope besides you. Maybe the fact actually is you are FOS. Just why is it you work alone anyway bob?

"This isnt about money. This is about doing the job correctly. 30 years don't mean squat in new construction. It means you are knowledgable of the industry. It does not mean you are doing the best job."

Hello Bob,

It seems the weather in Texas would never cause the need for A/C. It appears to be cloudy and rainy 24/7.

There are lots of people who work and never excel to god-like levels and get through this life. They do what they can do and the world keeps turning. Most people don't even know what a good job is, anyway. Look at it this way: We are only here for 70 years, going from point A to point B and then on to point H, upper level, or point H, lower level.

Let him go on with his rediculous rants baymee, the guys a self proclaimed air conditioning god. Take a look at this quote "In the 1970's NASA developed computers to help design rockets. Prior to this all engineering was done using a sliderule and by drafting on paper." Does that sound like something a rational person would say? I don't like using this forum asa sounding board but this guy really needs to have his meds adjusted.

My post has prompted a lot of irrelevant comments that I simply will ignore.

I'm not building in a subdivision with a stock plan. I'm doing one of the first tear down/new constructions in our town. My builder and his HVAC sub have not built this floorplan before.

The HVAC sub has said he will do the calcs, so I'm going to push for him to do it before we make any decisions.

Regardless of the acrimonious comments, BX is typical of the guys in the field. As I win small concessions from him and others, I realize what I am fighting. If I met with him on a regular basis, I would probably cherish his acquaintance. I am not the typical HVAC service guy, so I am not wedded to previous ways of doing business. Although I serviced several generations of equipment, I only spent about 20% of my time with HVAC and refrigeration as a service guy. I performed service for many years, but only spent a small portion of time with HVAC.

I did not believe in many of the NEW techniques for installation, until I understood why they are necessary. If I had the money or desire for continuing education, I am sure that I could learn new tricks.

What I do not like are old-timers that just donÂt want to keep up with new design techniques. Texas has attempted to force continuing education of contractors. This is a hindrance to the small guy, and large companies do not typically train the rank and file worker. The real problem is the availability of junk courses that can be taken that do nothing for training or learning of new information. Furthermore the requirement is only for licensed contractors. It does not include the workers. When a major company has multiple trucks, I would imagine the information gleaned is not passed to the workers. Since management is a small subset of the workforce, the training of workers does not exist in Texas. As long as they have a CFC certificate, this is all that is needed.

As to duct installation, training is on-the-job. The typical worker is non-English speaking; non green card entitled foreigners. This group of people is NEVER trained for more than is necessary to do a minimal job. Furthermore when they are knowledgeable, they are too old to do the job quickly, so they are replaced.

I am sure this is not the norm in all geographic areas, but in the southern areas this is typical. The hot climate areas are where design is so important for proper operation of equipment, but is done by minimal trained workers. When you see a legal worker, he is probably at the same level as the other group discussed.

Last summer was proof of the need for trained installation workers. The typical cooling bill for my area was double of previous years. Complaints of bills higher than mortgage payments were common. One story that emphasizes this issue was on TV and about an owner of a 3000 sqft house that had $900+ monthly electric bills, which were more than his mortgage payments.

If I sound too authoritative, sometimes it is needed, to wake up the general population. We tend to ignore problems until faced with them ourselves. Then when we look for help; it just isnÂt there.

What inspiration!!!! Makes me proud to hail from northern New England where men are men,,,,,,,not hombres! ;-)

You can do the load calc yourself with some software:

http://www.hvacloadcalculations.com/software.php

Here is a link that might be useful: Manual J