Why We Need a Simpler HVAC Design Methodology


First of all, let me make one thing perfectly clear.  The ACCA Manual J/S/D residential HVAC design methodology is the premier methodology available today, and has been for many years.  It is the most precise, accurate and refined process for designing residential HVAC systems in the world.  It has gone through the rigorous ANSI certification process and has been reviewed and scrutinized by many of the greatest experts in the field.  It’s not perfect, but it is the best, hands down.

That being said, there is a problem.  The ACCA Manual J/S/D methodologies and the software programs that are based on them (Wrightsoft and Elite) are very complicated and have a very steep learning curve.  I first started doing HVAC design back in 1988 using handwritten ACCA worksheets.  That was probably an advantage because it forced me to understand each and every calculation and to be very careful about every assumption made along the way.  If you made a mistake at the very beginning but did not discover it until the end, you spent a lot of time re-doing the worksheets.  It was very tedious, but very educational.

Computer software programs have made the calculations much easier and allow the user to do multiple “what if” scenarios instantaneously.  ~~ What if they added ceiling insulation? – CLICK – answer.  What if they used low-E windows? – CLICK – answer. ~~  The computer software also allows users to make very BIG mistakes very quickly.  There are many, many seemingly innocent little input fields scattered throughout the programs that have huge impacts on the final results.  There are also a large number of input fields that have absolutely no impact on the final result.  A large portion of the learning curve is figuring out which is which.

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This is a screen shot of a project in Wrightsoft’s Right-Suite Universal software. It makes me think of a control panel in a nuclear power plant. This is very intimidating, even to someone who is relatively computer literate. It takes years of experience and dozens of projects before someone can become comfortable with this level of complexity.

Both software packages cost over $1000 when you get most of the important features.  The software programs have also added a lot of really fancy features such as pull-down equipment libraries, estimating tools, and parts lists to name just a few.  In my opinion, these “features” sometimes clutter up the software and make it easier for people to make mistakes.

I truly, truly wish that every HVAC designer in the country used ACCA Manual J/S/D on every new home, addition, renovation and even most equipment replacements.  I honestly believe that 90%+ of existing homes would be well served to have their systems evaluated and re-designed based on ACCA Manual J/S/D.  Unfortunately, that will not happen.  Despite some really excellent training, much of it subsidized, Manual J/S/D is beyond the ability of the vast majority of HVAC contractors.  I don’t mean “ability” in terms of aptitude or intellect, but in terms of time and resources.  They don’t have the time to learn it or the time to perform it.  A good introductory J/S/D class is at least two full days long.  I personally have taught three-day classes that seemed like they only scratched the surface.

I forgot to mention that there are other manuals in addition to J, S, and D:

  • Manual H (Heat Pump Systems)
  • Manual P (Psychrometrics)
  • Manual T (Air Distribution Basics)
  • Manual 4 (Perimeter Heating & Cooling)
  • Manual TT-102 (Understanding the Friction Chart)

Again, don’t get me wrong.  I am a huge proponent for more ACCA J/S/D training.  It’s just that after teaching these classes multiple times and having performed about two thousand designs myself, I don’t think it is an appropriate level of precision for the vast majority of designs out there.

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 To do a full-blown Manual J/S/D design from start to finish on a typical home would take the average user 4-6 hours.  From the time you hand them a set of plans and they turn on their computer, to the time you get back a design detailed enough to install from, takes at least that long.  This is a very expensive investment in time and energy, especially if it is just for bidding purposes and does NOT include the time it takes to draft up a presentable, full size set of plans that could be turned in to a building department for review and approval.

I strongly believe that most designs could be accomplished using a methodology that takes about one-fifth of the time.  I’ll stick my neck out and say that 80% of the residential HVAC systems being installed today could be accomplished using a far more simplified approach and result in a system that is just as good as one designed using a full J/S/D approach.

When you step back and realize that residential HVAC equipment only comes in a few sizes and residential ducting only comes in a few sizes, it makes one wonder why we are being so precise in the calculations.  Think about it.  The difference between a 3-ton system and the next larger size, 3.5-ton system, is an increase of 25%!  The difference in airflow between a 7” duct and the next larger size,  8”, is about 40%!  Why are we spending so much time on calculations that only have small impacts on the total cooling load and even smaller impacts on room loads?

I have two sayings that I use a lot in training, and in daily life for that matter.  The first I heard a long time ago and I don’t know who to attribute it to:  “Don’t waste time splitting hairs when you need to be shaving heads.”  The other is attributed to John Maynard Keynes, a famous British economist from the early 20th century:  “It is better to be approximately right than exactly wrong.”

They both relate to the need to put an appropriate amount of time and effort into what you are doing and realizing how that will impact the final result.

I believe that the current approach to HVAC design results in far too much hair splitting and results in answers that are very precise, but often wrong.  Not because the methodology is wrong, but because it is being applied wrong.

I am a firm believer that if you want to change an industry you have to do it in baby steps.  You can’t expect even a portion of contractors to suddenly start using a process that requires such an investment in time, effort and money.  That is why we need something in between the horribly inadequate design process used by MOST contractors today: a combination of rules-of-thumb and trial-and-error, and the full-blown ACCA J/S/D process.

We desperately need a more simplified design methodology.  One that is not intended to replace ACCA J/S/D in any way, but is intended to be a stepping-stone to learning the full process.  I’ve referred to it as a “gateway drug”.  The goal is to get people used to following a formal design process, albeit a greatly simplified one.  Once they get “hooked”, then we lay the “heavy stuff” on them.

I think we all want the same thing: to have homes that are comfortable, efficient, and affordable to operate.  We need to be able to make a decent living designing and installing systems, and homeowners should get what they pay for.

No, It’s Not a “Pee-Trap”. So, Please Don’t . . .


An interesting little part of the condensate drain in a residential air conditioning system is the p-trap.  Note that it is called a p-trap because of its shape and that it is not a “pee-trap”.  That is something completely different. So, NO, that is not what that little vent pipe is for.  You’re just going to have to climb down the ladder and use the restroom like a civilized person.

I don’t know why they don’t call it a “u-trap”.  Yes, it would make more sense.  I had no say in the matter.

The p-trap traps condensate (water) so that air cannot pass through. Because the coil is under positive pressure when the system is running, air would rush right out of the condensate lines.  The p-trap helps prevent this.   Condensate trickles in from the coil side causing an equal amount to trickle out the other side and down to a sewer drain or some other acceptable location.

Code requires a vent-T that allows air to get in behind the escaping water.  Someone must have thought that relatively large amounts of water would be passing through, prompting the need for the t-vent.  Normally a vent like this is required to prevent the drain from gurgling or trapping air, much like vents used in sewer lines.  I’m quite sure that condensate drains would work just fine without the vent-T, but it is required by code.

The sad thing is that even though this is a fairly simple concept to understand, all too may times it is installed with the vent-T on the wrong side of the p-trap, making the p-trap completely irrelevant.  While not a big deal (the leakage out of the t-vent is only a few cfm), it does say a lot about the installer.  I can’t tell you how many times I’ve seen this.  I would guess probably 30% of the time.  If they don’t understand how a p-trap and vent works, how are we supposed to trust them around gas piping and refrigerant lines?

By the way.  I did my BPI field exam in a friend’s house a while back.  They had a brand new furnace in their attic.  The p-traps were wrong.  We also discovered that the gas line leaked where it was attached to the furnace.  It was only finger-tight.  They never used a wrench to tighten it down.  Coincidence?


Here is a little quiz for you.  If the pressure inside the coil is 90 Pascals, how much higher will the water level be on the right side of the p-trap compared to the left side in the diagram above?  Answer below.


If 249 Pascals equals one inch of water column (iwc), then 90 / 249 = 0.36 iwc.  So the water would be displaced by 0.36 inches.  In the lower digram it would not displace much, if any, because the pressure is escaping out of the vent-T.

School of Though #4: High Sidewall Register


As I mentioned in my previous post, the Four Schools of Thought for Ceiling Register Placement are 1. Register Over the Window, 2. Register interior to room., 3. Register in Center of Room, and 4. High Sidewall Register.  All four schools of thought can work just fine (in terms of comfort), when done correctly.  Comfort, however, is not the only factor to consider.  Energy efficiency, materials efficiency, ease of installation, and aesthetics are all things to consider as well.  This post will look at all of those factors for this particular school of thought: High Sidewall Registers.  By the way, unless I say otherwise, I’m focusing on cooling mode on a very hot day. 

If I were designing my own house and had to choose between one of the four schools of thought, this is the one that I would probably choose.  Actually, the house I’ve designed in my head that I would like to build for myself would have floor registers, but between the four schools of thoughts for ceiling registers, this is the one I would choose.  Ok, Ok, I already admitted that high sidewall registers are not ceiling registers, but they fall into the category of having ducts overhead.

Sidewall registers should always be the “bar type” registers.  These are designed to throw the air roughly perpendicular to the surface they are mounted in, as opposed to ceiling register that have a throw distance measured parallel to the surface they are mounted in.  Bar type registers are designed to handle roughly twice the airflow of a low-end stamped face register of the same size and at a similar sound rating and pressure drop.  You also get much better throw distances.

The air can be directed across the room toward the load.  It travels in the upper unoccupied zone of the room and has plenty of time to mix with the room air.  This helps prevent cold air from blowing directly on people.  Something else interesting occurs called “entrainment”.  This is when the stream of air coming out of the register pulls room air up toward it, improving mixing and distribution.

On the negative side, the worst part about high sidewall registers is getting the duct to the back of the register.  I cheated on my diagram.  I confess.  I do not show the duct that serves the register.  In the previous three examples, true ceiling registers, it is obvious.

There are two basic ways to get the duct to the back of the high sidewall register, one works very well and one does not, but both require some extra steps that some architects and/or framers will not like.

The most common method is to drop a short rectangular can down the wall, in between the studs.  This is not a good idea for a lot of reasons.  1. The fittings are expensive.  2. There are a lot of extra feet of equivalent lengths in those fittings.  3. The typical stud bay is 3.5 x 14.5 inches.  A rectangular sheet metal can of that size is barely equivalent to a 7” duct and that’s if you don’t insulate the metal.  4. The top plates of the wall have to be cut out.  This weakens the wall structurally.  5. The sheet metal fittings can make noise when they heat up and cool down.  This is called “oil canning”.

The better way to run ducts to the back of a high sidewall register is to have the room being served have a higher ceiling than the adjacent room and run the duct above the lower ceiling. 
For example.  If the bedroom had 9’ ceilings and the hall had 8’ ceilings, this leaves a 1’ area at the top of the wall that the register can poke through and the duct can run straight into the back of a standard boot.  Another idea is to drop the ceiling of closets.  All of these, of course require a cooperative architect who is willing to do this.

So that wraps up the four schools of thought on where to put ceiling registers.  Don’t hesitate to leave a question or comment.

Coming up next:  Duct Size vs. Air Flow – Misconceptions Shattered Here.  STAY TUNED!