FREE On-line Residential HVAC Design Training


NOTE: These classes are done. A very similar class was recorded and posted on our YouTube Channel, here: https://www.youtube.com/watch?v=1IrMBbjEgjE&list=PL_287eMrjGiH1w1YTsWbAj83lxrhcUioC

Hi Everyone,

Just wanted to pass this along. I’ll be teaching a couple classes in the next couple of weeks.

The great folks at Southern California Edison’s (SCE’s) Energy Education Center in Tulare, CA are hosting free residential HVAC Design Training

Featuring the Award-Winning Kwik Model with EnergyGauge Loads!

Not only is Kwik Model an awesome 3D design tool, but its 3D virtual environment makes it the best training tool available.

The first class is an introductory class, followed the next week by two classes (Parts 1 and 2), one on Tuesday and one on Wednesday.

The introductory class is optional, but recommended.

Part 2 is a continuation of Part 1 – please register for both

Anyone can attend these classes and participate from anywhere.

Check out these FREE LIVE (on line) classes – class descriptions below:

Intro to Residential HVAC Design in 3D

Tuesday, March 1, 2022

2:00 p.m. – 5:00 p.m. (Pacific)

Register Here

3D Residential HVAC Design (No CAD Required) – Part 1

Tuesday, March 8, 2022

2:00 p.m. – 5:00 p.m. (Pacific)

Register Here

3D Residential HVAC Design (No CAD Required) – Part 2

Wednesday, March 9, 2022

2:00 p.m. – 5:00 p.m. (Pacific)

Register Here

Class descriptions:

Introduction to Residential HVAC Design in 3D

This class will be an introduction to ACCA Manuals J, S and D (load calculations, equipment sizing and duct design) using a new 3D HVAC design software. Rather than drawing the house in a CAD software, this software “builds” the house out of scalable 3D boxes. The benefit of 3D design is that it helps make sure that the system being designed will actually fit in the house and gives a better indication of duct length, surface area, bends and fittings. Attendees should have a good working knowledge of HVAC terminology and concepts.

Course Objectives: 

• Understand the basics of building geometry.

• Understand the basics of heating and cooling load calculations.

• Understand the basics of heating and cooling equipment selection.

• Understand the basics of duct layout and sizing.

• Understand the importance of good airflow on cooling equipment capacity and efficiency.

• Understand the importance of proper register/grille location, sizing and type.

Target Audience:

• HVAC Contractors

• HVAC Designers / Architects

• Energy Consultants

• HERS Raters

Learning Level:

Basic Class: Content is introductory in nature and requires no prerequisite knowledge or experience to grasp the concepts or participate in exercises. Basic educational activities and materials are meant to establish a foundation of knowledge and competence that will be expanded upon in practice or in higher level seminars and workshops.

Prerequisites: Attendees should have a good working knowledge of HVAC terminology and concepts.

3D Residential HVAC Design – Parts 1 and 2

Part 1 will cover load calculations and equipment sizing according to ACCA Manuals J and S (duct design according to Manual D will be covered in Part 2). The training will be based on a new 3D HVAC design software. Rather than drawing the house in a CAD software, this software “builds” the house out of scalable 3D boxes. The benefit of 3D design is that it helps make sure that the system being designed will actually fit in the house and gives a better indication of duct length, surface area, bends and fittings. Attendees should have some basic experience using an HVAC design software and/or knowledge of ACCA Manuals J/S/D, and a good working knowledge of HVAC terminology and concepts. It is highly recommended that you take Part 1 before taking Part 2. Part 2 will be held at the same time the following evening.

Course Objectives: 

• Understand the basics of how building geometry affects load calcs.

• Understand the basics of heating and cooling load calculations.

• Understand the basics of heating and cooling equipment selection.

• Become comfortable with the basic commands of Kwik Model with Energy Gauge Loads software.

Target Audience:

• HVAC Contractors

• HVAC Designers / Architects

• Energy Consultants

• HERS Raters

Learning Level:

Intermediate Class: Content is appropriate for individuals who possess a fundamental understanding of the topic and have familiarity with basic terminology and methodology of the subject matter. Attendees should have the capacity to participate in instructor-led exercises requiring synthesis and application of concepts.

Prerequisite: Basic experience in HVAC design software and/or knowledge of ACCA Manuals J/S/D and an understanding of HVAC terminology and concepts.

Don’t miss out.

Mark your calendars now.

We hope you can make it!


Relieving the Pressure: Bypass Grilles, Jumper Ducts, and Dedicated Returns


(Image borrowed from the internet.)

Disclaimer:  The vast majority of my HVAC design experience is for new construction, production homes, in hot-dry climates (CA and NV).  I’ve estimated that I designed at least a couple thousand plans, each plan built many times in multiple orientations.  I never had any significant comfort complaints result from my designs.  We always used Manual J/S/D.  We designed to high airflow (>400 cfm/ton) low-ish external static pressure (<0.6 IWC) and we were careful not to oversize the air conditioners.  We also field tested most of these designs.  This article is based on that experience.  I don’t claim to be an expert on the topic of room pressurization, nor have I done a whole lot of research on the topic.  I’m just sharing my experience and opinions so that, good or bad, others may add it to their knowledge base and make informed decisions. Please feel free to comment and give your opinion and experience.

To start, let’s define the problem.  First, imagine a house with no interior doors and one central return grill in a hallway.  The return path from a supply register in a bedroom is out of the room through the door and down the hallway.  The rooms and hallways are essentially return ducts – nice, big, open ducts with little to no resistance to the air.  I think (I hope) we can all agree that in this situation, there would be no need for ducted return grilles in every bedroom.

Now add the interior doors.  These doors are essentially dampers in our nice big return ducts.  When these door/dampers are closed it creates significant blockage in our nice big return “ducts”.  I think (I hope) we can all agree that it’s the doors that are causing the problem.  Theoretically, they stop the air from leaving the room, which prevents supply air from entering the room, potentially causing under-conditioning of the room.

  1. My first question is:  How often and when are these doors closed?
  • My second question is: When a door is closed how big is the impact on the supply air flow to that room?
  • My third question is: How should we address the problem caused by closed doors?

My experience designing and testing HVAC systems in homes leads me, personally, to the following answers:

#1. Not often enough during peak load conditions to be a problem.

#2. Not big enough during peak load conditions to be a problem.

#3. What problem?

Keep in mind that the homes I design are very energy efficient (compared to other homes built at that time), the systems are properly sized, and the ducts are well designed for good airflow. This means that during peak loads (very hot days) the system is running almost continuously, the air is mixing well, and the supply air is not coming out of the registers super cold or super hot.  I never really made a big effort to check, but I recall that most of the bedroom doors in these homes had decent undercuts, say, 1” above the flooring.  I should also mention that I always insisted on a dedicated return in the master suite. If all these boxes are checked, I contend that pressure relief in secondary bedrooms is not necessary for comfort reasons.

I always try to make a point to say this in every class I teach: “A well-designed system will forgive a lot of sins.”  What I mean by that is a house with a properly sized system (not oversized) and good airflow will work fine despite:

  • Less than optimal register locations
  • Less than optimal room by room air balancing
  • Less than optimal thermostat location
  • Less than optimal owner behavior, and
  • Some room pressurization

The reason I emphasize comfort a couple paragraphs earlier is because there may also be energy efficiency reasons for pressure relief.  The Florida Solar Energy Center (FSEC) did a study on how room pressurization increases infiltration (exfiltration) and therefore energy consumption.  I have not read this study yet and do not know how tight these homes were, how often the doors were closed, how effective the door blocked airflow, how well the ducts were sized, etc.  Assuming that there is increased energy consumption, a cost effectiveness evaluation is warranted to justify pressure relief strategies.  As we will see, some strategies are more expensive than others.

I have tested houses designed by other people where there were severe comfort issues and room pressurization seemed to be part of the problem.  They installed pressure relief strategies and the problems mostly went away, but unfortunately, they also did other fixes, such as increasing return air capacity, sealing ducts, etc., so it is impossible to know how much impact the pressure relief strategies had or how much of a problem they were to begin with.

I guess you could say that my most used pressure relief strategy is to have big supply air ducts.  By “big” I mean a lot bigger than those installed by people who do not used Manual D.  I might put an 8” duct in a room where someone else might think a 6” duct is fine.  In very simplistic terms, when your duct system is big, the air is moving slower, the pressure drop into the room is lower and the overall system can compensate more easily to parts of the system being blocked off.  For example, if you were to close off a register in a room in a house with “big” ducts, that back pressure is easily absorbed by the rest of the system.  If the house had small ducts, the new back pressure is felt all the way back at the fan.  In other words, the pressure behind the closed off damper at the register is greater in a system with small ducts than in a system with large ducts.  I have not personally tested this, but if that damper is now the bedroom door, rather than the register, the pressure in the room should also be less when the ducts are big, all else being equal.  On the other side, one could argue that a system with big ducts is more likely to reduce airflow to a room when a door is closed because there is “room” for it to go elsewhere.  True, but the room is less pressurized, so the energy impact is reduced.

So, other than big ducts, what are the more common pressure relief strategies?  I categorize them as follows:

  1. Louvered bypass grilles
  2. Jumper ducts
  3. Dedicated ducted returns

I will evaluate them based on the following qualities (or lack thereof):

  • Balance/Comfort –Does it reduce the impact of opening and closing doors and thereby increase comfort?
  • Noise/Privacy – We have doors on rooms for a reason.  Does the pressure relief strategy allow outside noise to disturb the room occupants? Does it allow private conversations inside the room to be eavesdropped on from outside the room?  Note that this is very hard to quantify and varies widely from family to family.  I personally think this issue gets more attention that it deserves, but I never lived in a large family.
  • Aesthetics – Is it ugly?  Is it visually conspicuous?
  • Cost – Is it expensive to install relative to the other options?
  • Energy – Does it increase energy consumption?

Here are the three strategies, in detail:

  1. Louvered Bypass Grilles

These basically just provide additional pressure relief much like the undercut of the door.  They are usually the simplest and easiest to install.  They can be as simple as a louvered door, or back-to-back grilles in an interior wall (one side to room, one side to hallway), or ducted high/low grilles on either side of an interior wall.

There are some very nice-looking louvered doors available.  Personally, if I could design my own house from scratch, it would have fully louvered doors everywhere but on the bathrooms.  I just like the look and the good air circulation.  Doors can be fully louvered or partially louvered.  Even off-the-shelf panel doors from the big box stores have room at the bottom to install a 6” tall louvered panel.  These louvered door panels are common in commercial applications

Image from www.trudoor.com

Back-to-back louvers are similar but, in a wall, rather than a door.  See drawing, right side.

High/low louvers are similar but to mitigate sound transmission one side is up high and one side is down low.  See drawing, left side.  I have never seen these installed.  I heard of someone who wanted to, but a sharp building inspector pointed out that the section of the code that prevents us from using unducted building cavities as return ducts could apply here and that the stud bay should probably be lined with an approved ducting material.   In a hot-dry climate, I would not have any issues with passing room air through a well-sealed, unlined, stud bay, but in a humid climate it could be a mold issue. 

Sizing these louvers is tricky.  It depends on a lot of things.  I’m sure there are fancy equations for calculating the pressure drop of the various options, but I prefer empirical data and experimentation.  A target pressure differential across the door that I have seen referenced many times is 3 Pa.  I’m not sure how this number was arrived at, but it seems reasonable.  If you don’t like it, pick a target – the lower, the better.  It would be quite easy to build a test chamber using a calibrated fan, such as a duct tester and test different size louvered configurations at different airflows.  If the grilles you are using have good performance data, such as for supply registers, you could use that to predict pressure drop too. Be sure to account for the door undercut, or just use that as “safety margin”.

Here is my “scoring” of louvered bypass grilles as a pressure relief strategy.

  • Balance/Comfort – Very good.  Assuming proper sizing these should perform fine.
  • Noise/Privacy – Probably not good, especially fully louvered doors.  With louvered grilles I can imagine a younger brother being caught with his ear to a louvered register spying on his big sister’s phone conversations.  (It’s amazing what you can hear with your ear to a floor register when the system is not running, but no one complains about that.)
  • Aesthetics – I love the look of louvered doors, but then, I also like jalousie windows because they remind me of living in Hawaii as a child and the windows on our old VW camper.  On the room side, louvered grilles can be hidden behind furniture, but they might not look great on the hallway side.
  • Cost – Louvered doors can be pricey.  Back-to-back louvers installed in walls (or doors) are probably the cheapest of all options mentioned here.  High/low louvered would be expensive if you had to duct the wall cavity, otherwise pretty cheap.
  • Energy – Great.  No negative impact on energy use.
  • Jumper Ducts

These are very common in some parts of the country and in some energy efficiency programs.  They are probably the most common pressure relief strategy.  They are similar in function as louvered bypass grilles.  They provide an alternative return path past the door in addition to the door undercut.  The main difference is that they are in the ceiling rather than a wall and they are ducted.  There is one register in the room and then there can be a shared register or individual registers in the hall or common area.  When the system is running and the door is closed the air goes up into the register in the ceiling of the room, through the duct and out the register in the hall and back to the return.  See diagram.

Again, sizing of the ducts and registers can be tricky, but I have seen some sizing charts that people have put together specifically for this application.

Here’s my evaluation of jumper ducts:

  • Balance/Comfort – I think these are very good too.  Assuming proper sizing, they should work very well.
    • Noise/Privacy – Definitely better than the louvered bypass grilles.  Being up in the ceiling and ducted limits most sound transfer.
    • Aesthetics – Probably better to have the grilles in the ceiling than in the walls.  The ability to share the hallway grille helps too.
    • Cost – Substantially more than louvered bypass registers.  You have a ceiling boot at both ends, the duct material and the labor to install it all.
    • Energy – If the bypass duct is in conditioned space, there should be no energy impact, but this is probably rare.  In the more common scenario of jumper ducts in a vented attic, I think the energy impacts are their biggest disadvantage. They increase surface area for conduction, and unless perfectly sealed, they increase building infiltration.
  • Dedicated Ducted Return

This is basically putting a return duct in every bedroom.  Each bedroom will have a supply and a matching return, presumably sized to handle the same amount of air.  As I mentioned earlier, I insisted that all my designs had a dedicated return in the master bedroom.  This was because in new homes the master “suite” was quite large.  They usually included the master bedroom, master bath, toilet room walk in closets and sometimes a separate retreat area. The amount of supply air going to that side of the main door was quite large and the pressure across the closed door could be substantial.  I had good success with this strategy, but occasionally I found that if the owners kept the master bedroom door closed a lot, like all day, that room was not well monitored by the thermostat out in the hall and sometimes the temperature drifted away from the thermostat setpoint.  In larger homes that had more than one system, we often put the thermostat in the master bedroom for that reason.

  • Balance/Comfort – Poor.  Now, I’m admitting a bias here.  I only designed one project where the builder insisted on returns in every bedroom, and it was a nightmare.  This was a subdivision of large (3500-5000 sf), one-story production homes.  There were three or four models.  The builder was new to Las Vegas and had previously built homes on the east coast.  Despite my objections, he insisted that a return in every bedroom is what made a house a “quality” house.  The problem was that the City of Las Vegas required balance testing on the sales models of all subdivision projects.  This meant you had to measure and report all supply and return airflows at every register and they had to be within 10% of design.  This was a very hard criteria to meet for regular systems.  I had no idea how hard it was going to be for multiple-return systems.  At that time they did not specify if the test was to be performed with bedroom doors open or closed, so we tested both ways in hopes that one would pass more easily. 

I won’t go into the specifics, but even though we very carefully sized the ducts according to strict Manual J/S/D protocols, it was a nightmare to balance these systems.  Here’s just a small example:  We would measure the supply and return airflows to a bedroom with the door open.  They would be different despite being the same size ducts, registers, duct length, everything.  It is impossible to intentionally make the total equivalent length (TEL) of a supply and return duct be the same.  We could adjust the registers and maybe some balancing dampers until they were close, but then we would close the door and measure airflows.  They would both change by different amounts!  Without making any further changes, we would open the door and measure again.  Would they go back to the original measurements? NO! They would be totally different than before.  No other changes were made.  It was infuriating.  It has to do with the fact that there are two pressures acting on the room instead of just one and that there is more than just static pressure forcing air down certain pathways in the ducts.  There is also velocity pressure.  Think of it as the momentum of the air.  When you change velocity pressure it changes the way the air “wants” to go.  When you change the static pressure back, the air might say, “Nah, I kind of liked going this way.” 

I suspect that few contractors who install systems with returns in every bedroom ever had to balance them like we did.  If they had to, I’m sure they would find an easier way to achieve pressure relief.  Once we finally got them dialed in I told the builder that because he insisted on dedicated returns against my advice, we would not be responsible for any additional “fine tuning” requested by homeowners.  There weren’t many requests, fortunately, but the systems weren’t as trouble free as one would have expected for all the extra expense and work that went into them.

Despite that bad memory, I also think there are other problems caused by this design strategy.  I alluded to it earlier where I mentioned putting the thermostat inside the master suite. Everything, of course, depends on the system and layout, but for the “typical” system I have found that comfort is best achieved when the thermostat is measuring a good representative sample of all the air in the house.  This depends greatly on the thermostat location and that is a whole other topic to discuss later.

When a system is running, you want the air passing by the thermostat to be a good representation of air from all the rooms.  When you give a room its own return you effectively take that room out of them mix.  You take away its “vote”, so to speak.  If the thermostat is in the hallway where the bedrooms are and all bedrooms have their own returns, that hallway can actually become stagnant with no moving air.  Thermostat location is very important in these types of systems.

  • Noise/Privacy – Good. No sound transmission
  • Aesthetics – Good. Fewer registers than the other options.
  • Cost – Poor.  Much more expensive than the other options.
  • Energy – Poor.  Unless ducts are located within conditioned space, this strategy will greatly increase duct leakage and convection.  Also, note that longer return ducts add significant equivalent lengths to the entire system, which changes the friction rate for every run.  This potentially could require larger ducts on both the supply and return sides to compensate for this additional resistance.


I can’t emphasize enough that a good design (not oversized equipment, good airflow, low design ESP) greatly reduces the need for these types of “enhancements” to a ducted HVAC system. 

Secondly, I don’t want to discount the induced infiltration and energy problem.  It is worth evaluating the energy savings vs the cost of these strategies on a case-by-case basis.  More research is needed here.  I suspect that in tighter homes with better designed duct systems, the impact is greatly reduced.

Lastly, until I have seen some detailed temperature vs time data logger analysis of systems with and without different pressure relief strategies, I will be skeptical of their overall benefit to comfort.  Here’s why: If closing a door supposedly creates pressure, which reduces supply air to a room, then the result would be an underconditioned room (hot in the summer, cold in the winter).  What I have actually heard from as many homeowners as not, is that the rooms where the doors are often closed get over conditioned.  In fact, in my own house, in heating mode, if we close our master bedroom door, it gets too hot at night.  If we leave the door open, it’s just fine.  We do not have a return in that room, btw.  This makes no sense to me.  Clearly there are other factors at play besides just a pressure difference across the door and it will take more to remedy than just pressure relief.

Ceiling Registers vs. Floor Registers


Which is better for distributing heated air to a house, ceiling registers or floor registers?

This seems like an easy question. Hot air rises so blowing the air up would improve the flow. This makes sense on the surface, but let’s look deeper.

First of all, let me make it clear that if the system is properly designed, both will work just fine.  But, all things being equal, is one better than the other, even if only slightly?

Recall that the purpose of blowing heated air into a room is to maintain a constant temperature over time and an even, consistent temperature everywhere in the room.  That temperature is whatever the thermostat is set at. Let’s say that’s 70 degrees.  When the heating system stops, the room begins to cool off.  Hopefully the thermostat will sense that and turn the heating system back on. This cycling on and off can cause problems.

The air that we are blowing into the room is substantially hotter than the air in the room.  In other words, we are adding concentrated btus into a volume of air to replace the btus that the air has lost.  It’s sort of like adding red food coloring to white frosting, but the red keeps fading away and we have to keep adding more concentrated red coloring.  We want the frosting to have a very even color, no dark streaks (hot spots) and no light streaks (cold spots).  To do this we have to mix as much as we can.  Mixing is the key to even temperature distribution in a room.

The next thing to look at is the register itself.  What is the purpose of the register?  Take a typical stamped-face 2 way ceiling register and a similar floor register.  Why are there 2 directions?  To send the air to different parts of the room, of course.  Why do we want to do that? So we don’t have hot spots and cold spots.  In other words, the register is designed to distribute the air around the room, which is another way of saying to mix the air

Also notice that the registers are angled to direct the air away from whatever surface the register is mounted in.  Ceiling registers throw the air down and floor registers throw the air up.  Also notice that they have a horizontal direction, parallel to the ceiling or floor.  This horizontal distance the air travels before slowing down to a certain velocity is what is referred to as the “throw distance”, but there is also a significant vertical component.  Register manufacturers provide specifications for their registers, including throw distance, static pressure drop, and noise criteria, at different face velocities and flow.  Again, supply registers are intended to push the air to all parts of the room to ensure even temperature distribution.  So, hopefully you will agree, that the key factor for selecting a good register location (and type) is to promote mixing

Another issue that comes into play is that warmer air is less dense than colder air.  Notice the “-er” at the end of those two important words, warmer and colder.  It’s not correct to say that “hot” air rises, but of course when people say that they usually mean “hotter”.  Hotter air rises in the presence of colder air.  It’s relative.  Most people would consider 120 degree air “hot”.  I could make 120 degree air come out of a wall register and sink to the ground like fog at a Transylvania cemetery.  How?  Make the room 160 degrees first.  Not very practical, but you get the point.

How do we reduce stratification? By reducing the temperature difference (delta T) between the room air and the supply air.  How do we do that?  One way is to reduce the supply air temperature by increasing cfm.  You can do this by increasing ducts sizes and reducing restrictions.  You can also do it by increasing the speed that the air handler runs on in heating mode.  Other than that, the easiest and best way to reduce the temperature difference between two masses of air is the mix them.  The sooner the air mixes together, the less chance there will be of stratification.

So, how do we mix the air?  A giant blender in each room would be great.  That’s basically what a ceiling fan is.  Ceiling fans are awesome! Make sure it is blowing up in the winter and down in the summer.  They beat the air like a scrambled egg, virtually eliminating stratification. Unfortunately, they use electricity and home owners tend to leave them on too much. Other than ceiling fans, we can help the air mix with register placement and selection.  Mixing is helped by turbulence.  Turbulence is created by making the air do things that it doesn’t really want to do.  Blowing the air the opposite direction that it wants to go can create turbulence, like a bunch of people going out the entrance of a building while other people are trying to come in, like cars going the wrong way on a freeway.  If hotter air wants to rise, blowing the air up will only get it up to the ceiling faster, where it will stay.  Blowing hotter air down will make it go down through the colder air and then fight its way back up, by that time it has mixed and cooled off: lower delta T = less stratification.

Note that there are two types of air movement in a room that is caused by the incoming supply air.  The primary airflow is caused by the force and velocity of the air coming out of the register.  The secondary airflow takes over when the air has lost its momentum and other forces take over.  These forces are usually stratification (buoyancy pressure) or the fact that the room is being pressurized, assuming there is no return grille in the room, the air has to leave the room and is being pushed out by the air coming in behind it.

Note that higher face velocity of the air coming out of a register can improve mixing but it can also have other negative affects, such as higher static pressure drop (resistance) and noise.  It’s very important to realize that face velocity is completely different than the velocity of the air in the duct.  You can have extremely slow air in a large duct and very high face velocity if the air is coming out of a small register.  Velocity is cfm/area.  The area of the duct is usually very different than the net free area of the register.

The image below shows what happens when hotter air is blown up into a colder room.  The primary airflow sends it up toward the ceiling and there is little secondary airflow to make it go anywhere else.  This exacerbates stratification.

Image from HVAC 1.0 – Introduction to Residential HVAC Systems

This next image shows what happens when hotter air is down into a colder room.  The primary airflow sends it down toward the floor and the secondary airflow causes it to want to rise back up toward the ceiling.  This promotes mixing and reduces stratification.

Image from HVAC 1.0 – Introduction to Residential HVAC Systems

Based on this and with all else being equal (airflow, delta T, face velocity, etc.) registers in the ceiling are more likely to promote mixing of heated air blown into a room and the ceiling is therefore a better location for supply registers in heating mode than floor registers.

A Quick and Easy DIY for Improving Air Flow in a Home


My friends just moved into a new (to them) home and invited us to the housewarming party. I made the faux pas of critiquing their HVAC system. This embarrasses the heck out of my wife and happens far too often. It’s very hard not to say something when you know so much about how these homes were built. In our area, I can look at the type and location of the supply registers and tell you which HVAC company designed and installed it.

Probably 95% of production homes in CA (and likely all over) suffer from undersized ducts, which results in airflows below 350 CFM per ton or so. Some much less. In the 2013 version of CA’s energy code they mandated a minimum of 350 CFM per ton and 0.58 watts per CFM. Think of 350 CFM per ton as a D- grade. One CFM less is a FAIL. The other way to think of it is as the very worst airflow you can have and still meet code. When I was designing a lot of production homes, I designed to an absolute minimum of 400 CFM/ton and they regularly tested out at closer to 500 cfm/ton because I was pretty safe sizing ducts. More airflow is generally better, especially in hot/dry climates.

A real quick and easy way to improve airflow in these types of homes is to replace the cheap “stamped face” registers with a “bar-type” register. These may go by different names but, basically, a stamped face register is the most common style. The entire face and the fins are all from one piece of sheet metal that was stamped and the fins were bent in or out. Bar type registers have a rectangular frame, but each fin is a separate piece of metal that can be individually adjusted (without bending anything). Both Lowe’s and Home Depot sell both kinds. (Search “ceiling registers”on their sites.) The easiest way to tell them apart is price. Bar type registers are roughly twice the price of the same size stamped face, which explains why stamped face are the most common in most homes. But even at $15-$25 each, it’s a cheap way to really improve airflow. A bar type register is rated for roughly twice the airflow at the same pressure drop and sound rating as a stamped face. I’ve often measured up to 20% increase in airflow by replacing a stamped face register with a bar type, occasionally more. When I lived alone in an apartment, I took all the registers off completely and it made a huge difference! Only an bachelor engineering nerd can get away with that, though. (No, “bachelor engineering nerd” is not a redundant term.)


Bar Type Register – photo from homedepot.com


Stamped Face register – photo from homedepot.com











Here is link to a 10×6 bar type register sold by Home Depot: bar type register

Here is link to a similar one sold by Lowe’s: bar type register

Note that the size 10×6 refers to the size of the register boot behind the register. The dimension of the register itself is about 1 3/4 inch bigger in both dimensions. So if you were to go through your house and measure the outer frame dimension of all your registers, you would subtract about 1.75 from each dimension to get the nominal size (round to the nearest inch). They come in pretty standard sizes, usually even numbers, 12×4, 10×6, 12×6, 8×4, etc. They might also come in steel or aluminum. Aluminum is a bit more expensive. Steel is fine unless you live in a humid area. They perform about the same.

You can also sometimes buy directly from your local HVAC supply house. Tell them you want something comparable to a “Shoemaker 950 series (aluminum) or 951 series (steel) bar type register”.

The only tools you need are a screw driver and maybe a razor knife if the registers are caulked in place. Only do this project if you are comfortable working over your head while on a ladder and the registers are easily accessible. Be super careful. I’ve seen registers located 20′ above the floor. Leave those alone. Hopefully the screws holding the registers in place are going into wood and not just sheet rock. If not, which happens too often, you may have to use some sheet rock anchors.

I suggest only replacing the registers in the more important rooms, such as family room, master bedroom, etc. Smaller rooms like bathrooms and laundry rooms usually are getting plenty of air. If you have rooms where you’ve closed down a register, no need to replace those. Also, if you live in a two story house served by a single, non-zoned system (one thermostat) try replacing just the downstairs registers first. See if you notice a difference.

While you’ve got the registers off, take some caulk or expansive foam and seal the gap between the sheet rock and register boot (sheet metal box that penetrates the sheet rock and that the register slips into). Make sure you can get the register back in before the caulk or foam dries.

If you do this let me know how it came out! Good luck. Be safe.

How a House is Like a Tank of Water


Happy 2014, everyone. 2013 was a good year for me. It certainly did not go the direction I would have expected with the California Energy Commission work lasting all year, but it was a blessing and I’m very grateful. I realized that I only posted two blogs last year. Even though those two blogs generated a ton of feedback and even a little controversy, I resolve to do much better this year.

Great news! SMUD has generously offered to sponsor my “HVAC 1.0 – Introduction to Residential HVAC Systems” for FREE! Obviously, it is based on my book of the same name. You even get a free copy of the book (a $29.99 value). Here is a link to sign up: https://usage.smud.org/etcstudent/ClassDescription.aspx?Id=895 Right now it is to be offered on March 6 at their headquarters. If the demand is high and the response good, they could very well offer it again. If you can’t make it on March 6, be sure to tell them that you’d love to see it offered on a different date.

I’ve been experimenting with making this class an on-line class. I’ve taken some of the power point slides and some audio files of me speaking and created a short movie. We all hate the way our recorded voices sound and I’m no exception. I speak much more slowly and sound a lot more like Mr. Rogers than I do when I teach live.

As an experiment, I started with Appendix A. This is the “Tank of Water Analogy” that I’ve been using for years and getting excellent feed back. It’s amazing how a simple analogy can really help explain something that’s much less intuitive. It’s definitely the most basic part of the book. Other sections are far more technical. This was a good section to experiment with.

There are a lot of different ways to do on line training. For me, the most effective is the one that you can easily pause, rewind, replay. My plan is to take a class that can easily go 8 hours live and condense it down into about 5-6 hours worth of videos, none of which are more than 20 minutes long (hopefully).

Please take a look at this sample. It is about seven minutes and let me know what you think. I suggest that you frequently hit the pause button and let what was just said in the video sink in for a few seconds. Otherwise, I have found that minds tend to wander . . . Squirrel! (I watched “Up” over Christmas break. Great family movie.)


School of Thought #1: Register Over the Window


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: Register Above the Window.  By the way, unless I say otherwise, I’m focusing on cooling mode on a very hot day.

Putting a register above the window seems to be one of the most common locations in homes for many, many years.  It also seems to have the most ardent and dedicated (aka, stuck in their ways) practitioners.  Having put about 2000 residential HVAC designs to paper, I’ve received a lot of, shall we say “comments” about my plans.  No matter where I put a register, there was always an HVAC contractor who did not like that location.  The one location that most contractors would insist on was over the window.  The reasoning went from logical (this directly addresses the major load in the room), to rule of thumb (I was always taught that you had to “wash the windows”), to experience based (I’ve been doing it this way for 30 years and it has always worked fine), to nutty (it pushes the heat/cold back out the window).

When done correctly it can be very effective and maintain good comfort, but it does have some serious drawbacks.  The correct way to do this option is to use a two-way register oriented parallel to the window.  alternatively and bar-type register can be used with the air directed in a manner similar to a two-way register.  Using the wrong register can seriously screw this option up.  I’ve seen three way registers located here, but blowing back into the room or worse, blowing directly on the window.  Both of these can result in serious comfort and energy issues.

The down sides to this school of thought include:

  • compared to other locations, it requires the most ducting, which increases materials costs, conductive losses, and pressure drop.
  • If the roof pitch drops down over the window, the register boot can be very close to the roof decking.
  • Because the air only comes out in two directions it doesn’t mix as well and can cause cold spots if directly in the path of the airflow.
  • If located too close to the window, it can blow air directly on the window.  This increases the delta-T across the window, increasing conduction through the window.

Next Post:  School of Thought Number 2 – Interior to Room