Problem Solving

HVAC/R Service is about practical problem solving. All the tools, technology, training, and literature are just there to help us identify and solve problems. The primary skill that any service tech needs is problem solving. The most important tool at your disposal is your mind. I am frequently asked why we make students do BTU calculations of ice turning to steam, series parallel ohms law problems, gas law calculations, or any host of other primarily mental exercises that nearly all HVAC/R students must suffer through. Usually, the students asking are doing the most suffering. Although I can justify all of the above as an endeavor to garner a deeper understanding of the principles which make HVAC/R systems work, I usually tell them that you can’t become a champion weight lifter by lifting marshmallows. Service techs are not paid to connect gauges or take voltage readings, they are paid to solve problems. Obviously techs need to be familiar with all the tools at their disposal, but we should never overlook the fact that their primary tool sits on their shoulders. I am afraid the current focus on standardized testing throughout our educational system has not prepared our students for practical problem solving. Rather, they are used to selecting the best solution from a very limited set of answers. My prescription? Lots of work that requires students to recognize and define the problem, systematically find the cause, and offer a solution. Assign work that requires students to provide written answers. Ask questions whose answers have not been explicitly stated, but require students to put two pieces of information together. If you get an answer that is way off base, try and ask leading questions to help the student work their way through a solution to the problem. The exact answer is not as important as the process. Encouraging students to use their minds to solve problems is crucial. The shop is a great place to work on problem solving because real life problems are never as simple as a, b, c or d. But don’t accept “the part is bad” as an answer. The student should be able to explain what the unit is doing wrong, what is the cause, how they determined the cause of the problem, and what their proposed solution is. Truth be told, I believe this approach could work in a lot of fields besides HVAC/R. The leaders in any field are always people who have recognized and defined problems and then devised solutions.

Teaching Capacitive Reactance

A capacitor is basically an electrical spring. It stores an electrical charge when the voltage increases, and it releases that electrical charge when the voltage starts to drop. Capacitors are used smooth out the output of DC power supplies. When a capacitor is connected in an AC circuit it alternately charges and discharges with the AC current. This has the effect of causing voltage to peak after the current peaks. Note that this is exactly the opposite of inductive reactance which causes current to peak after the voltage peaks. Adding a capacitor in series with an inductive load offsets the inductive reactance so that the voltage and current peak together. Capacitors are used with PSC motors to improve their efficiency by offsetting the inductive reactance of the motor windings. This is why PSC motors became the standard motor for compressors and fans. Several years ago I had a furnace with a shaded pole indoor blower motor. When it died, I decided to replace it with a similar size PSC motor. The replacement PSC blower motor was slightly larger in horsepower than the original shaded pole motor, but drew only half as many amps while operating. Of course today PSC motors are being replaced with brushless DC, or ECM motors for efficiency savings that are equally dramatic.

You can do a demonstration of the effects of inductive and capacitive reactance by wiring a 60 watt light, a 120 volt 10 watt unit bearing shaded pole motor, and a 10 MFD run capacitor in series to a 120 volt power source. Wire a switch in parallel to the shaded pole motor and another switch in parallel with the capacitor. The switches will allow you to take the motor or capacitor out of the circuit by simply bypassing them. Ideally, you want to use an oscilloscope to show the source voltage and the voltage across the light. The light will remain in phase with the circuit current since the light is a resistive load. The phase relationship between the source voltage and the voltage across the light will show the relationship between voltage and current. Operating the switches will show the effects of inductive reactance, capacitive reactance, and the combination of the two. Even without an oscilloscope you can show that the voltage across the three devices adds up to more than the applied voltage. This shows why run capacitors must be rated for a higher voltage than the voltage to the unit.

I did a powerpoint presentation for the National HVACR Educators & Trainers Workshop this past March that includes an interactive virtual lab to show inductive and capacitive reactance. You can download the presentation at the HVAC Excellence web site. It is listed in the free downloads as "Practical Labs." Students can click on the switches to control them and see the results. For more ideas on teaching alternating current fundamentals check out Unit 29 Electrical Power and Circuits in Fundamentals of HVAC/R.

Dry Out!

Its spring, the weather is warming up, flowers are blooming, and our thoughts turn to dehumidification. In humid climates like the southeast, removing humidity from the air in warm weather is just is just as important to comfort as reducing its temperature. Being warm blooded, our body normally produces more heat than it needs and then regulates our temperature using different cooling mechanisms. The primary cooling mechanism is evaporation of perspiration from our skin. Dry air makes us feel cooler because it accelerates the evaporation from our skin. Humid air makes us feel warmer because the evaporation process is slowed down. Dehumidification can be the difference between being comfortable and being uncomfortable at 78°F. Dehumidification can save energy by reducing the amount of sensible cooling required for comfort. Many people with oversized air conditioning systems essentially over cool their house to be comfortable because their systems do not run long enough in mild weather to reduce humidity, so they do not feel comfortable until they reach temperatures of 70°F in their house. It takes several minutes for most air conditioning coils to get cold enough to sweat. An over sized system will often satisfy the thermostat and shut off shortly after the coil reaches dew pont. A properly sized air conditioning system will run longer, allowing longer operation with an evaporator operating below dew point and removing more water from the air. Systems with ECM blowers and thermidistat controllers have a special dehumidification mode that reduces system airflow for dehumidification. This increases the latent system capacity and decreases its sensible capacity. Two stage cooling systems can help by allowing longer system operation at moderate loads. All of these are a big improvement over the typical oversized single capacity system with a PSC blower. However, an air conditioner is still not a dehumidifier.

If you are really serious about dehumidification you need a dehumidifier. In a nutshell, a dehumidifier is an air conditioner with a single blower that moves air first over the evaporator, and then over the condenser. The air first passes over the evaporator where it is cooled to dew point to remove water, and then the same air passes over the condenser where it is reheated to a temperature slightly above its original temperature. If you have a basement in the southeast you NEED a dehumidifier. My basement stays below 80°F all the time without air conditioning, but it feels warm without my dehumidifier operating. In the past most dehumidifiers have been small console types that are noisy, inconvenient, and typically undersized. Several companies now offer whole house dehumidifiers that can be integrated into a complete comfort system for your house. They have enough capacity for a house, are far quieter, and do not require you to empty a bucket twice a day. Therma-Stor has a good short animation on why basements have high a relative humidity and how a dehumidifier addresses this problem. You will find other articles on their website which help address specific dehumidifier applications. They also have a psychrometric chart in a round format that looks like a ductulator that are great for teaching psychrometrics. To read more about the effect of humidity on comfort, check out Unit 61 Fundamentals of Psychrometrics in Fundamentals of HVAC/R. To download an interactive pschrometric chart for free, go to HandsDown Software.

Free RSES Jounal for Students

I truly believe that one of the best career decisions anyone can make is to choose something they love. After all, most of us are going to spend the majority of our conscious lifetimes at work. So if you hate your work, your life will be miserable. By contrast, if you enjoy your work, your life will be more fulfilling and enjoyable. However, young people often have not experienced enough of the world to really know what they will like. Unless they grow up in a family that works in the HVAC/R field, most teenagers really know very little about it. We should be cultivating a love of the HVAC/R field. One way is to expose your students to the wide array of careers in the HVAC/R field through professional journals. I bring in my RSES Journals to school and strategically place them around the class and lab for the students to pick up and read. OK, so maybe I leave them laying around – the effect is that students do read them. They will ask me about articles they read and are interested in. Many students use them as resources in other classes. Students have used RSES Journals for research articles and even for speech class. One student discovered that he could stand up and give an extemporaneous speech on air conditioning anytime based on RSES articles he read. After a few speeches on the refrigeration cycle, scroll compressors, and heat pumps the teacher told him the next speech could not be on air conditioning. She later told me that she never met anyone before who loved his field of study so much. That should be our goal – to instill respect for the craft and pride in learning and practicing it. RSES would like to help us out. They are offering FREE one year subscriptions to HVAC/R students. To get their free subscription they need to go to the RSES website http://www.rses.org/studentjournal.aspx The good folks at RSES did stress that this offer is for students only.

I think getting the RSES Journal for free is a great start at getting a whole new generation of HVAC/R technicians interested in a great organization and a rewarding career. Right after getting all your students signed up for a free RSES Journal subscription, consider thanking RSES by joining JOIN RSES. If you are already a member, thanks!

Flooded Equipment

Large portions of the country are experiencing flooding from unusually large amounts of rain falling in a short period of time. Areas that have experienced flooding typically have a big cleanup job and lots of rebuilding ahead. Make sure your students know to be careful when working on equipment that may have been flooded. Even after drying out, the corrosion and debris left behind can make the controls and motors inoperable. Worse, they can be dangerous. Safety controls that have been under water cannot be relied on to work. Silt and debris can create potential shorts. Another concern is for the health of the building occupants. Floodwaters contain all types of things that you really don’t want in your air conditioning system: chemicals, gasoline, dead animals, and sewage to name a few. Coils , equipment cabinets, and insulation all will retain some of these undesirable things even after the water has receded. Technicians may be asked to repair flood damaged equipment, but in most cases the proper repair is more costly than replacing the equipment. It is hard to tell someone who has just lost most of their possessions that you cannot fix their flood damaged equipment, but that is exactly what you should do. Then there is the matter of ductwork: obviously a good place to catch things. Again, cleaning may not really be practical. A danger to technicians is the muck they will often be working in. They will be walking, crouching, crawling, sitting, and laying on this muck when they go in the house to look at the equipment. Again, this is not just mud. This is a combination of nasty stuff that is definitely hazardous to your health. For more information on flood damaged equipment check out the AHRI page on flooded equipment.