Measuring resistance: The difference between 0 and OL.
by:  Eric Shidell July 25, 2017
Measuring the electrical resistance (Ohms) of motors, compressors, and other electrical devices in HVAC systems may seem pretty straightforward at first. But when the measurements of 0 or OL come up, it becomes clear that there is quite a bit of confusion around this topic.
The answer to this one question holds the key to understanding resistance measurements…
Almost every thing has the property of electrical resistance.  This includes electrical devices such as motors, switches, wires, etc., and even the human body.  This property of resistance is measured in units called “Ohms” and can be measured with a standard Digital Multi Meter.
  The question to ask is,
“What does electrical resistance resist?”
The answer is “Electrical resistance resists the flow of electric current.”
Things that have a very high amount of electrical resistance (large number of Ohms) resist the flow of current almost completely.  These are known as insulators.  Substances like rubber, plastic, Glass, ceramic, and air are good insulators because they have a very high property of electrical resistance.
Things that have a very low amount of electrical resistance (small number of Ohms) such as Gold, Silver, Copper, Nickel, and even Steel provide very little resistance to the flow of electrical current.  These are known as conductors and they are used to build electric current carrying devices like wires.
The typical wire you are familiar with is actually a conductor (copper, low ohms) encased in an insulator (Plastic or rubber, high ohms).  This allows current to flow where it is wanted and prevents current from flowing where it is not wanted.
Electric Loads
Somewhere in between an insulator (Very High Ohms) and a conductor (Very Low Ohms) is an electric load.  Motors, Solenoids, Contactor Coils, Light Bulbs, and Heater Elements are examples of electric loads.  Electric loads are constructed with a carefully calibrated amount of electrical resistance.  When the proper amount of voltage is applied to this specific resistance, a certain amount of work will be performed, and a certain amount of electric current will flow.
When a motor experiences damage or malfunction, frequently the motor’s electrical resistance will deviate from what it’s normal value is.
This is where confusion often sets in.
“Ohming out a motor” is the process of measuring the electrical resistance of the motor windings and comparing that resistance to normal values.  One problem with this is that “Normal Values” will vary widely depending on what type of motor you are looking at and you usually do not have access to what the normal values are supposed to be for your particular motor.
Two measurements that often come up are 0 and OL.   Each of these measurements is very different from one another and they mean entirely different things.
A measurement of Zero, or very close to zero (less than .5 OHM) indicates a very low resistance to current flow.  Applying voltage to this low level of resistance will result in extremely high current flow.  In fact, the power supply will happily provide all the current it possibly can when there is  very little to resist it.  The result will usually be a blown fuse or breaker, or melted wires or something similar.
(Please note that very high power motors that normally draw high levels of current will naturally have windings with very low resistance.)
While you may not know exactly what the resistance of your particular motor should be, you do know that it should be more than zero!  A resistance measurement of less than .5 ohms usually indicates a short circuit in the motor winding.  A short circuit is literally an alternative path for current to flow that “shortcuts” the usual path and avoids all the normal resistance.
A measurement of OL is something else all together.
Most technicians these days use an auto ranging meter.  These meters automatically adjust themselves to the correct scale range depending on what they are measuring.  When your meter is adjusted to measure “OHMS or Ω”, and you don’t have your test leads connected to anything, your meter automatically adjusts itself to its highest scale of measurement and reads “OL”.
It is easy to think that you aren’t measuring anything at this point, and you are not measuring any OHMS.  In truth, you are.  Remember that Air is a very good insulator.  You are now measuring the resistance of the air between your two test leads and the resistance of that air is very high.
In fact, it is so high that it is more resistance than you meter is capable of measuring.  It is literally off the scale.  Your meter is experiencing so much electrical resistance that is literally OVERLOADED (OL)!
OL actually means that the meter is experiencing more Ohms than it can count.
The other thing you know about your motor is that under normal circumstances, it does flow some current, so it’s windings must have a normal amount of resistance that can actually be counted by your meter.  OL represents an open winding which usually means one of the fine wires that create the winding is actually broken.  No current can flow under this condition, and the motor cannot run.
When measuring between any motor terminal to Ground, or to the case of the motor, or to a refrigerant pipe of a compressor, you always want to see OL.  This means that there is no path for current to flow to ground.
If you see 0, or anything other than OL, there is some path for current to flow to ground.  That motor is experiencing a short to ground and must be replaced.
Many standard high quality DMMs measure resistance up to about 50 MegOhms.  (Meg is short for Million.)  To measure higher levels of resistance accurately, you will need a MegOhm meter. MegOhm meters (aka “megger”) are essentially Ohm Meters that have a much higher scale of measurement.  They are used to test the integrity of insulation.  No insulator is perfect, and as insulation begins to break down, it can start literally leaking small amounts of current to ground.  This is a useful test in very large horsepower electric motors and compressors.
There is a lot more to discover about measuring resistance, but for now, this should clear up any confusion between 0 and OL.
0 means short, OL means open.

Summertime and the Living is Easy!

Summer time and the living is easy. Unless you’re an HVAC tech.

For many HVAC techs, summer is the busiest time of the year, and many companies bet the bank on a busy and productive summer season.

Without a doubt the busiest months should also be the most productive months. But with extreme conditions, high call volume, and long hours comes increased potential for errors. During this busy time, a call back or return trip caused by misdiagnosis is even more damaging than at other times of the year.

Here are some tips on how to make the best of the busy season and reduce or eliminate no charge calls.

Don’t take shortcuts.

Getting through calls quickly is always a priority, but in busy times, the pressure can seem even higher. Even though call volume is high and the pressure is on, it’s no time to rush through any kind of job. Taking shortcuts only leads to two things: Callbacks and injuries. Don’t skip the steps that make a job thorough and don’t skimp on safety to save time.

Don’t Rush.

Trying to reduce call times by working faster frequently leads to either a mis-diagnosis which takes even more time to correct, or to identifying a symptom only to miss the underlying cause of a problem. In either case, revenue is lost and time is wasted. Remember, it is not your job just to clear the dispatch board. It is your job to maximize the opportunity on every call and to serve every customer in your highest capacity.

If you’ve ever watched a seasoned pro in the busy season, they don’t really look all that busy. Sometimes they look like they are working in slow motion without a concern in the world. This is because they have learned a special time saving technique which is…

Think before you act.

I’m fond of saying that the last thing you want to do on a service call is do something. All too often, we as hands – on mechanical people rush into grabbing some tools and taking something apart. The expert troubleshooter knows that even though it feels slow, stopping to think problems all the way through before reaching for the tool kit actually takes less time. This is why it sometimes looks like they’re working in slow motion. All the heavy lifting is taking place in the mind. The key to doing that is…

Implement a proven troubleshooting process.

Troubleshooting is not a physical activity, but it is a decision making process. In tech school, they teach you a lot of the physical activities involved in troubleshooting, but most of the techs I work with never learned a formal logical process of troubleshooting. The better and more organized your decision making process is, the faster and more accurate you will be. Unfortunately, too many of us make decisions unconsciously. If you have ever found yourself at your wits end with a problem you just can’t seem to figure out no matter what you do, this is usually the reason.

Please join me this Saturday, July 15, at 9AM MDT for a free 30 minute training on troubleshooting processes through Facebook Live. Attendance is totally free, but you must pre-register to attend.

This could be the best thing you can do while you’re eating breakfast. If you are working at that time, or otherwise busy, make sure you register anyway! The training will be recorded, and replays will be available to all registered participants. Plus, everyone registered will get a special FREE bonus at the end of the training!

Go to to register!

Measure temperature the right way!

Most HVAC technicians have grown up being taught that when checking air conditioning systems, supply air temperature should be measured.  Naturally, this seems like a perfectly logical step.  Since an air conditioning system is meant to supply cold air, measuring supply air temp is a good idea.

But what supply air temp should be considered acceptable?  When I pose this question to my students during air conditioning advanced troubleshooting classes, I get a variety of answers!  The most common of which is 55 deg. F.  When I ask if 55 deg F is good, lots of heads start nodding, and at least one student will want to throw up a “yeah, but” and insist that because of the altitude, 50 degrees is really the number we want to look for.

So, I will say, “What if the temp is higher than 55?”

“Must be low on refrigerant” is a popular response.

“What if the temp is lower than 55?”

At this, some folks will shrug and say, “Colder is better than warmer any day”.

All of these statements reveal a long line of air conditioning myths that have been handed down from generation to generation.  Let’s look into these a little further:

First, let’s start with 55 degrees.  The “well known fact” that 55 degrees is the preferred supply air temperature comes to us from the commercial world.  For generations, large commercial air conditioning systems have been designed around a constant supply air temperature setpoint of 55 degrees.  Even today, many modern control systems will default to a supply air temperature setpoint of 55 degrees, and this is also the setpoint used to control compressor staging on variable capacity and variable air volume systems.  If the rest of the system was designed around a 55 degree supply air temp, this works great!

Compared to the whole of air conditioning, variable capacity and variable air volume systems are relatively uncommon.  Most air conditioning systems in operation today are fixed capacity constant volume types.  On these systems, 55 degree supply air doesn’t necessarily mean anything!

On these systems, the ∆T is the value we are interested in.  The symbol “∆”, or “Delta” means “Change in Value”.  When combined with the “T”, the meaning is “Change in Temperature”.  Another familiar term would be “Temperature Drop”.  These terms refer to the amount the air drops in temperature as it passes over the evaporator coil.

For example:  If the air entering the evaporator is 80 degrees, and the air leaving the evaporator is 60 degrees, the difference in temperature, or the ∆T, is 20 degrees.

Most HVAC techs will agree that 20 degrees is a good ∆T.  The best techs when asked what the ideal ∆T for air conditioning should be will say, “It depends”.  Here is why:

In every operating air conditioning system, there are two types of heat transfer taking place between the air stream and the refrigerant.  Latent heat transfer and sensible heat transfer.  Sensible heat transfer will be measured on a thermometer while latent heat transfer will not.  Latent heat transfer takes place when moisture held in the air condenses on the surface of the evaporator.

For every pound or water that comes out of the condensate drain, 970 btu of heat is absorbed by the refrigerant in the evaporator.  These btus are the latent heat load on the evaporator.  The higher the relative humidity of the air, the greater the latent heat load on the evaporator.  The greater the latent heat load on the evaporator, there is less sensible heat transfer that can take place.  Therefore, the higher the relative humidity of the air entering the evaporator coil, the lower the sensible ∆T will be.

The following chart details the relative humidity and ∆T relationship:

Air Conditioning airflow chart
20% RH 26
25% RH 25
30% RH 25
35% RH 23
40% RH 22
45% RH 20
50% RH 19
55% RH 19
60% RH 18
65% RH 16
70% RH 15
*based on 400 cfm per ton of cooling
© 2014 Eric Shidell; HVAC Service Mentor

You will see that the “standard” ∆T of 20 degrees is only correct if the relative humidity of the air entering the evaporator is 45%.  As the RH changes to a higher value, the ∆T changes to a lower value.

The above chart is a great way to do a quick check on the condition of an operating air conditioning system.  First, measure the relative humidity of the air stream entering the evaporator coil using a digital psychrometer or similar device.  Next, accurately measure the temperature of the air entering the evaporator coil and the air leaving the evaporator coil.  Subtract the two to find the temperature difference or ∆T.

There are four major variables that contribute to the final ∆T in an operating air conditioning system.  They are:  Relative Humidity of the entering air, Refrigerant Charge, Air Flow Rate, and mechanical integrity of the system (compressor, metering device, coils, etc.)

In an operating system that is perfectly correct, the measured operating ∆T value should match the chart for the measured relative humidity within two degrees.  If the value does not match, something is wrong and further investigation is needed.

Cold Weather Air Conditioning Service

April, 2017
Eric Shidell; HVAC Service Mentor
If you are reading this in most parts of North America, you may be facing a string of cool, mild weather. This is definitely not air conditioning weather. For air conditioning service techs who need to be out doing service calls and planned maintenance visits, that is not good news. When there is cold conditions outside, and little to no demand for cooling inside, air conditioning systems just don’t perform worth a darn. It is very difficult to determine whether or not a system is performing adequately or has the correct amount of refrigerant in it under these conditions.
This causes a lot of technicians to stay home, watch cable, and worry about how they are going to make ends meet when there is no work to do. They also know that when the weather breaks, there is going to be hell to pay because the scheduled calls that got cancelled due to weather still need to get done somehow.
If you want to get out of the house and get your PM calls done even in poor weather conditions, here are some hot tips to make your life easier.
First, you need to have a dry outdoor coil. Even in a light rain, there is a good chance your outdoor coil is still dry. How do you know? If your liquid line temperature is lower than the outdoor air temperature, your coil is wet. For this reason, you want to make sure that washing the outdoor coil is the last thing you do on your PM visit in cold weather.
Manufacturer’s charging charts and slide charts will be accurate and helpful down to about 55 degrees outdoor temp. Below that, you need a little help.
First, when you are about to begin travelling to the job location, call ahead and ask your customer to adjust the thermostat to 80 degrees in Heat mode in order to cause the furnace to run and heat. This will allow some btus to start to accumulate in the house and give the air conditioning system something to do when it turns on. Many homeowners may express concern about that, but if you explain that it is helpful to give the AC something to work on, and that that will help your visit to be more accurate and productive, most people will happily do as you ask.
Next, do what you can to keep the heat running while you are there. If you can, perform your electrical inspection of the outdoor unit while the heat is on. Replace or clean the filter while the heat is on. Once it becomes time to turn the AC on and do the running tests including check refrigerant charge, switch off the heat and switch on the AC. Apply your refrigerant gauges and liquid and suction line temperature probes.
Let the unit run normally for about five minutes and allow the heat from the furnace to dissipate.
The next step is to simulate warm outdoor conditions. Find something you can use to restrict air through the outdoor coil. You can use spare pieces of cardboard, empty trash bags, sheets of plastic, or your jacket and a few convenient rocks.
You want to partially block air through the condenser in order to drive head pressure to a more normal level. Ideally, you want to achieve a consistent liquid saturation temperature of 100 degrees. For R-22 this is a head pressure of 200 psig. For R410a this is a head pressure of 320 psig. You will need to adjust your condenser blockade so that the unit runs and maintains that head pressure.  Let the unit run for about 10 minutes so that it reaches a stable operating condition.
Now, you can run your superheat and subcool and indoor temp drop like you normally would. For fixed metering devices, use 80 degrees for your outdoor temperature value when determining required superheat.
If there is an undercharge or overcharge situation, you will find it. You can be reasonably certain that if the unit is working well now, it will continue to do so in the heat of the summer. If it is having problems now, it will have problems later, too.  You can add adjust the refrigerant charge, find airflow problems, metering device problems, compressor problems just as if it was a nice, warm day.  You can also give the unit a clean bill of health if that is the case too!
Once you have finished all your checks, now you can wash the outdoor coil at the end.
This method is not exactly perfect. You may find there about 5% of the units that you just can’t make a definite conclusion about how they will perform in warm weather. Those are the ones you want to schedule a return visit to double check when it’s warm.
That’s a whole lot better than rescheduling all of them!
-Eric Shidell


Nearly all conventional split system air conditioner condensing units employ a contactor to energize the compressor and condenser fan motor.  A contactor is essentially a heavy duty relay.  When the indoor thermostat calls for cooling, it sends 24 VAC to the contactor coil.  When energized, the contactor coil creates a magnetic field which attracts the contact bar and draws the high voltage contacts together.

These contacts when closed, will complete the circuit for the compressor and condenser fan motor.  Contactor contact points carry the full amperage drawn by the motors.  As such, they must be designed to handle the load.  The larger the compressor and fan, the higher the amp rating of the contacts.

We don’t normally associate electrical components with mechanical wear.  Contactor contact points are a big exception to this.  Contact points are in fact wear items similar to the brake pads on your van.  They are meant to wear out and get used up.

Every time the contacts close or open, a small arc is created between them for a brief instant.  This arc is similar to a welding arc, and the results are similar as well.  A small amount of the metal contact material vaporizes and burns away.  Over time, more and more of the material is worn away, and the contact point begins to change shape.

Both mating surfaces of the contact points are convex, or rounded outwards.  Essentially, they are both shallow domes that come together on the round parts.  As they wear, the surfaces become flatter and flatter.  As the contact points flatten out, the arcing is augmented and the wear accelerates.

As contact points experience this normal wear and tear, a number of bad things become more and more likely to happen.

As the wear becomes pronounced, the contact point surfaces become pitted and rough.  Due to the flattening, it becomes more likely that the contact points may not draw together evenly.  Both of these conditions will create a high resistance path for current flow.  This can cause three potentially damaging conditions.

First, as the current flows through a high resistance path through the contact points, heat is generated.  This heat can build in the contact points and lead to a total meltdown of the contactor and a no cooling call.

Second, the additional resistance of the contact points can create a voltage drop across them.  In some cases, the compressor will receive voltage that is too low as a result.  This leads to additional heat in the compressor motor windings and to compressor failure.

Third, the contacts can become so worn that even though the contactor bar is down, there will still be  no current flow across the contacts, and the unit won’t run at all.

A fourth problem can also develop.  As the contact points flatten out and the arcing becomes more pronounced, the likelihood that they can weld together increases.  In a single phase unit, this can lead to the condensing unit running wild and either overcooling or freezing the coil.  If not detected soon enough, this can also lead to compressor failure.

In three phase units controlled with a two pole contactor, if one set of contacts welds shut but the other breaks at the end of the cycle, the compressor will experience a single phase condition and may be damaged.

In short, contactors should be replaced proactively whenever they become pitted, flattened, and worn.  This practice will prevent a no cooling call and a compressor failure.

I once had a chance encounter with a retired engineer from the Cutler Hammer company.  He had begun his career there in the mid ‘50s and stayed there until he retired, designing contactors and motor starters.  He gave me this bit of advice which I pass along to you :  He said that if you replace a contactor with one that is the next amp rating higher (As in replace a 30A contactor with a 40A contactor), the new contactor will have lifespan that is five times longer than the original.

I have used this advice to great success on water source heat pumps that seem to wear out contactors very quickly.  And now it is yours.


The great debate

There is a great debate in the HVAC contracting and service world.  One side says that online training for technical people is the wave of the future.  The other says that distance learning for people who primarily work with their hands cannot be effective.

Recently, the ACHR News tackled this subject in the July 11 issue with an article written by Nicole Krawcke.  I was pleased to be quoted in the article and I’m glad to see that online training is becoming more recognized.     Click here to check out the full article.

I began to explore the idea of presenting training online when the demand for my live training grew to be intense.  I was teaching in 9 different locations across the state of Colorado and was getting a bit ragged.

I took a minute to think about what was going on and how strong the demand was for my training programs.

I asked the question:  What if instead of teaching the same class 9 different times in 9 different places, I could do the class for everyone at the same time through the internet?   I had received valuable training through the internet myself when I was involved in PV Solar.  It seemed possible.

I began to poll my live class students.  I asked them if they would be interested in receiving the same instruction we did in class only over the internet.  After several hundred surveys filled out, the results came back with 70% yes, 30 % no.  I was encouraged to press on with the idea.

I began looking for an online teaching platform and was quickly led to the format that Harvard University uses for their distance learning programs.  If it is good enough for the ivy league, it’s good enough for me.

The first class on combustion analysis turned out to be a fantastic success.  The class was well attended by students from a very wide area spanning hundreds of miles including some remote rural areas that are typically underserved.  The feedback afterwards was outstanding, too.  Every attendee felt as though the experience was as good if not better than coming to class like usual.  Several students I have worked with since have reported that they were able to take the training right into the field the very next day and they have been using it every day ever since.

HVAC Service Mentor Online was born.  I quickly realized that the online format was not limited to just my home state of Colorado, but also the entire nation.  Also, I now could produce more in depth programs spanning several weeks.  The Boot Camp six week training format was created.  My first AC Boot Camp Online course debuted to a national audience and students enrolled from as far West as California, as far East as New York, as far North as Wyoming, and as far South as New Orleans.

The enthustiastic response from the students and their supervisors clinched it.  Online training programs have real lasting value and I have been providing online programs ever since.  Check out the feedback from one recent Masterclass participant:

“I think what you’re doing with these mini seminars is incredible and I think there needs to be more of it!  It’s one thing to take full time extensive training course but when one can hold a full time job and custom build a knowledge base specific to them; it proves to be an incredible resource.”

While I am committed to the online training format, I have not abandoned live training.  I still appear in Denver several times a month, and providing live private training sessions is a very important part of my work.

HVAC Service Mentor serves busy contractors and facility maintenance teams by boosting the technical capability and profitability of their field technicians with convenient and cost effective technical mentoring and training programs.  Online training programs work well for students who cannot be present in person for whatever the reason.  Click here to find out about my current online programs as well as live training sessions!

Let’s talk shop…

What if you could have access to the knowledge and experience of a true expert in the HVAC service trade?  Someone who has spent nearly two decades running calls, and working on every type of heating, cooling and hydronic system imaginable.  Someone who is the “end of the line” for HVAC service problems when no one else can figure out what is wrong.  Someone who knows what kinds of mistakes techs make that lead to callbacks and what to do about preventing them.

If you had the chance to sit and talk shop with an expert in the HVAC Service trade, what would you ask?  What could you learn?

I am Eric Shidell, the HVAC Service Mentor, and I am that kind of expert.  From 1997 to 2015, I was a top HVAC service technician running calls and getting things running when no one else could.  Not only that, but also mentoring and educating other technicians and molding them into experts in their own right.

A true HVAC nerd, I eat sleep and breathe HVAC systems.  I lived it for nearly twenty years.  In an attic on the hottest day of the year.  On the roof in the middle of a blizzard.   Covered in soot in the boiler room at 3AM.   I have learned the secrets of what it takes to be at the top of the service field.  I now spend all my time teaching these secrets to others, so they too may rise to the top.

As a leader in the service field, I am in a unique position to understand what it is that technicians need to know in order to be truly excellent.  I also know what it is that holds technicians back and prevents them from being truly excellent.  I call these things “knowledge gaps”.

Knowledge gaps are the parts of a technician’s training and knowledge base that are incomplete.  For example, most techs know “how” to replace an ignition control, but don’t actually understand what it is that determines whether or not the control is at fault (even if they think they do!)

The technicians I work with already have jobs in the trade.  They don’t need me to show them how to use wrenches, torches, or other tools.  They need help filling in their knowledge gaps so they are better able to decide where, when, why, and how much to use those tools.   Harnessing the communication power of the internet, I can share my considerable wealth of knowledge and experience with technicians from all over North America in a convenient easy to use format.

Some people prefer the in person experience.  For them, I still offer live training events around my home state of Colorado, and many private contractors invite me in to their shops to work with their service and install teams directly.  But I can’t be everywhere.  Offering internet based training programs allows me to help a much larger group of people in a much larger way.

You would be surprised how much you can learn from a veteran field tech through the internet.  My students are able to experience a dramatic shift in their understanding which allows them to be more productive, more profitable, and more accurate.

In the words of a recent internet student, “I think what you’re doing with these mini seminars is incredible and I think there needs to be more of it!  It’s one thing to take full time extensive training course but when one can hold a full time job and custom build a knowledge base specific to them; it proves to be an incredible resource.”

A service manager of an HVAC contracting firm who is a 22 year veteran himself, had this to say after completing a recent HVAC Service Mentor online course with his team:  “The knowledge presented by the HVAC mentor gives a basic technician not only the confidence to perform his work but actual understanding of the refrigeration process and all of its components. I would say with the knowledge presented, that it would save it technician a minimum of two years of experience and increase the ability for a technician and company to increase its profits while taking care of customers. It is phenomenal to have such a resource that is an experienced, qualified, working instructor.”





Why won’t it cool?


The other day, I was having a conversation with an experienced HVAC technician.  Let’s call him “Joe”.  Joe was telling me about a problem he was having on an air conditioning system that he had installed earlier in the year, explaining that everything was running, but the system would not maintain setpoint.

He told me that in the first part of the cooling season, the system ran great, but now that it was very hot, it wouldn’t keep up.  He had performed a load calculation before selecting the equipment and he was certain it was correct.  He even did it a second time when the customer called to complain.

“Is the coil clean?” I asked.

“Yup” Joe said.  “That’s the first thing I tried.  I put in a new filter, too, just to be sure.”

Before I had a chance to ask about any other details, Joe added “The Suction line is nice and cold, and it’s blowing cold air and everything.”

“What was your temperature drop?” I asked.

Joe responded with “I didn’t measure that” and then added that he felt cold air blowing from the supply registers.

“What do you think it could be?” Joe asked.

This conversation is typical, and is very similar to many other conversations I’ve had over the years with many technicians.

As HVAC techs, we are involved in the process of moving heat energy from one place to another.  Heat energy is a little hard to pin down.  I don’t know anyone who has held a handful of Btu.  You can’t put a bunch of Btu in a container and put it on the scale and weigh them.  You can’t pull out a tape measure and measure heat energy.

In fact, the transfer of heat energy from one place to another can’t really be measured directly.  The only thing we can do is measure the effect that heat energy has on something else as the heat is absorbed or rejected.  When a thing absorbs or rejects heat, either its temperature will change (sensible heat) or it will change phase (latent heat).

Test instruments are used so that you can visualize things you can’t see.  Things like electricity and heat energy.  When we’re talking about heat energy (which we usually are), we’re talking about measuring temperature.

By itself, a temperature measurement isn’t all that useful.  For example, it’s not very helpful to know that the supply air temperature is 60 degrees unless you also know that the return air temperature is 80 degrees.  That means something.  If the supply air temperature was 60 degrees when the return air temperature was 70 degrees, that would mean something very different.
Temperature measurements are comparative.  When we compare one temperature to another and calculate the difference between those temperatures, this is giving us an indication of the movement of heat energy into or out of the thing we are measuring.

I asked Joe to go back to the job and measure the return air temperature and the supply air temperature and report back.  The question he needed to answer was this: Is the unit operating properly and is undersized, or is the unit sized properly but having an operational problem.

“The weather report says we have a cold front coming through.  I don’t think we’re going to see the same temperatures we saw before.”  Joe said.

“That won’t make a difference.”  I told him.  “If you have a problem when it’s hot, you’ll have the same problem when it’s mild.”

Joe called back a couple days later.

“Supply air temp is 47 degrees!” Joe reported triumphantly!  “I knew it was blowing cold!”

“What about the return air temp?”  I asked.

“Looks like 74 degrees.”  Joe said.  “The thermostat was satisfied so I had to turn it down to get it to come on.”

“The difference between 74 and 47 is 27 degrees.” I said.  Under the conditions you had yesterday, you should be running a temperature difference between 20 and 22 degrees.  27 degrees is too much.  You may have an airflow problem.”

As air moves across the evaporator more slowly, it has more opportunity to fall in temperature.  Also, as air moves across the evaporator more slowly, fewer Btu are delivered to the evaporator and fewer Btu per hour get moved through the system.  Thus, you have nice cold supply air, but less refrigeration capacity.  This explains why it couldn’t keep up in the hotter weather.

“Calibrated hands” are not enough to measure temperature accurately.  Technicians must be using accurate calibrated digital thermometers, using proper measuring techniques, and comparing the results to calculated norms based on current operating conditions.

As I always say, “Btu and Temperature are not the same thing.”  Colder isn’t always better.

Joe went back in and looked for an airflow problem.  He called back a while later.

“I’m feeling a little silly.”  Joe said.  “I had the dip switches on the variable speed furnace set backwards.  As soon as I set them right, that fan took off and I started seeing a temperature difference of 22 degrees.”

“Don’t feel bad,” I said, “Everyone makes mistakes.”  It’s important to have a backup plan in order to catch your mistakes before you leave the job.  Check it, then correct it.

Word to the wise:  Failing to measure temperature appropriately is one of the main causes of missed diagnosis in air conditioning systems!

Summer is officially in full swing and temps are heating up around the country.  Many areas are reporting higher than normal temperatures and others are seeing high temps linger longer than they are used to.

By now, most AC problems related to a failure to start have been discovered and sorted out.  Now the service calls start to take a turn in a different direction:  The emergency call.  Something that was working satisfactorily has stopped doing so and the customer is hot, bothered, and in need of relief.

Many techs focus on finding and solving the immediate problem and moving on to the next call as soon as possible, and may not be thinking about planned maintenance programs right now.  In fact, the busy service season is one of the best times to grow your maintenance customer base.  As you will see, there are several valid mechanical reasons why this is so.

Most summer service calls take one of three forms:

  1. Neglect related. These are new customers or folks who only call when there is a problem and they do not perform regular necessary maintenance.  They will report a no cooling, not enough cooling, or a water leak problem.  A brief investigation will reveal a maintenance related issue that was neglected such as a dirty filter, coil, or clogged condensate drain.  Each of these problems could have been avoided all together with a planned maintenance schedule.
  2. Age related failure. Sometimes things get old and wear out.  While planned maintenance can’t erase the effects of years of use, experienced techs can often see the effects of wear before a total failure occurs.  If the customer had been participating in a planned maintenance program, worn and failing components could have been detected, addressed, and dealt with before an inconvenient and expensive breakdown occurs.  This pleads the case for planned maintenance programs and you should absolutely offer one here to help this customer avoid this uncomfortable situation in the future.
  3. Chronic operational problems. Equipment mismatch, oversized, undersized, improper refrigerant charge, slow refrigerant leaks, airflow issues and the like are problems that exist all the time in many systems.  When the weather gets extreme, and these units are called upon to do their best, they prove to be unable to perform, even though they were providing satisfactory performance in milder weather.

Hot tip!!!  Most compressor failures are a result of a chronic operational problem that was never detected!!!  Compressors don’t just die, they are killed!

A system that has a performance issue on a hot day will also show a performance issue on a mild day.  Done correctly, planned maintenance calls will detect an ongoing operational problem and present the opportunity to handle it before it becomes an emergency situation.

As you are going about your days in the coming heat of summer, consider the type of breakdown you are looking at and ask yourself:  Could this problem have been avoided if I had done a complete PM on this unit two or three months ago?  You will find that the answer will be “Yes” a vast majority of the time.

This is the perfect time to offer your planned maintenance program and bring this customer into the fold.  Your customer is faced with the consequences of neglecting their equipment, and is currently “feeling the pain”.  You hold the solution to that problem and relief from that pain with your maintenance program.  Offering a preferred customer discount that you can apply to today’s invoice when they enroll in the program can help spread some soothing ointment to the burn they already feel and make the idea of planned maintenance even more attractive.

Not only is planned maintenance good for the customer, it is good for the techs and the contractors, too.  When you’re out there in the heat working hard and long hours, remember that most of these problems could have been avoided if these people were part of your planned maintenance program.  Plus, the company would have the advantage of a secure customer base and the ability to plan the work flow better in order to break free from the feast or famine cycle of weather driven service.

Given the choice, I would much rather do a PM call than replace a compressor any day.

My Commercial RTU service class is happening on July 13 at 5:30 PM at Johnstone Supply Denver, Colorado.  This class will teach how to provide a complete PM and service adjustment on commercial packaged rooftop units in order to prevent many of the most common types of failures.  Dinner is served from 5:00 PM til 5:30 PM.

On July 20, also at Johnstone Supply Denver at 5:30 PM, I’ll be teaching all about refrigerant leak detection techniques.  We’ll explore the wide variety of leak detection techniques and equipment available to technicians and discuss how to get the most out of each one to ensure that you find that stubborn leak every time!

On Friday, July 22, I’m presenting an all day hands on class beginning at 8 AM about No Cooling Diagnostic techniques, also at Johnstone Supply Dener.  We’ll be using Johnstone’s live fire lab to do hands on troubleshooting of actual no cooling problems.  Students will learn how to locate and identify the cause of many breakdown scenarios quickly and accurately.  Special attention will be put on proper troubleshooting processes and working with electrical systems.  Lunch will be served.

Contact Chris Smith at Johnstone Supply Denver to enroll for Johnstone classes at 303-573-5626.  Classes are held at Johnstone University training room upstairs at 2710 W 7th Ave Denver, CO 80204.

In the online environment, the Technician Acceleration Program is in full swing.  “Understanding Superheat and Subcool” and “ACR Compressor Troubleshooting and Replacement” are the video training classes that are currently available, and “Using Digital Multimeters” and “Refrigerant Recovery: Charging, Recovery, and Evacuation” will become available in July.

The Technician Acceleration Program helps busy contractors, service managers, and technicians uplevel productivity and profitability with convenient and cost effective technical mentoring programs delivered through the internet.

Looking forward to fall, the Boiler Basic Training Online course and the Gas Heating Boot Camp Live Online are both open for enrollment now.  Both of these powerful programs begin in August and the smart money is on signing up early to get your places reserved before they are all taken!

Go to for full details on all the online training happening now!

Remember, stay hydrated and protect yourself from heat stroke and sun burn!

-Eric Shidell

HVAC Service Mentor

Superhero Service Technicians

Every winter season, several Coloradoans are killed in their homes by carbon monoxide poisoning. Scores more experience sickness or injury. Frequently, the cause is traced back to a faulty gas burning appliance such as a furnace or water heater.

Recently, I was conducting a training session about planned maintenance procedures at a wholesale supply house. I was training a group of field service technicians about how to perform a complete and thorough “furnace tune up”. At one point of the session, I was talking about testing for carbon monoxide. I mentioned how HVAC technicians are really in the position to save lives by detecting and preventing dangerous carbon monoxide leaks.

After the session was over, one of the students told me this story. It is so poignant for this time of year that I decided to share it with you all.

The technician, we’ll call him “Jim”, was dispatched to an elderly woman’s home for her annual “furnace check up”. As Jim introduced himself to his client, she began to tell him a little bit about herself. He learned that she was 83 years old and that her health had taken a turn for the worse. Doctors really couldn’t tell what was wrong with her and she was taking twelve pills per day.

She looked weak, she had no energy, and her skin had a grey tinge. She felt that she was dying and she wasn’t able to leave her home. It was Jim’s practice to routinely test for carbon monoxide during his planned maintenance calls. During his inspection, he noticed ambient carbon monoxide levels rise to about 9 ppm (parts per million). Further inspection revealed five cracks in the heat exchanger of the gas fired forced air furnace.

Jim knew that long term exposure to low levels of carbon monoxide can cause mysterious health problems and ailments that are difficult for health care providers to diagnose. He also knew that elderly people are more at risk for these types of ailments.

Jim explained the situation to his customer, and together, they called her doctor. Jim explained to the doctor about the carbon monoxide levels he had measured in her home. The doctor immediately called an ambulance and the woman was taken to the hospital. After a few tests, it was discovered that the woman was suffering from chronic carbon monoxide poisoning.

Jim replaced the furnace and installed carbon monoxide alarms in the home. Later that year, in the spring, Jim visited the same woman’s home for her annual air conditioner check up. He said that it was as if someone had turned on a switch. The elderly woman had vigor about her accompanied by a healthy glow. She said that she was down to only three pills per day and that two of them were vitamin supplements. She was once again able to tend to her flowers and yard.

She thanked Jim for saving her life. She was certain that if it weren’t for him that she would have died. I believe that to be true.

For those of you out there in the field, please make sure that you are testing for carbon monoxide on every call. Make sure you are equipped with a professional quality carbon monoxide analyzer and that it is properly calibrated and in good working order.

If you do not have a carbon monoxide tester on your truck, here are three choices that I routinely recommend: The TPI model 707, The UEI model CO95, and the Bacharach Monoxor Plus. All three of these are professional quality and suitable for high temperature applications such as measuring in the supply air plenum, or in the flue.

Smaller instruments (I call them “shirt pocket testers”) such as the Bacharach Snifit 50, the UEI CO 71, the Testo 317, and the Fluke CO220 are great for measuring levels of CO in the ambient air. They are not, however, suitable for measuring inside ducts or flues.

Lastly, combustion analyzers are available at affordable prices and all of them will measure carbon monoxide in ambient air and in flues.

Join the conversation. Leave your own story below.