Month: July 2018


This article is the second in a 5 part series by Senior refrigeration and HVAC tech Jeremy Smith 

The Ground rules

I’ve spent some time thinking about troubleshooting and the processes and procedures that
I use to find problems. Not the “why isn’t my air-conditioner running?” problems but the “Things
just aren’t quite right.” type problems. The really difficult ones.
I’ve boiled it down to a sort of flowchart to simplify things and we’ll take the flowchart
step-by-step, explaining each step as we go along.
Something to keep in mind as you read this. There is no step by step, color by numbers guide
to troubleshooting. I’m not trying to give you a magic wand to wave at broken air conditioners
because such a thing doesn’t exist. Troubleshooting is more of a “can do” attitude combined
with experience and some applied critical thinking.
First thing, let’s start with a couple of “Dont’s” when troubleshooting.

#1. Don’t rush

Yes, I know that many of us get piled up under a load of calls and can
be pressured to rush through them to get home to the family. Yes, I know the boss or dispatcher (or both) are calling
you every 10 minutes asking if you’re done and ready to move. Yea, I know the customer is
breathing down your neck to get the machine running. This is probably the hardest part of
troubleshooting. You NEED TO block that stuff out. You need to take your time and work
through the problem methodically.
#2. Don’t assume

Follow your troubleshooting procedure through to the end. Taking
shortcuts is almost as bad as allowing yourself to be distracted.
Over the course of a couple of articles, I’m going to share my troubleshooting processes and
procedures and hopefully give you some tips to build a process that will help you to be better.

Part 3 is coming tomorrow

— Jeremy


This article was written by Senior Refrigeration tech Jeremy Smith. Big thanks to Jeremy for his contributions to HVAC School and the tech community.

Having spent many years in the trade and many years reading posts from techs on forums and social media, a big issue that I see is that troubleshooting is something of a lost art.

Troubleshooting is where the rubber meets the road for a service technician. Nobody cares what certifications you have, what union you belong to or anything else. If you can’t find the problem and solve it in a timely fashion, your customer and employer are not going to be happy.

One of the things that I think most guys struggle with is the mental aspect of troubleshooting. I’ll relate this in the form of a recent call I was sent on to “clean up”. It was a no heat call in a small convenience store. Trane RTU on a zone sensor.

The tech called me and related that the unit had a call for heat at the unit but the ignition sequence didn’t start. We talked a little about the problem, he checked some limits and a few other things. He wound up ordering an Ignition board and limit sensors. These were replaced late that night and the unit still didn’t work.
I was sent the next morning. Now, we get into the mental part of troubleshooting.

I met the tech so that he could communicate the basics of what he did. We talked for about 10 minutes and he went on to his job and I went to have a chat with the trouble unit.

20 minutes later, I had the problem solved. I found a failed RTRM board. Now, you guys that do Trane all the time probably aren’t surprised, but let’s analyze what went wrong and how this could have been handled on a “one stop” basis.

What did I do that the first Tech didn’t?

For starters, I took everything that I was told about the unit, what it was and wasn’t doing and what everybody and their brother thought was wrong with it and I threw it all out. Put it in a box in my head, closed the lid and locked it.

I dug out the basic Trane “Service Facts” book and started the troubleshooting procedure from Step 1 and followed it to the end.

Now, I can make these arrogant claims about how I’m a Billy Badass service guy and how I’m more awesome than anyone else, but the simple fact is that I’m not. I do things a little differently and think a little differently than many others  and that sets me apart.

What did the first Tech do wrong? While I’m not in his head, I think that he focused on why the heat didn’t work instead of taking the unit AS A WHOLE and diagnose it as a whole. Kind of like the guy who can’t figure out why the fridge is warm and spends an hour working on it only to find the plug pulled.

So, the the mental aspect of troubleshooting cannot be ignored.
Start at the beginning, work the process and troubleshoot the entire system. Being willing to read the manufacturers troubleshooting info isn’t a newbie move, it shows maturity.

Work on the troubleshooting mindset, don’t be a parts changer.

— Jeremy

(Edited by Bryan Orr, any mistakes are my fault)

In HVAC and electrical school, one of the first things you learn about electricity is Ohm’s law:

Volts = Amps x Ohms 

Pretty simple Right? and Watt’s law is just as easy:

Watts = Volts x Amps

With this new found knowledge the student walks confidently into the real world with two equations and some elementary Algebra skills expecting to be able to predict Volts, Amps, Watts and Ohms using this secret knowledge.

Then they Ohm out a residential compressor or other single phase motor windings for the first time…

They might read something like:

Start to Common = 4.2 Ohms

Run to Common = 2.6 Ohms

Start to Run = 6.9 Ohms

So they measure their voltage, grab a calculator and calculate.

Run Winding 243V ÷ 2.6Ω = 93.4A

Start Winding 243V ÷ 4.2Ω = 57.85A

Well, THAT makes no sense…. so they stop using Ohms law and settle with believing that electricity is magic and ohms law is broken.

In actuality, Ohms law does work it’s just that the loads we measure vary and don’t all behave in the same way.

Inductive Loads

In the case above, a compressor/fan motor/contactor coil etc… is an inductive load. That means that its job is to convert electrical energy to electromagnetic force. While an inductive load does have SOME resistance when de-energized that can be measured with an Ohmmeter, the majority of the electrical resistance only shows up once current is applied.

In an inductive (magnetic) load, this resistance that shows up once it’s powered on is called inductive reactance and is still measured in Ohms.

In the case of the example above, the run winding is connected directly between L1 & L2 and when the circuit is completed by the contactor in the example above the run winding WILL actually draw really high amps for a split second (first electrical cycle) until the inductive reactance kicks in.

In the start winding the current is limited by a combination of resistance, inductive reactance and the capacitance of the capacitor. Add in the fact that the applied Voltage across the run winding is actually higher than the L1 – L2 voltage due to back EMF (CEMF) and its enough to confuse anyone.

Then you throw in Power Factor to the mix in an inductive load… this means that even when we multiply Volts x Amps in an inductive AC circuit what we see in VA isn’t necessarily what you get in Watts.

It takes a lot of measurements and math to figure this all out and unless you are an engineer it’s much easier to measure what you have on a functioning component rather than attempting to calculate amperage and wattage based on voltage and ohms.


We are taught that resistive loads (loads that create light and heat) are much more simple. There is none of this inductive reactance or power factor nonsense in a resistive load like a light bulb or a heat strip.

But wait… there’s more

So this light bulb is measuring 12.2 Ohms confirmed with two different meters and it is rated at 14.4 volts. So we do the simple math:

Amps = 14.4v ÷ 12.2Ω 

Therefore Amps = 1.18
So we put it to the test by feeding this bulb exactly 14.4V from a DC power supply

Annnnddd… Not even close

We expected an amperage of 1.18 and got an amperage of 0.1


Incandescent light bulbs are a resistive load but they are also made with a filament of the element tungsten which has PTCR (positive temperature coefficient resistor) properties. This means that the resistance of a tungsten incandescent light bulb goes up by 10 to 15 times from its cold temperature to its hot temperature. In the case of the bulb above we now its cold resistance is 12.2 ohms, but by working ohms law backward we can also tell that its HOT resistance is 142.57 ohms which means that the hot resistance is 11.6 times higher than cold in this particular instance.

Not all resistive loads behave in this way though. Let’s look at a heat strip.

Amps = 240v ÷ 15.5Ω

Amps = 15.48 

Watts = 240v x 15.48

Watts = 3715 based on Ohms & Watts law

Don’t worry… This is a 7.2kw (7200 Watt) heat strip divided into two 3600 watt strips and we are only reading one half (3.6kw)

To bench test it further I applied one-tenth of the designed voltage (24 Volts) to see how it would respond.

1.532a @ 24v = 15.32a @ 240v 

The reason the math still doesn’t line up perfectly is that even in a heat strip the resistance increases as the temperature increases but in a much smaller fashion than in a light bulb.

When cold the math predicts it is a 3.7kw heater, when warm at 24v applied it predicts 3.67kw and at 240v the resistance increases to its full rating at 3.6kw.

All of this to say that Ohms & Watts law are useful and accurate but they are impacted by real “under load” forces in such a way that it much more realistic to make measurements on a functioning device and work backward than to use ohms to work upwards from an ohm bench test and a voltage.

— Bryan




As a technician gains skill they will learn that regularly testing your tools is a huge part of success. It isn’t long in the field before techs find out that just because a meter or gauge gives a particular reading it doesn’t ALWAYS mean it is correct. Vacuum is one of these areas.

Everything in an air conditioning and refrigeration system leaks to some extent, our job isn’t to eliminate all leaks, our job is is to reduce the leakage rate to as low as possible. When using a sensitive micron gauge we find that isolating an assembly and checking the “decay” or standing leak rate is a great way to test and ensure that a system has minimal leaks and moisture. The challenge is that all of the connections in your rig leak and even the vacuum gauge itself leaks.

Some techs attempt to test the leak rate on micron gauge by connecting it to a core tool and then straight to the pump, evacuating the gauge down to very low level and then valving off. If you do this, you will find that every commercially available vacuum (micron) gauge shoots up pretty quickly. This is because the VOLUME of the gauge and coupler are so low that ANY leakage whatsoever has an enormous effect.

In this video Ulises Palacios shows us how to use an an empty recovery tank to better test the leak rate of a vacuum gauge rig.

It is certainly important to test all of your vacuum rig components, just remember that volume makes a huge difference when decay (standing vacuum) leak testing.

— Bryan

We all learned how to read the tonnage off of a model number within a few weeks of beginning in the trade. What you may (or may not) have learned is that just because something has an 036 in the model number does not mean it actually produces 36,000 btu/hr even during RATED conditions let alone real world conditions.

Some of you may be used to pulling up an AHRI rating to find the true capacity of a system match. This is a good start and often you will find out that the the system produces slightly less to up 4,000 btu/hr less than the nominal rating. Here is the AHRI ratings for the system I have on my home.

You will notice that the 2-ton matches actually produces 24,000 btu/hr at the rated conditions, which are REALLY WARM temps inside and out by the way. However the 4-ton match produces 46,000 btu/hr at the same conditions.

Here is an example of some real world capacity readings I took on my Carrier VNA8 4-ton system with the Testo Smart Probes app and two 605i thermo-hygrometers.

This is a 4 ton unit with a proper charge (right at 11.6 subcool like the Infinity stat calls for) a 0.45 TESP and it’s been running for 30 minutes at high stage. You might be tempted to think something is wrong with the measurement or the unit, but we need to look closer.

You will notice pretty quick that my indoor temperature is low (68.3db)with a low indoor RH (54%) which equates to a 57 degree wet bulb indoor return.

Also, the outdoor temperature is only 72 degrees DB. In order to tell if 41,000 btu/hr is within range or not we will need to look in detail at the manufacturers expanded performance data located in the product data.

Here is the expanded data for this particular match and we lucked out. My air handler, condenser and suction line size are the match that the rating is based on. In some cases you will need to use a multiplier based on an alternate match or smaller copper sizes which can further reduce the rated capacity and possibly the efficiency as well like in the case of the FE4ANF003 or 002 below.

Now let’s zoom in on the performance data that applies to our actual conditions and see how we did.

The highlighted figure is the closest this chart comes to our actual conditions, though our indoor dry bulb is actually significantly lower than the 75 degree DB on the chart. So now the real world 41,223 btu/hr actually stacks up pretty well with the 42,870 btu/hr on the chart.

All of this to say that when sizing equipment and when testing capacity there is a LOT more to it than just the nominal tonnage in the model #. The only real way to know is to dig into the manufacturer product data and really understand that piece of equipment of equipment.

— Bryan

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