Troubleshooting: How Stuff Works…and Breaks

So, maybe you’re thinking troubleshooting heat pumps is about Ohm’s Law, psychrometric charts, Mollier diagrams, sensible heat ratios, COP’s and…etc., etc…? There are a lot of scientific principles explaining how heat pumps do what they do. And at some point in our training we eventually have to deal with some of the science in understanding heat pump operation…electricity, magnetism, thermodynamics, fluid flow dynamics…stuff like that. But fluency in scientific principles generally won’t get you to a diagnosis. I can remember a time early in my career when I could recite a lot of science to heat pump owners, but couldn’t tell them why it wasn’t working like it’s supposed to…and then there’s the engineer who posted the following comment at this website:

“..Brilliant blog! I’m just starting to wade through this stuff. Ended up here after looking for answers on diagnosing reversing valves…I’m the engineer learning how to be the tech (geothermal) for the last 3 years and I find practical/applied stuff like this invaluable. Your strength is being able to recall the time when you didn’t understand the problem and relating to that person now trying to figure it out…”

So, just what does one need to know, in order to troubleshoot? Let’s look at a hypothetical situation…

If you walk into the bathroom and flip the light switch, most of the time the light comes on. That’s a simple result of an enormous amount of science and engineering. In order for that light to come on, Ben Franklin had to discover electricity, Edison had to invent the lightbulb, Tesla and Westinghouse had to develp AC power generation…but all you and I have to know is simple circuitry: when the light switch closes, a circuit is completed allowing voltage to be supplied, causing current flow which makes the light burn. And as long as the light comes on each time the switch is flipped, we don’t generally give it a second thought.

But when the light doesn’t come on, what are the options? Well, you could go to the breaker panel and check for a tripped breaker or even take a  meter and confirm the breaker is actually transferring voltage to the circuit. If that didn’t reveal the problem, you could move to the switch and see if there is a problem there…voltage in, voltage out. If you still don’t find the problem, checking the fixture for voltage would be the next thing to do. If after all that, you haven’t found the problem, then your last test would be continuity through the bulb…

But what do we usually do when the light doesn’t come on, after the  switch is flipped? Just change the bulb, right? And why do we change the bulb  before doing all that other stuff? Because most of us know the bulb is the weak link in the chain, the most likely candidate for failure. And how do we know that? Simply from years of replacing bathroom lightbulbs…

So the conclusion of the story is…? Troubleshooting residential heating and cooling equipment is, most of the time, all about knowing how stuff works and how stuff breaks. In the  bathroom light scenario, we use our knowledge of how stuff breaks to get us to a quick and probable diagnosis. But on the few occasions where the bulb isn’t the problem, we can use our knowledge of how stuff works and “test” our way to a solution.

Let’s look at another example closer to reality for us:

Suppose you go on a 10 SEER / R-22 system service call in July, and after making some pressure and temperature measurements, come up with the following numbers:

Suction pressure… 58 psi

Head pressure…….200 psi

Superheat……………2 F

Subcooling………….15 F

An experienced technician can look at the numbers and immediately see a problem with the suction pressure / superheat values,  know what the problem is and the most likely cause…low airflow across the indoor coil, due most likely to a dirty coil. His conclusions are part science and part experience. Normal suction pressures are closer to 70 psi by design with normal superheats closer to 10 F. Most 10 SEER equipment is fixed orifice, meaning superheats  vary with operating conditions. Low suction pressure, with low superheat, is the classic symptom of low evaporator air. There’s not enough heat available to boil off the refrigerant inside the coil, resulting in less refrigerant vapor produced and excessive liquid remaining in the coil. Dirty coils being the cause is mostly a product  of experience.

Knowing how things work tells us what heat pumps are supposed to do and what it takes for them do it. With that knowledge, you can always figure out what’s wrong with a failed system. Knowing how things break quite often tells you where to start the troubleshooting process…

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For Technicians Only…

..and I should add, techs who are a little weak in diagnostics. We always credit experience for expertise in some skill, regardless of the skill. Experience has always been the best teacher whether you’re working with heat pumps or airplanes…but experience is a somewhat generalized term leaving the inexperienced individual with little assistance in improving his skills level. In preparing presentation material recently for some “Troubleshooting Heat Pumps” seminars, I  came up with a better way (I believe) to illustrate just what experience teaches us, relative to troubleshooting.

It helps to understand the mentality needed for troubleshooting equipment as opposed to building equipment. Heat pump technology is a product of several sciences. But a heat pump unit is the application of those same sciences. And if the unit was designed and manufactured around the scientific principles, it will do what it’s supposed to do…the science is built-in at the factory. So, as technicians, we aren’t required to verify or confirm the science, just the designed operation. So…whether or not we really understand all the implications of the Second Law of Thermodynamics isn’t really an issue. The Second Law of Thermodynamics may explain in scientific terms how or why a heat pump can cool / heat the house, but it won’t tell you the filters are dirty…So just what is it we need to know in order to troubleshoot equipment? For lack of a better expression, I’m going to say, knowing how stuff works, or knowing what stuff is supposed to do. And then you can extend that definition a little and add, …and what it takes to make it do it.

As an illustration, the electricity / magnetism principles that explain why/how a motor “runs”, won’t really help you diagnose a motor that isn’t running…if it isn’t running, some of the science is obviously missing. And if it isn’t running, then it’s not doing what it’s supposed to do. If you know or can figure out what it takes to make it run, you can diagnose the failure….and what does it take to make it run, or do what it’s supposed to do? Voltage source? that’s a good place to start. Closed switch? yep. Unbroken wires/circuit connecting the voltage to the switch to the motor? absolutely. Decent bearings? a must. Good capacitor? of course.

So if you find a motor not doing what it’s supposed to do, now you know what items to test, check, etc., to determine why the motor isn’t doing what it’s supposed to do. Isn’t that far simpler than than trying to measure or analyze the magnetic fields? I would think so…

I believe you will eventually discover diagnostics is more about understanding what the engineers intended the equipment to do, than understanding the scientific principles behind just how it does it. So, when the situation has you stumped, think in terms of what the device, part or system is supposed to do and what might keep it from doing it. Some skill with that approach will help you get you through the service calls…

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TROUBLESHOOTING HEAT PUMP SYSTEMS…INDOOR / OUTDOOR TEMPS VS SUPERHEAT W/ FIXED ORIFICE SYSTEMS

..If you look at a superheat charging chart for a fixed orifice system, you quickly see the required superheat varies with outdoor and indoor conditions. As the outdoor temperatures vary, so does the required superheat…pretty much the same relationship for indoor temperatures. Why? The net force pushing liquid through the metering device is the difference in the head and suction pressures, more or less. And I would guess the designers figure in some maximum outdoor temperature in conjunction with some minimum indoor temp and come up with a minimum superheat value for “worst case” scenarios.

The point being, if the outdoor temperature is 75F you don’t won’t want a “beer can cold” suction line…because by the time the afternoon temperature hits mid-90’s, the increased head pressure will have increased the “net force” pushing the liquid through the orifice, and the system will be overcharged, resulting in a lower than desired superheat.

Likewise, if the indoor temps are “high”, superheats will be high. Most charging charts use indoor wetbulb as the control variable, since wetbulb temps include the humidity factor. As indoor wetbulb goes down, the superheat will decrease, everything else being equal. The following clip demonstrates variations in superheat with outdoor conditions.

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TROUBLESHOOTING HEAT PUMP SYSTEMS…OPEN CONTROL VOLTAGE CIRCUITS

One of the more frustrating and difficult situations with heat pump diagnostics is open circuits in the control wiring. There is a logical process to follow when attempting to locate the failure…

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TROUBLESHOOTING HEAT PUMPS SYSTEMS…REFRIGERANT LEAK DETECTION DEVICES

No doubt, my most frustrating service issue has been locating refrigerant leaks. And I’ll be the first to admit it was due to my own ignorance, from simply not doing a little research. I started out with a cold sensor technology electronic detector, bought a second cold sensor electronic detector and eventually concluded electronic detectors were pretty much worthless, at least for my desires and needs.

Next , I let someone talk me into the ultrasonic detector method. I never found the first leak using it. In fact, I couldn’t find a leak in my truck tire with the thing…so much for ultrasonics.

When I discovered the fluorescent dyes, I thought my leak search headaches were over. And to a certain extent, locating some leaks did prove to be much easier. So long as the black light would shine on the leak area, and it was reasonably dark around the suspect area, and the dye was actually coming out of the leak, I was in pretty good shape. But then there’s the waiting period between injecting the dye, and actually seeing it exit the puncture…and the mess…and all the paraphernalia required to actually locate a leak…

At some point I walked into my favorite wholesale house and told the manager, “Today is the day I buy my last leak detector…if it doesn’t do what I need it to do, I’m just gonna’ slit my wrists, and let my wife collect the insurance…” I bought another electronic detector with heated sensor technology…I had done a little research. That turned out to be one of the finer moments in my service career. It worked so well and was so reliable, I didn’t believe it for a while. But once I finally gained some confidence with the tool, my leak search issues were mostly a thing of the past. I can find most leaks now about as fast as I can access the equipment…especially those pesky indoor coils. Add to the detector’s capabilities my knowing where to look, and my batting average is close to 1000. The biggest problem I’ve had recently was a 410A leak that didn’t want to sniff out well.

Before I get too many people overly irritated with my conclusions, let’s back up a minute or two and pay some due respect to the aforementioned devices and methods. I’m sure there is some useful purpose for electronic detectors that use cold sensor technology. I just don’t believe it’s the residential sector. They will indeed detect refrigerant…but they also detect other stuff, so you never know for sure if the alarm is refrigerant or some other unknown something.

The ultrasonics are revered by some folks, who claim good success in finding leaks. I’m not gonna’ call those same folks liars.

The dyes are absolutely an option for some situations. If for whatever reasons you need to pinpoint the location of a leak, that’s the way to go, unless you want to try the bubble solutions.

But for me, most of the time, I just want to know if a coil is leaking, or an accumulator, or a service valve, or a liquid line filter or whatever. If the coil is leaking, I’ll replace the coil…if the accumulator is leaking, I’ll replace the accumulator.

Most of the repairable leak sources are visible via oil deposits. The electronic detector will usually get you in the general vicinity, and the oil, along with some bubble solution will show you the target.

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