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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