On a direct ignition gas furnace service call, you find the furnace goes through the firing cycle as designed until the circulating blower starts and then the flames “blow” out into the area around the in-shot burners. The problem is due to:
a) blocked B-vent
b) failed combustion chamber
c) restricted combustion chamber
d) excess manifold pressure
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The engineering explanations of superheat and subcooling are…well they’re probably above my level of intellect. But for those of us who calculate the numbers for analysis and diagnostic purposes only, they simply tell us how much liquid refrigerant is in the evaporator and condenser coils. And there is an engineered amount of liquid in each coil for a given system. Since there’s no way to directly measure the amount of liquid, we have to use the temperature method. In order to understand how this works you have to visualize what’s going on inside either coil.
Inside the condenser coil, the hot gas enters and begins moving through the tubing, all the while giving up heat to the cooler condenser air. At some point the vapor temperature equals the saturated (condensing point) temperature of the refrigerant at whatever value the head pressure is running. When the vapor reaches that temperature, it condenses to the liquid phase. At some location near the exit point in the coil, the refrigerant volume is 100% liquid. Since it is still exposed to the condenser air, it will give up additional heat and cool down below the condensing temperature. We measure the liquid line tubing temperature and take that value as the liquid temperature. The difference in the liquid and condensing temperatures is what we call subcooling (the definition of “sub” here is “below”). If an R-22 system head pressure is 225 psi, the saturated (condensing) temperature is 110F. If the liquid line temp is 95F, the subcooling is 15F.
Inside the evaporator coil, the liquid enters and begins moving through the tubing, taking in heat from the warmer evaporator air. For the particular operating suction pressure, the saturated temperature of the refrigerant equals the boiling point temperature of the liquid. Whatever heat is absorbed by the liquid is used in changing its phase from liquid to vapor, rather than increasing the liquid temperature. At some location before the exit point of the coil, all the liquid will have changed to the vapor phase, but will continue to take in heat from the evaporator air. So the temperature of the vapor will increase above the saturated refrigerant temperature. We measure the suction line tubing temperature and take that value as the temperature of the vapor. The difference in the vapor temperature and the boiling point (saturated) temperature is the superheat (the definition of “super” here is “above”). So, if our R-22 system suction pressure is 70 psi, the boiling point is about 40F. If the suction line temperature measures 55F, the superheat is 15F.
Contrary to what we sometimes get accustomed to working with, refrigeration systems aren’t designed to operate at certain pressures. They are designed to operate with certain evaporator and condenser coil temperatures. The resulting system pressures are dependent on the design coil temperatures and the particular refrigerant chosen for the particular application. But we can’t easily or accurately measure the refrigerant temperatures inside the coils, so we measure the pressures and convert those values to saturated refrigerant temperatures, using the temperature scales on the gauges or a refrigerant P-T chart.
And as said earlier, refrigeration systems also have design values for superheat and subcooling. If we know the design coil temperatures, the design superheat / subcooling values, the type refrigerant, have a set of gauges and a thermometer suitable for measuring tubing temperatures, we can begin to analyze a system.
To be continued…
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No, it’s not any sort, or form of, sacrilegious intentions…just a unique handle for a very unique guy…my good friend Tony B. He stopped by today and offered up some of his insightful comments at the “R-410A vs R-22″ post. He’s the Founder and Top Gun over at HVACPROTech.com…PROTech is the epitome of HVAC technical forums and arguably the best and most comprehensive you’ll find on the world wide web. I’ve already invited Tony to provide a Guest Post and enlighten everyone about the advantages of membering up at HVACPROTech…in the meantime, I’ll just say when you tire of reading my stuff, you can surf over there and discover a virtual limitless supply of topics, discussions and technical information…
..OK, try this one. Heat pump system (R22) operating in cool cycle. Indoor coil fed by a TXV . Measured suction pressure is 50 psi, superheat is 12F. What’s wrong, most likely, with the system?
Diagnostics, or troubleshooting, is a non-profit activity. There is a service charge that includes the cost of getting the tech to the job, and some time for the diagnosis. Notice I said “some time”…that doesn’t translate to half a day. Now admittedly, even the best of us have the occasional service call that takes more than 10 minutes to diagnose…but if you need an hour or two on every service call, something is amiss. And you’re not gonna’ last too long with that kind of batting average.
I like to define troubleshooting as simply, “finding the real problem, real fast…” and in case you didn’t notice, that definition doesn’t allow the options of guessing, parts swapping or excuses…you might guess and get lucky…we all do it sometimes…but nobody’s interested in whether or not the sun got in your eyes…just whether or not you caught the ball…so, to exist in the service business, sooner or later you have to learn how stuff works and how it breaks, and in some cases, the quickest way to correct the problem.
Seasoned service people develop an “instinct” for diagnosing equipment. Years and years of servicing the same or similar equipment, has given them a sixth sense for knowing what to expect to find on service calls. In spite of all the possible failures that can occur with a system, there is a relatively small number of failures that occur over and over: run capacitors, leaky indoor coils, furnace ignitors, condenser motors… to name a few. Eventually the veteran tech finds himself no longer troubleshooting systems, but simply confirming failures he’s seen before. He will go on a cooling season service call expecting to find low charge, a failed run capacitor or maybe ants in the contactor. So he doesn’t usually start at “square one” in the diagnostics procedure.
This is of course, a huge time saving ability. The newb will likely want to “flex his muscles” by showing off his knowledge and massive array of tools/instruments, and consequently spend a lot of time locating the problem. The senior tech just wants to correct the situation and move on to the next chore of the day, or maybe just go for coffee…
