Understanding Superheat and Subcooling

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Superheat and subcooling are the terms used to describe two of a heat pump system’s operating characteristics. We, in the service business, generally rely on these numbers to evaluate system performance as well diagnose system problems. The values essentially provide us with information about what’s going on inside the evaporator and condenser coils. And depending on the metering device used in the system, one or the other number is the value used to determine optimum system charge.

If you wanted to define the words non-mathematically, superheat is the increase in temperature of the refrigerant vapor in the evaporator before it exits the coil, and subcooling is the decrease in temperature of the refrigerant liquid in the condenser before it exits the coil. The diagram below offers a visual illustration.

The two numbers are actually calculated temperature values, using simple arithmetic with saturated temperatures and tubing temperatures.

Normal operation always results in some percentage of the evaporator coil filled 100% with vapor and some percentage of the condenser coil filled 100% with liquid. Since the vapor starts out at the same saturated suction temperature, the vapor will take in heat or warm up, before it exits the evaporator coil. Likewise, the liquid starts out at the same saturated condensing temperature, so it will give up heat or cool down before exiting the condenser coil.

So to calculate the superheat, subtract the saturated suction temperature from the suction line temperature. Which in the diagram is 50 – 40 = 10 F superheat.

To calculate subcooling, subtract the liquid line temperature from the saturated condensing temperature: 110 – 100 = 10 F subcooling.

You can get more explanation of heat pump charging details in the refrigerant system video for rent:

Troubleshooting Heat Pump Refrigerant Systems

 

Charging TXV Systems

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Somewhere in a previous post there is some discussion and pics relative to charging heat pumps in the cool cycle. The method of charging depends on the metering device feeding the evaporator coil. Fixed orifice systems have to be charged by the superheat method, TXV systems by the subcooling method. The video below illustrates charging by the subcooling method…

You can get more explanation of heat pump charging details in the refrigerant system video for rent:

Troubleshooting Heat Pump Refrigerant Systems

 

Replacing A Reversing Valve

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Replacing reversing valves is one of the more taxing repairs to heat pumps. I’m sure every tech has his preferred method for doing it, which probably depends on his personal level of confidence, skills and dexterity with a torch. The primary objective (for me) is getting the new valve in, without destroying the internal heat sensitive components. The valve body can only withstand temperatures approaching 250 F degrees…which ain’t much considering the 1400 F or so necessary to get a good braze joint.

And you have to consider in most cases, the valve is in a position where you can’t just “sweat” it out. There has to be some “slack” available and usually isn’t. Even if you have the special torch attachment that can heat all three tubing connections simultaneously, you still need room to pull the valve away from the tubing. Even if you manage that, you got to consider brazing in the new one without burning it up.

So, I usually study the “geometry” of the valve and try to cut the tubing in places I can easily access on the re-braze…Then, once you get the “assembly” out, you can heat the connections in whatever manner you prefer and remove the old valve.

Then, you only have to braze in the new valve one tube at a time, allowing you to use wet rags or whatever, to keep the valve body cool.

Once you complete that step, using swaged ends or couplings allows you to reconnect the cut joints.

 

R-410A System Pressures

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Heat pump system pressures are indirect measurements of saturated temperatures…the pressures are simply the result of the particular refrigerant used in the system. If you know what the saturated temperatures are supposed to be, you can easily determine the pressures.

Lets assume we’re connected to the R-22 heat pump system below running in the cool cycle, measuring pressures, tubing temps and calculating superheat and subcooling.

So, we’re looking at a head pressure of 225 psi, a suction pressure of 75 psi, liquid line temp @ 100F and suction line temp @ 55F. If we convert the head and suction pressures to saturated temperatures, the results are a condensing temp of 110F in the outdoor (condenser) coil and a boiling temp of 45F degrees for the liquid refrigerant in the indoor (evaporator) coil, giving us 10F degrees of subcooling, and 10F degrees superheat.

Now, suppose for the sake of discussion, we recover all the 22 refrigerant from the system, replace the oil, replace the 22 TXV with a 410A TXV, re-charge the system with 410A refrigerant, and start the equipment back up…Well, guess what? The system temperatures would be the same values (or at least near the same values). The only thing different would be the head and suction pressures.

We couldn’t actually convert a 22 system to 410A that simply…but you could have a 22 system and 410A system side by side, operating under the same indoor and outdoor conditions and see about the same operating temperatures. The gist of the post is to illustrate the fact that heat pumps (mechanical refrigeration systems) are designed to produce or generate, temperatures. The subsequent system pressures are simply the result of the saturated pressure-temperature relationships for a particular refrigerant at a particular temperature…

You can get a full explanation and illustrations of heat pump operating pressures and temperatures in the refrigerant system video for rent:

Troubleshooting Heat Pump Refrigerant Systems

 

Troubleshooting TXV Systems

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This short video illustrates the symptoms of some of the problems associated with TXV systems…can you diagnosis the numbers?

You can see a more in depth explanation of TXV operation and illustrated failures in the “Troubleshooting Heat Pump Refrigerant Systems” and “Troubleshooting TXV’s” rental videos:

Troubleshooting Heat Pump Refrigerant Systems

Troubleshooting TXV’s

 

Superheat…or Subcooling?

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So, here’s the question: How do you charge fixed orifice and txv systems? Same way? No. Fixed orifice systems can only be charged via superheat. TXV systems have to be charged by the subcooling method. The explanations for the answers are related to how TXV’s do what they do and how fixed orifice devices do what they do. Fixed orifice devices deliver a rate of refrigerant flow dependent on the pressure differential across the orifice. TXV’s are active devices that “look” at the suction line temperature and evaporator temperature and maintain a fairly consistent superheat value…Following are illustrations of both. (You can “click” the pics for a slightly larger view)

Fixed Orifice System…R-22

I started a service call on this system knowing there’s a small leak and that the system needed topping off…

An early reading of suction pressure and superheat…

Then I got carried away…”0″ deg superheat.

Removed some refrigerant, and got a number that was about right for the particular indoor and outdoor conditions.

Charging fixed metering systems by superheat is a delicate process, requiring an indoor wetbulb temperature, outdoor drybulb temperature and some “tool” for determining the required superheat for the given system and conditions.

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TXV System…R-410A

This one is a new system start-up.

Early reading…subcooling top right value.

After adding a few ounces of 410A…

You can see a complete explanation of superheat and subcooling in the “Troubleshooting Heat Pump Refrigerant Systems” rental video:

Troubleshooting Heat Pump Refrigerant Systems

 

Overcharged System

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I was called on this unit, found some electrical problems and noticed the compressor sounded not quite right. On checking the compressor amp draw, it was running right at RLA. I messed with it a while and finally connected the gauges and temp sensors. What you’re seeing are the system pressures, saturated temps and liquid line temp. The net result is 50 degrees subcooling.

You can see an explanation and demonstrations of all the common refrigerant system failures in the “Troubleshooting Heat Pump Refrigerant Systems” rental video.

Troubleshooting Heat Pump Refrigerant Systems

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Diagnosing Low Voltage Failures

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The system is a split heat pump, with the air handler in the attic. The HO had reported nothing was running. On arrival I flipped the stat fan switch to “ON” and nothing happened. Neither indoor nor outdoor sections running, usually indicates a low voltage failure or loss of high voltage to the transformer. Since the transformer is usually located in the air handler, that’s where to start the troubleshooting process.

Once I accessed the electrics, the problem was quickly discovered…

Blown fuses all too frequently indicate a short somewhere in the control voltage system. Maybe conductor failure or component failure. While at the air handler I ran some resistance checks and found the resistance low in the “Y” circuit going to the condenser unit.

At the condenser, I accessed the stat wire connections…

This one turned out to be easy. The stat cable had been lying against a section of tubing for a long time. The long term contact between the two eventually led to the insulation wearing through and “grounding” the “Y” conductor to the copper, creating the short and blowing the control voltage fuse.

You can get a full explanation and illustrations of all the common electrical failures in the “Troubleshooting Heat Pump Electrical Systems” videos for rent:

Troubleshooting Heat Pump Electrical Systems

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Liquid Line Restrictions

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I started the service call guessing the problem was low charge, due to a leak somewhere, which isn’t a bad guess. The system is in heat cycle and you can see the frosted distributor lines, which can be caused by low charge. But after adding a pound or so of refrigerant, the suction pressure didn’t come up significantly, so I figured the problem was something else…

After connecting a temp sensor to the liquid line, it was fairly obvious the subcooling was much higher than it should be, which would indicate a restriction in the liquid line somewhere.

After looking over the refrigerant tubing and not finding any temperature drops ahead of the orifice distributor head, I knew the restriction had to be there.

And after disassembling the distributor and accessing the orifice, I found a piece of “something” partially blocking the orifice bore opening.

You can see an explanation and demonstrations of all the common refrigerant system failures in the “Troubleshooting Heat Pump Refrigerant Systems” rental video.

Troubleshooting Heat Pump Refrigerant Systems

(The content of this post is intended for consideration by trained service personnel only)

 

Learning To Troubleshoot…Maybe Not

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We all start out in the service business thinking we have to learn how to troubleshoot and hoping we will some day. And if we stay in the business long enough, we eventually do. But the fact of the matter is, we didn’t really learn to troubleshoot because the truth of the matter is, you can’t learn to troubleshoot. What we end up learning is how stuff works…

If we were to try to define troubleshooting, the definition could be something like:

Troubleshooting is the process of collecting and analyzing information, then drawing conclusions from the information about how a system is operating.

What we’re doing when we troubleshoot is simply comparing the actual operation of a system to the expected or intended operation of the system. When we see the actual operation is different from the intended operation, we do whatever is necessary to determine the cause for the unexpected behavior. Which usually amounts to making some kind of electrical, pressure or temperature measurements. And the only way we can do that is by knowing what the expected or intended behavior is supposed to be, in the first place…knowing how stuff works.

If you can ever learn what heat pumps and furnaces are supposed to do, and what it takes to make them do it (learn how stuff works), troubleshooting becomes an instinctive, or maybe subconscious, response to an unexpected behavior. There’s nothing to learn about troubleshooting.

Let me illustrate what I’m saying…Say we go on a heat pump service call to investigate a “no cooling” complaint. In order for the system to initiate a “call for cool”, what’s supposed to happen?

We know a switch (or switches) inside the thermostat has to close and send 24 volts to the contactor coil, the reversing valve solenoid (with most equipment brands) and the blower motor control. Then the contactor coil is supposed to “pull-in”, completing the high voltage circuits to the compressor and condenser fan motors. The blower motor control also closes a switch, completing the high voltage circuit to the blower motor and the reversing valve solenoid causes the pilot valve to shift, which in turn results in the main valve shifting. If all that takes place as designed, the compressor motor, condenser fan motor and blower motor start, and the reversing valve directs the refrigerant flow in the right direction.

At that point, the intended/expected operation amounts to the correct or design volume of air being moved by the condenser fan and blower wheel, the compressor pumping the design volume of refrigerant, the reversing valve channeling the refrigerant flow in the right direction and the TXV maintaining the correct amount of liquid entering the evaporator coil. And of course, we confirm whether or not all that has happened simply through visual observations, system pressures and temperature measurements.

If our visual observations tell us the condenser didn’t start, we would logically begin troubleshooting for some electrical issue. The first thing we would do is check whether or not the contactor has pulled in. If not, the troubleshooting process begins with an analysis of the low voltage circuits. We would check for voltage on the “hot” side of the contactor coil. If the meter reads zero, we start “backing up” towards the low voltage source, which is the thermostat in this case, looking for the open circuit.

If the contactor has pulled in, then the high voltage circuits are suspect. We would begin by checking for high voltage at the contactor L1-L2 connections for power coming into the condenser unit. If there was no voltage there, you’d begin “backing up” towards the high voltage source, which is a breaker somewhere in the panel box.

In either case, it is our knowledge and understanding of what is supposed to take place, and our ability to reason through information, that tells us where to check for voltages.

If on the other hand, everything appears to be running, you initially begin the troubleshooting process with the assumption there is some problem with the refrigerant system. We confirm that by measuring system pressures, superheat and subcooling.

The expected behavior of an R-22 system would generate a suction pressure in the 70-80 psi range and a superheat in the 10-15 degree range. If you measured 50 psi and 30 degrees, you’d know something is wrong. If you understand what has to take place in order for the numbers to be 70-80 psi and 10-15 degrees, you would know there is not enough refrigerant entering the evaporator coil. And you would also know that has to be due to an undercharged system, a faulty TXV or liquid line restriction. At that point you have to evaluate the head pressure and subcooling to decide which of those possibilities is the problem. If the head pressure and subcooling are low, the problem is low charge. If the head pressure is in the normal range and the subcooling on the highish side, you know the problem is one of the other possibilities.

But to emphasize my point again, we didn’t learn to troubleshoot the problems…our knowledge of what the heat pump is supposed to do, instinctively led us to follow a logical procedure of observations and measurements, which resulted in a conclusion explaining the unexpected behavior of the heat pump.

And restating what I said earlier, once we learn what heat pumps and furnaces are supposed to do, and what it takes to make them do it (learn how stuff works), troubleshooting becomes an intuitive, subconscious response to an unexpected behavior. You just know what to do, without thinking about it. There’s nothing to learn about troubleshooting.

(The content of this post is intended for consideration by trained service personnel only)