Tag Archives: hvac training

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.

More discussion: HvacR Professional.Com

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

Troubleshooting Heat Pump Refrigerant Systems

You can post questions and get answers from professionals here:

Hvac/R Professional.Com

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

You can post questions and get answers from professionals here:

Hvac/R Professional.Com

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…

More discussion: Hvacr Professional. Com

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?

More discussion: Hvacr Professional. Com

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

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.

The videos I’ve made available take a practical approach teaching “how stuff works”. If you already have some training in the fundamentals, the video content will help you apply your knowledge to real world situations in the field.

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

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

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..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.

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.

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)

TROUBLESHOOTING HEAT PUMP SYSTEMS…OPEN CONTROL VOLTAGE CIRCUITS

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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…

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

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