The Diagnostic Method

The Diagnostic Method

This diagnostic method is a generally accepted diagnostic method used by the HVAC industry. Measuring temperatures and pressures and doing calculations and comparing the outcomes of the calculations to what are acceptable operating ranges is what makes the technician skilled in refrigeration cycle diagnosis.

What are acceptable operating ranges for the important performance indicators, evaporating temperature, superheat, subcooling and condensing temperature over ambient? There is a wide variation in the outcomes of the diagnostic methods used by skilled technicians because of differing opinions as to what is an acceptable operating range, what is the meaning of an indicator and what are the standard service procedures used by the company or the technician.  There are technologies available to help.  Selecting a technology that has been tested by an independent third party is important.

Some objective criteria when determining acceptable operating ranges

Evaporating temperature- If the evaporating temperature gets much below 32°F, the condensate on the coil can start to freeze.  If the evaporating temperature gets much above 47°F, the system is very likely to not control humidity.  A low efficiency unit under low load may have an evaporating temperature goal near 32°F and a high efficiency unit under high load could have an evaporating temperature goal at the upper range.  Keep the coil cooler if humidity control is important and try to maintain higher evaporating temperatures when humidity control in unimportant

Superheat- High and low superheat leads directly to premature compressor failures.  With fixed orifice equipment, the superheat expectation is found on the units charging chart.  Expect higher superheat goals when it is cool outside or when there is a large load inside.  With TxV equipment, the superheat expectation is around 20°F

Condensing temperature over ambient- Normally expect the COA to be under 30°F. Higher efficiency units have lower COA expectations.  The COA expectation of fixed orifice equipment is strongly influenced by the load on the evaporator.  Low COA is an indication that relatively little heat was absorbed on the low side.

Subcooling- Subcooling is the best way to charge TxV equipment.  The subcooling goal for TxV equipment is a manufacturer provided value. If the manufacturer’s specification is not available, the generally accepted best guess is 10°F.  The normal range across all makes and models is 5°F to 20°F.  That is not to say that anything between 5°F to 20°F is acceptable, but that there is equipment where the goal is 5°F and others that have goal values as high as 20°F.  With fixed orifice equipment, subcooling is useful in troubleshooting, but the expected value is not stated.

The expected values for the performance indicators evaporating temperature superheat, condensing temperature over ambient and subcooling given above cannot be considered precise for any individual machine.  The range of acceptable performance for low efficiency equipment at low loads has no overlap with the range of acceptable performance values for high efficiency equipment under high loads.  I use a technology developed by my company, Field Diagnostic Services Inc. (https://www.fielddiagnostics.com/).  There are other methods used by other people.  Independent third-party research has proven that not all fault detection and diagnostic methods are equally effective. The task of considering all the variables that go into determining acceptable goal values is probably not something people can do without a technological tool.

Diagnostics

If you can achieve the superheat and subcooling goals within an acceptable range of evaporating and condensing temperature, the system is likely to operate dependably over a wide range of conditions. Use fault detection and diagnostics when the superheat and subcooling goals cannot be achieved within the limits of acceptable evaporating and condensing temperature.

There is an alternate concept of superheat and subcooling that can be useful when puzzling out refrigeration cycle problems.

An alternative concept of superheat is that superheat is the relative amount of refrigerant on the low side, given the heat transfer capacity of the evaporator. What does that mean?  It means that if you want to lower the superheat you can add refrigerant, or you can lower the heat transfer capacity of the evaporator. How would a technician lower the heat transfer capacity of the evaporator? A technician can lower the heat transfer capacity of the evaporator by reducing the fan speed or restricting the airflow in some way..

An alternative concept of subcooling is that subcooling is the relative amount of refrigerant on the high side, given the mass flow rate of the refrigerant through the system. What does that mean? It means that if you want to raise the subcooling you can add refrigerant, or you can restrict the mass flow rate of the refrigerant through the system. How would a technician restrict the mass flow rate of the refrigerant through the system? While some technicians rightly point out that adjusting the expansion valve is a way of adjusting the mass flow rate, most technicians understand that it takes a greater than ordinary amount of skill and time to effectively adjust a TXV, and that is often not the answer to the problem anyway. The TxV is a superheat controller and should be adjusted to the superheat goal and not be used to adjust subcooling.  Having said that, an expansion valve that has be miss-adjusted to stay open may not allow subcooling to become established, almost regardless of how much refrigerant is added to the system.

Technicians don’t generally add restrictions to a system as a service procedure. However excessive restrictions to refrigerant flow are characterized by high subcooling in combination with high superheat and low evaporating temperature.  What this leaves us with is that when considering the level of charge in a system, subcooling is driven by adding and removing charge.

So, if working on a TXV system, charge to achieve the desired subcooling and then adjust the fan speed to get the superheat correct. When working on fixed orifice equipment, set the fan speed and charge by superheat.

When doing fault detection and diagnostics, the conditions we are left with is superheat can be high and superheat can be low and subcooling can be high and subcooling can be low.

Some examples

             

Let’s consider the case where the subcooling is high and the superheat is also high. If we apply the alternate idea about subcooling, we see that when there is high subcooling, there is a lot of refrigerant on the high side backing up into the condenser; when we add the alternate superheat idea we see that when the superheat is high, there isn’t much refrigerant on the low side. A technician may think of this as an undercharged low side and an overcharged high side.

What kind of problem would we have if the system has a lot of refrigerant on the high side and not much on the low side?

Refrigerant flow restriction

In order for an air conditioner to operate properly, refrigerant flow through the metering device (which ultimately is capacity) needs to match the amount of heat absorption capacity of the evaporator and it needs to keep up with the pumping capacity of the compressor.

The diagnostic method described here is more sensitive to restrictions than the commonly held belief that refrigerant flow restrictions cause a temperature drop at the point of the restriction.  It’s true that if the restriction is severe enough to result in a pressure drop large enough to cause the refrigerant to start to flash, or become a vapor, a temperature drop will be noticeable.  With a more sensitive detection method, restrictions will be diagnosed that will require a decision about when and whether to fix the restriction. Sometimes, with relatively minor restrictions, it’s better to overcharge the system, raising the head pressure and increasing the flow through the restriction a bit and live with the slight restriction because the cost to repair restrictions is high. Restrictions cause reduced capacity and reduced efficiency of the system.

             

Severely restricted refrigerant flow makes it impossible to have enough flow to pressurize the evaporator (low suction pressure). Restricted refrigerant flow makes it impossible to cool down the suction line (high superheat).

Under-charged

Another case where the superheat is high is when the system is under charged.  This time however, the subcooling is low. Using the alternate ideas of superheat and subcooling, low subcooling means a lack of refrigerant is on the high side. High superheat means too little refrigerant on the low side. What would cause that? A restriction causes the superheat to be high by not allowing the refrigerant in the system to flow into the evaporator and causes the subcooling to be high by filling the condenser with the refrigerant.  When the system is undercharged, the superheat is high for the same reason; the refrigerant is not flowing into the evaporator.  The difference is that the reason is for the refrigerant flow problem is the fact that there is just not enough refrigerant in the system, as confirmed by the low subcooling calculation.

Under-charge can cause greatly reduced system capacity but it has a smaller effect on the reduction in efficiency of the system because under-charged systems have very low head pressure.

             

Very low refrigerant mass flow because of under-charge also makes it impossible to have enough refrigerant flow to pressurize the evaporator (low suction pressure). Low refrigerant flow again makes it impossible to cool down the suction line (high superheat). This time however, the subcooling is also low.

Over-charged

             

This is the case when the superheat is low and the subcooling is high. Using the alternate ideas of superheat and subcooling, high subcooling means a lot of refrigerant is on the high side. Low superheat means too much refrigerant on the low side. What would cause that?

Excessive refrigerant charge fills the condenser with refrigerant resulting in high subcooling and high condensing temperature over ambient, or as technicians would say “high head pressure”.  Excessive high side pressure increased the pressure drop across the metering device.  This over-feeds the refrigerant into the evaporator and over-cools the suction line. This will be seen as low superheat.

Overcharge has a much smaller impact on capacity, but the elevated high side pressure caused by overfilling the condenser with refrigerant increases the amount of energy used by the compressor to pump against that high head pressure; thus  reducing system efficiency.

             

Low side heat transfer problem, miss-matched equipment or an overfeeding metering device

             

That leaves us with the last case; low superheat and low subcooling. The system with low subcooling is undercharged, however a system with low superheat it is running too cold on the low side. You can’t remove charge, as you might want to do to raise superheat, because the subcooling is already low and you can’t add charge to raise the subcooling, because the superheat is low. Maybe charge isn’t the first issue that needs to be dealt with here.

Low side heat transfer problem

The refrigeration cycle cannot tell why there is insufficient heat being absorbed.  It might be that there is not enough air flowing across the evaporator.  That may be caused by the fan running too slow.  It also may be caused by a restriction to airflow caused by dirty filters.  Airflow can be restricted by undersized ductwork or too few return or supply grills.  There may be dampers in the ductwork that are closed.  A dirty evaporator coil also causes the system to absorb too little heat given its refrigeration capacity.  The dirt on and inside the evaporator not only restricts air flow, but dirt is also an insulator.  Insulators are not desirable in a heat transfer system.

A low side heat transfer diagnosis is the proper diagnosis whenever the evaporating temperature is low and the superheat is low, without regard to subcooling.  When the subcooling is also low, this just indicates that a previous technician tried to resolve the low superheat condition by removing charge and then leaving the job without solving the problem.

Low superheat and low subcooling is not a single problem. It is called a “multiple coincidental faults” situation.  Meaning that more than one fault exists at the same time.  Some would argue that nearly every system has this condition.  When the fault or combination of faults become severe enough to be noticeable as a comfort problem, or has the performance indicators out of acceptable ranges they must be dealt with.

Solving multiple coincidental faults

Since all systems may have multiple coincidental faults at some level, the resolution is to break the problem up into pieces.  The principle I would propose is to do the maintenance activity first, then make adjustments, and if that isn’t effective, you’re left with a hard fault.

Do your maintenance first

This means if heat exchangers, the evaporator and condenser are fouled, clean them.  If the fan has a belt drive, check the belt and sheave for wear, replace dirty filters.  Make sure the condenser fans are running and running in the right direction. Doing your maintenance first is an important principle.  It starts the process of actually solving the problem, instead of just covering it up or adapting the system to the problem.

Then make adjustments

This means that when the fundamentals are resolved and the system can operate properly, make the necessary adjustments to the system so it does run properly.  This means if the subcooling is low, add charge.  If the subcooling is high, remove charge.  When the subcooling is correct, meaning something like the right amount of refrigerant is in the system, there still might a superheat or evaporating temperature or condensing temperature over ambient problem.  At least you know it’s not a charge problem.

Then tackle the next issue.  If the condensing temperature over ambient is high, look into the condenser fans and look at the condenser again.  Some condenser coils have two or ever three condenser coils sandwiched together.  They need to be split apart to clean them.  If the superheat is still low, look into the air flow.  Does the fan need to run at a higher speed?  Is there dirt crusting the blower wheel? Is the ductwork adequate?

There is a situation that rarely occurs but can happen.  This is when no matter how much refrigerant you add to the system, no subcooling is established.  This can be a real puzzler if you never saw it before.  In that case, the refrigerant flow through the metering device must be so excessive that no liquid can stay in the condenser.  In a correctly designed system with an adjustable TxV, the TxV may be adjusted to almost no spring pressure allowing the valve to stay wide open all the time.  Adjust the TxV to attempt to resolve that.  We will discuss adjusting the TxV in the next installment of this series.

Hard faults

If the fundamentals are right and adjustments aren’t solving the problem, there may be a hard fault.  Hard faults are problems that are only resolved by replacing something.  There are three common hard faults.  The compressor can be inefficient, the can be a refrigerant flow restriction or there can be non-condensables, or contaminants in the refrigerant.

Inefficient compressors

Inefficient compressors or compressors with broken pistons or valves will have very high suction pressure and low head pressure.  If an inefficient compressor is suspected, do an efficiency test by closing a valve and stopping refrigerant flow while running the compressor.  If the compressor has a suction service valve, use that. Otherwise close the liquid line valve.  If no valves are available the efficiency test cannot be performed.  With the valve closed an intact compressor will pump down into a vacuum.  Many technicians like to see the compressor pull down to 20 inches of vacuum.  When the compressor pumps down, shut it off.  The intact compressor will hold the suction side vacuum.  If the compressor does not pump down or hold the vacuum the cause may be an inefficient compressor. 

There are exceptions to this that must be investigated before arriving at a final diagnosis. A compressor that has cylinder head unloading or hot gas bypass unloading is designed to reduce its pumping capacity.  Those systems must be disabled prior to testing.  If the compressor pumps down, but the low side pressure slowly rises, pump it down again. 

If each time you pump it down the pressure rises more slowly and raises less, you are very probably pulling refrigerant entrained in the oil out.  This means the compressor was slugging, meaning liquid refrigerant was entering the compressor.  Continue to pump the system down to “clean” the liquid out of the oil and then find the root cause of the low superheat operation.

Non-condensables

Non condensables are materials that contaminate the refrigerant.  Common non-condensables are air, nitrogen and water.  The most common cause of non-condensables is sloppy service and installation procedures.  A system must have the air removed from it with a vacuum pump prior to charging.  Proper evacuation procedures include replacing the oil in the vacuum pump prior to use on each system and using a vacuum gage to measure that a deep enough vacuum was achieved.  Detailed evacuation procedures are a topic in themselves.

Testing for non-condensables means having the all refrigerant in a system, to the extent possible, at the saturation temperature. This means the system should have not been operating for long enough that the liquid line is at the ambient temperature, and in the case of split systems, the evaporator coil is at return air temperature.  When that condition exists, measure the suction line temperature and suction line pressure and compare them to what the T-P chart says it should be.  Almost all contaminants in refrigerants have higher vapor pressures than refrigerant.  If the pressure in the system is higher than the chart says pure refrigerant will be at that temperature, there are non-condensables in the refrigerant.  Take measurement errors into consideration.  The rule I use is that if the pressure measured corresponds to a temperature 3° or higher than the current measured temperature, non-condensables are confirmed to a level significant enough to cause a high head pressure condition.

Restrictions

High superheat and high subcooling indicate a restriction to refrigerant flow.  The problem is determining where the restriction is.  The most common places for a restriction are the liquid line drier or the metering device.  From a contractor’s business perspective, since the cost of removing the refrigerant, evacuating and re-charging is so high, the prudent course of action is just to replace them both when that is practical.  There are many good reasons what that might not be practical in any given case, but replacing one without the other increases the risk of spending all that time and money and not resolving the problem.

Summary

Refrigeration cycle diagnostics requires collecting the six important measurements.

SP - suction pressure

LP - liquid pressure

ST - suction temperature

LT - liquid temperature

AMB - outdoor ambient temperature

RAWB- return air wet bulb temperature

From these measurements we can calculate these performance indices:

COA (condensing temperature over ambient)

ET (evaporating temperature)

PD (pressure drop across the metering device)

SH (suction superheat)

SC (liquid subcooling)

The problems a working technician sees most commonly.

1.    There can be too little heat absorbed into the low side.

2.    There can be too much heat absorbed into the low side

3.    It can be too hard to reject heat from the high side.

4.    There can be too much heat rejected from the high side

5.    There can be too little refrigerant in the system

6.    There can be too much refrigerant in the system

7.    There can be too little refrigerant flow through the system

8.    There can be too much refrigerant flow through the system

9.    There can be contaminants in the refrigerant

10. The compressor can be pumping less than it was designed to

An effective diagnostic method eliminates the need to guess and produces reliable and efficient HVAC units.