Applying diagnostics to HVAC units in the field

 Applying diagnostics to HVAC units in the field

There is a concept called “the three-legged stool” of refrigeration cycle performance.  What it means is that a unit that is running well balances three factors. These are efficiency, capacity and reliability. 

My experience is that people responsible for HVAC units primarily want reliability.  They bought the unit and are willing to spend money on energy to run it and will even pay to repair it when it is not working.  They generally will not think about efficiency until they have reliability.  They are not interested in any more capacity than they need to maintain comport.  Reliability is the important issue to most people responsible for air conditioners.

When an air conditioner is reliable, those responsible for them often would like them to be efficient.  Efficiency means how much capacity it is delivering for the energy it is using.  Air conditioners that are running poorly very often use about the same amount of energy as those that are running well, they just deliver less capacity. Therefore have to run longer to satisfy the set point and shut off.  The additional runtime uses more energy and reduces the unit’s total efficiency.  There are some refrigeration cycle faults and degradations that cause an air conditioner to use more energy while it’s running.  I have found that the units that are the most efficient tend to also be the most reliable.  My experience is that units that perform as similar as possible to the way they performed when they were new produce the best balance between efficiency, capacity and reliability.

The cost of running commercial HVAC goes well beyond the cost of energy.  The cost to maintain and service the equipments is often considered. Sometimes people consider the cost of lost business or production when the equipment fails.  When all the financial aspects of owning HVAC are analyzed, the best argument for effective maintenance is really extended equipment life.  The cost of maintenance is low compared to the cost of replacing equipment.  The accountants would say that the net present value of a few years of extended equipment life produces an astronomical return on investment from effective maintenance.

Proposed field modifications to equipment to achieve energy efficiency

There are some that might be thinking about ways to modify a unit to make it more efficient.  I hear about schemes like that all the time.  Add something to the oil; put something by the metering device to get more even flow, etc, etc.  I am not a big fan of these.  They may have some demonstrate-able positive impact, but those that are promoting them don’t talk about the costs and negative potential impacts, especially anything that increases the probability of refrigerant leaks.  Field re-engineering equipment, especially by service technicians is not something I would support. 

There are systems for units in hot-dry climates that are being sold now.  They put an evaporative pre-cooler before the condenser in the air stream.  That is not a fundamental change to the system; it merely simulates a cooler ambient temperature to the unit.  But care must be taken with them too.  They require more maintenance, the pre-cooler can get dirty and impacted with minerals from the water, and if it is placed too close to the condenser coil, the water can cause corrosion.  When servicing it, the technician must be sophisticated enough to measure the ambient between the pre-cooler and the condenser coil.  When I think about the cost to install it and run water to it, the cost of the water used, the additional maintenance costs and the consequence in additional energy use if the maintenance isn’t done, and the chance that a technician will not account for it when servicing the unit and charge it badly, I am just not sure it’s worth the cost and risk.  But it may be for some people in some places.

Maintenance program design

On average, about 20% of all equipment has about 80% of the energy savings potential. It has been seen that the units at a facility are often all in about the same condition. They are often the same age, they have been maintained in about the same way and generally would all have about the same run time. So, expect the units at a facility to all be operating more-or-less similarly. This is not to say that there won’t be an individual unit here and there that has a situation that is different than the rest, but if the first few units you work on are running pretty well, it is highly probable that they are mostly running well. If there are lots of problems with the first few, there is a good chance there will be lots of problems all over the roof. 

Having said that, the point remains that finding and repairing the unit with the serious problems is where the major benefit from maintenance comes from.  It is not a good assumption from my experience to expect that a program of doing the same task to all units will produce the most benefit. The most benefit for the least cost comes from evaluating each unit separately and applying the maintenance tasks that are appropriate for the needs of the individual unit and circuit.

The equipment inventory

The maintenance process starts with a survey of all the HVAC units. Good documentation is essential if outcomes are to be reported. The survey should include an equipment inventory. This means the make, model and serial number as well as the equipment age, size and type and it’s characteristics like the refrigerant type, metering device type, fan drive type and if it has an economizer.

When the inventory is complete, a series of observations about the condition of the equipment should be made.  The next series of blog posts I will make are an investigation into maintenance tasking and the use of the ACCA180 maintenance standard. That will go into detail about what condition observations are useful.

Conditions for a valid test

Whenever a technician attempts to diagnose a refrigeration cycle, there are several criteria that must be met before a refrigeration cycle is diagnosable.

The unit must be operating in steady state. This means there must be no condenser fans cycling, no TXV hunting and the compressor should be running continuously for 10-15 minutes prior to testing. A good indication of steady state operation is when the liquid temperature is steady for a few minutes.

The air conditioning unit should be running for some reasonable time before tests begin to assure they are in steady state operation. If there are several units at a facility to test, they may be started and running prior to testing so that they are already in steady state operation when the technician starts to work on them. I call this the “production method.” Jumping out the next few units when testing a unit and then moving quickly to the next is the best way to get many units done quickly while maintaining the quality of the outcomes.

Another important factor is that it’s always best if tests are run with the units fully loaded. This suggestion is not related to the load in the building. A fully loaded unit means that all compressors in the unit are running and that if the unit has the ability to unload, like cylinder head unloading, hot-gas bypass, multiple compressors in the same circuit or multi-speed compressors, that the circuit is fully loaded. There may be times when there just isn’t enough load because of testing on a cool day to run fully loaded.  It is not too much of a problem to test one fully loaded refrigeration cycle with others in the unit off. But if the circuit must be unloaded, a complete test is not possible because the condenser cannot be tested under load.  There may be times when testing part load circuits is required, like when doing service work under lower load conditions.  An improper loading scheme can be a problem that can only be found under part-load conditions.  For maintenance performance testing, only fully loaded circuits can be comprehensively tested.

There are some other things the technician must know clearly when testing units.

Be sure that you know what refrigerant you are using.

Be sure you know the design efficiency of the unit. The design efficiency is expressed as EER or SEER. The EER is preferred for performance testing.  The design efficiency will impact the evaporating temperature and the condensing temperature over ambient expectation.

Be sure that you know what metering device is used.  The diagnostic method is different for TxV equipment and fixed orifice equipment

Be sure that the choice of where the high-pressure information is coming from, discharge or liquid pressure, is taken into consideration when estimating subcooling.

Lastly, there is a range of driving conditions where the unit is testable. The minimum temperature condition is a 55°F ambient temperature and a 50°F minimum return air wet bulb temperature. The maximum temperature condition is a 115°F ambient temperature and a 76°F maximum return air wet bulb temperature. These are the conditions under which the many manufacturers publish performance data.

Performance testing

Performance testing of the refrigeration cycle requires 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 are:

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

There can be too much heat absorbed into the low side

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

There can be too much heat rejected from the high side

There can be too little refrigerant in the system

There can be too much refrigerant in the system

There can be too little refrigerant flow through the system

There can be too much refrigerant flow through the system

There can be contaminants in the refrigerant

The compressor can be pumping less than it was designed to

Some things make refrigeration cycle performance analysis more complicated.

Using indoor air ΔT (RA-SA) in diagnostics

I am not a proponent of using indoor air ΔT as a diagnostic tool.  I understand that it is a calculation that many people like to make and use.  If you choose to use indoor air ΔT I offer these suggestions:

The amount indoor air ΔT, or the change in temperature of the air through the unit is dependent on the units relative capacity, the amount of airflow through the unit, how much of the air is coming from the outside and the humidity or latent load of the air entering the evaporator.  Measuring the airflow through the unit and the amount of the air that is coming from the outside is very difficult to do precisely and accurately in the field.  Understanding the impact of humidity on the expected ΔT also requires advanced knowledge and is probably only practical using a computer.

Extra care should be taken when analyzing air ΔT with a partly loaded machine.

All the latent work must be done before any sensible work can be done. What this means is that a unit running at one half capacity will probably have less than one half the expected ΔT because most of the first stage capacity will be used to condense humidity from the air.  Air going through an inactive coil will dilute the ΔT of the active coil causing a lower than expected ΔT. This effect will mask heat exchanger fouling and low airflow problems in partially loaded units.

Compressors with unloaders

If the unit cannot be, or should not be run fully loaded, be careful on how the diagnostics are interpreted:

Compressors with unloaders that are controlled properly may produce acceptable test results. Compressors that are loaded under low load conditions will sometime show a low side heat transfer problem diagnosis, and sometimes show a restriction diagnosis depending on how well superheat is being controlled. Compressors that are unloaded under higher load conditions will show an inefficient compressor diagnosis.

Compressors running unloaded have, in effect, relatively larger evaporator and condenser coils (compared to the capacity of the compressor) than they will have when the compressor is fully loaded. This will mask heat exchanger fouling and low airflow problems. Another interesting effect is that because the heat exchangers are large compared to the capacity of the unloaded compressor, units with unloaded compressors test as extremely efficient.

Multi-zone equipment

When working on multi-zone equipment, performance testing becomes more complicated. When a VFD is controlling the indoor fan motor, the practical technique would be to allow the static pressure to be controlled as it would be normally and then to diagnose each refrigeration cycle one at a time. This is because multi-zone units are often oversized, running all compressors fully loaded and running the fan to its maximum capacity may be un-natural for that unit.

Conclusions about working on multi-stage equipment and equipment with unloaders under part load conditions

By and large, it is recommended to test equipment under loaded conditions. Technicians working on partly loaded machines can probably feel comfortable with the superheat and subcooling expectations and with the charge diagnosis. The airside diagnosis, especially the “Low side heat transfer” issue as well as the condenser air high ΔT test will be masked by lower compressor capacity. Unloaders that are not set correctly may cause a diagnosis that is misleading.

Making HVAC units run well

The tasks described here, when taken together, comprise what some may call a unit tune-up.  A tune-up consists of performance testing, or documenting the condition of the refrigeration cycle prior to work being done to it, cleaning the evaporator and condenser coils as needed, adjusting the refrigerant charge and airflow if indicated and then testing out, or documenting the performance of the refrigeration cycle after the work is done.

Some simple ideas to make HVAC equipment perform better

High condensing temperature over ambient

One of the most common causes for low efficiency in a refrigeration cycle is a high COA problem. What would we do to drop the condensing temperature?

It depends on what is causing the high condensing temperature, of course. There are three potential causes for high condensing temperatures, or as it is better known to technicians, “high head.” Those three potential causes are a high side heat transfer problem, meaning a condenser fan problem or a dirty condenser coil, an over-charged system, and non-condensables, which is probably air in the refrigerant. Non-condensables is a hard fault.  This means that something, in this case the refrigerant, must be replaced. A unit tune-up, done well, will probably solve any of the problems that can be solved by doing maintenance.

A unit tune-up may include cleaning the coils, adjusting the refrigerant charge and adjusting the airflow through the evaporator. The part of the unit tune-up that is most directly aimed at lowering condensing temperature is condenser coil cleaning.

Low side heat transfer problems

One of the most common causes for low system capacity is a low evaporating temperature. What would we do to raise the evaporating temperature?  Controlling superheat is important to avoid premature compressor failures. What would we do to raise the superheat? Low evaporating temperature and low superheat may both be the result of a low side heat transfer problem.

What would we do to raise the evaporating temperature? What would we do to raise the superheat? One part of the unit tune-up that is directly aimed at a low side heat transfer problems is evaporator coil cleaning. Low side heat transfer problems are among the most common refrigeration cycle problem.  This is because it can be caused by several common problems, and solving a low side heat transfer problem may involve tasks that service technicians don’t like to do and customers don’t like to pay for.  However, low side heat transfer problems cause HVAC units to use more energy and it risks compressor failures.  Compressor failures are a primary motivator for unplanned unit replacements.  Emergency unit replacements are the most expensive way to replace equipment.  This is one of the reasons that effective maintenance that avoids premature compressor failure produces such a high return on investment from effective maintenance.

Evaporator coil cleaning, as many technicians know has many problems and difficulties. The most obvious problem is access. Many evaporator coils in split systems are nearly inaccessible either because the whole air handler is difficult to access or because the evaporator is encased in a plenum or installed in a way where accessing the evaporator for cleaning requires dismantling the entire unit. A common problem in rooftop units when cleaning evaporator coils is that the return air duct is directly below the coil and when water, from cleaning or any other reason, drips down the coil it can run down the return air duct and ruin ceilings or drip directly on people. Another problem has to do with the occupant’s reaction to the smell of the coil cleaner. A third problem is that to effectively clean an evaporator coil there must be access to the face of the coil.  In rooftop units this may mean removing the economizer or taking off the top of the unit.

All these problems are real, but they don’t take away from the fact that if the coil is dirty, the unit will not operate well until it is cleaned. There are ways to deal with all these problems but it might be time-consuming and therefore expensive. However, dirty evaporators cause air conditioners to be unreliable and sometimes kill compressors. Blower wheels can get packed with dirt, too. Dirty blower wheels don’t push as much air as they were designed to.

Another part of the unit tune-up that is directly aimed at low side heat transfer problems is solving airflow problems. The second step in precision tuning is to make adjustments. Adjusting the fan speed to increase airflow is sometimes just a matter of adjusting or replacing a motor shive or replacing the belt. Sometimes return and supply ductwork is the problem, it can be undersized, blocked in some way, or a have a number of other problems. Ductwork problems may not be in the scope of a tune-up and therefore may be considered unfixable for the purposes of this kind of job.

Adjusting refrigerant charge

Once the coils are clean and obvious airflow problems are corrected it may be time to think about the refrigerant charge level. Under-charged systems may appear very efficient because, without any other problems, they will run with low head pressure. Since the compressor uses about 80% of the power used by an air conditioner and all the power used by the compressor is used to pump against head pressure, units with lower head pressures seem very efficient.  They actually may be very efficient if slightly undercharged because the loss in capacity is countered by a reduction in compressor energy use.  From a service perspective the main problem caused by a minor undercharged situation is the range of conditions under which the system will function properly is more limited.

Under-charged units, pose a reliability problem when it’s hot outside.

When the load on the evaporator goes up the refrigerant flow rate goes up; this is because a TxV opens to allow more refrigerant flow.  In a fixed orifice system, the increased head pressure caused by the fact that there is more heat to reject in the condenser because the greater load on the evaporator caused more heat to be absorbed, pushes harder on the refrigerant through the metering device. In either case, when system can no longer deliver an adequate liquid refrigerant supply to the metering device because of an under-charged situation, the capacity of the unit will rapidly diminish and the superheat will spike, endangering the compressor and making it less likely the unit will satisfy the load in the building, making people uncomfortable.

Over-charged units are less efficient and pose a reliability problem when it’s hot and when it’s cold outside

An over-charged system becomes less efficient because of the power it takes to pump against the resulting higher head pressure. The energy problem becomes more noticeable when it is warm outside. When it’s cold outside over-charged units may run with lower than expected superheat, which is dangerous to compressors.

Refrigerant leaks are a common cause of an under-charged unit. Leaks must be repaired if the unit is to maintain its charge. Look for oil as an indication of a leak.

There is only one cause of an over-charged unit: someone over-charged it. Knowing the subcooling goal is critical to properly charging a TxV unit. When working with fixed orifice units, the charging chart requires measuring the return air humidity and the ambient temperature to arrive at the required superheat that is the guide for charging fixed orifice units.

After maintenance and adjustments are made, hard faults may remain. Non-condensables, refrigerant flow restrictions and inefficient compressors greatly reduce both the efficiency and capacity of the refrigeration cycle.

Precision tuning is in one sense, about getting the four performance indices, evaporating temperature, superheat, condensing temperature over ambient and subcooling closer to their goal values. Precision tuning is in the end about creating both energy savings and non-energy savings that come from fewer service interruptions, fewer compressor replacements and longer equipment life. Calculating the value of energy savings is easier than non-energy savings. Effective maintenance is critical to controlling the life cycle costs of operating HVAC equipment.