A/C System Testing Basics
Use Critical A/C System Tests to Promote Your Business and Build Trust /Credibility with Customers
By Russell Harju – Product Manager, Fieldpiece Instruments

A couple years ago a co-worker saw an ad in the paper for an $80 A/C tune up.  Although we have several certified technicians on staff at Fieldpiece, sometimes it’s just easier to have somebody that tunes A/C systems everyday to work on our home systems.  He came out, replaced the filter, cleaned the condenser and hooked up his low side gauge to the suction line.  He looked at his pressure gauge and told our guy (his customer,) “That’s about right, I’ve been doing this twenty years.”

No questions were asked, he received his $80, a mental note was made to tune our A/C systems ourselves and off he went.  The unit in question was a R-410A system with a TXV.  These systems are properly charged using the subcooling test which is done on the liquid line side of the air conditioner.  As you may expect, there was very little comfort that the air conditioner was working any better than before he got there.

The next day the head engineer from Fieldpiece Instruments came out and performed a full efficiency analysis on the system.  He was testing out some of the new products Fieldpiece was developing and tuning up the A/C in the process.  The long and short of the analysis was that the system was starved of refrigerant and a healthy dose of R-410A needed to be added.

Proper testing in the field is one of the important jobs for a technician to perform and the only way to diagnose system problems.  Knowing how and when to perform the tests and what to do with the results is essential to being a good technician.  In this article I’m simply giving a refresher course on critical A/C tests, namely superheat, subcooling and target evaporator exit temperature AKA temperature drop.  This overview gives technicians the basic “knowledge tools” they needs to get his job done easier, faster and at a higher quality.

Superheat - Target and Actual


Superheat is the temperature rise in the refrigerant after it boils in the evaporator.  The refrigerant leaves the throttle valve (fixed orifice, capillary tube, TXV) and enters the evaporator as a low pressure, low temperature liquid.  As air from the building is blown across the evaporator, the heat is taken out of the air and added to the refrigerant.  As heat is added to the liquid refrigerant the refrigerant boils and changes to a gas.  The temperature at which a substance (in this case a refrigerant) changes from a liquid to a gas is known as the boiling point or saturation temperature.  The change of state happens in the evaporator.  As the refrigerant continues out of the evaporator and onto the compressor heat is added to the refrigerant.  The added heat to the refrigerant between the boiling point and the compressor is the superheat.

Superheat is used for properly charging fixed orifice systems.  Superheat is important for two reasons:

  1. The first reason is to know if the evaporator is doing its job and the A/C system is running efficiently.  If there is no superheat, very little heat is being taken out of the return air and hence the air being supplied to the space is the same temperature as the air being returned to the evaporator.
  2. The second reason is to save the life of the compressor.  The compressor is designed to compress (add pressure) to gas refrigerant.  It is not powerful enough to compress liquid refrigerant and it will burn up the compressor.  It’s like pushing on a brick wall.  The compressor is also designed to be cooled by the refrigerant it compresses.  When the superheat is too high the compressor is working at higher than normal temperatures and can cause it to overheat.

To measure the superheat, first you place a thermocouple (there are pipe clamps specifically designed for this) on the suction line near the compressor and record the measured temperature.  Next attach a pressure gauge to the suction line service valve, measure and record the reading.  Using a pressure/temperature chart or pressure temperature calculator find the boiling point (saturation temperature) of the refrigerant at that specific pressure.  Subtract the saturation temperature from the temperature measure from the thermocouple and that is the total system superheat.

We now have the actual superheat but how do we know that is an acceptable superheat?  Using the target superheat table from California’s Title 24, which can be found at www.energy.ca.gov/title24, we can look up the target superheat based on the indoor and outdoor conditions.

Important: Rules of thumb are out.  Between 8 and 12 degrees of superheat is out.  Sophisticated target superheats derived by the experts are in!

Measure and record the ambient air temperature going into the condenser.  Make sure to measure in the shade and get the average temperature going into the condenser.  Next measure the wet bulb temperature going into the evaporator.  Place the thermocouple in the center of the airflow in the duct near the evaporator.  If using a wet sock thermocouple, watch the temperature stabilize before recording it.  Use the outdoor ambient air temperature and the return air wet bulb to look up the target superheat on the chart.

Compare the actual superheat to the target superheat.  If the actual is greater than the target, add refrigerant.  If the actual is less than the target, remove refrigerant.  Adjust the charge until the actual superheat is within ±3 degrees Fahrenheit of the target superheat.  How much refrigerant to add or remove will be based on the size of the system and how far off the superheat is.  A good way to dial in the correct charge is by using a digital superheat tool such as Fieldpiece’s ASX14 accessory head, or SSX34 superheat subcooling standalone meter.  This way you can watch the superheat live rather than continually recalculating the superheat.

Subcooling and Systems Regulated by TXVs


A TXV (Thermal Expansion Valve) is a device between the condenser and evaporator that regulates the pressure of the refrigerant and how much refrigerant is let into the system.  It is designed to keep the superheat constant and because of this a TXV system is charged using subcooling.  Subcooling is the temperature decrease after refrigerant condenses in the condenser.  The refrigerant leaves the compressor as a high pressure, high temperature gas.  Air is blown across the coil, removing heat from the refrigerant.  This is the same heat that was added to the refrigerant in the evaporator coil and it is now being moved outside where it is not making anyone uncomfortable.  As heat is removed from the refrigerant in the condenser the refrigerant condenses and changes to a liquid state, known as the condensing point or saturation temperature.  As it travels to the TXV valve it is still giving off heat and decreasing in temperature.  The temperature decrease after the condensing point is the subcooling.

Much like superheat the subcooling needs to be adjusted to the proper level in order to ensure maximum capacity of the condenser and efficiency of the system.  If there is too much subccoling the refrigerant condenses too early and wastes the capacity of the condenser.  Too little subcooling and the refrigerant does not fully condense and enters the TXV as a liquid/gas mixture with causes low system efficiency.

To measure the subcooling, attach a thermocouple to the liquid line after the condenser (again a pipe clamp is recommended) and record the measured temperature.  Next attach a pressure gauge to the liquid line service valve, measure and record the reading.  Using a pressure/temperature chart or a pressure/temperature calculator, find the condensing point (saturation temperature) at that specific pressure.  Subtract the condensing point temperature from the actual liquid line temperature to find your actual subcooling.

Important: Unlike superheat, the target subcooling varies from system to system.  Manufacturers usually provide a target subcooling chart on the name plate of the system.  If it is not on the name plate, call the manufacturer.  If you cannot find a target subcooling, as a last resort use 12 degrees Fahrenheit of subcooling.  Compare the actual subcooling to the target subcooling.  If there is too much subcooling, remove refrigerant.  If there is too little subcooling, add refrigerant.  Adjust refrigerant charge until the subcooling is within 3 degrees of the target subcooling.

Target Evaporator Exit Temperature - Diagnosing Potential  Airflow and Capacity Problems


TEET_Situ_WetBulb_brighter.jpgAirflow is crucial to properly running A/C systems.  A good way to check airflow is to look at the air temperature exiting the evaporator vs. the target evaporator exit temperature.  To find the target evaporator exit temperature measure and record both the wet bulb and dry bulb temperatures going into the evaporator.  California’s Title 24 provides a chart for the proper temperature drop across the coil.  Use the wet bulb and dry bulb temperatures to find the temperature drop on the chart and subtract this from the actual dry bulb temperature entering the evaporator.  This figure will be the target evaporator exit temperature.  To find the actual evaporator exit temperature, use a thermocouple and measure and record the air temperature exiting the evaporator at the center of airflow.

Compare the actual evaporator exit temperature to the target evaporator exit temperature.  If the actual is greater than the target the airflow could be too high or more likely the evaporator has a low capacity.  If the actual is less than the target there is a low airflow.  Adjust the airflow until the actual evaporator exit temperature is within ±3 degrees Fahrenheit of the target.



Proper Testing vs. “Winging It”


Proper testing is just the start for quality service and can ultimately lead the technician to more business and make more money.  Going back to the tech that was hired last year, if he were to have demonstrated proper testing practices he would have been at my house this year getting my A/C unit ready for this summer.  Following the simple, yet critical, practices outlined above can also be used to sell yourself as a “green” technician because they will make every unit you touch more efficient – saving the customer money and reducing overall energy demand. And we need that more than ever with the ever-increasing price and demand for energy. The lower the demand, the less we all pay (just look at gas prices).
Customer service is also a way to gain more business.

If you can create customer loyalty by using these tests to educate your customer, they will likely call you back next year for routine maintenance or next time they want a new system.  If they trust you, they are yours...forever.

There are endless ways to sell and market your services but proper testing is the only place to start.

Russell Harju is a product manager at Fieldpiece Instruments and a certified HVAC technician. He can be reached at 714.634.1844 or at This e-mail address is being protected from spambots. You need JavaScript enabled to view it

 

Back