HVAC Buddy
Superheat and Subcool
The best way to properly charge an AC system is to use Superheat or Subcooling. Use Superheat on systems with a fixed capillary (orfice) restrictor. Use Subcooling on systems with a TXV/EXV valve. Used together Superheat and Subcool values are helpful for diagnosing certain conditions. For example, on a TXV equipped system knowing the Superheat value can help you determine if the TXV valve is functioning correctly and the evaporator coil is getting the proper amount of refrigerant.
Superheat and Subcooling can be measured independently but some diagnostic suggestions can be done when both values are available. Superheat and Subcool are just one component of servicing a unit and are a waste of time if other factors such as proper airflow, coil cleanliness, etc. have not been addressed.
Tools Needed
• Refrigerant gauges
• Wet and dry bulb thermometer
• Thermocouple or method of measuring line temperature
It's desireable to have the manufacturer's superheat or Subcool chart specific to your unit. The chart may be with the unit or on the manufacturer's website. In the event the chart cannot be located generic tables are used to calculate target superheat and Subcool values. Again, make sure the coils are clean and airflow is good.
Taking Measurements
Take readings when the system is in a steady state condition, usually after it has been running for 15 to 20 minutes. Make sure the unit stays on during testing by setting a low temperature on the thermostat. If the system temperatures and pressures are no longer noticeably changing the system is at steady state.
To obtain valid measurements try to take all temperature and pressure measurements within a short period of time. Outdoor afternoon conditions can change significantly affecting your target superheat subcooling number over a thirty minute period.
Imperial and Metric
In the upper right hand corner of each tab are two buttons labeled Imperial and Metric. Use these to toggle back and forth. All values are instantly updated and redisplayed. In "Imperial" mode temperatures are displayed in degrees Fahrenheit and pressures are in pounds per square inch (psi). In "Metric" mode temperatures are displayed in Celsius and pressures are displayed in KiloPascals (kPa). All calculations and table lookups are done in their respective units of measure. If you decide to enter Manufacturers Superheat and Subcooling targets these are absolute values. So if you enter "10" as a target Superheat in Imperial mode and decide to switch to Metric mode your "10" will be left alone and not converted to Celsius.
Reset Slider
If the slider is at the edge and you still need to move the values "Reset Slider" adds the currently displayed value to the base value and re-zeros the slider.
Calculating Superheat
When calculating Superheat you will need to measure indoor wet bulb temperature and outdoor dry bulb temperature. Take indoor wet bulb measurement as close to the coil as possible. You may take the temperature just outside the return air duct. Wet Bulb means the bulb of the thermometer is kept wet with pure water and a cloth covering the bulb to keep it wet. Take Dry Bulb or ambient temperature of the outside air entering the coil. Keep the thermometer out of the sun.
Target Superheat
Using wet and dry bulb temperatures look up the target superheat on the manufacturers chart. If a chart is not available a target value is automatically calculated from your inputs using an internal generic chart. The Superheat chart range is valid for wet bulb temperatures from 50°F (27.7°C) to 80°F (44.4°C) and outdoor ambient temperatures from 40°F (22.2°C) to 115°F (63.9°C). Values entered outside these ranges will be displayed with a yellow dot. Target superheats can vary from 5°F (2.8°C) to 45°F (25°C) depending on the conditions. Higher loads demand higher target superheat.
1) Enter Indoor Wet Bulb Temp
2) Enter Outdoor Dry Bulb Temp
3) Enter the Refrigerant Type
If you do not know or are unsure of the manufacturers target superheat value the entry can be left at zero. Leaving the value at zero indicates you want to use the calculated target superheat. Any non-zero value will cause that value to be used for target superheat. If your indoor and outdoor temperatures are outside the range of the lookup chart the values will be displayed with a red dot. If within range a green dot is displayed.
4) If known, enter Manufacturers Target Superheat
Determine the pressure and temperature of the refrigerant going into the the compressor. This is the pressure of the suction, evaporator, or low side as shown on the blue dial. Take the temperature of the line as close to the compressor as possible but at least 6 inches away. Best results can be obtained by making sure the line is clean and using a thermocouple wrapped with insulation held in place with Velcro.
5) Enter the Suction Pressure
6) Enter the Suction Temperature
Target vs. Actual Superheat
At the top of the screen you should now see three values. Target Superheat at the top; either the manufacturer's number or one derived from a formula. Immediately below is Actual Superheat, the actual calculated superheat value given the input pressures, temperatures, and choice of refrigerant. The third value is the difference between target and actual superheat. Ideally this number is zero but can be 3°F (1.7°C) to 5°F (2.8°C) off in either direction. A green dot next to the difference indicates the system is within the 3°F (1.7°C) target. Yellow indicates it is within the 5°F (2.8°C) target and may be acceptable. A blue or red dot indicates superheat is outside the acceptable range and may be undercharged (blue) or overcharged (red).
Adjusting Values
As you are working with temperature and pressure changes adjust adjust them in either direction using the sliders located in the upper left side of the screen. If the change is greater than five and the slider is at the limit re-enter the value.
Subcool
Navigate to the Subcool screen by pressing the Subcool tab. The refrigerant, indoor, and outdoor temperatures are carried forward from the Superheat screen and may be updated if needed. In the next step best results will be obtained by using the manufacturers data for target Subcool.
7) Enter Manufacturers Target Subcool
If the target Subcool data is not available a generic table is used as a fallback. The Subcool chart range is valid for wet bulb temperatures from 57°F (31.7°C) to 73°F (40.6°C) and outdoor ambient temperatures from 75°F (41.7°C) to 115°F (63.9°C). Values outside these ranges will display with a yellow dot and the value at the limit is used. Manufacturers determine target Subcool recommendations based on several factors not known in the program such as lineset length, compressor design, etc. which will not be reflected in the generic table.
Liquid Leaving Pressure and Temperature
Next determine the discharge pressure and temperature of the liquid leaving line of the condenser. This is the pressure as shown on the red dial. Best results can be obtained by making sure the line is clean and using a thermocouple wrapped with insulation held in place with Velcro.
8) Enter Leaving Liquid Line Pressure
9) Enter Leaving Liquid Line Temperature
Again you see three values. Target Subcool is at the top, either the manufacturers number or one derived from a table. Immediately below is Actual Subcool, the actual calculated Subcool value given the entered pressures, temperatures, and refrigerant. The third value is the difference between target and actual Subcool. Ideally this number is zero but can be 2°F (1.1°C) off in either direction. For accuracy use the manufacturers recommendation.
A green dot next to the difference indicates the system is within the target. A blue or red dot indicates Subcool is outside the target and may be undercharged (blue) or overcharged (red).
As you are working with the unit to get the proper charge the resulting temperature and pressure changes can be reflected in the application by moving the sliders located in the upper left side of the screen. The sliders can make a maximum adjustment of 5 units. If the required change is greater than five re-enter the value. Be sure you allow enough time for the system to settle down adter adding refrigerant before drawing any conclusions.
Airflow
The airflow tab is designed to help evaluate system airflow by comparing a targeted temperature difference across the coil versus the actual difference. The target value is obtained by taking dry and wet bulb temperatures as close to the coil as possible. These values are then used to look up a targeted value from the publicly available table RA3.2-3 of the California Energy Commissions document CEC-400-2008-004-CMF. The targeted value is then compared against the actual temperature split (return dry - supply dry) to come up with a differential. This is also called Delta-T or Targeted Evaporator Exit Temperature (TEET). The table has certain temperature limitations that are adhered to within the application. As such, the sliders for airflow have fixed ranges that do not allow you to exceed the given ranges. To calculate airflow:
8) Enter the Return Wet Bulb Temperature
9) Enter the Return Dry Bulb Temperature
10) Enter the Supply Dry Bulb Temperature
The differences are then calculated and displayed. You may use the slider to adjust values. If the values are within 3°F (1.7°C) then a green dot will show. If the airflow is high a blue dot will show (unlikely), red indicates low airflow.
System Status Report
The email tab lets you email the values entered along with calculated results. Some basic information about the client and location can also be entered. These values are saved from session to session. This allows you to set yourself or your company up as an email recipient for establishing a permanent record. You can also enter text in the form of a system status report to be included in an email to the client. This is also retained from session to session.
The first screen on the email tab is the "Defaults" screen. Every field on this sheet is used in generating the email. The actual email is created by pressing the "Send Email" button in the upper right hand corner. Once this is pressed the defaults are combined with the actual data from the application in tabular form. You may perform additional edits, such as entering any nameplate information or comments about the service. You may also add any email addresses from your contact list to the distribution.
Diagnostics
The diagnostics section is intended to give qualified service personnel a starting point to identify problems. It is neither all-encompassing nor can it pinpoint exactly the problem and may indicate problems which do not exist. It is not a substitute or replacement for accurate diagnosis and repair by a technician.
A black icon to the left of the diagnostic means there is not enough information to calculate and diagnose a result. Go to the relevant page and fill in the missing values. Missing data is indicated with red icons.
Green icons mean the check did not find anything, rows with red or blue icons mean a value may need attention. Select these rows for more detail.
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