Common AC Problems in an Air Conditioner in Houston
COMMON AC/REFRIGERATION PROBLEMS, SYMPTHOMS, DIAGNOSIS
Condenser Sub-cooling – is defined as the different between the measured liquid temperature and saturation temperature at a given pressure. A forced air condenser should have from 6 degree to 10 degree F of liquid sub-cooling if charged properly.
Compressor Superheat – should always be between 20 degree and 30 degree F. This ensures that the compressor will not see any liquid refrigerant at lower loads. This superheat range will also prevent compressor overheating and high amp draws.
Suction Line Restrictions
A restricted suction line will cause low suction pressures and a starved compressor and condenser. A starved compressor will lead to low compressor amp draw because of its lightened load. Restricted and/ or dirty suction filters are the major cause of suction line restrictions
Compressors are responsible for circulating refrigerant throughout a system; therefore, inefficient compressors will decrease the heat transfer ability of an air conditioning system.
Air and water vapor are probably the known noncondensibles in a refrigeration or air conditioning system. Noncondensibles usually enter a system through poor service practices and/ or leaks. A technician forgetting to purge hoses can let air and water vapor into a system.
Restricted Metering Device
A restricted metering device acts similarly to a liquid line restriction that occurs after the receiver. A restricted TXV will cause the evaporator, compressor, and condenser to be starved of refrigerant, causing low suction pressures, high superheat, low amp draws, and low head pressures
Oil may gather in the evaporator because it is the coldest component with the largest tubes, thus the slowest refrigerant flow rate. Oil in the evaporator will coat the inner wall of the coil and reduce heat transfer through the walls, causing a loss of capacity and poor performance. The compressor will be robbed of some of its crankcase oil and run with a lower than normal oil level. This may score or ruin mechanical parts in the compressor.
Restricted Air Flow Over Evaporator
Whenever the evaporator coil sees reduced airflow across its face, there is a reduced heat load on the coil. No airflow will cause the refrigerant in the coil to remain a liquid and not vaporize. This liquid refrigerant will travel past the evaporator coil and eventually reach the compressor, causing compressor damage from flooding and/or slugging.
Inefficient Compressor from Bad Valves
A system that has bad valves will have a high evaporator (suction) pressure along with a low condensing (head) pressure. There is no other situation that will give a system both low head and high suction at the same time other than worn piston rings causing blow-by of gases around the rings. Anytime the gauge reads low head with high suction pressure, there must be a valve problem, worn rings, or cylinder damage.
HVAC/R TROUBLESHOOTING DATA SHEET
ITEMS TO BE MEASURED MEASURED VALUES
Compressor Discharge Temperature
Condenser Outlet Temperature
Evaporator Outlet Temperature
Compressor Inlet Temperature
Box Temperature(Room Temperature)
Suction (evaporator) Pressure
Discharge (condensing) Pressure
Expansion Valve inlet Temperature
ITEMS TO BE CALCULATED CALCULATED VALUES
Flow Rate of Refrigerant
Note: Convert Refrigerant Pressures to Temperature
HVAC/R SYSTEM OPERATING RANGES
Evaporator Superheat–should always be between 8 to 12 degree F for Air conditioning/Refrigeration and Heat pump.
Condenser Sub-cooling 6 degree F to 10 degree F
Compressor Superheat 20 degree F to 30 degree F
Evaporator Superheat 8 degree F to 12 degree F
Max Compressor discharge temperature 225 degree F
Recommended Low side Temperature For R-22 35 degree to 40 degree F (62 to 70 psig)
Approximate Discharge Pressure: Ambient temperature + 30 degree F converted to pressure
Condenser split 25 degree F to 35 degree F
Condensing temperature = High side pressure converted temperature
Condenser Split = Condensing temperature – Ambient temperature
Condenser Sub-cooling = Condensing temperature – Condenser Outlet temperature
Evaporator Superheat = Evaporator Outlet temperature – Evaporator Refrigerant Temperature (Pressure Converted to Temperature)
Compressor Superheat = Compressor Inlet temperature – Low side pressure (converted to temperature) High Superheat—No cooling to the compressor motor
Low Superheat—Liquid refrigerant going to the compressor
High Sub-cooling—Too Much refrigerant, Restriction in the liquid line, Bad Valves
Low Sub-cooling—Low Charge, Restriction
SIZING REFRIGERANT PIPING
The three prime considerations in designing a refrigerant piping system are.
A- The cost of the tubing installation.
B – Friction losses. (Pressure drop)
C -Oil return.
A. Cost is an obvious consideration and dictates that the smallest tubing possible be used that will result in, a system with acceptable friction losses.
B. Pressure Drop or friction losses are important from a performance standpoint. The following general statements point out the effects of pressure drop in the various components of the refrigerant piping system.
1 – Pressure drop in the suction line reduces system capacity significantly and increases power consumption per ton. The most generally accepted value for pressure drop in a suction line is a pressure drop equivalent to 2 Degrees F. (approx. 3 PSI with R-22 in the air conditioning range of evaporating temperatures).
2 – Pressure drop in hot gas lines reduces system capacity to a somewhat lesser degree and increases power consumption to a slightly lesser degree than does pressure drop in suction lines. Since the only hot gas lines we are concerned with are in heat pump systems where they also serve as suction lines, we will treat them as suction lines.
3 – There is no direct penalty for pressure drop in a liquid line provided that 100% liquid is being delivered to the expansion device, and that the liquid pressure available to the expansion device is adequate to produce the required refrigerant flow. Pressure drop due to vertical lift must be added to the friction loss in liquid lines to determine the total pressure drop. The acceptable pressure drop in a liquid line (standard system without a receiver) is 35 P.S.I.
C. Oil return must always be considered since some oil is continually being circulated with the refrigerant and must be returned to the compressor. If the recommended suction line sizes are used, no oil return problems should be encountered with split systems and no traps are required if tubing sizes are kept within the recommended limits.
The following pages cover brazed liquid and suction lines for split cooling systems, connecting lines for split heat pumps and quick-attach systems (cooling and heat pumps). It is recommended that the user read the entire book to better understand the charts and tables used therein. The chart on page 5 together with the examples given enables the user to calculate the maximum allowable pressure drop in liquid lines, the pressure drop due to friction, and the pressure drop due to vertical lift. (The velocity of the liquid refrigerant in liquid lines is also indicated.)
QUESTION AND ANSWER
John, Pittsburg PA
Question: What could cause the discharge pressure higher and at the same time the current draw on the fan motor is lower than system specs?
Jacob, Hampton VA
Answer: John, first let’s talk high discharge pressure situation. For most instances high discharge pressure should not affect the condenser fan motor amp draw significantly except for instances where the condenser is blocked or dirty. Below are some examples:
Dirty/blocked coils–high discharge pressure–high fan motor current draw
Liquid line restriction–high discharge pressure–normal fan motor current draw
Noncondensibles–high discharge pressure–normal-slightly high fan motor current draw
Overcharge–high discharge pressure–normal-slightly high fan motor current draw.
However, if the condenser panels are taken off or missing while the unit is running the discharge pressure will rise and the condenser motor amp draw may be lower than normal. The discharge pressure will rise due to the fan not rejecting sufficient heat. The fan amp draw is lower due reduced resistance–air is being pulled through openings where panels are missing instead of through the condenser coil.
If you’re looking for an air conditioner in Houston TX please give us a call today at (281) 376-5600