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This basic HVAC and air conditioner troubleshooting step will be bases on refrigeration cycle. Learning what is superheat and subcooling will help. This troubleshooting step is for air conditioning unit problem. It is based upon following the step by step process of listening, looking, measuring, and analyzing the HVAC unit in prescribed order. The process is valid for most air conditioner units; we need to ask the primary question that will lead us to determine the problem the air conditioners have. This air conditioner troubleshooting step will require that you following it with open mind – take it as an opinion. Temperature controls and power supply Temperature controls can be electric, electronic, or pneumatic. (In any case) Nothing will happen until the temperature controls tell the HVAC to start, or if there is no power, this will be the logical starting point for our diagnostic process. Be sure the thermostat or other control is telling for the air conditioner unit to run and it has power to run. Condensing unit The condensing unit consists of the condenser fan, coil, and compressor. The fan must be operating properly and the airflow through the coil must be adequate.Inadequate condenser airflow will cause confusing symptoms to appear in various parts of the air conditioner unit. At this point we need to have the compressor running. It will be checked for proper function later on in the procedure. Evaporating unit The evaporating unit consists of the evaporator fan, coil, expansion device and (in some cases) associated ductwork. Proper fan operation and unobstructed airflow through the coil is essential. Any obstruction to airflow will substantially affect central ac operation and will confuse your diagnostic efforts. Heat Measurements Subcooling and superheat measurements are most accurate under design conditions. In cold ambient air temperatures it is desirable to raise the condensing temperature to at least 100°F before taking these measurements. The discharge measurements will tell us how much refrigerant is in the HVAC system (relatively), where it is, and what it is doing. These measurements are meaningless unless both the evaporator and condenser have adequate airflow. THE FOLLOWING SECTIONS ARE THE PRIMARY STEPS AND QUESTIONS REQUIRED TO ENSURE ACCURATE DIAGNOSIS OF THE SYSTEM These steps and questions must be done in proper sequence. Before we puts gauges or thermometers on a central ac unit it is best to first listen and look . Listen! To what the customer has observed about the operation of the central air unit. To the sounds of the central ac unit. Look! Is the control system telling for the HVAC to run? Is the evaporator fan running? Is the air filter clean? Is the evaporator coil clean? Is the compressor running? Is the condenser fan running? Is the condenser fan turning in the correct rotation and is the fan pitched for the direction? Is the condenser coil clean? Is there any apparent damage to the condensing unit or line set? By looking and listening before we put the gauges and thermometers on, we have eliminated dirty air conditioner part, possible electrical problems and physical damage as the reasons the HVAC unit is not operating properly. Measure! Now it’s time to put our gauges and thermometers on and gather some data to analyze the central air unit. Measure the Superheat, Subcooling, the temperature drop across the Evaporator, the temperature rise across the Condenser, Voltage and Amperage at the compressor. Be sure to insulate the sensing elements of the thermometers measuring the suction line temperature and liquid line temperature. Heating measurements refer to the subcooling and superheat measurements taken at the receiver outlet and the compressor inlet. Keep in mind that a central ac unit which does not have a separate receiver is using the bottom of the condenser for the purpose. Therefore, the condenser outlet on central ac is the receiver outlet. During cold weather the condensing temperature should be raised to approximately 100°F – 110°F to simulate condensing temperature under design ambient conditions. Analyze! After we have gathered the necessary information it is time to analyze the HVAC unit. It is important to remember to analyze the entire central air unit. If we find a problem, do not ASSUME that we have found all of the problem. Is the subcooling TOO HIGH TOO LOW OR NORMAL? Is the superheat TOO HIGH TOO LOW OR NORMAL? Is the temperature drop across the evaporator TOO HIGH TOO LOW OR NORMAL? Is the temperature drop across the condenser TOO HIGH TOO LOW OR NORMAL? Condenser Analysis We start our analysis at the condenser because that is where the expansion device and evaporator get their refrigerant. If there is not enough refrigerant in the condenser, the expansion device and evaporator cannot function properly. Condensers (It may be helpful to review the pH chart and basic refrigeration cycle) The purpose of the condenser is to remove heat from the refrigerant and reject it outside the space being conditioned. When heat is removed from a vapor, it changes state (condenses) into a liquid. As the vapor circulates through the condenser, it loses some of its heat and condenses into a liquid. If the airflow through the condenser is adequate, the efficiency of the condenser (at design ambient) will be determined by how much condensing surface area it has to work in. As shown on the pH diagram basic refrigeration cycle diagram, the first part of the condenser removes superheat, bringing the refrigerant to its saturation point. The second portion of the condenser removes latent heat, causing the vapor to change from the saturated vapor state to the saturated liquid state. The final portion of the condenser removes more sensible heat lowering the liquid refrigerant temperature to a point below its saturation point. This is called subcooling. The condenser also acts as a storage area to excess refrigerant in HVAC units designed without receivers. Most systems are designed to carry excess refrigerant. Remember, subcooling is the result of excess refrigerant. However, if there is too much refrigerant in the high side, excess liquid will back up into the condensing area, limiting the ability of the condenser to reject its heat. In other words, liquid refrigerant will be occupying space in the condenser that should be doing latent heat removal from refrigerant vapor. Because of this the condensing temperature quickly rises and at the same time, the liquid has more time to cool before reaching the receiver outlet. The difference between the condensing temperature and the receiver outlet temperature increases if there is too much refrigerant in the high side of the system. This will result in a high condensing temperature and too much subcooling. Of course, if the hvac unit is way overcharged, it will most likely be shutting down on high pressure safety control. The difference between the condensing temperature, which is read off the gauges, and the receiver outlet temperature which is read from a thermometer attached to the liquid line, is the subcooling temperature. Saturation Temp – Liquid Lines Tem = subcooling The higher the subcooling is, the more refrigerant contained in the high side of the central ac unit. Some subcooling is needed to assure a steady supply of refrigerant to the expansion device, and subsequently the evaporator. Too much refrigerant will reduce the capacity of the condenser to reject heat. Condenser Air Flow If we are following the troubleshooting process, we have checked the condenser fan and coil long before we measured the subcooling; therefore, we assume the airflow is adequate. Is it adequate? What if a replacement fan motor or blade was improperly sized? What if the coil appeared to be clean, but really wasn’t? Perform one more test to confirm condenser airflow. Measure the temperature of the air entering the condenser and the air leaving the condenser. If the airflow is inadequate, the condensing temperature is higher. Since the slower moving air has more time to absorb sensible heat, so the temperature difference across the coil is higher. If the difference between entering air and leaving air is more than 30°F, this indicated low condensers airflow. Most air cooled condensing units in some area are selected based on a 95°F ambient temperature and 120°F condensing temperature. Knowing the capacity of the condenser in BTUH and design CFM, you can calculate the design temperature rise using the formula: ?T= BTUH/ 1.08 x CFM Where CFM is the airflow across the condenser, 1.08 is a constant and ?T is the temperature rise across the condenser. You can then calculate the actual CFM using the formula CFM = BTUH/ (1.08 x ?T), inserting the actual temperature rise for ?T. Subcooling Measurements Measuring Subcooling Most refrigeration gauges have both pressure and temperature scales. Get in the habit of reading the temperature scales rather than the pressure scales start thinking in terms of condensing temperature instead of discharge pressure. This will make diagnosing hvac units a lot easier. Subcooling is measured by subtracting the liquid line temperature at the receiver outlet from the condensing temperature. Keep in mind that we are comparing two temperatures. Either temperature by itself will not tell us what we need to know. Saturation Temp – Liquid Lines Temp = subcooling Be sure the thermometer strapped to the liquid line is adequately insulated. Cold Ambient Temperature Low ambient temperature has the same effect as over sizing the condenser. A hvac unit which has sufficient subcooling during cold weather may be grossly overcharged during warm weather. In cold ambient it is necessary to block off part of the condenser until the condensing temperature is 100°F – 110°F before taking subcooling or superheat measurements. Excessive subcooling Excessive subcooling is caused by non-condensable (usually air), over-charge, or restriction. After determined that the HVAC units does not contain non-condensable, we simply remove refrigerant until the subcooling is no longer excessive. If the central air unit was overcharged, we just took care of it. If the ac unit is restricted, it will show up in the superheat measurements. In most ac units anything over 15°F subcooling should be considered excessive. In those rare central ac units which specify more than 15°F (some as high as 20°F), 15°F will work just fine, but will not be quite as energy efficient. Consult the manufacturers specifications when possible. If in doubt, 15°F is a reasonable limit for subcooling. Non-condensable Under normal conditions, the discharge pressure will tell us the condensing temperature. However, air other non-condensable gases, add pressure to the central air system without raising it temperature, thereby distorting the pressure and temperature relationship of the refrigerant. If possible, pump down the hvac unit and run the condenser fan without the compressor for 15 minutes. Read the temperature of the last coil in the condenser. Then convert the temperature reading to pressure using a pressure and temperature relation chart. Now compare the conversion pressure to the actual pressure. If the actual pressure is higher than the converted pressure, there are non-condensable in the system. If a pump down is not possible, shut down the central ac unit and wait for at least 15 minutes. If possible, run the condenser fan without the compressor running and then perform the previous comparison. If it contains non-condensable, remove them using EPA approved procedure before continuing this procedure. With all the various refrigerants used today, it is possible the central air conditioner system is contaminated with different refrigerants. Excess Refrigerant If the central air unit does not contain non-condensable and the subcooling is more than 15°F, this tells us that the condenser contains excess refrigerant. There may be too much refrigerant in the ac unit (overcharge), or the excess refrigerant has been borrowed from the low side (restriction). Overcharge At this point in the procedure it is not necessary to know whether the hvac unit is overcharged or restricted. We simply remove refrigerant until the subcooling is less than 15°F. If it was overcharged, we have just taken care of it. If it is restricted we will find this when the superheat is checked. Low or No Subcooling Low or no subcooling may be due to low refrigerant charge, a compressor problem, or an expansion device problem. Before charging the hvac system, check the operation of the compressor and expansion device as describe in the following sections. If the central ac system has no subcooling, then by removing excess refrigerant we have, in effect, decreased the flow rate through the evaporator to match the capacity of the compressor. Assuming that the compressor was properly sized, we must question whether the compressor has become incapable of handling the output of the evaporator (inefficient compressor). It is also possible that this ac system was designed to operate without subcooling. At this point we must check the compressor efficiency. Charging the Ac System I had see most service technicians charge refrigerant into a central air system until the sight glass is clear. MOST OF THOSE HVAC ARE OVERCHARGED, filling the bottom passes of the condenser with liquid. This causes the condensing temperature to rise dramatically during warm weather, adding to the power consumption and shortening the life of the compressor. When we start checking subcooling temperatures you will begin to realize just how widespread this problem is. Central ac system should be charged to normal subcooling at design ambient and, then, on cap tube systems the charge should be trimmed to achieve normal superheat. Expansion Device Analysis When we add heat to a liquid it boils or, in other words, evaporates. This is the purpose of the evaporator. The liquid refrigerant circulating through the evaporator absorbs heat from the conditioned space, and in doing so, becomes a vapor. For the evaporator to do its job, it must have a steady supply of liquid refrigerant form the condenser. This is why we check the subcooling before measuring the superheat. Whenever the liquid evaporates (reaches complete vapor saturation), the vapor temperature will rise above the saturation temperature. If there is less refrigerant in the coil, the evaporating temperature will drop and, at the same time, the vapor temperature will have more time to rise before it reaches the compressor inlet. In a properly designed hvac unit, the compressor is designed to pull just enough refrigerant vapor out of the evaporator to maintain the desired saturation temperature. Typically 40°F for air conditioning units. When the evaporator cannot produce enough vapor, either due to inadequate heat transfer or lack of liquid refrigerant, the compressor will still pull the volume of its cylinder (s). When this happens the pressure in the evaporator is reduced and, based on the pressure temperature relationship of refrigerant, the temperature is also reduced. When the central ac unit is low on refrigerant there is not enough liquid available to cover the area in the evaporator that was designed to be covered. Therefore, superheat starts taking place in the evaporator closer to the expansion device than it was designed to. Look at the pH chart at the basic refrigeration and picture moving the refrigerant cycle to the right and down to get an illustration of what is happening. If there is more refrigerant in the coil, the saturation temperature will rise, while at the same time, the vapor temperature will have less time to rise before it reaches the compressor inlet. Then the difference between the saturation temperature and compressor inlet temperature (superheat temperature) will tell us if there is not enough refrigerant in the low side of the hvac unit (high superheat) or too much refrigerant (low superheat). The superheat must be low enough to flood the evaporator and cool the compressor, but not low enough to flood the compressor. Cold Ambient Temperatures Low head pressure (low condensing temperature) will reduce the rate of refrigerant flow through the metering device, causing the superheat to be higher than normal. Metering devices are designed to provide enough refrigerant at a specified pressure drop across them. In cold ambient it is necessary to simulate condensing temperature to design ambient conditions by blocking off part of the condenser until the condensing temperature is 100°F – 110°F before taking superheat (or subcooling) measurements. Measuring HVAC Superheat Most refrigeration gauges have both pressure and temperature scales. Get in the habit of reading the temperature scales rather than the pressure scales. Start thinking in terms of saturation temperature instead of suction pressure. This will make diagnosing hvac units a lot easier. Superheat is measured by subtracting the evaporator saturation temperature from the suction line temperature taken approximately 6 inches from the compressor inlet. Keep in mind that we are comparing two temperatures. Either temperature by itself will not tell us what we need to know. Suction Line Temp – Evaporator Saturation Temp = superheat Be sure that thermometer mounted to the suction line is adequately insulated. Heat Load The superheat temperature tells us if there is excessive or inadequate refrigerant in the low side of the hvac units. Anything which adds heat to the evaporator coil will boil off more of the refrigerant, thereby increasing the superheat. Conversely, anything which interferes with the coil’s ability to absorb heat will lower the superheat, possibly flooding the compressor. For this reason we check the evaporator fan motor and coil before measuring the superheat and recheck the superheat after reaching design space temperature. Evaporator Airflow Since we are following this HVAC troubleshooting process, we have visually determined that the airflow through the evaporator is adequate. Is it, really? Let’s test to confirm evaporator airflow. Measure the temperature of the air entering the evaporator and the air leaving the evaporator. If the airflow is inadequate, the evaporating temperature is lower than design and the slower moving air has more time to cool, so the temperature difference across the coil is greater. If the difference between entering air and leaving air is more than 20°F, the airflow is inadequate. Note: Colder air leaving the evaporator means less refrigeration, not more. One way to check air flow across the evaporator is to put the hvac units in the heat mode and raise the thermostat to a point that will bring the heat on. Place the fan switch in the on position. This should place the fan motor in the high speed setting if the motor is variable speed. Next measure the temperature rise across the heat exchange. Find the BTUH output on the furnace name tag. Insert the known values in the following formula to solve for CFM. CFM = BTUH/(1.08 x ?T) For example, if the furnace is rated 100,000 BTUH output and the actual temperature rise is 50 °F, the CFM would be 100,000/(1.08 x 50) = 1852 If you are working with resistance heat, you can convert the Kw of resistance heat to BTU by multiplying the Kw x 3.414 BTU/KW. For example, 10 Kw X 3.414 BTU/Kw = 10,000 X 3.414 = 34,414 BTU’S. How much air is too much and how much is too little? Most furnaces have a design temperature rise noted on the nameplate for heating. For example, one may see a range of 35° - 36° on the nameplate. This means that at maximum air flow in the heating mode the temperature rise would be 35°F and at minimum flow it would be 65°F. For air conditioning, the air flow through the furnace should be close to or above the maximum heating air flow. A good rule of thumb for air conditioning air flow through the evaporator is 400 CFM per ton of refrigeration. Conditioned Space Temperatures Warm air flowing through the evaporator coil increases its ability to absorb heat and therefore increases its capacity. When the conditioned space temperature is above its design temperature range we must expect the superheat temperature to be higher than normal. This is particularly true on cap tube ac unit. For this reason we must recheck superheat measurements after the hvac units has reached its design temperature range in order to obtain true readings. Suction Line Insulation Suction lines should be insulated before measuring superheat. By insulating the suction line we eliminate condensation on the suction line and help keep the refrigerant vapor temperature low enough to provide adequate compressor cooling. Exceptions We may assume that a compressor is refrigerant cooled until proven otherwise. One exception is the open compressor, which does not need cold vapor to cool its windings since its motor is separate from the compressor. Another exception is a hvac units that has a suction/liquid heat exchanger. By purposely installing a device which adds superheat to the suction line, the manufacturer is telling us that the compressor is not as sensitive to inlet vapor temperature. High superheat High superheat is caused by undercharge (leak), or restriction. If we add refrigerant until the subcooling is normal and the superheat is still high, the hvac units is restricted. Note: for diagnostic purposes we may consider an underfeeding metering device to be a form of restriction. If the superheat is no longer high, the air conditioner unit was undercharged. Locate and repair the leak before proceeding. In most central air unit anything over 30°F superheat at the compressor should be considered too high. In general, lower superheat is better for the compressor, however some manufacturers specify higher than 30°F superheat for their central ac unit, usually low temp refrigeration. Consult the manufacturers specifications when possible. If in doubt, 20°F at the compressor, at design space temperature, is a reasonable limit for superheat. High superheat is caused by undercharge or restriction. Restriction By holding back the flow of refrigerant, a restriction causes refrigerant to accumulate in the high side while limiting the amount of refrigerant in the low side; therefore, a restriction is identified by a combination of high superheat at design space temperature with subcooling at design ambient. If we add refrigerant to the central air units until the superheat is 20°F and the subcooling rises above 15°F, the hvac units is restricted. The point of restriction can be found by taking temperature measurements on the liquid line at the condensing unit and the point where it enters the expansion device and/or across each of the liquid line device, such as driers, valves, etc.. If the temperature drop from one end of the liquid line to the other is less than 5°F, the liquid line and its devices are not restricted. Note: A temperature drop across a heat exchanger is normal. If the temperature drop across the liquid line is more than 5°F, check for a temperature drop across each of the liquid line devices. If the liquid line devices are not restricted and there is no temperature drop, the restriction is in the liquid line. Check for kinks in the line or an undersized line. If the temperature drop across the liquid line minus the drop across the heat exchanger is less than 5°F, the metering device is restricted. Restricted Metering Device Once we have traced the restriction to the metering device, we must determine whether the device is in fact restricted or simply out of adjustment. In a cap tube central ac it is definitely a restriction, but in a TXV ac unit the inlet screen should be examined before blaming the valve. If the screen is clear, the problem is in the TXV. Unless you have reason to believe that the TXV has been improperly adjusted, replace the TXV. Long Suction Lines Generally speaking, TXV may be unable to maintain accurate control with less than 3°F superheat at its sensing bulb. On a hvac units with a long suction line this could make it impossible to maintain less than 30°F superheat at the compressor inlet. If the compressor is air cooled, higher superheat at the compressor is permissible. If the compressor is refrigerant cooled, it will need a de-superheating metering device. This is an extra metering device which feeds a small amount of liquid refrigerant into the suction line near the compressor. Undercharge Undercharge of refrigerant is identified by a combination of high superheat and low or no subcooling. If we add refrigerant until the superheat at the compressor is 20°F at design space temperature and the subcooling does not exceed 15°F, the hvac unit was undercharged. Unless you have reason to believe that the hvac was not charged properly, it is reasonable to assume that there is a leak. Locate and repair the leak before continuing the procedure. Flow Rate The amount of refrigerant in the low side is also affected by the rate of refrigerant flow through the metering device. Anything which slows the flow rate will decrease the amount of refrigerant, thereby increasing the superheat, and anything which increases the flow rate will increase the amount of refrigerant, thereby decreasing the superheat. Since the condensing temperature (discharge pressure) affects the rate of flow, we checked the condenser fan motor and condenser coil, raised the condensing temperature to simulate design ambient, and checked the subcooling before measuring the superheat. Insufficient Superheat Low superheat (floodback) is caused by overcharge, overfeeding metering device, or inefficient compressor. If we remove refrigerant until the superheat is normal, and the central air still has measureable subcooling, the central air was overcharged. If the hvac units has no subcooling, check the compressor efficiency as described later. If the compressor is OK, the metering device is overfeeding. Low Superheat (cap tube system) Although we have determined that the amount of refrigerant in the air conditioner systems is not excessive for the condenser (less than 15°F subcooling), on a cap tube system this may be excessive for the evaporator. In other words, an overcharge is still possible even though the subcooling is not excessive (under 15°F), on a cap tube hvac units. Anything less than 15°F superheat at the compressor should be considered too low (floodback). If we remove refrigerant slowly until the superheat is normal (over 20°F), and we still have measureable subcooling, than the hvac units has enough refrigerant to satisfy the needs of the evaporator and has refrigerant in reserve (subcooling) to handle heavier heat loads. In other words, the hvac units was overcharged, but is now functioning normally. Low Superheat (TXV system) Like the cap tube system, anything less than 15°F superheat at the compressor is too low. Unlike the cap tube system, a TXV is capable of opening and closing it’s orifice, thereby regulating the amount of refrigerant held in reserve (subcooling). Removing refrigerant at this point would do nothing more than to deplete the subcooling, while proving nothing. Therefore, at this point we must check the compressor efficiency as described later. Metering Device Overfeed From a diagnostic viewpoint, the main difference between a cap tube system and a TXV system is that a TXV is capable of overfeeding (or underfeeding) the evaporator (and compressor). The sensing bulb must be in the proper location, fastened tightly to the line, and insulated. If this does not solve the problem, and if you have no reason to believe that the TXV has been improperly adjusted, assume that its internal parts are worm out and replace the TXV. Since the TXV is capable of regulating subcooling, be sure to check the subcooling after locating and repairing the problem. COMPRESSOR ANALYSIS Compressor Vacuum Test A compressor efficiency test involves shutting off the flow of refrigerant to the metering device or, even better, the compressor inlet, and the running the compressor to see how far into a vacuum it can pump. If the hvac units does not have valves with which to shut off the flow of refrigerant and you are fairly certain that the compressor is inefficient, pinch the liquid line at the drier tightly with a pinch-off tool to stop the flow of refrigerant. Be careful in doing this. Keep in mind that you will have to repair the liquid line and replace the drier after this test. The question is not whether the compressor is efficient, but whether it is efficient enough to do its job. If the compressor cannot pump at least 15 inches of vacuum, it is not efficient enough for a normal heat load and should be repaired or replaced. If it pumps 20 inches, it is efficient. This test will identify most of the compressor pumping problems but not all. We have tested its efficiency but not its capacity. |