System Charging Essentials
With consideration to modern, high efficiency comfort systems, proper
refrigerant charge is more important than ever! Symptoms of poor charging can range from higher energy costs to premature equipment failure. Some symptoms are noticeable to the end user while others are much more covert. Manufacturers create a great deal of resources for technicians to ensure their equipment is dialed-in correctly. Some have even gone as far as creating a “self-contained” charging system within the equipment itself. The bottom line is, manufacturers have lost confidence in the technician’s ability to accurately charge the system.
Why do technicians find it difficult to properly and accurately charge equipment? Firstly, there is a lack of education and training. Many technicians learn from the last person on the job. Ten or fifteen-year-old systems are not the same as today; older equipment was much more tolerant of minor charge anomalies. Ignorance, which is sometimes followed by pride, is a dangerous combination. Compressors do not usually die of natural causes; it is a homicide, so keep up with manufacturer training! If there are any questions regarding procedure or refrigerant quantities, call the manufacturer! That is why they supply a technical support line or site. Most companies will provide training classes available at your local supply house.
Here are some of the effects and symptoms of improper system charge:
Slightly under charged: May have little effect on comfort, but annual operation
costs will be higher than expected. The cooling capacity may also be impacted on hotter days. The evaporator will be slightly starved for refrigerant, thereby causing the superheat to be higher than normal. The temperature difference across the coil (the return air versus the supply air), will be lower than expected. The compressor may run hotter internal temperatures, possibly hastening its demise.
Severe under charge: Will adversely impact comfort and cooling capacity,
particularly on hot days. The system’s evaporator will be starved for refrigerant, causing a much higher superheat. The temperature difference across the coil will be very low or close to zero. The evaporator’s saturated pressure will be low, perhaps below freezing, causing the coil to ice up from the bottom up. The compressor’s internal temperature will be much higher, causing the motor to overheat. The compressor’s death will be much sooner than its expected ten-year life span*, and the annual operational costs will be extremely high.
All of the refrigerant is gone: Can be extremely detrimental to the system
should it run in this condition, or if the system is “flat” as a result of a leak. This means there IS air and moisture in the system and it’s killing the compressor. There will be no cooling whatsoever in addition to the damage already done. Many manufacturers are installing low pressure safeties to prevent this occurrence. Low system charge can be a result of a number of factors: A leak could be caused by a failure on the manufacturing side or it could be a result of poor workmanship during or after installation and commissioning procedures. It is highly recommended to prepare a “commissioning sheet”, which is similar to a maintenance report. This is done at start-up to prevent and correct any potential problems with the equipment or it’s installation. If the system requires a charge adjustment, it is done while commissioning the equipment. Many manufacturers require a commissioning sheet to support warranty claims. Searching for leaks can be a costly endeavor for the technician and customer alike. The age of the system and the customer’s financial resources should determine whether a leak search is necessary. A system over ten years old utilizing R22 is a likely candidate for replacement rather than repair. Always gauge these kind of decisions with this question: What will best serve the customer? Although there is no law prohibiting the “topping off” of a system below a certain size**, it is doing the customer a disservice!
As the refrigerant charge depletes, the system will run less and less efficiently,
thus destined for a premature failure.
Over Charged :
Slight over charge: May have little effect on comfort or capacity, and may not
even be noticed with consideration to the temperature difference across the
evaporator. The evaporator, however, will be flooded, driving the expected
superheat down. On cooler days, there is a possibility of flood-back to the
compressor and a low evaporator load. The condenser may also be slightly
flooded. The effect of a flooded condenser will be realized in higher saturated
pressure and high sub-cool temperatures, The efficiency of the flooded
condenser will be compromised, which will undoubtedly result in higher
Severe over charge: Will have a much greater detrimental effect on the system. Comfort and capacity will be negatively affected. The system’s pressure will be much higher than normal, driving the evaporator at a higher temperature. The condenser will also be flooded and barely able to reject heat. There is a greater opportunity for liquid flood-back and fatal compressor damage. The compressor’s amperage will be much higher than expected, and if left unchecked, the system will be destined to fail. Keep in mind this may NOT be covered under any warranty as this is likely not a manufacturer’s failure.
Over-charging a system is actually quite easy. If the ambient temperature is low on the day of start-up, the technician may mistake the low system pressures for low charge. On cooler days, special procedures like covering the condenser coil are necessary to “trick” the condenser into thinking it’s warmer. Another common mistake is not allowing the system to achieve a “steady state”. It is imperative to wait 15-20 minutes after initially starting the system for the pressure and temperature to stabilize. A classic axiom suggests that the suction line needs to be “BCC” (Beer Can Cold) but that measure does not apply to modern equipment. In today’s world we must run a full diagnostic to complete the job.
Charging procedures depend on a number of variables including system type, metering device characteristics, refrigerant class and manufacturers’ requirements. It is always a good practice to consult the manufacturer with regard to their equipment and requirements. Manufacturers such as Trane, have taken steps to improve the performance of their equipment by designing an on-board charging system (Trane’s Charge Assist System), but this is NOT an autopilot method of charging! The technician will still need to be aware of the system’s performance and charge!
As technicians, we should always be on a mission to restore manufacturers’
confidence in our ability to maintain their systems. Most importantly, our
customers deserve our attention to their system’s performance and efficiency.
Keeping these goals in focus can only benefit our industry!
* According to energy.gov
** According to epa.gov/section608