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Question & Answer

The Questions

Below are some of the "Questions and Answers" from the most commonly asked technical questions that we receive. Ask your question in the comment section of any video posted on our YouTube channel. To date, we have answered over 12,000 comments/questions posted under our technical training videos. Is your's next?

Charging and Checking the Charge

Questions

Charging and Checking the Charge

 

Why does R-410A have to be charged from the bottle as a liquid while R-22 can be charged as a liquid or gas? 

R-22 is a single component refrigerant whereas R-410A is actually made up of 50% R-32 and 50% R-125. To keep the refrigerant mix of these two refrigerants equal, R-410A must exit the bottle as a liquid.

Why can't you charge liquid refrigerant into the liquid port while the system is running?

While the system is running, the low side vapor pressure is lower than the refrigerant bottle pressure while the high pressure liquid pressure is a higher pressure than the refrigerant bottle pressure. Because of this, refrigerant would exit the system and enter the bottle if the two were connected. This is how we recover a partial refrigerant charge while the system is running while using a reusable recovery bottle. Refrigerant must be charged into the low pressure side while the system is running in order to add refrigerant.

How do you check the refrigerant level inside an air conditioner?

On single or two speed air conditioning systems, I use the Total Superheat or Subcooling method unless the manufacturer recommends another method for that particular unit. To know which method to use, view the type of metering device the air conditioner has. If the unit has a fixed orifice such as a piston or capillary tube then use the total superheat method to check the charge while the system is running after the first 10-15 minutes of run time. If the system has a TXV as the metering device, use the subcooling method to check the refrigerant charge after the first 5-10 minutes of run time.

Why can't you just set the refrigerant pressure to the same pressure on every system to get the charge level correct?

Refrigerant pressures change due to the indoor WB temperature, outdoor DB temperature, indoor airflow, the seer rating of the unit, outdoor airflow, the metering device, and other factors. Only use subcooling, total superheat, or manufacturer recommended charging method to verify the charge.

How do I know if a unit is low on refrigerant?

Determine the type of metering device. If the unit has a fixed orifice such as a piston or capillary tube then use the total superheat method to check the charge while the system is running after the first 10-15 minutes of run time. If the system has a txv as the metering device, use the subcooling method to check the refrigerant charge after the first 5-10 minutes of run time. These methods are typically used on systems with a single or two speed compressor. These methods may not be used on Micro-channel units.

How do I determine what type of refrigerant is in a system?

While the system is off and has remained off for an hour or more, measure the equalized pressure from the ports. Convert this pressure to the saturated temperature of known refrigerants. Compare the saturated pressure of known refrigerants to the oudoor air to see which one matches the temp of the outdoor air. If there is not a match, the system may have air or nitrogen mixed with the refrigerant, the system may have a different refrigerant inside, or the system may be holding a nitrogen pressure test instead of refrigerant.

Is ambient plus 30 a charging method?

Ambient Plus 30 is more of a rule of thumb that is seen as a pattern after checking the charge of a 10 seer r-22 unit. It can apply to other refrigerants used for air conditioning but was primarily conceived from working with r-22 outdoor condensers. This rule of thumb will get you close to the correct refrigerant charge but it does not give the ability to set the charge very accurately or to set it for the most efficient rate. It also does not give any insight into the compressor safety since it does not show the superheat. Additionally, it does not allow troubleshooting of the system. Total Superheat, Subcooling, or another manufacturer charging method should be used for checking the charge, adding refrigerant, and troubleshooting.

Why don't technicians use superheat instead of total superheat to check a refrigerant charge?

There usually is not a port on the vapor line as the refrigerant exits the indoor coil of a split system. Therefore instead of a pressure reading at this location, the technician would need to take the evap cover off, locate where the saturated state would be in the coil, tape a sensor at that location and close the door before starting the unit up. For checking the total superheat, the technician usually has a quick access to the vapor pressure at the outdoor unit service port. The total superheat can be taken right there using pressure converted to temperature along with the tube's temperature within 6 inches of the port. Also total superheat lets you know if there is only vapor entering the compressor which is a crucial thing to check for compressor safety.  Total superheat is measured at a location right before the refrigerant enters the compressor.

What is the total weight charging method?

This is when the technician breaks the vacuum with the total "factory charge" that the system needs. This is also when the vacuum is broken with the amount of refrigerant needed for an extended amount of line set length.

Why can't you add R-410A into a system as a vapor?

R-410A must come out of the refrigerant bottle as a liquid because the refrigerant is made up of two separate refrigerants in the bottle. Keeping the refrigerant as a liquid as it exits the bottle will keep the mix correct.

Why can't you feel the suction line of an air conditioner to tell if the charge is correct?

This will not tell you if the refrigerant is completely in the vapor state before entering the vapor compressor. If the refrigerant is saturated, it will damage the compressor. This method does not tell you how efficient the system is operating or if the unit has the proper airflow needed.

Why is the refrigerant low pressure when it is superheated in the vapor line?

Superheat means the amount of degrees the refrigerant has risen after changing from a saturated state into a complete vapor. The refrigerant is still low pressure since it is between the metering device and the compressor during cooling mode. Refer to the "refrigeration cycle video" for a deeper explanation.

Refrigerant Leaks

 
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How do you find a refrigerant leak in a system?

The technician can take an initial glance to see if there are any oil stains indicating a leak. Joints and obvious spots can be leak checked by adding non-corrosive bubble leak detector to the outside of the tubing. An ultrasonic leak detector can be used to listen for the ultrasonic noise that is made when refrigerant is escaping out of a small hole. An electronic sniffing tool can be used to sniff the location of the leak. Fluorescent refrigerant dye can be added to the system and an ultraviolet light can be used to search for leaks after the dye has had a chance to circulate through the system for a day or more. Another leak detection method that can be used. The outdoor unit can be pumped down. The line set and evap coil can be pressure tested for leaks. If there is a leak on this side of the system then the line set and evap coil can be replaced or separated to pressure test individually.

After fixing a refrigerant leak, can you top off an R-410A charge or do you need to recover and weigh in virgin refrigerant?

Since R-410A is a near azeotropic mixture of 50% R-32 and 50% R-125, the mixture tends to leak at pretty even rates especially if the leak occurs when the system is predominantly running. The boiling points of the two refrigerants that make up R-410A are only .03 degrees off from each other. If the leak occurs when the system is off and the leak is severe, then recover and weigh in virgin R-410A. If it was a small leak that occurred while the system was predominantly running, then virgin R-410A refrigerant can be added to the existing refrigerant in the system.

Why do you say not to use dish detergent and water to create a bubble solution to check for gas leaks?

Present day dish detergents have too many corrosive chemicals compared to dish detergents from 30 years ago that were used for leak checking. These corrosive chemicals can eat through joints and tubing to create leaks and therefore should not be used. Only use non-corrosive bubble leak detector.

What pressure should you pressure test a system to?

Only pressure test to a pressure below the "max design pressure" on the low side of the system. This max design pressure can be found on the evaporator coil box. If the unit is older, pressure test at an even lower pressure to avoid creating a leak due to the corrosion of the tubing.

Brazing & Soldering

 

Why do you mainly braze instead of silver solder?

Manufacturers of HVACR equipment always braze their coils and systems together. Because of this, I simply follow suit and do the same. However, on some sensitive components that are susceptible to heat, I do use silver solder due to the lower melting point.

Can silver solder withstand the pressure of normal R-410A systems?

Yes 5% and higher silver solder can be used to sweat copper tubing together for refrigerant lines. A lower temperature torch can be used to silver solder line connections. There are two main drawbacks to this. Acid is introduced into the system due to the acid flux used to clean the joint for solder to adhere. The other drawback is the clearance between the copper tube and the swage. The tubing swage must be tight and not have any slop in order to maintain a good silver solder joint. High temperature 15% silver braze rod can be used on joints that have more play when inserting the tube into the swage. However, for best joints always try to the smallest clearances between the tube and the swage or coupling.

Why do you flow nitrogen through the tubing while brazing?

Nitrogen is used to purge the air from the inside of line set tubing. If air is left inside the tubing while the tubing is heated up copper oxidation will occur. This is bad for the system since it will be trapped inside the tubing and will recirculate through the system over and over or possibly clog up the filter drier and/or reduce lubrication at the compressor.

Troubleshooting

 

Why does the evaporator coil freeze up?

This could be due to a low airflow problem, a low refrigerant charge or a liquid line restriction. Use superheat and subcooling to determine the problem after the coil is melted. While the unit first starts up and after the first 5 minutes, if the superheat is high then it is not a low airflow problem. If the subcooling is high then the problem is a liquid line restriction. If the subcooling is low and the superheat is high then the unit is low on refrigerant.

Why does the heat pump freeze up when the temperature is above 32 degrees?

The refrigerant in the outdoor coil must be at a temperature below 32 degrees in order to absorb heat from the outdoor air. The outdoor unit draws air across the coil in order to absorb heat. The humidity in the air collects on the fins creating frost and then gets thicker.

Is my thermostat bad? It won't turn the heat on.

Test with a multimeter from w to c at the indoor furnace control board to see if the thermostat is actually sending a signal to turn heat on. If it is, test the control board output voltage to the inducer motor. If the board is sending voltage, turn the power off to the furnace and test the capacitor if equipped, see if the inducer motor wheel is stuck and test the inducer motor itself to see if it is shorted out.

Why is my thermostat blank?

The thermostat could be blank due to a blown 3 amp or 5 amp 24v fuse, the batteries could be bad, the power to the furnace or air handler could be turned off, or there could be a condensate safety switch that is tripped.

Why does my blower fan not shut off?

There could be a limit switch that is tripped or a loose wire connection at a limit switch. The g thermostat wire and the r thermostat wire could be shorted, the thermostat could be bad and connecting the r and g terminals, the blower relay on the control board could be stuck in the closed position, the blower motor module could be bad if it is an ECM blower motor.

Why does the blower motor not turn on?

First check to make sure there is a low voltage signal on the control board of the furnace, air handler, or packaged unit, telling the control board or relay to turn on the blower motor. Turn the thermostat fan button to the on position. Measure from g to c on the control board with the multimeter to see if 24v is present. If there is no voltage then the thermostat is not sending the signal. The thermostat or thermostat wiring could be bad. Obvious things such as if the unit has power should be checked and testing. Make sure that there is 24v from R to c on the control board first.

Why does the air conditioner only blow warm air?

The outdoor condenser fan may be running but the compressor may not be running. Turn the outdoor unit off and check/test the capacitor. If the capacitor is good, check the electrical connections leading to the compressor and then the compressor itself.

Miscellaneous Questions

 

Why do you use valve core removal tools for recovering refrigerant?

Removing the valve cores allows for an unobstructed pathway for the refrigerant to flow through. Because of this, the refrigerant will be recovered faster and the pressure or vacuum read on the refrigerant gauges will be more accurate during the recovery process.

Why do you remove the valve cores to vacuum a system?

This allows for a larger pathway for the non-condensables to be removed from the system. Leaving the valve core in the port while vacuuming will lead to a much slower vacuum and inaccurate vacuum measurement while the vacuum pump is running. If the valve cores are left in the ports, the vacuum may take 20 to 40 times as long to reach the same vacuum level during a vacuum in which the valve cores are removed.

How do you find out what type of metering device is in an AC unit?

The indoor unit rating plate of a split system will state which type of metering device is factory installed in the unit. It is best to visually check this metering device by removing the indoor coil box cover in case the technician has replaced the factory installed metering  device with another device.

Should I install a piston or txv in an air conditioning system?

The piston only allows a fixed amount of refrigerant through. This means that when the system needs more refrigerant in the evaporator coil when it is hot in the building, the piston can only deliver the normal amount of refrigerant and no more. The TXV on the other hand is able to throttle the refrigerant flow and allow more refrigerant into then evaporator coil to absorb the extra heat crossing  it. The TXV can also allow less refrigerant into the evaporator coil during a low heat load or low indoor airflow problem.

What is a Seer rating?

Seasonal Energy Efficiency Ratio. When the Seer rating is higher, the surface area of the fins on an outdoor unit is larger which equates to a greater heat transfer. The surface area of the fins on the indoor coil is also larger. A high Seer rating means that there is greater means to transfer heat energy at a lower energy usage.

Got More Questions?

Check back soon as we are always updating our website with the most relevant information possible or ask your own technical questions in the comment section on our AC Service Tech YouTube Channel!