Frequently Asked Questions (FAQs) About Air Conditioning and Heating

Common HVAC Questions & Answers



How do I braze a service valve?
Make sure the valve is mid-seated before brazing. The heat from brazing a fully front-seated or back-seated valve can cause the button of the valve stem (inside the valve) to ‘weld’ to the seating area on the inside of the body of the valve.


What are compressor electronics?
Compressor electronics use the compressor as a sensor to monitor an AC or refrigeration system and diagnose performance. They deliver more compressor and system information so that end users can identify performance issues quickly and accurately for greater reliability and cost savings, giving them freedom to refocus time and money on other priorities.
How do compressor electronics protect the compressor?
Compressor electronics monitor the basic operating conditions within the compressor or system, such as voltage, temperature, oil or pressure. When conditions exist that lead the compressor down a path towards failure, the electronics respond with active protection, thereby shutting down the compressor.
When should a contactor be replaced?
Contactors have a limited life and should be inspected during routine maintenance and replaced every time a compressor is installed.
What does a crankcase pressure regulator do?
Crankcase pressure regulating valves (CPRs) regulate the downstream pressure to a maximum value. These are commonly installed in the suction line ahead of the compressor to limit the inlet pressure to the compressor. Limiting the inlet pressure prevents the compressor from “stalling” during start-up if the compressor is over loaded.



How do I use electronics to troubleshoot a system?
Compressor electronics deliver a range of fault codes that can identify the root cause of a problem and protect the compressor from adverse system conditions. These fault codes are based upon software algorithms that specifically protect the compressor motor. This diagnostic capability enables greater speed and accuracy of troubleshooting activity, especially with lesser-skilled technicians, reducing costly call-backs due to improper diagnosis.
Where should the external equalizer be installed?
The external equalizer line should be installed on top of the suction line before any traps and located within 6 inches of the sensing bulb position. If this is not possible, and a different location is required, it must first be confirmed that the pressure at the desired location is identical to the pressure at the bulb.
What happens if the equalizer tube becomes kinked?
If the equalizer line becomes “kinked,” the pressure sensed at the underside of the diaphragm will no longer correspond to the evaporator outlet pressure and the valve will not be able to operate as intended.
I have seen some equalizer tubes frosted. Is that normal?
Frost on the equalizer line is an indication that the packing seal has failed, allowing higher pressure refrigerant to leak past and expand into the equalizer line. Depending on the valve type, either the cage assembly or the entire valve should be replaced.
What is the purpose for evacuating a refrigeration system?
Evacuating a refrigeration system serves two primary objectives. It removes non-condensable material (air) and dehydrates (removes water vapor).
Does evacuation actually pull liquid water out of the system?
No. Evacuation will not pull liquid water out of the system. When you evacuate a system you are actually dropping the pressure sufficiently to allow water to “boil” at room temperature. As the water boils, it changes to a vapor state and this vapor is drawn out by the vacuum pump.


What are the signs of a flooded start?
Signs of a flooded start are worn or scored rods or bearings, rods broken from seizure, and an erratic wear pattern on the crankshaft.
What causes a flooded start?
Flooded starts are the result of refrigerant vapor migrating to the crankcase oil during the off cycle. When the compressor starts, the diluted oil cannot properly lubricate the crankshaft load bearing surface causing an erratic wear or seizure pattern.
What can be done to avoid flooded starts?
Locate compressor in warm ambient or install continuous pump down and check crankcase heater operation.


What is global warming?
In 2012, the Environmental Protection Agency (EPA) defines global warming as “an increase in the near surface temperature of the earth.”
What is meant by direct global warming??
Direct global warming is the measure of global warming potential (GWP) that each greenhouse gas contributes to the warming process if it is released “directly” into the atmosphere.
What is meant by indirect global warming?
Indirect global warming considers the amount of contributing effect to global warming by the manufacture of greenhouse gases and their efficiency of operation. In other words, it takes energy from power plants, which also emit greenhouse gases, to manufacture the gases and operate the equipment that the greenhouse gases are used in.
What is total equivalent warming impact (TEWI)?
TEWI is the sum of a greenhouse gas’s direct and indirect GWP. This value takes into consideration both the direct factor of release of the gas into the atmosphere and the indirect factor of the manufacture and lifetime operation of the system in which the gas is used.


What are the signs of high discharge temperature?
Signs of high discharge temperature are discolored valve plates, burned valve reeds, worn pistons, rings and cylinders, or a stator spot burn from metal debris.
What causes high discharge temperature?
High discharge temperature is the result of temperatures in the compressor head and cylinders becoming so hot that the oil loses its ability to lubricate properly. This causes rings, pistons and cylinders to wear resulting in by, leaking valves, and metal debris in the oil.
What can be done to avoid high discharge temperature?
You should correct abnormally low load conditions, correct high discharge pressure conditions, insulate suction lines, and provide proper compressor cooling.






What is the standard metering device?
Many air conditioning systems incorporate a TXV style metering device as the standard. To adjust evaporator coil superheat, follow the manufacturer’s recommendations. If these are not available, the following guide lines could apply, depending upon the system design temperature:
High Temp 8°F – 12°F
Medium Temp 5°F – 8°F
Low Temp 2°F – 6°F
What is a micron?
A micron is a metric measure and is defined as 1 millionth of a meter or 1 thousandth of a millimeter. Most people in the U.S. think of a perfect vacuum as 30 inches of Mercury (Hg). The last inch (from 29-30) of vacuum is equal to 25,400 microns. The micron then is a much more precise method for measuring deep vacuums. Micron = 0.001 mm Hg = 0.000039 inches of Hg.
What is migration?
Migration is the term used to describe when refrigerant moves throughout the system during the compressor off cycle. As an example, in a split air-conditioning system with the compressor/condenser outside, liquid refrigerant from the evaporator will migrate to the compressor during winter months due to the compressor being off, and colder than the indoor (evaporator) temperature. If this is not prevented, then upon start-up in the spring, compressor damage may occur.
What does the MOPD rating on solenoid valves mean?
Solenoid valves are rated in terms of Maximum Operating Pressure Differential (MOPD). The MOPD rating for a valve is the maximum pressure differential against which the valve will reliably operate. For example, with the valve closed against an inlet pressure of 250 psi and an outlet pressure of 50 psi, the pressure differential across the valve is 250-50 or 200 psi.
Why do some valves have a Min OPD rating and what does this mean?
Min OPD stands for Minimum Operating Pressure Differential. All pilot-operated valves and require a small amount of pressure differential to enable the piston or diaphragm to rise off the main seat. Typically, 2 to 5 psig differential is needed to accomplish this. If the pressure differential is less than the Min OPD, then the valve will not open when actuated or will fail to remain open.



What is the right oil to use with the new refrigerants?
With the introduction of HFC refrigerants as alternatives to CFC and HCFC refrigerants, the question of the proper oil to use still comes up. The generally preferred oil for use with HFCs is a polyol ester (POE) that has an additive package for refrigeration applications. Mineral oil (MO) is not recommended, because oil return is considered to be compromised.
What about alkyl benzene oils?
Most interim HCFC refrigerants can also use alkyl benzene (AB) oil, if approved by the compressor manufacturer.
Do I have to remove all mineral oil from the system when retrofitting?
If retrofitting a system to an HFC refrigerant, current recommendations are to remove the mineral oil so that 5% or less is all that remains in the system before changing to POE oil. The percentage can be measured by using a refractometer.
What is the purpose of oil separators?
Oil separators are used on refrigeration systems where it is difficult for the oil to return from the evaporator. Oil separators are installed in the compressor(s) discharge line. They are usually a vertical container with the discharge gas connections at the top and an oil return port at the bottom.
What are the signs of loss of oil?
Signs of loss of oil are worn or scored rods and bearings, a uniformly scored crankshaft, rods broken from seizure, or little or no oil in crankcase.
What causes loss of oil?
Loss of oil is a result of insufficient oil in the crankcase to properly lubricate the load bearing surfaces. When there is not enough refrigerant mass flow in the system to return oil to the compressor as fast as it is pumped out, there will be a uniform wearing or scoring of all load bearing surfaces.
What can be done to avoid loss of oil?
Check oil failure control operation if applicable; check system refrigerant charge; correct abnormally low load conditions or short cycling; check for incorrect pipe sizes and/or oil traps; check for inadequate defrosts.


What are pressure controls?
There are two main categories of pressure controls – high pressure and low pressure. These controls may be individual or combined into one control and are designed to protect the compressor.
What is a pump down system?
A pump down system consists of a normally closed solenoid valve installed in the liquid line and a low-pressure control that senses suction pressure. The system operation is design to prevent “off cycle” migration.
What is the advantage of the pump down system?
The advantage of a pump down system is that the liquid refrigerant is stored in the receiver and condenser when the compressor is not operating. This prevents liquid migrating to the compressor crankcase during the off cycle and the ensuing possibility of liquid refrigerant in the oil during compressor start-up.



What oil do I use with R-410A refrigerant?
The Air Conditioning industry has converted to the environmentally friendly refrigerant, R-410A. Since R-410A is classified as an HFC, the only recommended oil is polyol ester (POE) oil.
What does a refrigerant distributor do?
The purpose of the distributor is to provide equal feeding of the refrigerant to each individual circuit. Distributors are used on multi-circuited evaporator coils. By using multiple circuits in evaporators, the pressure drop through the evaporator is minimized.
What is refrigerant flooding?
Flooding is the term used to describe the condition when liquid refrigerant reaches the compressor. This typically occurs when the amount of liquid fed to the evaporator is more than can be evaporated.
What is refrigerant flood back?
Refrigerant flood back is a result of liquid refrigerant returning to the compressor during the running cycle. The oil is diluted with refrigerant to the point it cannot properly lubricate the load bearing surfaces.
What are the signs of refrigerant flood back in an air-cooled compressor?
Worn pistons and cylinders and no evidence of overheating.
How does refrigerant flood back happen in an air-cooled compressor?
The liquid washed the oil off the pistons and cylinders during the suction stroke causing them to wear during the compression stroke.
How does refrigerant flood back happen in a refrigerant-cooled compressor?
The liquid dilutes the oil in the crankcase and the refrigerant rich oil will be pumped to the rods and the bearings through the crankshaft. As the refrigerant boils off, there will not be enough oil for sufficient lubrication at the bearings farthest from the oil pump. The center and rear bearings may seize or may wear enough to allow the rotor to drop and drag on the stator causing it to short.
What are the signs of refrigerant flood back in a refrigerant-cooled compressor?
The center and rear bearings are worn or seized; there is a dragging rotor and shorted stator, a progressively scored crankshaft, and worn or broken rods.
What can be done to avoid refrigerant flood back?
Maintain proper evaporator and compressor superheat, correct abnormally low load conditions, and install accumulators to stop uncontrolled liquid return.


What is SEER?
The SEER (seasonal energy efficiency ratio) is a measure of how efficiently an air conditioner or heat pump will operate over an entire cooling season.
What are the signs of system slugging?
Signs of slugging are broken reeds, rods, or crankshaft, and loose or broken backer bolts, as well as blown head gaskets.
What causes slugging?
Slugging is the result of trying to compress liquid refrigerant and/or oil, in the cylinders. Slugging is an extreme flood back in air cooled compressors and a severe flooded start on refrigerant cooled compressors.
What can be done to avoid slugging?
Several things can be done to avoid slugging. Maintain proper evaporator and compressor superheat. Correct abnormally low load conditions. Install accumulators to stop uncontrolled liquid return and locate the compressor in warm ambient air or install continuous pump down.
What is meant by sub-cooling?
Sub-cooling is the condition where the liquid refrigerant is colder than the saturation temperature. The amount of sub-cooling, at a given condition, is the difference between its saturation temperature and the actual liquid refrigerant temperature.
Why is sub-cooling desirable?
Sub-cooling is desirable for several reasons. It increases the efficiency of the system since the amount of heat being removed per pound of refrigerant circulated is greater. It is also beneficial because it helps to ensure that liquid refrigerant can reach the expansion valve.
What is the purpose of a suction accumulator?
A suction accumulator is used to prevent liquid refrigerant flooding back to the compressor. Accumulators are commonly used on heat pumps, and any place that liquid refrigerant is a concern.
What is superheat?
The term superheat refers to the number of degrees a vapor is above its saturation temperature (boiling point) at a particular pressure.
How do I measure superheat?
Superheat is determined by taking the low side pressure gauge reading, converting that pressure to temperature using a PT chart, and then subtracting that temperature from the actual temperature measured (using an accurate thermometer or thermocouple) at the same point the pressure was taken.
Why is it important to know the superheat of a system?
Superheat gives an indication if the amount of refrigerant flowing into the evaporator is appropriate for the load. If the superheat is too high, then not enough refrigerant is being fed resulting in poor refrigeration and excess energy use. If the superheat is too low then too much refrigerant is being fed possibly resulting in liquid getting back to the compressor and causing compressor damage.
What is evaporator superheat?
Superheat varies within the system depending on where it is being measured. The superheat that the thermal expansion valve is controlling is the evaporator superheat. This is measured at the outlet of the evaporator. The refrigerant gains superheat as it travels through the evaporator, starting at 0 as it enters the evaporator and reaching a maximum at the outlet as the refrigerant travels though the evaporator absorbing heat.
What is system superheat?
System superheat refers to the superheat entering the suction of the compressor. Some people confuse system superheat with ‘return gas temperature.’ It should be remembered that superheat varies as the saturated suction pressure of the refrigerant varies. Return gas temperature is a temperature value measured by a thermometer or other temperature-sensing device. It does not vary because of pressure changes.


What is a TXV?
The Thermal Expansion Valve (TXV) responds to the superheat at the outlet of the evaporator and as a result is more responsive to the actual load resulting in a more efficient system.
How do I find the TXV selection?
Calculate the pressure drop across the TXV by subtracting suction (low side) pressure from condensing (high side) pressure. Also subtract pressure drop from a distributor, if one is used. The difference is pressure drop across the TXV.
What is the correct placement for the TXV sensing bulb?
The sensing bulb should be attached at 12 o’clock on any suction line of 7/8” diameter or smaller. On lines larger than 7/8” diameter the bulb should be placed at either 4 or 8 o’clock. The bulb should never be placed at 6 o’clock.
What is an internally equalized TXV?
An internally equalized TXV uses evaporator inlet pressure to create the “closing” force on the valve.
What is an externally equalized TXV?
An externally equalized valve uses the evaporator outlet pressure, thereby compensating for any pressure drop through the evaporator.
When should I use an externally equalized TXV?
On any large system, generally over 1 ton capacity; also, on any system utilizing a distributor.
Will an externally equalized TXV allow system pressures to “equalize” during off cycles?
No, an externally equalized valve will NOT allow system high and low sides to “equalize” during the off cycle. The only way this can be accomplished is through the use of a “bleed” type TXV.