Learn to Troubleshooting HVAC Units

Hvac units troubleshooting part 1

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.


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