Month: July 2019

Service factor is an interesting motor rating that you will see on many motor data tags. It simply means how much additional “work” a motor can do or “load” it may be placed under for short periods of time without failure or overload.

For example. The FLA or Full Load Amps of the motor above is 10.8 amps at 115 volts

The Service Factor or S.F. is 1.5, which makes the Service Factor Amps 16.2 (rounded down to 16 on the motor tag)at 115v because 10.8 x 1.5 = 16.2

Don’t confuse SFA with LRA (Locked Rotor Amps). LRA is the current the motor will draw when the rotor is stationary, such as during startup. Service Factor is simply a short term “fudge factor” that the motor has for short periods of higher than normal load.

When a motor is running above its Full Load Amps and in the Service Factor range it may function but its operational life will be shorter and it will generally run at lower efficiency and power factor.

In other words, only go into the “service factor” range when necessary, not as a matter of normal operation.

— Bryan

There is a common belief in the trade that the higher the subcooling the better the system efficiency because lower liquid line temperature means less flash gas.

This statement is only partially true and can lead to some confusion among techs.

Subcooling is temperature decrease below the condensing temperature of the refrigerant that occurs once the refrigerant is 100% liquid. Our objective is to provide subcooled liquid to the metering device at the lowest temperature possible while maintaining the minimum pressure drop required across the metering device.

We do not want to accomplish high subcooling by driving up the condensing temperature artificially unless there is no other option to provide the minimum pressure drop across the metering device.

The “subcooling is the answer” mistake occurs when a technician overcharges a system to get a higher subcooling number at the expense of higher head pressure rather than lower liquid temperature.

The job of the condenser is to reject heat from the refrigerant to the condensing medium (generally air) and the liquid temperature cannot drop below the temperature of whatever it is rejecting it’s heat to. So as refrigerant is added to a system the quantity of liquid contained in the condensing coil increases resulting in a higher subcooling number but ALSO resulting in higher head pressure, condensing temperature and compression ratio.

As more refrigerant is packed into the condenser coil there may be some decrease in liquid line temperature but much of the increased subcooling will come from an increase in condensing (liquid saturation) temperature rather than actually cooler liquid.

Take a look at the system readings above, the outdoor temperature was 80° and the liquid temperature was 82.4° but the head pressure and subcooling were astronomically high due to a severe overcharge.

The liquid line temperature was limited to just above the outdoor temperature and as more and more refrigerant was added to the system the head pressure and therefore the condensing temperature was going sky high resulting in poor system efficiency.

While it is true that a lower liquid line temperature entering the metering device does reduce the amount of “flash gas” converted directly from liquid to vapor as it leaves the metering device this is not an efficiency gain when the subcooling is artificially increased due to overcharge or other method of increasing head pressure.

— Bryan

 

We do this exercise when I teach electrical basics where we sit down and connect a 10 watt bulb to a power supply and through a switch. A SUPER SIMPLE circuit, the kind you might have learned about in high school science class.

But then I grab another 10 watt bulb and tell them to connect it in line with the other 10 Watt bulb (series circuit) and BEFORE they can turn the switch on I ask them a series of questions.

  • Will the two lights be twice as bright as the one? the same? or half as bright?
  • Will the circuit draw twice the amps as before? The Same? or half the amps?

Before we move on, I want you to make your choice.

So everyone makes their choice.. we turn on the switch…

AND THE MAJORITY OF THE CLASS IS WRONG

The bulbs combined are half as bright, using half the amps and thus half the watts. On my quizzes this is an area where experienced techs and electricians will even get frustrated “If you have X2 10 Watt bulbs that is 20 watts” they will say.

The science is actually really simple. In a light bulb, they may be stamped with a rating wattage but that wattage is just a rated wattage when the full rated voltage is applied. The constant in a light bulb is the resistance in Ohms, not the wattage. When you double the resistance of a circuit by adding in another 10 watt bulb in series you are cutting the amperage in half and therefore also cutting the wattage of the circuit in half.

an electrical circuit is a path between two points that have a difference in electrical potential (Voltage) the amperage (and by extension the wattage) is a function of the total resistance of that circuit between those points. If the resistance goes up, the amperage goes down and vice versa. It doesn’t matter if that resistance is added by a bulb, resistor, thermistor, pitted contactor points, motors etc…

Now when we mix in inductive reactance in motors and other inductive loads that resistance is bit less cut and dry to understand… but we will save that for another tip.

— Bryan

Suction pressure, head pressure, subcooling, superheat, Delta T

Taking all five of these calculations into account on every service call is critical. Even if further diagnostic tests must be done to pinpoint the problem, these five factors are the groundwork before more effective diagnosis can be done. I would also add static pressure as an important reading that should be checked regularly (Keep TESP between .3″wc and .7″ wc on most systems) but I would still place it slightly below these five as far as fundamental HVAC technician measurements.

Some of these are “rules of thumb” and obviously are for reference only. Refer to manufacturer recommendations when setting a charge.

Suction Pressure / Low Side
Suction pressure tells us several things. The first thing it tells us is what the boiling temperature of the refrigerant in the evaporator is. If the suction pressure is below 32° saturation temperature, the evaporator coil will eventually freeze.

As a general rule, the higher the temperature of the air passing over the evaporator, the higher your suction pressure will be. A good rule of thumb for suction pressure is 35°  saturation below indoor ambient +/- 5° (Return temperature measured at the evaporator coil). This temperature differential is often called an evaporator split or design temperature difference (DTD). When calculating DTD a “Higher” DTD means lower suction pressure in comparison to the return temperature, a lower DTD means higher suction pressure.

This means that when the temperature of the air passing over the evaporator is 80°, the low side saturation temperature should be 45° when the system is set for 400 CFM per ton output. Remember the temperature scale next to the pressure scale on the gauge represents saturation or if you don’t have the correct sale on (or in your gauge if you have a Digital manifold) you would need to use a PT chart.

This 35° rule only works at 400 CFM per ton, when a system is designed for 350 CFM per ton the DTD will be closer to 38° – 40° +/- 5° 

Make sure you know the actual CFM output of the system before you calculate DTD. It can vary significantly based on the setup of the particular blower. Also, keep in mind that oversized evaporator coils that some manufacturers specify for efficiency can also result in slightly lower DTD (higher suction). If you don’t know all the details it is my experience that using 35° is the best bet.

Head Pressure / High Side
When used in conjunction with liquid line temperature, we can know what state the refrigerant in the liquid line and that the compressor is pumping/operating in the required compression ratio. We can also know something about the state of the metering device as to whether or not refrigerant is “backing up” against the metering device. A good rule of thumb for head pressure is a 15° – 20° saturation above outdoor ambient +/- 3° for most modern systems. These saturation / ambient calculations are only indicators; they are not set in stone. Keep in mind, when I say ambient; I am talking about the air entering the evaporator for suction pressure and the condenser for head pressure.

Jim Bergmann points out that different equipment efficiencies will have different target Condensing Temperature Over Ambient (CTOA) readings. Keep in mind that these date ranges don’t guarantee the SEER but rather give the date ranges that these efficiencies will be most likely. The larger the condenser coil in relation to the volume of refrigerant being moved the lower the CTOA will be.

6 – 10 SEER Equipment (Older than 1991) = 30° CTOA

10 -12 SEER Equipment (1992 – 2005) = 25° CTOA

13 – 15 SEER Equipment (2006 – Present) = 20° CTOA

16 SEER+ Equipment (2006 – Present) = 15° CTOA

Superheat
Superheat is important for two reasons. It tells us whether or not we could be damaging the compressor and whether we are fully feeding the evaporator with boiling, flashing refrigerant. If the system has a 0° superheat, a mixture of liquid and vapor is entering the compressor. This is called liquid slugging and it can damage a compressor. A superheat that is higher than the manufacturer’s specification can both starve the evaporator, causing capacity loss, as well as cause the compressor to overheat. So how do we know what superheat we should have? First, we must find out what type of metering device the system is using. If it is using a piston or other fixed metering device, you must refer to the manufacturers superheat requirements or a superheat chart like the one below.

If it is a TXV type metering device, the TXV will generally attempt to maintain between a 5° to 15° superheat on the suction line exiting the evaporator coil (10° +/- 5°) 

TXV target superheat setting may vary slightly based on equipment type.

Subcooling
Subcooling tells us whether or not the liquid line is full of liquid. A 0° subcool reading tells us that the refrigerant in the liquid line is part liquid and part vapor. An abnormally high subcool reading tells us that the refrigerant is moving through the condenser too slowly, causing it to give up a large amount of sensible heat past saturation temperature. A high subcool is often accompanied by high head pressure and, conversely, a low subcool by low head pressure. Subcool is always a very important calculation to take because it lets you know whether or not the metering device is receiving a full line of liquid. Typical ranges for subcooling are between 8 and 14 degrees on a TXV system, but always check the manufacturer’s information to confirm. in general, on a TXV system using 10° +/- 3° at the condenser outlet is an acceptable “rule of thumb” in the absence of manufacturer’s data.

On a fixed orifice/piston system the subcooling will vary even more based on load conditions and you will see a range of 5° to 23° making subcooling less valuable on a fixed orifice system. In my experience during normal operating conditions the subcooling on a fixed orifice system will still usually be in the 10° +/- 3° range.

Evaporator Air Temperature Split (Delta T)
The evaporator air temperature split (Delta T) is a nice calculation because it gives you a good look at system performance and airflow. The air temperature split during typical conditions will be between 16 and 22 degrees difference from the return to the supply. Keep in mind, when you are doing a new system start-up, high humidity will cause your air temperature split to be on the low side. Refer to the air temperature split and comfort considerations sheets for further information.

For systems that are set to 400 CFM per ton, you can use a target Delta T sheet like the one shown below

 

If the leaving temperature/delta T split is high it is an indication of low airflow. If it is low it is an indication of poor system performance/capacity.

Again, this only applies to 400 CFM ton. Systems set at 350 CFM per ton or less are more common today than ever, especially in humid climates and in those cases the above chart won’t apply and the delta T will be higher.

Diagnosing With The Five Pillars
The way this list must be utilized is by taking all five calculations and matching up the potential problems until you find the most likely ones. A very critical thing to remember is that a TXV system will maintain a constant superheat, and fairly constant suction pressure. The exceptions to this rule are when the TXV fails, is not receiving a full line of liquid or does not have the required liquid pressure/pressure drop to operate. This situation would show 0° subcooling and in this case, will no longer be able to maintain the correct superheat. Before using this list, you must also know what type of metering device is being utilized, then adjust thinking accordingly. Also remember, in heat mode, the condenser is inside and the evaporator is outside.

Low Suction Pressure
• Low on charge
• Low airflow /load – dirty filter, dirty evaporator, kinked return, return too small, not enough supply ducts, blower wheel dirty, blower not running correct speed, insulation pulling up against the blower, etc.
• Metering device restricting flow too much – piston too small, piston or TXV restricted, TXV failing closed
• Liquid line restriction – clogged filter/drier, clogged screen, kinked copper
• Low ambient (Low evaporator load)
• Extremely Kinked suction line (after the kink)
• Internal evaporator restriction

High Suction Pressure
• Overcharge
• High return temperature (Evaporator Load)
• Metering device allowing too much refrigerant flow – piston too large, TXV failing open, piston seating improperly
• Too much airflow over the evaporator (Blower tapped or set too high)
• Compressor not pumping properly – leaking suction valve, leaking discharge valve, other compression issues
• Reversing valve bypassing
• Discharge line restriction

Low Head Pressure
• Low on charge
• Low ambient temperature / low load
• Metering device allowing too much refrigerant flow – piston too large, TXV failing open, piston seating improperly
• Wet condenser coil
• Compressor not pumping properly – leaking suction valve, leaking discharge valve, other compression issues
• Reversing valve bypassing (heat pump units)
• Kinked suction line
• Restricted discharge line
• Severe Liquid Line Restriction
• Wet Condensing Coil

High Head Pressure
• Overcharge
• Low condenser airflow – condensing fan not operating, dirty condenser, fins bent on the condenser, bushes too close to the condenser, wrong blade, wrong motor, blade set wrong
• High outdoor ambient temperature
• Mixed / incorrect refrigerant/retrofit without proper markings
• Non-condensables in the system
• Liquid line restriction + overcharge (someone added charge when they saw low suction) – piston too small, piston or TXV restricted, TXV failing closed, restricted line drier

Low Superheat
• Overcharge
• Low air flow / load – dirty filter, dirty evaporator, kinked return, return too small, not enough supply ducts, blower wheel dirty, blower not running correct speed, insulation pulling up against the blower etc.
• Metering device allowing too much refrigerant flow – piston too large, TXV failing open, piston seating improperly
• Low return air temperature
• Abnormally low humidity
• Internal evaporator restriction
• Very Poor Compression (Compressor, reversing Valve Issues) but will also be combined with VERY HIGH suction

High Superheat
• Low on charge
• Metering device restricting flow / underfeeding / overmetering – piston too small, piston or TXV restricted, TXV failing closed
• High return air temperature
• Liquid line restriction – clogged filter/drier, clogged screen, kinked copper

 

Low Subcooling
• Low on charge
• Metering device allowing too much refrigerant flow – piston too large, TXV failing open, piston seating improperly
• Compressor not pumping properly – leaking suction valve, leaking discharge valve, bad or broken crank
• Reversing valve bypassing
• Discharge Line Restriction
• Compressor not pumping

High Subcooling
• Overcharge
• Metering device restricting too much flow – piston too small, piston or TXV restricted, TXV failing closed
• Liquid line restriction – clogged filter/drier, clogged screen, kinked copper
• Dirty Condenser Coil on New High-Efficiency Condensers (Increased Condensing Temp Can Actually Result in Higher Subcooling)
• Having an H.R.U. in the discharge line (old school I know)
• Internal evaporator restriction

High Evaporator Air Temperature Split
• Low air flow – dirty filter, dirty evaporator, kinked return, return too small, not enough supply ducts, blower wheel dirty, blower not running correct speed, insulation pulling up against the blower etc.
• Abnormally low humidity (WB Temp)
• Blower not running the correct speed or running backward

Low Evaporator Air Temperature Split
• Undercharge
• Severe Overcharge with fixed orifice metering device – because saturation temperature is increased with overcharge
• Metering device not functioning properly – restricting too much flow or allowing too much flow
• Too much airflow through the evaporator – blower not running correct speed
• Heat strips running with air
• Abnormally high humidity
• Liquid line restriction
• Compressor not pumping properly – bad suction valve, bad discharge valve, bad or broken crank
• Reversing valve bypassing
• Discharge line restriction

 

This is an incomplete list designed to help you. Always keep your eyes and ears open for other possibilities. Diagnosis is an art as well as a science.

The MeasureQuick app is a great free app that can help you in making a complete diagnosis using these 5 pillars and more.

— Bryan

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