Tag: static pressure

Measuring airflow is easy… measuring airflow accurately is quite a bit more difficult. In many cases when we as technicians measure airflow we are trying to get to the almighty CFM (Cubic Feet per Minute) volume measurement. You can take CFM readings fairly easily with a hood like the Testo 420 shown above, but even a hood has some limitations when the goal is to measure total system CFM vs. register / grille CFM.

In this series of videos Bill Spohn from Trutech tools demonstrates all of the tools you can use to measure airflow from hot wire and rotating vane anemometers, to flow hoods, to smart grids and pitot tubes, all the way down to using a GARBAGE BAG.

I had the privilege of seeing this presentation in person (I am the one behind the camera) and I wanted to share it with you. It is well worth your time.

— Bryan

In my recent classes with my employees at Kalos, we are going over finding target pressures and temperatures for an air conditioning system with the goal being to get techs to have “target” readings in mind before they start connecting tools. This step is an important part of being able to “check a system without gauges” like we have talked about so often. Much of this list makes more sense if you are already familiar with our 5 pillars of diagnosis.

It is important that we start using these terms when speaking to one another, writing notes and diagnosing because these will translate better between systems and between A/C and Refrigeration. Some readings we take in HVAC like static pressure and delta t do not apply to refrigeration, while others like target condensing and evaporator temperature are key in both disciplines.

Keep in mind that when I give a “rule of thumb” you should always consider manufacturer specs, charts, panels, install and service manuals as superior to a rule of thumb. You will be amazed what you might learn reading the service and installation manuals of the systems you install and work on.

 

Target Evaporator Temperature or DTD (Design Temperature Difference) = The temperature the evaporator coil should be based on the return temp (A/C 35°F(19.25°K) below ambient) or below box temp in the case of refrigeration (10°(5.5°K) below box on a walk in 20°(11°K) below box on a reach in). on a typical A/C system with a 75°F return temperature, the DTD would be 35°F(19.25°K) which means the target evaporator temperature would be 40°F. The DTD will vary based on airflow and evaporator coil size.

 

Measured Evaporator Temperature or TD (Temperature Difference) = The suction saturation temperature (not pressure) as measured on the suction gauge for that particular refrigerant it can then be compared to box or return temperature to calculate your measured or actual TD

 

Target Condensing Temperature Over Ambient (CTOA) = This is the target temperature the liquid saturation (condensing temperature) measured on the gauge SHOULD be above the outdoor air temperature measured in the shade entering the condenser coil. This will be 30°(16.5°K) over ambient on VERY old units, all the down to as low as 15°(8.25°K) on new very high-efficiency units. This is LIQUID LINE ONLY the discharge line will be higher pressure.

 

Target Condensing Temperature – The outdoor temperature + the CTOA = Target condensing temperature Example: Outdoor Temp 95° + 15° for a 16 SEER system = 110° target condensing temp.

 

Target Superheat – This is the superheat you SHOULD have and it varies based on if the system has a TXV or Piston metering device. If a piston you MUST use a use a superheat chart as well as a thermo-hygrometer / psychrometer to measure the indoor wet bulb and dry bulb because those charts require those readings. If the system is a TXV then set your target at 5-15°(2.75°K – 8.25°K) if measured inside and 10° – 20°(5.5°K – 11°K) if measured outside

 

Measured Superheat – The increase of the suction line temperature when compared to the suction saturation.

 

Target Subcool – The subcool you wish to achieve. Many units will have this marked on the data tag, if not then use 10°(5.5°K) subcool on TXV systems and 5° – 15°(2.75°K – 8.25°K) on piston systems recognizing that on a piston system this rule will not always apply.

 

Measured Subcool – The measured difference between the liquid line temperature and the condensing temperature (liquid saturation temp) off of the high side gauge. This is liquid line only, not the discharge line.

 

Outdoor Ambient – the outdoor dry bulb temperature, in the shade entering near the center of the condenser coil

 

Return DB – Return dry bulb. The temperature of the return air without taking evaporation or humidity into account. Best taken in the return right before the unit and not in the space.

 

Return WB – Return wet bulb. The temperature of the return air + the evaporative effect. Lower WB, when compared to DB, means lower relative humidity. Wet bulb and dry bulb will be the same at 100% RH. Best taken in the return right before the unit and not in the space.

 

Return RH% –  The relative humidity of the air in the return. Relative humidity is the percentage of moisture in the air compared to how much moisture is in the air. Hotter air can hold more moisture than the same air at a lower temperature.

 

Target Supply Air Temperature – Target supply air temperature is calculated using a delta t chart and comparing the return DB and the return WB temperatures. The target supply air temperature is dry bulb and can be compared to the return DB to calculate the target delta t.

 

Target Delta T (Air Temp Split) – Don’t confuse TD or Evaporator split above with delta t or air temp split. Keep in mind that the 18°- 22°(9.9°K – 12.1°K) rule that many use only applies to homes with 45% to 55% relative humidity. As RH% goes up the target split will go down and as RH% goes down the split will go up.

 

Measured Delta T – The measured difference between the supply and return air DB. Keep in mind this should be taken a few feet before and after the unit to allow for air mixing and reduce radiant gains/losses.

 

Delta H – Delta H is an advanced measurement that calculates the change in enthalpy (heat content) of the air between the return and the supply. You can do this with two digital thermos-hygrometers like the 605i and it takes into account the temperature and humidity of the air entering and leaving the evaporator.

 

Delivered Capacity – Delivered capacity is the calculation of BTUs of heat being removed from an air stream which combines the Delta H with the CFM of air to give you the total “work” being done across the evaporator coil.

 

Discharge Temperature – The measured temperature (with a line clamp) of the discharge line leaving the compressor, not the liquid line

 

Target Liquid Line Temperature  –  When “checking a system without gauges” the target liquid line temperature is the target condensing temperature minus the target subcool. This is usually measured at the condensing unit.

 

Target Suction Line Temperature –  When “checking a system without gauges” the target suction temp is the target evaporator temperature plus the target superheat. This is most accurate when measured inside but is also valuable when measured outside.

 

Approach – Approach is just another name for target liquid line temperature and it a reading that Lennox publishes a target for on many of their units in the installation manual and on the back of the panel of the condensing unit. Systems with larger or more efficient condenser coils tend to have a lower approach (cooler liquid line) while those with smaller, less efficient coils ten to have a higher approach (warmer liquid line)

 

Suction Line TD (Temperature Rise) – The difference between the suction line temperature inside after the evaporator coil and outside by the condensing unit. When a suction TD is more than 10°F(-12.22°C) compressor overheating and oil carbonization can occur under some load conditions.

 

Liquid Line TD (Temperature Drop) – The difference between the liquid line temperature outside by the service valve and inside before the metering device. ideally, the liquid would have VERY LITTLE temperature drop and any drop of more than a few degrees should be looked into. Long line length, vertical risers, running the liquid line through a low ambient space, contact between the liquid line and the suction line or restrictions can lead to higher than normal LL TD.

 

Static Pressure – The positive or negative pressure exerted on all surfaces equally within a duct system. Static pressure does not measure flow, it is like the pressure inside a balloon that inflates or deflates. Static pressure is generally measured in inches of water column in the USA (“wc)

 

Design TESP – This is the total external static pressure, both positive (supply) and negative (return) that a particular furnace or air handler are designed to work under external to the appliance. Most typical residential units are designed for 0.5”wc(124.5 pa).

 

Measured TESP – This is the total external static measured using a manometer or magnahelic gauge and static pressure tips. On a furnace this would be measured inside the furnace before and after the blower but before the coil. In an air handler or fan coil it will generally be measured before and after the unit in the ducts. This is the total difference between the negative and positive reading so if return static was -0.2”(-49.7 pa) and supply static was +0.3”(74.65 pa) the total would be 0.5”wc(124.5 pa) TESP.

 

Static Pressure Drop – This is the measured pressure change across a portion of the air system. For example across a coil, filter, duct etc… This is helpful in diagnosing air flow issues and changes over time.

 

While this may seem like a long list, most of it is pretty common sense. One thing to mention is the fact that if you do not have a thermos-hygrometer like the 605i and accurate temp clamps you cannot properly check a Delta T on any system or set the superheat on a fixed orifice system. In order to properly set a charge or diagnose a system, you need a way to accurately test line temperatures and measure return / indoor Wet Bulb, Dry Bulb and Relative Humidity.

 

Step one on diagnosing a refrigerant issue, checking or setting a charge should be to get an accurate return (or box) DB, WB and RH as well as the outdoor ambient temperature and then working from there taking appropriate readings. When calling a senior tech or your manager please be prepared will all relevant readings to make a quick and correct diagnosis.

— Bryan

 

This article is written by Neil Comparetto. Neil is one of the smartest and most thoughtful techs I know online. Thanks Neil.


Why measure static pressure? Because it’s fun

I enjoy drilling holes in things. I rarely leave a house without drilling a hole in something. I also believe it’s an essential step to commissioning and diagnosing a forced air piece of equipment. Let me explain why.

Commissioning 

I think we all can agree that proper airflow is necessary across the indoor coil. You should set the airflow before adjusting the refrigerant charge, right? Yes. Well, how do you know what fan speed to set the blower at?

Whether it’s a PSC, X-13, or an ECM motor you have fan speed options. The easiest way to set the blower speed is to measure TESP (total external static pressure), cross reference the TESP to the manufacturers blower chart in the installation manual, and adjust the blower speed. Sure, there are other ways of estimating or measuring airflow, but for commissioning a system in cooling, static pressure and a blower chart is easy and accurate enough.

350-400 CFM(594.65 m3/h – 679.6 m3/h) per ton works in my neck of the woods. If you are in a very dry climate, or at high altitude the CFM per ton requirements may be higher, often 450 – 500 CFM( 764.55 m3/h – 849.51 m3/h) per ton.

Even when commissioning a furnace in heating I like set up my airflow first, or at least know how many CFM the blower is moving. Typically it’s between 130 and 150 CFM(220.87 m3/h – 254.85 m3/h) per 10,000 input BTUH.

How many times have you serviced a system installed by others, there is no evidence of airflow being measured, and the blower speed is set too high? My guess, everyday.

There’s a better than good chance the airflow is wrong, and has been since day one.

Benchmarking

The airflow is set, now what? Take it to the next level, benchmark your pressure drops.

The pressure drops across the return duct, air filter, indoor coil, and supply duct can be valuable pieces of information when servicing the equipment in the future.

Imagine knowing exactly what the pressure drop across the coil was when commissioned 7 years ago. In a typical arrangement of the evaporator coil on top of a furnace a visual inspection of the coil for cleanliness can be difficult.

Knowing the original pressure drop can save time diagnosing, and justify taking further action. TESP by itself will not give you this information, only that there is an issue somewhere in the supply air side of the system. (It is recommended to record the dry and wet pressure drops across the coil, they will be different).


Same applies for the air filter. I typically install 5” media filters on our installations. On a furnace I aim for a .10” pressure drop across the filter when new.

Air handlers can typically handle a higher pressure drop across the filter (because the coil is included in the TESP rating), but at the cost of filter efficiency. Generally these filters are good for 6-12 months. Knowing the before and after pressure drop of the filter in this system will help you determine how frequently it needs to be changed.

Knowing the supply and return duct pressure drop can be useful as well. It’s not unheard of for vents to be closed, return grilles blocked, internal liners to collapse, flex duct to get smashed, or even disconnected. A static pressure reading of the ducts referenced to the pressure drops when commissioned can quickly tell you if there are any discrepancies, and better yet what actions to take.

Does benchmarking lengthen the time it takes to commission the system? Yes. Does it give you the information necessary to quickly and accurately diagnose airside issues during future servicing? Yes it does. In reality, once you do it a few times and develop a system it doesn’t take much longer at all.

If you have not listened to Bryan Orr and Jim Bergmann’s podcast on checking the refrigerant charge without gauges please do. They make a case for an even more comprehensive benchmarking procedure. Listening to it was one of those ah-ha moments for me.


Servicing 

Take TESP to verify airflow against benchmarks and / or blower charts. In my experience most of the time airflow is incorrect. If this system is new to you and will be part of a service agreement I recommend that you check all four pressure drops (return, filter, coil, supply) for reasons mentioned earlier.

I find a lot of air filters that are too restrictive (small) on service calls. Air filters can be low hanging fruit if the equipment is not getting to proper CFM. It’s not uncommon to get .30” pressure drops on new filters.

If you find issues with the existing duct system, and it’s exposed, static pressure readings can help pinpoint where the restriction is.

Many times the restriction is obvious. A nasty reverse elbow then it turns twice, transitioning from 20” to 10” into some kind of cap-and-tap contraption. Sometimes the restriction is internal, or not obvious. Collapsed duct liner or a closed damper can be found with static pressure strategically measured across portions of the duct system.

If there are issues with the duct system let the homeowner know. This conversation might expose some comfort problems that they are experiencing. At the least it will make them (and you) aware that there are issues that may need to be addressed when it’s time to replace the system.

I’m not advocating to check static pressure every time you run a service call. I know that’s not always practical. I am advocating for installing the pressure ports, and benchmarking on commissioning as well as measuring airflow against the benchmarks during service when the call type calls for it. Future service techs will thank you and you will come to a faster and more accurate diagnosis.

— Neil


There is a big move in residential and light commercial HVAC toward measuring static pressure regularly during commissioning, service and maintenance. And don’t get me wrong

Measuring static pressure is VERY important 

The challenge comes in when techs begin taking measurements without understanding where to take them, what they mean, or worse… they use measurements as an excuse not to do a proper visual inspection.

So before we go on, let’s cut to the chase. You need to visually inspect blower wheels, blower taps and settings, blower direction, belts, pulleys, evaporator coils, filters and condenser coils as well as look for any other abnormal return or condenser restrictions. 

Do this BEFORE you take detailed measurements and you will save yourself a lot of time and heartache. 

So what is “static pressure” anyway? 

Think of the air side of the system like a balloon. Static Pressure is the inflating (positive) or deflating (negative) pressure against the walls of the ducts / fan coil / furnace in relationship to another point which is usually atmospheric pressure 14.7 PSIA (at sea level) or 0 PSIA. 

When you blow up a balloon there is a positive pressure against all sides inside the balloon in relationship to the atmospheric pressure around the balloon. 

Static Pressure in residential and light commercial HVAC is generally measured in Inches of water column. We often measure it with an accurate digital manometer zeroed out to atmospheric pressure before use.

Static Pressure is not air flow. You could have static Pressure and have no airflow whatsoever. If you think of it in electrical terms, you can read voltage (potential) between two points and have no actual movement of electrons. It is a measure of difference in energy states between two points not a measure of quantity.

If you took a blower, attached a duct to it and blocked the end of the duct with a cap and turned the blower on you would have 0 CFM of airflow in the duct and very high static pressure. The exact amount of static pressure would be based on the ability of that particular blower motor and wheel to build up pressure. 

So when we are measuring static we are measuring pressure in the duct system not flow.

The more powerful the motor, the more pressure it can create and the more pressure / resistance it can overcome.

Think of a blower motor like a compressor, when it is off the pressure on both the inlet and outlet are the same. In the case of a compressor the pressure when off will be the static pressure of the refrigerant (let’s say 132 PSIG static pressure for R22 at 75° ambient) in the case of a blower it will be atmospheric pressure.

When the compressor turns on the suction pressure drops below 132 PSIG and the head pressure rises above 132 PSIG. The compressor creates this difference in pressure both above and below the static, saturated refrigerant pressure.

When a blower turns on it also drops the pressure of the return side below atmospheric pressure (14.7 PSIA) and it increases the supply side pressure above atmospheric pressure. 

We measure this static pressure at various points to find out how much resistance to airflow there is at various points in the system. 

For example we may measure the pressure drop across the evaporator or the filter or a particular run of duct or across a fire damper (to see if it’s slammed shut)


We also measure at the top and bottom of the appliance (furnace or fan coil) to find the Total External Static Pressure (TESP) which helps us calculate airflow when we compare to fam tables as well as helps us understand if we duct or system issues.

On a brand new, perfectly functioning system this works great.

But on an older system with a dirty blower wheel or a fan coil with a dirty coil this no longer serves its original purpose. 

If the blower wheel is dirty, the blower loses its ability to move air effectively, and therefore also loses its ability to create the pressure differential between the return and supply. 

We are trained to think that low static equals good and high static equals bad 

In the case of a dirty blower wheel or a clogged evap on a fan coil the TESP will be LOW and there will still be low air flow (low CFM).

As far as the refrigerant circuit and capacity is concerned the static pressure is meaningless, it is all a matter of how many CFM of air are traveling over the coil surface area. We use static Pressure as a diagnostic and benchmarking tool when taken together with an understanding of the system, blower specs and settings and duct design. Static pressure by itself means very little in the same way that measuring voltage or head pressure by themselves mean very little.

The point of this article is not to downplay the importance of static pressure or to explain how to measure it. The point is to remind you of two important facts.

  1. Check your blower wheel, blower direction, coils, filters, blower settings and other obvious airflow restrictions and issues first. 
  2. Before measuring your static think carefully about where you are placing your probes and what you expect to see / what you diagnosing with the measurement.

— Bryan

For a detailed explanation of static pressure you can go HERE

Or see a great video by Jim Bergmann using a Testo manometer HERE or Corbett Lundsford on static HERE

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 relationship 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 the best “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. A typical air temperature split will be between 16 and 22 degrees difference from return to 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. 350 CFM per ton or less are more common today than ever and in those cases the above chart won’t apply.

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 a 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 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 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 – bad suction valve, bad discharge valve, bad or broken crank
• 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 – bad suction valve, bad discharge valve, bad or broken crank
• Reversing valve bypassing (heat pump units)
• Kinked suction line
• Restricted discharge line
• Severe Liquid Line Restriction
High Head Pressure
• Overcharge
• Low condenser airflow – condensing fan not operating, dirty condenser, fins bent on condenser, bushes too close to 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
• Wet condenser coil
• 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.

— Bryan

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