Our Main man Bill Spohn joins us again to talk specifically about combustion and how to select and properly utilize an analyzer
There are many examples of teaching using metaphor to help someone get a grasp of how something works without being EXACTLY correct.
Some examples are how we often use water flow to explain electrical flow or refrigerant circuit dynamics. It's enough like the way it works to get our heads wrapped around it but there are many differences and the metaphors eventually break down.
This is definitely the case with air and nitrogen “absorbing” water
I've done podcasts and videos about how air can “hold” less moisture when it is cooler and more when it is hotter. You have likely heard old school techs talk about triple evacuation and sweeping with nitrogen to “absorb” the moisture from the system.
News Flash, Air and Nitrogen DO NOT absorb or hold moisture… They ignore one another at parties and they certainly don't shake hands.
Water vapor in the air behaves much like all the other gasses contained in the air with the notable exception that water exists in both vapor and liquid states at atmospheric pressure and temperature.
When the temperature of water vapor is higher, a higher percentage of the air by volume can CONTAIN water vapor, but the air itself isn't what is holding it. It does interact with it as the molecules move and bounce around and the percentage of water vapor in the air does impact the mass/weight of the air by volume (water vapor weighs less than dry air) so there are certainly impacts to the makeup of the air based on moisture content.
The percentage of the air around us that is moisture can vary from almost zero In cold arctic & Antarctic climates to nearly 4% in hot, tropical climates.
When teaching it we speak as though the air is a sponge and the hotter the air the bigger the sponge. This certainly helps us remember but it isn't really how it works. In reality water in the air is all about the saturation temperature and pressure of the water and the air has little to do with it.
This is the same sort of thinking when a tech is having a hard time pulling a vacuum and they add dry nitrogen to the system to “absorb” the moisture. First off, you will want to sweep the nitrogen through the system, not just pressurize. Secondly, the nitrogen has no special properties that allow it to “grab” moisture. It can entrain the water vapor using Bernoulli's principle, it will warm up the system a bit, it will certainly add in a bit of turbulence which can help move the oil around and potentially release some trapped moisture… but nothing more than that.
Don't get me wrong, there is nothing wrong with sweeping with dry nitrogen, even better to use a heat gun and warm the compressor crankcase, receivers and accumulator and coils during a deep vacuum on a large system to help speed up the vaporization of moisture.
It doesn't change the fact that air and nitrogen don't “hold” moisture.
The most common and often most frustrating questions, that trainers and senior techs get goes something like this. “What should my ______ be?” or “My _____ is at ______ does that sound right?
Usually, when the conversation is over both the senior and junior techs walk away feeling frustrated because the junior tech just wanted a quick answer and the more experienced tech wants them to take all of the proper readings and actually understand the relationships between the different measurements.
In this series of articles we will explore the, “What should my _______ be?” questions one at time and hopefully learn some things along the way.
So what should the superheat be?
First, what is superheat anyway? It is simply the temperature increase on the refrigerant once it has become fully vapor. In other words, it is the temperature of a vapor above it's boiling (saturation) temperature at a given pressure.
The air around us is all superheated! Head for the Hills!
How can you tell that the air around us is all superheated? Because the air all around us is made of vapor. If the air around us were a mixture of liquid air and vapor air, first off you would be dead and secondly, the air would be at SATURATION. So the air around us is well above its boiling temperature (-355° F) at atmospheric pressure which means it is fully vapor and SUPERHEATED. In fact, on a 75-degree day, the air around you is running a superheat of 430°
But why do we care?
We measure superheat (generally) on the suction line exiting the evaporator coil and it helps us understand a few things.
#1 – It helps ensure we are not flooding the compressor
First, if we have any reading above 0° of superheat we can be certain (depending on the accuracy and resolution of your measuring tools) that the suction line is full of fully vapor refrigerant and not a mix of vapor and liquid. This is important because it ensures that we are not running liquid refrigerant into the compressor crankcase. This is called FLOODING and results in compressor lubrication issues over time.
#2 – It gives us an indication as to how well the evaporator coil is being fed
When the suction superheat is lower it tells us that saturated (boiling) liquid/vapor mixture is feeding FURTHER through the coil. In other words, lower superheat means saturated refrigerant is feeding a higher % of the coil. When the superheat is higher we know that the saturated refrigerant is not feeding as far through the coil. In other words higher superheat means a lower % of the coil is being fed with saturated (boiling) refrigerant.
The higher the % of the coil being fed the higher the capacity of the system and the higher the efficiency of the coil.
This is why on a fixed orifice system we often “set the charge” using superheat once all other parameters are properly set. Adding refrigerant (on a fixed orifice / piston / cap tube) will feed the coil with more refrigerant resulting in a lower superheat. Removing refrigerant will increase the superheat by feeding less of the coil with saturated (mixed liquid and vapor) refrigerant.
This method of “setting the charge” by superheat does not work on TXV / TEV / EEV systems because the valve itself controls the superheat. This does not negate the benefit of checking superheat, it just isn't used to “set the charge”.
#3 – We can ensure our compressor stays cool by measuring superheat
Most air conditioning compressors are refrigerant cooled. This means that when the suction gas (vapor) travels down the line and enters the compressor crankcase it also cools the motor and internal components of the compressor. In order for the compressor to stay cool, the refrigerant must be of sufficient volume (mass flow) and low temperature. Measuring superheat along with suction pressure gives us the confidence that the compressor will be properly cooled. This is one reason why a properly sized metering device, evaporator coil, and load to system match must be established to result in an appropriate superheat at the compressor.
#4 – Superheat helps us diagnose the operation of an active metering device (TXV / TEV/ EEV)
Most “active” metering devices are designed to output a set superheat (or tight range) at the outlet of the evaporator coil if the valve is provided with a full liquid line of a high enough pressure liquid (often at least 100 PSIG higher than the valve outlet / evaporator pressure). Once we establish that the valve is being fed with a full line of liquid at the appropriate pressure we check the superheat at the outlet of the evaporator to ensure that the valve itself is functioning properly and /or adjusted properly. If the superheat is too low on a TEV system we would say the valve is too far open. If it is too high the valve is too far closed.
#5 – Superheat is an indication of load on the evaporator
On both TEV / EEV systems and fixed orifice systems (piston / cap tube) you will notice that when the air (or fluid) going over the evaporator coil has less heat, or when there is less air flow (or fluid flow) over the evaporator coil the suction pressure will drop. However, on a TEV / EEV system as the heat load on the coil drops the valve will respond and shut further, keeping the superheat fairly constant. On a fixed orifice system as the load drops so will the superheat. It can drop so much on a fixed orifice system that when the system is run outside of design conditions the superheat can easily be zero resulting in compressor flooding.
When the load on the evaporator coil goes up a TEV / EEV will respond by opening further in an attempt to keep the superheat constant. A fixed metering device cannot adjust, so as the heat load on the coil goes up, so does the superheat.
When charging a fixed orifice A/C system you can use the chart below to figure out the proper superheat to set once all other parameters have been accounted for or you can use our special superheat and delta t calculator HERE
Using this chart requires that you measure indoor (return) wet bulb temperature so that the heat associated with the moisture in the air is also being accounted for as well. This is one of MANY target superheat calculators out there, you can use apps, sliderules etc… Here is ANOTHER ONE
Remember, this chart ONLY applies to fixed orifice systems.
So what should your superheat be in systems with a TEV / EEV? The best answer is… like usual… Whatever the manufacturer says it should be.If you really NEED a general answer you can generally expect
High temp / A/C systems to run 6 – 14 degrees of superheat
Medium Temp – 5-10
Low Temp – 4-10
Some ice machines and other specialty refrigeration may be as low as 3 degrees of superheat
When setting superheat on a refrigeration system with any type of metering you often must get the case / space down close to target temperature before you will be able to make fine superheat adjustments due to the huge swing in evaporator load. Once again, refer to manufacturer's design specs.
Jaden Lane joins us to discuss some best practices when using hoods and how the Dwyer Smart is innovating in the hood space
Sensors, Measurements, and Physics
As HVAC/R Technicians, we use tools and instruments to make measurements every day. In fact, 90% of our job could not be done efficiently without some kind of measurement.
“How do we measure?”
“With what instruments?”
“How accurate are these measurements?”
These are all questions a thoughtful technician should ask before spending money on a tool or implementing solutions to solve a problem.
Tools are our primary resource for measurement. Measuring tapes, scales, pressure transducers, thermocouples; the list goes on. But how exact are these measurements? How precise must our measurements be for us to use them to make decisions regarding the mechanical operation, occupant comfort, and occupant health? To answer these questions, we need a crash course in a little bit of physics.
Don’t worry; we aren’t going into the rabbit hole too deep. I simply want to introduce you to a concept called the Heisenberg Uncertainty Principle in quantum mechanics. The Uncertainty Principle states that the more precisely you determine a particle’s position, the less precisely you can determine that particle’s momentum. Basically, the Uncertainty Principle limits our ability to measure things exactly. In modern physics, there simply is no such thing. There is only the agreed-upon accuracy and precision everyone is satisfied with to make practical decisions. For example, if you asked me how tall I was, my answer might be 5’11”. But is that 5ft. 11in. exactly? The line on the measuring tape with which I measured my height has a thickness, doesn’t it? Where within the thickness of that line do I fall? This principle is, of course, much more noticeable in the quantum (atomic) scale than on the macro scale. However, when measuring airflow and trying to solve occupant health concerns by measuring indoor air quality, accuracy, and precision matters. Our margin of uncertainty matters.
A technician doesn’t have to lose sleep over the fact nothing can be measured exactly. In our trade, and most of life, it’s not necessary. We can use this knowledge, however, to be more critical about the types of tools to choose to use. A duct traverse with a rotating vane anemometer can provide a quick and dirty idea for system airflow. Still, it’s not accurate nor precise enough to make complex troubleshooting decisions based on its results. The margin of uncertainty is too high. A flow hood or The Energy Conservatory’s TrueFlow Grid would be required for more accurate measurements upon which to base airflow balancing solutions. So how do we quality check for accuracy and precision? First, here’s a diagram showing the difference between the two and various combinations of accuracy and precision:
It’s important to understand the differences between these different combinations of accuracy and precision. Take any three brands of micron gauges and do a vacuum pump test. It helps if you have a digital gauge on the pump itself. Pull only on the pump first, and record the level of vacuum achieved (a good vacuum with fresh oil should be able to pull below 50 microns). Next, add the gauges one at a time and pull a vacuum in three more separate tests. You will very likely record three different micron levels for each gauge. In this experiment, the pump was our reference. In reality, the pump gauge itself would also need to be scrutinized for accuracy and precision, but as a demonstration, this test works well. Looking at the recorded micron levels from the gauges relative to the pump, where on the graph would your test results lie? Most are either accurate, but not precise, or precise, but not accurate.
When thinking about buying a measurement tool, first consider what you are trying to accomplish with that measurement. If you aren’t planning on making accurate and precise capacity calculations, then you don’t really need the more accurate and precise hygrometers and airflow measurement tools. You may not need a digital manifold to charge a system to proper superheat and subcooling, because the margin of uncertainty is forgiving in that context. But a more accurate and precise sensor for a digital manifold or probe sure makes a pin-hole leak during a standing pressure test a lot easier notice.
Sensors are another factor to consider when choosing a measurement tool. Sensors are responsible for most of what makes a tool more expensive (not always, but most of the time). The product data for any instrument can be found either in the manuals or through a quick phone call to the manufacturer. In a recent podcast with Aeroqual’s Bernadette Shahin, we discuss some points to look for when researching the quality of a particular sensor.
- A sensor that can measure the target parameter only (humidity, pressure, CO, microns, VOCs, etc.)
- A quality sensor should be able to have low cross-sensitivity to other parameters and should be stable in its ability to measure the target parameter over time.
- A sensor has a published response time to change, but a manufacturer can confirm how long a sensor takes to reach an acceptable range of accuracy and precision expected from the sensor
Home automation is becoming very popular in the HVAC trade, and many are using Indoor Air Quality monitors to determine when and for long a mechanical system will operate, in order to maintain a comfortable and healthy home. To do this effectively, the accuracy and precision of the sensors in that monitor should be scrutinized. AQ-Spec is an excellent resource for this specific type of evaluation. Other manufacturers may have done their own third-party testing, and results can be released upon request.
Keep in mind we aren't referring to this concept as the “margin of error,” as many often call it. There is hardly ever any ill-intent when it comes to making a measurement, and manufacturers are not trying to create poor quality products. There are simply varying levels of uncertainty when we are using the tools at our disposal, and picking the right tool for the job is essential for quality solution implementation.
Another principle in physics, which all technicians should be aware of, is the Observer Effect. This theory states that the very act of observing a property's state of being will inevitably change that property's state of being altogether. In other words, by the time you have made a measurement, the state of whatever property you are measuring has changed, and no longer holds the same state of being it held before you started to measure it. A good example of this would be connecting hoses to a system. The mere act of attaching your hose has let out a little bit of pressure from the system; therefore, the pressure you will read is different from the pressure the system held the moment before you connected. This example might incentivize some to make the switch to probes, and ditch the hoses! Another example of the Observer Effect is checking electrical current on an indoor blower motor with the cabinet door removed.
One last point to make…accuracy and precision ≠ resolution. The resolution simply specifies to what decimal point the measurement is going to display. The resolution does not provide any direct information about accuracy, or precision. So next time you are comparing tools to see which is the best quality for the price, you may find a more accurate tool at a lower relative resolution. It is important to note, however, that precision and resolution are related. The higher the precision, the higher the resolution necessary to interpret the readings. It's all about what you need the measurement to do for the application in which you use it.
Here are some links, in case you want to gain more insight into sensors, the Observer Effect, and the Heisenberg Uncertainty Principle:
Piping support is covered in Section 305 of the IMC (International Mechanical Code) which once again, isn't binding but is the code that most local codes are based on.
|Piping / Tubing Material||Maximum Horizontal Distance Between Support||Maximum Vertical Distance Between Support|
|Copper Tubing 1 1/4″ & Smaller||6′||10′|
|Copper Tubing 1 1/2″ & Larger||10′||10′|
|CPVC 1″ & Smaller||3′||10′|
|CPVC 1 1/4″ & Larger||4′||10′|
You will notice pretty quick that 10′ is the vertical support distance in all of these common cases. When supporting horizontally It's also important to use supports that won't compress insulation on insulated suction lines and drain lines.
There are really nice saddles made nowadays for insulated lines like the B-line Snap ‘n Shield supports shown above. We recently re-insulated a large grocery store with overhead copper and replaced the existing supports with these to eliminate condensation.
Russ King joins us to discuss simplifying duct design for residential contractors
We keep 2 pole 40 amp 24v coil contactors on all of our vans. They are versatile, reliable and you can replace most residential A/C contactors with them.
There are a few things to watch for though, especially when you have a crankcase heater. Many brands power the crankcase heater constantly and shut it on and off with a thermostat, often mounted on the discharge line (here's looking at you Trane). When you replace a single pole with a two pole contactor in this type you need to make sure you connect BOTH sides of the crankcase circuit across the L1 and L2 line side of the contactor to ensure the heater can function when the compressor is off.
Even more confusing that that…. Look at the diagram at the top and focus on the top left part of the diagram where the crankcase heater is located…
How does that work do you think?….. I will wait while you think it through…. Don't cheat… Look at it.
This is a common Carrier Heat Pump crankcase heater configuration.
You notice that one side of the heater is going to L1 line side Terminal 1 and the other side is going to L1 load side terminal 2.
So the crankcase heater ONLY functions when the compressor contactor is OPEN and even then it does so by back feeding through the compressor common and back through the run winding of the compressor to the constant powered L2 side of the contactor.
This means if you replace this contactor wire for wire with a 2 pole contactor the crankcase heater will never work. You must put the compressor run wire (yellow) to the bottom of the contactor (L2 line side) instead of the top like it was if you want the crankcase heater to function in this situation…
All of this to remind you, DON'T BE A PARTS CHANGER! Know what you are replacing, why you are replacing it and what each wire and component actually does.
❶ Understand What a Run Capacitor Does
The run capacitor provides continuous phase-shifted current to the motor start winding allowing the motor to run
- With the design efficiency
- In the right direction
- With the appropriate torque
- With near “unity” power factor (power factor near 1.0)
If the run capacitor is failed often the motor won't run in the case of high torque motors like a compressor or in the case of fans they may run backwards or slowly or with high amperage or overheat.
❷ Understand Why a Run Capacitor Fails
A run capacitor fails due to
- Poor manufacturing quality
- Overtemperature (They got hot)
- Over Voltage (The voltage was too high)
Many will say a failed motor “takes out” the capacitor. In actuality a failed or weak capacitor can take out a motor
❸ Visually Inspect
If a capacitor shows physical damage such as the top bubbling or oil leakage then it should be replaced. Normal rust is not a reason to replace a capacitor. Note the MFD or uF rating listed on the capacitor. The voltage rating is also worth noting but you may use a HIGHER voltage rated capacitor but not lower.
❹ Decide if under load or bench testing is best
If the system is currently running then an under load test may be best. Do not do an under load test on blower capacitors due to the risk of the meter leads around a spinning blower wheel. If the system is NOT running then a bench test will be the best bet.
❺ Bench or Under Load Test?
Choose bench for simplicity or if the system is not running. Choose under load because it can be done in real load conditions on a running system.
Bench Test Go to step ❼
Under Load Test Go to step ❻
❻ Test Under Load
In order to test under load, you need to take measurements with the system running. Wear proper PPE and only do so when safe. You need to have an accurate multimeter that can measure Voltage and Amperage reliably. Often under load measurements may come out high if the amp clamp picks up interference from other circuits. Measure the amperage on the start wire with the wire centered in the clamp and multiply by 2652. Now measure the voltage across the capacitor and divide the amperage x 2652 by that voltage to find the capacitance in MFD.If the under load MFD is less than 10% low we suggest replacement. If it is over the rating it is often a mis-measurement
If a capacitor measures weak via under load test go ahead and perform the bench test.
❼ Bench Test
Bench Testing is simply removing both leads from the run capacitor after safely disconnecting power and discharging the capacitor. You then place a meter designed to test capacitance across the terminals and note the reading. Be careful not to touch the meter probes and to get a good solid connection to the metal connection spades on the capacitor.If the measurement is more than 10% we suggest a replacement
❽ Inspect the dual run capacitor and note where each wire is connected
Taking a picture is one of the easiest ways to remember before removing the wires.
❾ Remove disconnect
It is imperative that the disconnect is removed or the unit is off and without potential. Test using a meter that is pre-tested to a known voltage source and check L1 to L2 and L1 – ground and L-2 ground to ensure no voltage is present.
❿ Discharge capacitor
Before testing, touching or removing the capacitor, you need to discharge it. Do this using a high resistance resistor bridging HERM and Fan terminals across to C or across a single capacitor.
NOTE: It is actually very rare for a run capacitor to contain a charge on a normally running system because it bleeds off through the compressor windings UNLESS one of the windings is open. FOr this reason many techs opt to just use a screwdriver to discharge which is controversial but common practice.
⓫ Disconnect wires and remove used dual run capacitor
To remove, disconnect wires on the top of the capacitor and also remove the strap holding the capacitor in place.
⓬ Replace capacitor with same MFD rating
Apples to Apples: You must use the same MFD rating capacitor during this process. This will be located on the box and also the side of the capacitor. Mount the capacitor upright with terminals pointed up.TEST THE NEW CAPACITOR VIA BENCH TEST BEFORE INSTALLING
⓭ Re-sizing of strap
Sometimes the new capacitor might be larger or smaller. At this time, use the metal strapping and create a new strap for the capacitor, by cutting it to the correct size and using self tapping screws to attach to the correct area. Always check before using self tappers to confirm that you are not in any danger of puncturing the coil.
⓮ Attach the wires
At this time reattach the wires onto the top of the dual run capacitor. Make sure that the common is connected, the HERM (compressor) is connected and the FAN is connected. You should always double-check to make sure all wires are in their proper places. Make sure the terminal fit very snug, tighten them by squeezing with a needle nose before installing to make sure they are very tight.
⓯ Test system
Reconnect the disconnect or flip the breaker back on. Check to make sure that all aspects of the system are running and the compressor and fan are running at the proper amperage.Using a power meter and testing motor power factor to ensure it is near unity is a good additional practice.
⓰ Clean up and reattach panel on the condenser
Make sure you clean up the area and reattach the panel to the condenser