HVAC AC

Because HVAC work varies over the course of the year, we have divided our catalog into two distinct seasons: Spring/Summer and Fall/Winter. This division helps service techs and maintenance personnel to select the proper test equipment for each time of the year. The HVAC AC category includes testers used to troubleshoot problems in AC cooling units, circuits and ventilation systems, as well as find other issues in the refrigeration field.

The most common testers used for AC cooling and refrigeration include: AC manifold gauge sets, vacuum pumps, refrigerant recovery machines, refrigerant scales, leak detectors (both dye type and sensor based), AC charging hoses, electronic micron vacuum gauges, HVAC multimeters and clamp meters, recovery tanks, air velocity meters and CFM anemometers, and various types of thermometers.

HVAC AC Service Technicians now most often use digital manifold gauge sets as they provide more accurate readings and the ability to use the same piece of equipment for all refrigerants. They also use IR thermometers and thermal cameras to scan AC units for problems that are detected by temperature variations of various pieces of equipment. Techs also want to get more mobility and reporting features, so wireless or Bluetooth based testers are becoming a new trend.

Value Testers, as an authorized distributor of many quality brands, keeps up with these innovations in order to offer our clients the best and most cutting-edge test equipment. Our customer service techs receive training from manufacturers that we represent in order to help clients choose the best testers to meet their diagnostic needs. Please call us, and we will help you find best testers and save the most money on your purchase.

AC systems cool the air inside an enclosed space (e.g. vehicle or building) by essentially pumping heat from the air inside that space to the air outside of it. “Pumping” is an apt word since the vapor compression cycle which AC is based on has a compressor at its heart.

Inside an AC unit is a closed loop of circulating gas (the refrigerant) which undergoes a phase change from liquid to gas and then back again to liquid. These two phase changes occur in different sections of the AC system: the “high-pressure side” and the “low-pressure side”.

Let’s start with the high-pressure side, which begins at the outlet of the compressor. The compressor’s job is to draw in low pressure gas and squeeze it so that it exits at a higher pressure. When the volume is held constant, compressing a gas increases its temperature. Thus, the high-pressure side of the compressor produces hot (well above the outside ambient temperature) refrigerant under high pressure.

Upon leaving the compressor, the refrigerant then enters the condenser, which is the heat exchanger which transfers heat from the compressed refrigerant to air drawn in from outside and blown across the condenser.

The refrigerant then cools down as it travels through the long tubing of the condenser, until the gas reaches the boiling point for the pressure it’s under. This causes some of the refrigerant to change from gas to liquid, releasing heat (latent heat).

Under ideal conditions, the refrigerant becomes entirely liquid as it exits the condenser and enters the metering device (either a capillary tube or a TXV) which restricts (throttles) the transfer of the liquid refrigerant into the low-pressure side, where the cooling actually occurs.

The metering device is the second device which maintains the separation between the high-pressure and low-pressure sides. By creating a significant flow restriction, a large pressure drop is developed across the metering device.

The liquid refrigerant travels through the metering device and into another large heat exchanger: the evaporator. The pressure inside the evaporator is much lower than in condenser, and as a result, the refrigerant’s boiling point is too. This causes the liquid refrigerant transition from liquid to gas, absorbing the latent heat of vaporization it previously released in the condenser.

This evaporation cools the refrigerant as well as the tubing and fins of the evaporator. Air from the space to be cooled is blown across the evaporator, and thus that air gets cooled. As the liquid refrigerant travels through the evaporator, more and more of it becomes gas until (ideally) all of the refrigerant leaving the condenser is in the vapor (gaseous) phase.

The refrigerant gas is at will be relatively “cool” and at low pressure when it gets drawn into the inlet (low-pressure port) of the compressor, where it is compressed (and therefore heated) and the cycle begins again.

Using the process described above, AC units remove heat from the evaporator and transfer it to the condenser. At the condenser, that heat is transferred again to the outside air by blowing that air via a fan over the tubing and fins of the condenser. The now warmer ambient air disperses into the atmosphere, and is (practically speaking) the end of the story for the hot side of the AC system.

The cold side of the AC system, however, extends far beyond the evaporator. A fan circulates air from the space to be cooled through the evaporator and throughout that space by way ducts and registers (also called grilles or vents).

The ducts and grilles must be appropriately sized and placed to deliver the air flow necessary for uniform and optimal cooling. Just as degrees and psi units are employed to quantify the temperatures and pressures respectively inside an AC system, cubic feet per minute (CFM) is used to specify the air flow capacity of the AC registers and grilles.

The registers through which the cooled air returns often feature adjustable vanes which can both restrict and direct the flow of the cooled air in that particular area of the house.

For residential AC systems, the main control device is the thermostat. Thermostats measure the air temperature and electrically switch the AC to cool the air if that air temperature is above an adjustable set point, or switch it off if the temperature is below that set point.

In reality, a degree or two of hysteresis is added so that the AC unit isn’t constantly being turned on and off. By waiting not beginning cooling until the air temperature is one or two degrees above the set point (and conversely, not turning off the cooling until the ambient temperature drops a degree or two below the set point), the AC system and all of its components are spared excessive on-off cycling, extending their useful lifetimes.

Many thermostats now use electronic components for both the sensing the switching. They use some kind of temperature sensitive electronic component to feed an analog voltage signal to a chip which translates that signal into a number for comparison against the stored target temperature.

Originally, thermostats used relied on a bimetallic strip whose bend radius depended on the ambient air temperature. This strip is then used as one of the contacts for the switch which turned the AC unit either on or off. These older mechanical/electrical hybrid devices can still be found at work in houses today.

WiFi-enabled home AC thermostats are now on the market which can connect to the Internet via the home’s wireless network. This allows users to monitor and control their home’s temperature through their smartphone or tablet. For instance, someone who bumps up their thermostat during the summer while they’re away at work in order to save energy may choose to begin cooling their home before they even leave the office to ensure that their house is cool and comfortable the moment they walk in.

The AC cooling cycle described above is commonly implemented as a single self-contained unit—a large box which either hangs out of a window or is installed on top of a roof or on the ground next to a house or other building. In these AC units, both the high- and low-pressure parts of the cycle (are their respective components) are housed with the same physical box.

It is also possible, however, to physically separate those halves of the cycle: one unit containing the compressor and condenser (and its blower) while another unit houses the evaporator and cold side vent and blower. The unit containing the evaporator is installed inside, and the unit containing the condenser is usually located outside. The sections are joined by conduit containing tubing for moving the refrigerant to and from the outside unit to the inside one.

AC cooling systems which are separated in this way are referred to as “split” AC systems, in contrast to the “packaged” AC systems which contain both cycles in one unit.

It’s important to note that split AC systems don’t use ducts, due to the fact that more than one inside unit can be connected to a single outside unit, and thus each room can be cooled with ducts and grilles circulating air through the evaporator.

There are two main advantages to using a split AC system over a packaged one:

Even though split AC systems have advantages over packaged units, it’s seldom cost effective or practical to switch a building from a packaged AC system to split AC units. This is mostly due to the costs of installing split AC units exceeding those of simply replacing one packaged unit with another one.

The logic of course cuts both ways. Installing ductwork in a house where none previously existed adds considerably to the cost of installing a packaged AC system. This is why it’s recommended by AC pros to continue to use the same type of AC system that was previously used when replacing units.

Split AC systems can be a very efficient and practical AC solution for many buildings.

Each of these three functions requires different wire diameters (i.e. wire gauge sizes), lengths, as well as amount and type of insulation.

Thick (lower wire gauge) wires have less electrical resistance than thinner wires over a given wire length. That’s why wires and cables carrying lots of current have to be thicker. Due to Ohm’s Law, the the more current flows through a given resistance, the larger the voltage drop across that resistance. The larger the voltage drop across the wire, the smaller the source voltage available for the device consuming the electrical power. Wires which are not thick enough can thus cause a device to be starved for current.

Wire gauge size also determines the amount of heating that wire will experience when carrying a given current. As wire resistance increases, so does the amount of electrical power that is dissipated (lost) as heat. Therefore, wire size is a safety issue because overheated wires can melt or compromise their insulation and cause fires or dangerous short circuits. This is also the reason why wire size and insulation rating can be dictated by code.

Length also becomes a practical concern. Two wires of the same gauge but different lengths will have different resistances, with the longer wire having the higher resistance.

The term “ampacity” refers to the amount of current a wire or cable can safely and practically carry without overheating. Ampacity is a useful concept because any wire can carry any amount of current. It all depends on whether you want a make a long-lasting and safely operating system or a short-lived light bulb.

Of course, HVAC techs paid to ensure the former. That’s why the industry has developed tools to check, evaluate, troubleshoot, and diagnose electrical wiring.

There are two main areas of concern regarding electrical wiring:

Basic function of the circuit: Is there electrical continuity from one end of the wire to the other? Is the electrical resistance of that wire low enough to prevent excessive voltage dropout?

The condition of the wire’s insulation: Visually, the insulation may seem intact, but deterioration over time due to sunlight, heat, cold, and the absorption of moisture, dirt, and other contaminants. This deterioration can cause the insulation to fail suddenly and catastrophically. Even before catastrophic failure, insulation deterioration will can cause unacceptable amounts of current to leak out, which will create a safety hazard.

Both of these concerns apply to all 3 uses of wiring we listed above (i.e. delivering power, transmitting control signals, or magnet coil windings). Sudden insulation failures can create system malfunctions, unwanted current flows between different conductors in the same cable, rapidly heating short circuits between a current-carrying wire and ground, blown fuses, fire and expensive AC compressor, blower, or pump replacements.

There are many kinds of electrical testers an HVAC service professional can use to detect signs of insulation compromise and troubleshoot wiring problems.

One class of tool is a Digital Multimeter (DMM). DMM’s can perform a variety of useful electrical tests and measurements:

. . .and many others

When it comes to electrical cabling issues, the continuity and resistance modes are usually the most useful functions.

For continuity testing, the DMM applies a small direct current (DC) voltage difference to the probes. If there is electrical continuity between the probes (e.g. they are connected by an unbroken length of wire), a current will flow and the DMM will audibly “beep” if the resistance is low. Continuity tests show if two points in a circuits are electrically connected.

When a DMM is placed into resistance measuring mode, the DMM measures how well two points are electrically connected. The DMM passes a constant current through the resistance present between the probes and measures the resulting voltage (utilizing Ohm’s Law).

Whereas continuity testing is good for quickly determining if two points are connected by a wire, this mode usually has a max resistance limit of 40 ohms or less. The resistance measurement mode of standard DMMs can reach tens of megaohms (one megaohm is equal to a million ohms), a range which is very useful for a quick check of the integrity of wiring insulation. Since the very purpose of that insulation is to create a very high (ideally, infinite) resistance barrier to current flow, lower than expected resistance readings between two conductors separated by that insulation indicates that it’s wet, damaged, or degraded.

Due to gradual drying out and the absorption of moisture, dirt, dust, and other contaminants over time, wiring insulation can deteriorate and yet still give a high resistance reading right up to the moment it catastrophically breaks down and causes a problem.

This is why a second class of more specialized electrical testers exist to more comprehensively diagnose the state of the insulation.

Megohmmeters

This second group of testers are often called “insulation testers” or “megohmmeters”. They differ from DMMs in that megohmmeters have much higher internal resistances, produce much higher voltages on the test leads, and can accurately measure resistances well into tens or even hundreds or gigahoms.

Furthermore, theses testers are used for four related, but distinct insulation test methods:

Insulation Resistance (IR) test: The system is first de-energized and disconnected. A high voltage is applied across the insulation to be tested for a period of one minute. During that time, the resistance is monitored. Insulation in good condition should produce either no significant change or a slight decrease in resistance during the testing interval. The final value at the one minute interval is recorded. This test is impacted by temperature, and thus the ambient temperate will have to be recorded as well, since a temperature dependent correction factor has to be applied before comparing readings at different temperatures.

Step Voltage test: Essentially, two IR tests are performed on the insulation, each one at different voltages. The two reading are then compared. A difference of over 25% is a sign that the insulation has significantly aged or is damaged.

Polarization Index (PI) test: Like the Step Voltage test, the PI test requires two readings. The two IR readings of the PI test are taken at one and 10 minute intervals. Good insulation without absorbed moisture or impurities will show a lower IR value at one minute than at 10 minutes. During the test, good insulation will polarize like the dielectric layer in a capacitor, and there will be a small amount of current associated with that process. Over time, that current will actually decrease, as the insulation reaches full charge (again, just like a capacitor charging). Moisture and impurities absorbed in the insualtion, however will function as charge carries and thus the current flow due to that absorption will remain constant. The PI ratio of the 10 minute reading to the one minute reading can be a useful indication of the amount of both "polarization" and absorbed impurity current. The lower the PI ratio, the greater the contribution of the absorbed impurities and thus the greater amount of impurities absorbed into the insulation.

Dielectric Absorption (DA) or Dielectric Absorption Ratio (DAR) test: When the leakage current through insulation stabilizes within one minute, the PI test is not very helpful, since the ratio equals 1. For these instances, the DA is used instead. The DA ratio test uses 30 seconds and one minute as the IR measurement time intervals.

Since both the DA and the PI tests are ratios, temperature corrections are not needed.

With the right test tools and methods, HVAC techs can perform detailed and thorough electrical testing and troubleshooting of all kinds of HVAC/R systems.

Capacitors are devices which store electrical charge. When an external voltage source is applied to a capacitor’s terminals, charge accumulates on two separate, but very physically close metal plates inside the capacitor. These two plates are separated by a thin insulating layer, called a dielectric. Interesting fact: the amount of charge stored inside a capacitor increases proportionally with the applied voltage—doubling the voltage also doubles the number of electrons pulled off of one plate and moved to the other. In fact, the unit of capacitance, the farad, is defined as one coulomb of charge accumulation per one volt.

The material the dielectric is made of, the surface area of the plates, and the distance between them determines the capacitance. Since there are practical limits on the size, weight, and cost of useful capacitors for each application, most capacitors found electrical circuits—including AC systems— have capacitances in the microfarads (one microfarad is equal to one millionth of a farad), nanofarads (one billionth), and even picofarads (one trillionth). The main capacitor HVAC techs are concerned about, however, usually falls within the microfarad range.

There are other practical constraints on capacitor design. Every dielectric material has a breaking point at which the electric field generated by the difference in charge of the plates is intense enough to break the neutral atoms into ions and mobile electrons. At this point, the normally insulating dielectric starts conducting, and all the stored energy on the capacitor’s plates is released in a fraction of the second. This is called “dielectric breakdown” and results in catastrophic failure of the device as the extreme localized heating permanently damages the capacitor.

There’s one example of dielectric breakdown we’re all familiar with: lightning strikes. During a thunderstom, a difference in static electrical charge builds up between the cloud and the ground until the air between them breaks down into ions and electrons and starts conducting. Those ions and electrons are accelerated by the electric field created by the two large opposite static charges, and as they accelerate, the electrons bump into other neutral atoms in the air with enough energy to ionize them as well, making the air even more conductive and causing more ions and electrons to accelerate. These newly freed electrons then go on to knock still more electrons free, and so on.

Dielectric breakdown is worth explaining because it’s the reason why capacitors are given a second rating in addition to their capacitance: the maximum working voltage, which is low enough to avoid dielectric breakdown, with safety margin to spare. In general, the thicker the dielectric layer, the higher voltage that layer can withstand before it breaks down.

Thus, when HVAC technicians have to replace a start/run compressor capacitor in the field, the new capacitor must be of the same microfarads rating (for reasons we’ll explain below) and at least the same voltage rating as the previous capacitor, assuming the old capacitor met the specs of the AC unit manufacturer.

Small capacitors are found in lots of circuit boards, where they filter out electrical noise, serve as charge tanks for supplying momentary high current demand while keeping voltages stable, and produce delays as part of timing circuits.

In an AC cooling system, however, the most important capacitor is the large one connected to the compressor and fan. This capacitor increases the energy efficiency of the compressor, which is the biggest power consumer in the system by raising the power factor (the ratio of how much of the applied AC power does any useful work). In short, the large capacitance value of this capacitor cancels out the large inductance caused by a motor’s coil windings while the motor is running.

This capacitor is usually a “split” type, meaning that there are electrically two separate capacitors inside the can-like package: one with a large capacitance for the compressor, and a smaller one for the fan, whose smaller motor requires less capacitance to balance its inductance.

Because this capacitor must operate at high AC voltages (and thus has thicker dielectric layers) and have a large amount of capacitance (read: large plates), the main AC capacitor is physically large.

Even though the capacitor doesn’t physically move during AC unit operation, and thus doesn’t have any parts which wear out, the capacitor does degrade over time, leading to reduced capacitance and even catastrophic failure. Here are the most common failure modes of AC unit’s main capacitor:

Even if a capacitor seems to be fine electrically, it should be replaced if it has burst, or if is any visible bulging or leaking oil.

Although standalone capacitance testers are commercially available, most modern HVAC-focused DMMs have capacitance measurement built in. While this theoretically makes measuring a capacitor as easy as ohming out a resistor—just power off the equipment, disconnect the device from the circuit, place the probes on the terminal, and read the screen—in practice many HVAC pros choose to measure capacitance indirectly.

This indirect method involves using the meter’s amp clamp to measure both the start winding amps and the voltage between the capacitor’s terminals connected to the compressor. Using a simple formula, the technician can calculate the actual microfarad value of the capacitor as it is working in the circuit. Another benefit is that this method is often quicker because no terminals have to be disconnected and reconnected.

Regardless of the method used, if the capacitor’s actual value is outside the AC manufacturer’s specifications after factoring in tolerance, then it should be replaced.

On modern AC systems, the compressor motor is almost always housed together with the compressor in a single sealed unit. This packaging helps prevent dust, moisture, and other contaminates from infiltrating and then degrading the motor’s wiring or prematurely wearing out its mechanical parts, such as the seals and bearings.

In both the compressor and blower motors, alternating current (AC) electricity flowing through the wires of the motor coils creating alternating magnetic fields, and these fields then either induce secondary alternating fields in the rotor (as in the case of induction motors) or push and pull directly on the magnetic fields created in the rotor electromagnets or permanent magnets (synchronous motors). In either case, the interaction of these fields causes the motor rotor, which is attached to the output shaft, to spin.

One of the most common types of failure encountered in AC cooling system motors is a short between the coil windings. Although the thin wire which comprises the coils is covered by a layer of insulation, this electrical barrier can deteriorate over time, leading to a short circuit between two or more windings.

These short circuits are a problem because the electrical current will flow through this electrical shortcut instead of taking the longer path through the coil. The result is a weaker magnetic field and resultant drops in motor performance and efficiency. Since both the resistance and inductance of a coil decrease when there’s short present between windings, the motor will draw excessive current and may overheat.

A break in the coil wiring (i.e. open circuit) is another failure mode. If a part of the coil wire is broken, current stops flowing through the whole coil. This leads to a dramatic performance drop, and may even prevent the motor from turning at all.

Since motors are both electrical and mechanical devices, they exhibit mechanical failure modes as well. For instance, rusting can cause excessive friction. Bearings can degrade or wear out.

Problems external to the motor can also cause premature motor failure. A motor which is mechanically loaded beyond its specs will draw excessive current for long periods of time, leading to overheating. That overheating can accelerate the degradation of the insulation of the coil’s wires, leading to shorted windings and even more overheating. Excessive head pressure can cause such overloading.

In hermetically sealed compressor motors, the cool refrigerant helps keep the motor temperature within limits and prevents failure due to overheating (aka “burning out”). Therefore, if an AC system loses refrigerant, that can lead to motor failure.

If the AC voltage being fed to the motor drops too much, the motor will pull more current that what it’s rated for. Such “line drop” can be the result of inadequate wiring for the motor load. The wiring feeding the mains voltage to the motor must be sized to accommodate both the motor’s peak (startup) and running current demand.

Since the vast majority of AC system motor failures are electrical in nature (or have electrical causes), electrical tools and testers such as digital multimeters (DMMs), ohmeters, and megohmeters are used to inspect motors and troubleshoot their failures. Many HVAC-focused DMMs have the ability to accurately measure the low resistances typical of motor coil windings.