Month: January 2020

Capacitors are traditionally tested with a capacitance meter (commonly found as a function within a multi-meter) with the component taken completely out of the circuit. “Bench testing”, as this method is referred to, is hands down the safest method of checking capacitance in micro-farads. All other methods require the capacitor to be wired into the circuit with an applied load. To bench test, you simply take the meter leads and check across the terminals of the capacitor. For a dual-run capacitor, you would check between the Common terminal and whichever side (Fan or Herm) you wished to test.

Another popular test many technicians use is the “Under Load Capacitance Calculation”. This test is performed while the system is in normal operation. A technician would measure voltage across the terminals of the capacitor (again, Common and Motor terminals if dual-run), then current off the start winding of the motor to which the capacitor is attached. Next, you plug those values into a calculation, which uses a mathematical constant: (amps x 2,652) ÷ voltage. Finally, the product of that calculation is compared against the rated capacitance printed on the capacitor. As long as the calculated value is within +/- 6% of the rated value, the capacitor quality is acceptable.

Bench testing and capacitance calculations are pretty popular choices when verifying the capacitance of a capacitor against its rating. However, there is yet another way to test a capacitor under load you may not have thought of before. You can use a power quality meter to check the capacitor under load using power factor. In order to explain the validity of this measurement, here is a review of reactive power, inductive loads, and capacitors.

Reactive power is one of three different types of power in an alternating current circuit. True Power is the actual energy in watts dissipated by a circuit. In other words, the real work being done. Then there is Apparent Power, measured in Volts-Amps (VA). Apparent power is the RMS current multiplied by the RMS voltage. Reactive Power is the power dissipated as a result of either inductive or capacitive loads. Reactive Power is measured in Volts-Amps Reactive (VAr). When the current and voltage waveforms are out of phase with each other, that is reactive power.  Inductive loads, such as a condenser fan motor, are inductive by virtue of the fact their alternating current lags behind the alternating voltage as the current flows into the load. Capacitive loads have an alternating current waveform that leads the alternating waveform of the voltage.

For the purposes of this tech tip, inductive loads will be exclusively discussed, because they are most common in the residential field; i.e. condenser, blower, and compressor motors. Inductive loads use a magnetic field to cause physical movement. The magnetic field is generated as electric current flows through a coil. In other words, this current/energy used to generate a magnetic field is known as reactive power. Notice, however, there is no real work being done. The force of the magnetic field can cause physical movement (work), but it does no real work itself. Inductive loads need reactive power in order to do work, but by using more and more reactive power, the load uses also uses more current (usually from the utility company). Take a look at the “Power Triangle”. Pictured is a power triangle depicting an inductive load. The hypotenuse of this triangle is notated as Apparent Power (the available power in the circuit). The leg on the y-axis is notated as Reactive Power (magnetic field), and the leg on the x-axis is notated as Real Power (actual work being done). If you notice the Theta symbol in the left acute hypotenuse angle (𝜭), this is referring to the Power factor of the load. Power factor (cos𝜭) is the ratio of the average Real Power in watts to the Apparent Power in volts-amps . Ideally, the Apparent and Real Power would be the same, as in a Resistive load (i.e. a power factor of 1). However, inductive loads need a magnetic field.

If the reactive power leg on the y-axis were to increase and rise higher on the y-axis, the hypotenuse (apparent power) would also increase. The power factor, in this case, decreases, and moves closer and closer to the left acute hypotenuse angle, thereby increasing the distance between the Apparent power and Real power. This is counter-productive, because as the load uses more current, more heat energy is generated, and the energy used to do the actual work becomes inefficient.

Therefore, the goal of the engineer is to minimize the amount of reactive power the inductive load uses from the apparent power. Basically, the goal is to increase power factor back to as close to unity (1) as possible. This is when capacitors enter the scene.

Capacitors are generally accepted as reactive power generators. To understand more about how capacitors work, and some common misconceptions, check out these other tech tips/podcast episodes: Run Capacitor Facts You May Not Know (Podcast) 5 Capacitor Facts You Should Know

Capacitors, when applied to a circuit, decrease the amount of apparent power needed by the inductive load to generate the magnetic field. This effectively increases the power factor. Looking at the power triangle again, as the Reactive power on the y axis decreases, the hypotenuse (Apparent power) also decreases, moving closer to the Real power. This is the endgame for capacitors.

Therefore, it can be inferred from the understanding of inductive loads and capacitors: if a capacitor is attached to an AC circuit in an inductive load (like a PSC motor) for the purpose of bringing power factor back to as close to unity as possible, but the power factor is measured to be low, the capacitor must then be either sized incorrectly or failing/failed.

Using a power quality meter on an inductive load, a technician can judge the functionality of a particular capacitor. To do this, Voltage and Current must be measured simultaneously at the load. The Supco Redfish iDVM-550 is a great tool for this application.

It must be mentioned that using a power quality meter to measure power factor on a load is valid only when the load contains run capacitors like compressors and permanent-split capacitor motors. ECMs (electronically commutated motors) use a different type of technology altogether, and they are engineered for use with a lower power factor by design. Also, power factor testing is not practical for start capacitors either, since the capacitor is taken out of the circuit too quickly. This measurement is valid and practical only for PSC type blower, condenser motors, and most single-phase, single-stage compressors. 

First, let’s get straight that BRAZING is when you use a filler rod that isn’t the exact same material as the base metal but that melts ABOVE 840°F. Soldering is the same but at temperatures below 840°F.

With HVAC rods melting at around 1200°F it confuses me why we usually call it “silver solder” but we will often also call it brazing rod. The best term to call it a “brazing alloy” and I try to remember it but I often find myself calling it silver solder.

The most common rods used for typical HVAC brazing are 0%, 5% and 15% with several other levels mixed in there.

The percentage is the percentage of silver content in the rod. The only real reason to use lower silver levels is the cost and the difference can have many techs and owners wondering what the difference is.

So the big question is –

Is more silver worth the price?  

First, let’s establish that we are talking about copper to copper applications here because that is the most common use for these rods. In copper to copper none of these phos/silver/copper rods need flux or even benefit from it. The phosphorus allows the rod to self-flux on copper and flux when overused can get in the system and cause more harm than good. Flux is required when joining brass to copper using 15%, just make sure not to use too much flux, a thin layer on the male side of the tubing only is all you need.

The silver increases the “ductility” of the filler and allows it to flow at a slightly lower temperature. This results in a better flow of solder into the joint and a lower odds of cracking with thermal expansion and contraction or with vibration. The increased silver also allows the solder to remain strong when filling slightly larger gaps due to ill-fitting copper.

Have you ever seen a leak in a discharge line fitting that you SWEAR wasn’t leaking when the compressor was installed? This can be attributed to poor brazing practices (failing to pull solder into the joint) and often you will find that 0% or 5% rod was used.

The reason we went to all 15% rod is due to the costs of callbacks and refrigerant. With labor prices and refrigerant prices increasing and technician brazing skills on the decline, we want to give techs the best possible chance of making a connection that will stand up to temperature changes and vibration, especially when we are doing an install or making an expensive repair.

If you are going to use the less expensive rod make sure it won’t be in a location with a lot of vibration, that you get the fit between the tubing and the fitting REALLY tight and that more heat is used to “draw” the solder into the joint.

The biggest mistake new techs make is just “capping” the edge rather than pulling the solder into the joint for a solid bond.

— Bryan

PS – We are big fans of Solderweld products including the round 15% Sil-Sol rods. You can find out more at productsbypros.com

When you ask many people nowadays how to check the charge on a heat pump during low outdoor temps they will say that you need to “weigh in and weigh out” the charge. While this may be an effective method it isn’t always practical.

Now… If you are making a refrigerant circuit repair, weighing out and weighing in makes perfect sense, especially since microchannel condensers and scroll compressors make pumping down less viable anyway. But there are many cases where you just need to check the charge to make sure the system is working properly and in these cases, weighing in and out would be plain silly.

I originally wrote this guideline back in 2003 and truthfully, not much has changed since then in regards to checking a heat mode charge on a heat pump.

Step #1 – If there is any frost on the outside unit get it completely defrosted first.

Step #2 – Check all the obvious things first, filter, coils, blower wheel etc… If the unit isn’t clean it will be really hard to check.

When charging in heat mode Read manufacturer specifications first. Lennox gives specific instructions for charging their units in below 65˚ outdoor ambient conditions. It involves blocking off the condenser coil with cardboard (or even better using a charging jacket) while continuing to run the system in cool mode. Lennox gives specific instruction for how high to raise the head pressure, and what level of subcooling you should expect.

Remember that in heat mode on a heat pump the evaporator is outside, and the condenser is inside. This is important because in cool mode a dirty air filter caused low airflow on the evaporator. This would typically cause a low suction pressure, and a low superheat. In heat mode, a dirty air filter causes low airflow across the condenser. This can cause Extremely high head pressure. In heat mode, a dirty outdoor coil can cause a low suction pressure.

As an example, Trane includes a pressure curve chart with many heat pump condensing units. Be sure to use the scale all the way to the right that says heat mode. Indoor and outdoor dry bulb temperatures are necessary to use the Trane pressure curve. Carrier supplies many heat pump condensing units with a pressure guideline chart. Carrier only wants the heat mode pressure chart used as a guideline, not as a charging tool. Always reference manufacturer guidelines before setting any charge.

100˚ Over Ambient Rule of Thumb

Even though manufacturer specifications should be followed, there are some basic guidelines that will aid in charging and diagnosis in a pinch. The most widely quoted rule of thumb is the 100˚ – 110˚ over ambient discharge rule. This guideline states that a properly charged unit will have a discharge line temperature of 100˚ – 110˚ above the outdoor temperature. If the discharge line is too hot add refrigerant (If the charge is the issue and not another problem). If the discharge line is too cool remove refrigerant (again only if the charge is diagnosed as the issue).

Keep in mind that this rule only works if you are close to being in the correct zone. For example, an extremely overcharged system with an outdoor TXV can actually show a high discharge temperature. It’s just a rule of thumb and you shouldn’t reply too heavily on it.

First off, the photo above was taken in 2003 so give me some slack on my gauges. Nowadays I would be using my Testo 550’s.

To give a simple example using the 100˚ – 110˚ over ambient rule. If it were 60˚ outside you could say by the 100˚ – 110˚over ambient rule, the charge is about correct. If it were 30˚ outside the 100˚ – 110˚ over ambient rule would show undercharge (or other conditions that can cause high discharge line temp see this article) . If for example the discharge temperature were 210˚ with a 150 P.S.I. head pressure and a 10 P.S.I. suction with a 50˚ outdoor temperature; this would show an extreme undercharge. Subcool and superheat can still be checked in heat mode, the problem is since there are rarely any set guidelines it is difficult to tell when the charge is set correctly by simply checking subcool or superheat alone. Generally, you will see normal superheat (8-14) on a system with  heat mode TXV and the subcooling will generally be a bit higher than usual, especially when measured outside.

Suction Pressure / EVAP DTD Rule of Thumb 

Another common old school rule of thumb is suction pressure should be close to the outdoor temperature in a R22 system. However, this rule of thumb (obviously) does not work on an R-410A system. A more applicable guideline is 20˚-25˚ suction saturation below outdoor ambient. This means if it is 50˚ outside the suction saturation temperature should be between 25˚and 30˚ (on most systems).

Head Pressure / CTOA Rule of Thumb

Because the evaporator coil is substantially smaller than the condenser you will usually see higher head pressure (condensing temperature) in relationship to the condensing air, in this case, the indoor air. This can vary a lot depending on the age / SEER of the unit, the size of the coil and how the indoor airflow is setup but generally will be 30˚ – 40˚ condensing temperature over the indoor dry bulb.

Checking Without Gauges 

Here are some quick tests you can do on a heat pump to confirm it is operating close to specs without using gauges when the coil is frost-free and the outdoor temps are 65˚ – 15˚.

  • Check the discharge (vapor) line, it should be 100˚ – 110˚ over the outdoor ambient temperature
  • Suction line Temp should be 5˚ – 15˚ cooler than the outdoor temperature
  • Liquid Line should be 3˚ – 15˚ warmer than the indoor temperature
  •  Delta T indoors will vary greatly depending on the outdoor temperature.

If anything looks off, go ahead and connect gauges to verify further…. and like I said several times already, follow manufacturers guidelines.

The best way is to verify total system capacity (with heat strips off) using dual in duct thermometers and manufacturer specs but I understand how challenging it can be to ACCURATELY verify system airflow so it likely won’t always be your first move. We are a big fan of MeasureQuick around our business so I would suggest checking it out for this.

— Bryan

I’m far from a country boy but I did grow up in a rural area with animals, playing in the woods and cleaning out chicken coups. Like many of you, we would play most of the day outside without our parents knowing or worrying about where we were.

Was that an “unsafe” way to grow up?

I guess it wasn’t always perfectly safe but it did result in a lot of unintentional learning as we navigated the world around us and gained experiences and feedback via trial and error.

It’s undeniable that one of the quickest and most reliable ways to learn is to try and fail until we get it right.

It’s how you learned to ride a bike, rollerskate and probably how you learned to swim.

Like David Sandler said –

You can’t teach a kid to ride a bike in a seminar

We know it’s true but so often we attempt to teach topics through talking and reading and writing and watching rather than allowing people to learn through good old trial and error.

The problem with the “trial and error” method of learning professionally is the “error” part of the equation and the potential cost of those mistakes.

Learning From Mistakes Without Disaster

I have a friend who works as a nuclear analyst for power plants. He learned much of what he knows in the Navy while working on a nuclear submarine. On a nuke sub you can’t afford to “learn from your mistakes”, a mistake that could kill everyone and possibly bring an end to civilization isn’t a mistake you can risk. In these mission-critical environments, the military doesn’t resort to teaching the book over and over without practice. Instead, they do drills and work through redundant checklists with hands-on practice over and over and over.

It isn’t that they remove practice and trial and error… far from it. Instead, they allow the trial and error to occur in an environment where the mistakes are controlled in a way that can NEVER result in a mistake in real life.

In other words…

They don’t practice until they get it right. They practice until they can’t get it wrong.

Previous generations understood the importance of drills and practice where more modern education has focused on cognitive understanding as the foundation.

Understand it first and then you can do it. It’s as if understanding all about a bike, the chain and how it made, the gears, the brake mechanism etc… must be learned first before a kid should get on the darn thing and learn to ride it.

Often it’s nerds like me that try to force-feed new people a bunch of technical mumbo jumbo because it interests me rather than helping them get on the bike so they can learn how to turn and peddle.

Applying Trial and Error Into HVACR

We can all agree our trade faces an honest to goodness shortage of skilled workers that is only getting worse. We can sit in our ivory palaces and pine away about how to give everyone a perfect education with every detail listed out and taught in a nice clean classroom but it won’t work and it’s too little too late.

In order to get people trained fast we need to allow them to put their hands on tools and equipment in realistic situations and practice, practice, practice until they can’t get it wrong. We need to shorten the list of things we expect workers to know in their brains before we start to allow them to experience it.

After talking to world-class, innovative instructors like Ty Brannaman with NTI I am learning that narrowing down the curriculum and giving more tool time earlier in the training is leading to much better outcomes.

This doesn’t mean that we aren’t teaching safety and compliance but that we are teaching it as a part of drills and practice rather than separate from it.

It means practicing on equipment over and over with modern tools and techniques. It means charging, recovering and evacuating over and over until they can do it in their sleep. It means wiring and diagnosing electrical issues that will actually be seen in the field on the sort of equipment they are likely to see.

It means practice and trial and error before being so heavy-handed with books and theory.

What are your thoughts?

— Bryan

 

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