Author: Bryan Orr


Breakers are designed to trip anytime the circuit draws a current above the rating for a period of time. The time the breaker takes to trip is a function of how high the circuit amperage in comparison to the breaker rating.

The higher the amperage above the rating the faster the breaker will trip

Breakers can accomplish this either thermally, by tripping on increased heat or inductively, by tripping on increased magnetic field when amperage increases.

The majority of residential circuit breakers are thermal which means they are more prone to trip during high ambient temperature than during low ambient temperature. This is one factor in why you will receive more nuisance or intermittent breaker tripping calls on a hot Summer day.

Many times breakers get replaced just for doing their job and tripping when they should.

There are five common causes of breaker tripping. Improper circuit design, Overload, ground fault, leg to leg short and breaker issues

Improper Circuit Design

Improper circuit design can result in an overload condition when the circuit ampacity (amperage capacity) or the circuit breaker size is not properly matched to the load to begin with and / or someone added additional load to the circuit later on.

For HVAC equipment this means that the circuit size should be matched to to MCA (Minimum Circuit Ampacity) and the circuit breaker or fuse should be matched to the MOCP (Maximum Overcurrent Protection)

If the conductor is smaller than the MCA rating or the breaker is smaller than the MOCP rating It can result in a tripping breaker.

You will also see cases where more than one system will be connected to one circuit breaker which is incorrect unless the systems have additional, independent overcurrent protection.

These issues usually causes an intermittent trip as it takes time under load to show up depending on the severity of the issue.

Overload

An overload condition occurs when the loads draw more current / do more more work than they are designed for. Common overload conditions would be a compressors locking up, motor bearings binding, blower belts too tight or sheaves adjusted improperly. And overload generally occurs with inductive (magnetic) loads like motors in cases where the motor is either being placed under a greater torque load than it’s designed for or the motor itself is beginning to fail mechanically.

Overload conditions often don’t trip a breaker because the motor itself will usually have an overload that specifically protects the motor. This is why when a compressor is locked it is much more likely to shut off on thermal overload than it is to trip a breaker even though it will draw far higher amps than the breaker rating on startup. In these cases the thermal overload is designed to respond quicker than the breaker.

If a breaker is tripping because of an overload condition it will usually be after several seconds, minutes or even hours of operation. It will not be “instantaneous” unless someone installed the wrong breaker or fuse and used an “instantaneous trip” instead of a typical “slow blow” or slow acting type. This would be quite rare.


Ground Fault

A ground fault is a short circuit (no load path) between an energized circuit and equipment ground.

A ground fault is the most common cause of instantaneous breaker tripping

In most ground fault situations there will be very amperage, very quickly resulting in a breaker that trips right away.

Common cases would a shorted motor, such as a shorted compressor or a rubbed out wire.

A combination of visual inspection, isolation and ohm measurement to ground and megaohm / hi-pot tests or hot verification as needed is the best way to diagnose a short to ground (ground fault).

Leg to Leg Short (Bucking Phases)

When you have two legs of power that have different sine wave patterns such a 240V single phase or 3 phase power you must prevent the legs from coming into contact except through a load.

If they do come in contact there will be an enormous transfer of energy and a significant arc.

This can happen when two wires rub out, when switch gear becomes compromised or within a motor.


Many times techs will look for short circuits from “leg to leg” or “winding to winding” in a compressor or a motor without first measuring to ground.

This is not a good idea
Even when a motor does short “winding to winding” it is rare that it just stays shorted. Usually it will ALSO be shorted to ground or it will be open after the arc flash that resulted from the short.


Think of a circuit board. Circuit boards short out all the time and the result is a big black spot on the board and nothing works anymore (open). It rarely results in a continued short circuit because the arc from the short blew the connection apart.

The reason I encourage caution is because I have seen many junior techs condemn good compressors due to a “leg to leg” short just because the ohm reading between Run and Common appeared low to them.

The only way to know if a single phase compressor is shorted “leg to leg”  with an ohmmeter is to know what the windings should read in the first place.

On a three phase motor all three legs should read the same ohms leg to leg which makes it considerably easier.

When you do encounter leg to leg (only) short circuits it is more often on fan motors than on compressors.

Breaker Issues

Because most breakers trip due to heat, anything that causes the breaker to get hotter than normal can result in tripping.

This can be due to a poor connection inside the breaker itself, but often it is due to a poor wire connection on the breaker or a poor connection between the breaker and the bus bar.

Usually these types of breaker issues are caused by installation problems such as loose connection, wrong breaker type, failure to use anti-oxidation paste on alum to copper or excessive tripping / using the breaker as a switch.

Here are some tips for diagnosing a tripping breaker

Tripping instantly

  • Perform a visual inspection of all wires and connections. Look for signs of rubout, damage and arcing
  • Isolate components and ohm to ground
  • If you are unable to locate with an ohmmeter use a megohmmeter to ground (with caution especially on scroll compressors)
  • Finally, once you believe you have identified the cause, fully disconnnect the shorted component and power the unit back up and make sure everything else functions.

Tripping intermittently or after more than 3 seconds

  • Visually inspect all electrical connections and ensure they are clean and tight.
  • Inspect the breaker and bus bar connections
  • Check breaker and wiring size
  • Measure running voltage and ensure it is within +/- 10% rating
  • Measure for voltage drop during startup (less than 15%) as well as between the power source and right at the unit (less than 5% overall)
  • Measure component amperages while starting and running and compare to manufacturer specs
  • Measure motor and compressor temperatures and watch for temperature increase over time. Infrared and thermal imaging can assist with this
  • Watch for anything that can cause overload such as failing bearings, belts too tight, or sheaves adjusted for too much RPM
  • Measure current right at the breaker, if it remains below the breaker rating and the breaker STILL TRIPS, only then replace the breaker.

I also learned recently that AFCI (Arc Fault) breakers generate heat internally which mean that you will see a hot spot on them with a thermal imaging camera or IR thermometer.

Don’t replace a breaker unless you know it’s failed and don’t condemn a part as being shorted unless you can isolate it out of the circuit and every other component still functions (as possible)

— Bryan

First, a thermocouple is not a flame rectifier like a modern flame sensor. A thermocouple actually generates a milivolt potential difference when it is heated by a flame.. Just to get that out of the way for any of you newer techs who are used to modern flame sensors.

With higher efficiency gas fired equipment being the norm for replacement systems, thermocouples and standing pilots are becoming a thing of the past. Newer appliances do not typically utilize a standing pilot, opting instead for hot surface or spark to pilot ignition. These types of ignition systems have benefits over standing pilot, from increased reliability and longevity to higher efficiency numbers. But there are many appliances in the field that still use a standing pilot, and a good service technician should be able to diagnose a thermocouple issue.

Many of you will say-

“Why even check the thermocouple? It’s a 5 dollar part, just throw a new one in!”

“Why are you so lazy? Do you even HVAC in real life or just on the internet?”

Yes, I know thermocouples are cheap and I am all for replacing them when they need to be replaced, or while replacing a gas valve or pilot assembly. But over the years I have seen a lot of guys ( me included) go on calls for pilot issues, find a pilot blown out, relight the pilot, and then because it’s the easiest, quickest fix, replace the thermocouple, only to have the same customer call in a day or two later with the pilot being out AGAIN. And when the tech goes back and relights the pilot, then what? Is that brand new thermocouple bad after a few days? Probably not. There is probably some other issue, but checking the thermocouple millivolt production is the first step for a proper diagnosis.

So how does a thermocouple work? Well, I’m no scientist ( I’m barely a writer), but I’ll tell you what I know. When different metals are joined, and there is a temperature difference between them, a magnetic field occurs between the joints where the different metals meet. The heat of the pilot flame is the source of the temperature difference in a normal pilot system. Through this process, a small amount of current is produced, generally around 30 millivolts. This voltage is sensed by the gas valve and is used to keep the pilot valve internal to the main gas open. If the pilot goes out, the heat that is generating the potential (voltage) is lost, thus current stops flowing to the gas valve, and the pilot valve is closed, closing off fuel to the pilot assembly. The thermocouple is a safety device. If the pilot flame goes and the pilot valve doesn’t close, the burner compartment and potentially the room the equipment is in can fill up with gas. That the consequences of that would require a different article.

When should you check a thermocouple? I am in the habit of checking thermocouples when I encounter them, whether it’s on a maintenance inspection or a service call. If you are in the habit of checking them, it usually doesn’t take more than a few minutes. If the millivolt measurement is less than 26-27, I typically recommend replacement.

To check a thermocouple, you need a multimeter that is able to measure millivolts. It is typically shown as mV or is just the third decimal over on the DC voltage reading. Remember, the meter should be set to DC voltage.

It’s also helpful to have a extra set of hands, but it is very possible to perform this check by yourself if you hold your tongue correctly (or just use alligator clips). First, disconnect the thermocouple from the gas valve. Then light the pilot. Most gas valves have a turn knob that has to be set from On/Off to Pilot. There usually is a push button that is pressed to manually open the pilot valve, sending gas to the pilot assembly in order to light the pilot. The trick is to light the pilot, and position the meter leads in the proper place to read the voltage. The push button must be depressed through the whole check. With the thermocouple being disconnected from the gas valve for checks, the pilot valve should not stay open and the flame should go out when the push button is let up.

Put on meter lead directly on the gas valve side of the thermocouple. Put the other lead on the copper line as shown by my right hand in the picture above. While holding the meter leads in this position, light the pilot. The thermocouple needs to heat up for 30 seconds to 1 minute in order to obtain a proper reading.

30 millivolts is the desired reading, with a swing of plus or minus 5 millivolts. If the readings are in that range, and you have been having pilot failure issues, more than likely there is some other cause. Dirty pilot assembly/ orifice is the most common other issue I encounter, but it could be down draft/flue or combustion air issues, fuel pressure problems, or a failing gas valve. But as stated above, the thermocouple should be eliminated as a potential issue before moving on with a proper diagnosis. Don’t throw parts at a problem and see what sticks. With thorough troubleshooting, you can save a lot of time, headaches, and maybe the customer a little bit of money and frustration.

— Justin Skinner

I walked into a supply house the other day and I was looking at “universal” expansion valve on the shelf. The friendly guy behind the counter saw me and walked over, after saying hello he offered

“That’s a great valve, it’s even balanced port”.

Now I am a bit of a trouble maker, I should have just nodded and said “uh huh” but instead I asked, “what does balanced port mean?”. The counter guy sort of half shrugged and said “I guess it means it works on a lot of different systems?”

I would bet that most people in the industry have heard the term “balanced port” and figure it sounds like a good thing but don’t really know what it does. Not long ago, I would have been one of them.

We have all been taught that there are three forces that act on an expansion valve –

  1. Bulb Pressure is an opening force
  2. Evaporator Pressure (external equalizer) is a closing force
  3. The Spring is a closing force

while the system is within its design operating conditions these forces are the primary forces at work that allow the valve to “set” the evaporator outlet superheat.

There is a fourth force and that is the opening force applied by the refrigerant passing through the needle. When the inlet (liquid line) pressure is within the normal operating range this force is accounted for in a normal TXV. In cases where the liquid pressure is higher than usual the force will be greater allowing more flow through the coil and when it is less it will allow less flow.

The result of this effect is fluctuating superheat based on liquid pressure which may be acceptable in small amounts but can become unacceptable quickly on systems that require accurate evaporator feeding or systems that have a wide swing in condensing temperatures and pressures.

Sporlan largely solved this particular issue in the 40’s when they brought the “balanced port” valve to market. While the technology is nothing new it has been improved on over time.

Balanced port TXVs can vary in design but they solve this problem by allowing the inlet pressure to effect the top and bottom of the needle (orifice) equally. This eliminates (or reduces) the liquid pressure as an opening force and instead turns it into a “balanced” force that neither opens or closes the valve.

If you have an application where the head pressure is allowed to change or “float” over a wide range, the balanced port TXV is a great choice.

— Bryan

 

 


This quick tip was written by Daniel Andersen in the HVAC School Group. Daniel was one of my early encouragements to make the podcast even though he refuses to come on himself. Thanks Daniel!

ERV – What is it? 
An energy recovery ventilator allows fresh air from outside to be introduced into the conditioned space, and conditions it (recovers energy) prior to entering the space.

This is especially important on a structure that has a nice tight envelope, where you are required a certain amount of air exchange.

How does it work? You might ask 

Sensible Recovery

The core in the ERV allows the incoming air to be tempered by the outgoing air, either absorbing heat from the conditioned air in the winter, or the hot entering air in the summer is allowed to dissipate heat to the outgoing air in the summer


Latent recovery

By using a desiccant wheel or other desiccant media the ERV is able to pre condition the air by removing moisture from the incoming fresh air, and transferring it to the air exiting the space. This “latent” moisture energy transfer is what makes an ERV different than an HRV (Heat Recovery Ventilator)


What will affect its operation?

CLEANLINESS! (Like most things in HVAC/R)

Keep the prefilters clean. Entering air can bring in a lot of contaminants that can QUICKLY clog the pre-filter or inlet bug screen.

I commonly see leaves and airborne debris in the filter area. Cleaning is usually just a matter of using water on the wheel and / or core.

If it is belt driven, make sure the sheave and belt are in good shape and properly aligned and adjusted.

— Daniel

If you work in refrigeration you may have heard the term “hot pull down”. This phrase is used to describe a condition where the load on the evaporator is above design due to the box temperature and/ or the temperature of the product in the box being higher than it would normally be.

My grandpa called me a few month back all upset “I just slaughtered a bunch of chickens and I’m going to lose all my meat because this freezer you got me isn’t working” he gasped into the phone. Now I had helped him pick out a commercial freezer a year or two back and he put it in his garage (a less than ideal location to begin with). What I had forgotten to mention to him was the importance of only loading with meat that was already down to temperature.

I showed up to look at it and sure enough, there sat a bunch of freshly slain birds PACKED into his freezer and the box temperature struggling to get below 15° instead of the 0° we really needed.

Most refrigeration equipment is designed to only maintain the temperature of the product, not to bring it down to temperature all at once, at least not in large quantities. This is due to two aspects of the design.

  1. Capacity – Most freezers and refrigerators just don’t move enough pounds of refrigerant to generate the necessary refrigeration effect to “pull down” warm product in a timely fashion. In other words, just like many A/C systems don’t keep up on a freak 98 degree day in Indiana, refrigeration equipment won’t pull down quickly if you add in more BTUs of heat than it is sized to remove.
  2. Coil Feeding Range – In the case of a cap tube or other fixed orifice metering device, the amount of refrigerant fed into the evaporator is directly proportional to the amount of refrigerant pressure differential between the liquid line and the evaporator. With a TXV the valve responds to superheat in order to open and close, opening as superheat rises and closing as the superheat falls. In a hot pull down the load on the evaporator is so high that the expansion valve goes wide open but still,  the coil “starves” or underfeeds refrigerant. This results in high superheat, high suction pressure and high head pressure but will also often result in low subcooling because so much of the refrigerant charge will move to the evaporator coil.

During a hot pull down the compressor will draw higher than usual amperage due to the increased density of the suction gas, this coupled with high superheat can result in compressor damage if it is allowed to run outside of specs for an extended period (Sporlan has a great piece on compressor overheating you can read HERE).

The conclusion is that most equipment should be allowed to get down to temperature before being loaded with product and the product should generally be at or near the design temperature. There are freezers and refrigerators that are designed specifically for “flash freezing” or pulling product down to temperature often called a “blast freezer”.

In the case of my grandpa’s freezer, we moved some of the meat around to other freezers and got it down in time to prevent salmonella… at least I hope so… I was feeling funny after Grandma’s chicken soup for Sunday dinner…..

— Bryan

 

Condensate_Pump

One of my techs (Jim Walch) mentioned to me that another common “double trap” style issue that comes up often is techs and installers running a drain too far into a condensate pump.

When you run the system drain too deep into the pump reservoir the water level can rise high enough to cover the drain end. This can create the same type of “airlock” you get with a double trap.

When draining into a condensate pump only extend the drain tube 1″ or so into the reservoir of the pump to prevent the water from rising and covering the end. Also, make sure to wire up the overflow switch on the pump in series with your system condensate switch so that whether the pump itself fails or the system drain pan has a blockage it will shut the system off.

Simple stuff, but it can prevent thousands of dollars in damage.

— Bryan


In most cases when a low voltage circuit is blowing a fuse it’s because one of the circuits is shorted to ground or common. Rubbed out wires, shorted components and boards etc…

Less commonly you will see the low voltage circuit draw high amperage because of magnetic solenoids that are energized but the mechanical pin, stem or armature is stuck.

A common example is a contactor that is stuck open. This results in high amperage because the solenoid is energized without the magnetic resistance (reactance) provided by the induced magnetic field.

Another example is a reversing valve solenoid that is not mounted or is not properly on the reversing valve stem. You can see the same effect in any magnetic switchgear such as relays, pump down solenoids etc…

This occurs because the magnetic field in the coil isn’t reacting with the load so there isn’t enough inductive resistance known as “inductive reactance”. It’s essentially the same thing as locked rotor amps on a motor, if you keep that motor from spinning the electrical resistance in the windings remains too low and the windings overheat and go out on thermal overload.

When this does occur in low voltage circuits it often won’t blow a fuse / trip right away. A good way to catch it is to put an amp clamp on the low voltage wires feeding different components until you find the one pulling very high amperage in comparison with other low voltage components.

So check for short circuits first but also keep your eyes open for stuck or improperly mounted solenoids.

–Bryan

While there are various line lift, sizing and trapping guidelines depending on oil and refrigerant type there is one guideline we can all understand easily and remember to apply and this is to slope suction lines towards condensing units / compressors.

Keeping the suction line sloped toward the compressor, especially with long, overhead lines helps to return the oil back which assists with capacity and lubrication resulting in fewer issues.

A good general rule is to slope the suction line 1/2″ per 10′ of horizontal run.

Short and simple but often forgotten.

— Bryan

graph

There are all sorts of complicated refrigerant acronyms.. HFC, HCFC, CFC and well as the mythical Zeotropic, Azeotropic and near Azeotropic… Let’s simplify

CFC = Refrigerant that is really bad for the Ozone. They are almost all gone. R-12 and R-11 are examples

HCFC – Refrigerants that are bad for the Ozone but not as bad as CFCs. Most common is R22.

HFC – Refrigerants that aren’t bad for the Ozone but they do add to global warming through the greenhouse effect. Most common is R410a.

When it comes to the whole zeotropic, azeotropic thing the main thing you need to know is that older refrigerants were often just one type of molecule. That meant that they condensed and evaporated consistently and it didn’t matter if you added them to the system via vapor or liquid. These simple refrigerants were known as PURE refrigerants.

Today we mostly work with HFC and HCFC blends. These blends can be Azeotropic, which means they blend together and act as one refrigerant or Zeotropic which means they have “glide” resulting in different boiling and condensing temperatures of the refrigerants mixed in. Rubber meets the road in a refrigerant with high glide when you need a separate “condensing” and “boiling” termperatures on the PT chart. R407C is an example of a high glide zeotropic refrigerant where R410a has nearly 0 glide. While R410a is TECHNICALLY zeotropic is is so close to being Azeotropic that the industry coined the phrase near-azeotropic.

In all blends you must charge the refrigerant as a liquid to prevent the refrigerants from separating in the vapor state. As always, when charging liquid in the suction line, add it slowly and carefully, allowing all the liquid to boil off before entering the compressor to prevent flooding / slugging.

— Bryan

 

One of the most common mistakes I hear techs make is confusing Zero ohms with infinite ohms. The fuse above is showing near zero Ohms which means a good electrical path with very little resistance.

If there is a perfect path it would have zero ohms (which isn’t actually possible unless you happen to be testing a superconductor).

If there is no path, the circuit has infinite ohms. This would be shown as Open or OL or something similar.

Often when I ask what ohm reading a tech is getting they will say “none”… None could easily mean zero or infinite so it’s important to clarify.

Once again.

Zero ohms = shorted / closed / directly connected

Infinite ohms = open/ no path

Try to remember to say either Infinite or zero instead of  “no ohms” or “none” to avoid confusion

— Bryan

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