Month: January 2019

Relays can be used for many different control applications including controlling fans, blowers, other relays or contactors, valves, dampers, pumps and much more. A 90-340 is a very common, versatile relay that many techs have on their truck so we will use it as the example.

A relay is just a remotely controlled switch that opens and closes using an electromagnet. The electromagnetic portion that provides the opening and/or closing force of the switch is called the coil. Relay coils can come in many different voltages depending on the application, but in residential and light commercial HVAC 24-volt coils are the most common.

The portion of the relay that opens and closes can be called the switch, contacts or points. These contacts can either be closed meaning there is an electrical path or open meaning there is no electrical path. Often this open or closed circuit will be described as “making” a circuit, meaning the switch is closed or “breaking” a circuit meaning the switch is open.

It is important when connecting a relay to distinguish which two relay points connect the coil. In the case of the 90-340, it is the bottom two terminals of the relay. Even though the coil is unmarked on most 90-340 relays, you can find it easily by locating the terminals with the small strands of wire connected. These two points connect together through the electromagnetic coil. When 24 volts of potential is applied across the coil the switch portion of the relay will switch from open to closed and closed to open depending on the terminal. Keep in mind that in a normal 24v circuit one side of the coil is connected to a 24v switch leg such as the thermostat “G” circuit for blower control, and the OTHER side of the coil is connected back to common.

The other six terminals are switch/contact terminals and the relay has a diagram embossed right on the top for easy reference. The way the circuit is drawn shows the de-energized state of the relay, meaning the state of the switches when no power is applied to the coil. When power is applied to the coil the points that were previously open (broken) now become closed (made) and the ones that were closed become open. When two points are closed when no power is applied to a relay coil we call them “normally closed” when they are open when no power is applied they are called “normally open”.

So based on this embossed diagram on the relay 1 to 3 and 4 to 6 are open (normally open) with no power to the coil and closed when power is applied. 1 to 2 and 4 to 5 are closed (normally closed) with no power and they open when the coil is energized. There is never a path between 2 & 3 or 5 & 6 because between them, at least one of them is always open. There is also no path or circuit between the top three terminals and the bottom three terminals or between the switch and coil portions of the 90-340 relay.

The data tag on a 90-340 shows both the coil voltage as well as the LRA (locked rotor amps) and FLA (full load amps) that the contacts can handle at various voltages for inductive (magnetic) loads like motors. It also lists the amp rating if the relay is controlling a RES (resistive) load like a heater or an incandescent light.

This relay can control a 39.6 LRA and 6.9 FLA Motor or a 15 amp heater at 240 volts based on the data tag.

— Bryan

One of our techs called me the other day and gave me a story of woe.

He had been working on a system and he had the following readings

  • Low superheat
  • Low suction pressure
  • Low head pressure

He reassured me that the system airflow was correct and wondered what could have been wrong.

I asked him how he could be sure his airflow was correct and he told me that he had “checked everything”. By that he meant he has looked at the coil, blower wheel, filter and inspected the ducts, NOT that he had measured the airflow.

This isn’t a tip on how to measure airflow but there are many ways it can be done with varying levels of accuracy in the field. From a hot wire anemometer in duct to an air flow hood measuring airflow can be done and is certainly better than just guessing, especially when you get stuck on a diagnosis. My favorite way to measure airflow is to use factory fan tables and static pressure but that method just doesn’t work when anything in the system has been altered from factory test conditions (dirty blower wheel, wheel or motor replaced etc…)

While there is validity to visual inspection and to airflow measurement there are some issues that can be tough to notice that can lead to the symptoms the tech was observing.

Low Load

While we often think of the combo of low suction, superheat and head pressure as being caused by low airflow it actually falls under a larger heading of low evaporator load. This simply means that the quantity of heat being picked up in the evaporator is lower than the refrigerant mass flow rate requires for desired operation.

This can be caused by low air temperature passing over the coil, low air flow, or an undersized coil.

Here are some things to look out for that can cause these symptoms that are more uncommon.

Missing Blower Cutoff Plate

The blower housing cutoff plate helps to direct the airflow from the wheel out of the housing. It’s there so the blower wheel can be removed but if it’s missing it can greatly reduce airflow.

Incorrect Blower Wheel

We’ve seen several occasions where a homeowner or handyman has replaced a blower wheel with a wheel off of another system where it is too small. This will generally be visually obvious but is certainly worth looking out for.

Incorrect Evaporator Coil

We had one instance where we were consistently seeing symptoms of low load and later found that someone had put in an Evaporator coil that was a smaller tonnage than the original.

Oversized Compressor

Sometimes a compressor will be replaced with a compressor a size or two larger than the original. This will show low suction and superheat but will show higher than usual head pressure rather than lower like a typical low load evaporator condition.

Incorrect Blower Motor

In the old days you would simply match HP, RPM and Voltage on a Motor and you would get a fairly consistent result. There are now off the shelf ECM/X13 Motor replacement kits that can produce very different results from the original factory motors depending on how they are programmed.

Concealed Duct Issues

Issues like a collapsed inner duct liner or an old filter pulled deep into a return can be tough to find visually. I will generally use a combination of measuring total system airflow and measuring static pressure at various points in the duct system to help find these concealed issues.

Air Bypassing or Recirculating

Open bypass dampers are a common source of issues but there can also be cases where there are gaps around the coil where air can pull around the coil without adding heat to the coil like usual.

Blower Spinning Backwards

This is an extreme case but I’ve had techs chasing their tails on many occasions just to find out the blower was running backwards. Some older ECM motors would fail and run backwards though I haven’t seen that issue occur recently.

Oil logged evaporator

Over time an Evaporator can become logged with oil that can impede the transfer of heat through the tubing walls. This can look like a low load condition and often accompanies low refrigerant velocity CAUSED by low load over time. This was more common in older mineral oil systems especially when the system has had a compressor changed or oil added over time. The only way to fix it is to flush the coil internally or use an additive designed to help with oil return.

The way to find these more uncommon causes is to

  • Measure total system airflow against design
  • Use static pressure to help isolate issues
  • Look for signs of past repairs or newer parts and confirm the replacements are correct and setup properly

— Bryan

This article is written by Austin Higgins, an experienced commercial service tech from Iowa. Thanks Austin!


Ice machines and Limescale

Any seasoned Refrigeration technician knows that ice machines can be extremely finicky contraptions. Modern commercial ice makers have become a complex symphony of tubing, valves, pumps, and water directed by advanced microprocessor control boards. Newer technicians are often overwhelmed by the sight of all these different components somehow working together to produce something most people take for granted: frozen water. Ice machines have become a showcase of engineering and human ingenuity. However, no matter how technologically advanced the ice making process becomes, new and old designs alike have a common enemy: Scale buildup.


What is limescale?


Limescale goes by several names, such as calcium deposits, calcium carbonate, or simply “scale”. It is caused by hard water which contains higher concentrations of dissolved minerals such as magnesium and calcium. These minerals, when not filtered out by quality water filters, cause severe problems for any appliance or equipment exposed to the water supply. Fortunately, limescale is not harmful to humans- in fact, it is the main ingredient in many over-the-counter antacids. It is, however, harmful to ice machines and the pocketbooks of negligent owners.

Scale buildup and discoloration on an Ice Machine water distributor and ice thickness probe (photo by Austin)


How limescale can affect an Ice Machine

Commercial ice makers are akin to a symphony of different parts working in harmony with one another. Like an orchestra, one wrong note can ruin the entire piece. Limescale buildup in an ice machine is like replacing half of the London Philharmonic with 8th-grade band students from Nebraska and expecting them to play Tchaikovsky perfectly. It’s probably not going to work very well.

Limescale has the unfortunate quality of adhering to the plastic and metal components of ice machines and never letting go. Once scale begins to build up, it tends to have a ‘snowball’ effect- more and more piles on until it becomes nearly impossible to remove. Trying to remove limescale with abrasive material such as emery cloth or a knife is effective, but only makes the problem worse- as you’ve created more micro-crevasses for it to take hold. Scale buildup becomes apparent in all parts of the water distribution system: the inlet screen on the water fill valve can become restricted or clogged causing low water pressure and slow fill times. The water trough tends to collect a layer of white/chalky deposit which can build up over time and reduce the amount of water allowed into the sump; this is a particular problem with smaller Manitowoc machines, as they rarely have much water to spare at the end of the freeze cycle to begin with. From the water trough, the limescale is then sucked up by the water pump and brought to the water distributor and circulated over the evaporator grid. With enough time, the impeller can become laden with scale, causing higher amp draw and a hotter motor. In severe cases, the pump will become seized. The water distributor holes will clog, causing low water flow over the grid and forming ice that looks like a frozen waterfall. A small, hard to see layer of scale buildup on the ice grid itself can cause longer freeze times and longer harvest times.

Ice thickness probes and water level probes are especially susceptible to buildup.

Manitowoc water level probe caked in limescale (photo by me)

When this occurs, the limescale is often enough to ground or “short” the probes and the control board will then react accordingly. When a water level probe touches the water in the trough, it sends a signal to the control board telling it to shut off the water inlet valve. With enough scale buildup, it will short to itself instead of the water, and the water inlet valve will not open. The same goes for the Ice thickness probe (often used by Manitowoc Model Q and later as well as Scotsman Prodigy) and the freeze cycle will terminate before enough (or any) ice is allowed to form.  


Last, the dump valve and drain line. Common problems include clogged drains and leaking dump valves. Clogged ice machine drains often cause much more buildup in the water trough since the minerals cannot be carried away, thus compounding and causing many of the aforementioned issues. A faulty dump valve that won’t open will present the same issues as a clogged drain. Another way that the dump valve can be affected is when small flakes of scale become lodged in the valve, holding it open. This can cause low water pressure in the distributor and the water in the trough may run out before the freeze cycle is complete.


Prevention and Mitigation

The ball is in the customer’s court on this one. You can advise them to get a water softener, talk to a qualified water quality technician, install a good water filtration system dedicated to the ice machine (I like the Everpure InsurIce), and to schedule regular ice machine cleanings depending on the severity of the hard water. Ice machines should be cleaned a minimum of every 6 months; but every machine is different and developing a unique cleaning schedule with the customer based on water quality, usage, and age of the machine is recommended. I have seen some machines go years without proper cleaning, and I have seen machines in towns with terrible water require almost weekly cleaning. Follow the manufacturer’s recommendations on specific ice machine cleaning chemicals and concentrations, as well as their instructions on how to properly clean the machine. Be aware of whether the ice machine cleaner you carry is nickel-safe. If it isn’t, and you use it on a nickel-plated evaporator it will strip the nickel from the grid and render it useless.


Preventing limescale after installation of any new machine should be discussed with the customer before the first batch of ice ever drops. Informing yourself and your customers about the problems of scale buildup will save you from headaches and emergency calls, and will save them money over time if they invest in proper filtration and regular professional cleanings. Parts affected by scale are often not covered by warranty. When it comes to ice machines and limescale, diligence pays for itself.

–Austin Higgins

Note From Bryan:

Refrigeration Technologies makes an excellent Nickel safe ice machine cleaner that you can find more about HERE


This tip will be like an episode of Columbo, we will start with the what and who and then get to the why.

  1. Don’t pump down a scroll into a vacuum
  2. Don’t run a scroll in a vacuum
  3. Don’t run a high voltage megohmmeter or Hi-pot test on a scroll (As a general rule don’t go over double the rated running volts)
  4. Don’t do any megohmmeter test with a scroll under vacuum

These points have been confirmed with Copeland (Emerson) as being on the naughty list this Christmas.

Resistance / Megohm Testing

A scroll is like any other compressor in that it has a motor and a compression chamber “hermetically” sealed inside the shell. There are many differences between a scroll and a reciprocating compressor but let’s focus on the few that are pertinent to this conversation (or at least the pertinent ones I can think of).

  • In a scroll, the motor is located on the bottom, this means that the motor is immersed in refrigerant and oil. When the compressor has been off and is cold, there can even be some liquid refrigerant in the compressor.
  • A scroll is more compact and balanced design as there is no need for “suspension” like a reciprocating compressor. This results in closer tolerances/distances between the electrical components and the other metal parts.

The motor being located at the bottom is the biggest thing. Copeland states in bulletin AE4-1294 that megohm readings as low as 0.5 megohms to ground are acceptable. Besides that fact that this makes a scroll difficult to successfully meg (essentially impossible with a tool like the Supco M500 because it only reads down to 20 Mohms) it is a clear indication that a scroll compressor is running tighter resistance tolerances and a higher risk of internal arcing due to many factors. Another thing to consider is the scroll will read lower ohms to ground when it is cold than when it is running due to higher refrigerant/oil density at lower temperature and of course you are generally doing a meg test when a scroll has been off…. so that makes it tricky.

Some of the factors that can decrease resistance further and lead to problems are:

  • Moisture contamination
  • Free metallic particles due to copper leaching (acids), small metal pieces left from copper fabrication or metal from compressor breakdown due to other issues like overheating, flooding and improper lubrication.
  • Other contaminants

All of this to point out that tolerances are tight in a scroll to begin with.. add in some extra nastiness and you are at risk.

Pump Down 

First, many scroll compressors won’t even allow you to pump them down into a vacuum. Either they are equipped with a low pressure cut out or some sort of low pressure / low compression bypass like shown in this USPTO drawing


For example, in Copeland AE4-1303 it states “Copeland Scroll compressors incorporate internal low vacuum protection and will stop pumping (unload) when the pressure ratio exceeds approximately 10:1. There is an audible increase in sound when the scrolls start unloading.’ This is to prevent the compressor from pulling down into a vacuum.

In addition to that, there are lots of threats and warnings about running a scroll while it is in a vacuum, as in if you had just evacuated the system and then accidentally turned the system on. Which is a bad idea on any compressor, but worse on a scroll.


The totally obvious reason is that the compressor itself isn’t designed to run in a vacuum and it will overheat as well as fail to lubricate properly, but that isn’t the only reason or even the primary reason. All of the literature mentions arcing and I spoke to more than one tech rep who mentioned the “fusite” plug arcing or being damaged.

First, Fusite is a brand name and one of the companies in the Emerson family. So when we say “fusite” we are using a ubiquitous term for a sealed glass to metal compressor terminal feed through. There are many different types and designs of Fusite terminal just as there are many different types and designs of compressor. There are scroll compressors that use them, there are reciprocating compressors that use them, the ice cream truck that plays that obnoxious music driving through your neighborhood probably has one…. on the refrigeration compressor. Do certain fusite terminals short out more easily than others? I’m sure some are more susceptible than others. Is that what is going in here… maybe.. but if so it’s only part of the story.

What we do know about a scroll is the electrical tolerances are tighter… and when electrical tolerances are tighter there is a greater likelihood of arcing.

It’s about to get really nerdy here so if you don’t care just stop reading and go back to the very beginning, memorize the 4 points and move on with your life.

I can’t do that… because I’m broken.

Why is vacuum an issue? Isn’t vacuum the absence of matter and isn’t matter required for electrons to arc from one surface (cathode) to the other surface (anode)?

The answer is not really simple AT ALL but the summary is that under certain circumstances vacuum increases the likelihood of arcing and scroll compressor terminals inside the compressor happen to be one of those circumstances.

First thing to remember is that while electrons do travel through matter, electromagnetic fields do not require matter to exist and in either case.. we are incapable of achieving a perfect vacuum so no matter how deep we pull a vacuum, some molecules are still present.

I’ve heard some techs attribute this to the corona discharge effect which can occur due to the ionization of particles around a high voltage conductor. I really don’t see this as being the answer both because the voltages applied are not THAT high and corona discharge is not an arc or a short in the traditional sense, just a “loss” to the environment around the conductor and a pretty cool looking light (as well a decent Mexican beer).

My opinion (and this is an opinion, not a proven fact) is that the arcing is due to something called field electron emissions which can result in insulator breakdown in vacuum conditions (NASA has to deal with it all the time in space because space is a vacuum ).

The conclusion is that while this phenomenon can happen in ANY compressor, it is made more likely in a scroll due to tighter tolerence and “motor down” configuration. This means that doing a high voltage meg test, or any running/meg testing under vacuum is a bad idea.

If you want to read more about Fusite, Copeland scroll compressors and a great overall guide that includes evacuation procedures just click the links.

Nerd rant over.

— Bryan



Photo by Ulises Palacios

Refrigerant circuit restrictions can be common things like a plugged filter drier or a restricted metering device. They can also be more difficult to diagnose and exotic issues like a kinked liquid line, blocked evaporator feeder tube or a compressor connected improperly with a discharge line full of solder (I’ve seen it).

To start with let’s talk about the symptoms.

When an undesigned restriction occurs, refrigerant will “back up” against the restriction resulting in more refrigerant being present before the restriction and less afterwards than designed. Think of it like a refrigerant traffic jam with the refrigerant “road” being congested before the restriction and free and clear afterwards. This restriction will result in a pressure drop across the restriction with higher pressure being on the inlet side and lower pressure on the outlet side of the “traffic jam”.

First we must be aware that a restriction exists in the first place. In the case of the most common liquid line restrictions on HVAC equipment (with no receiver) we will see low suction pressure, high superheat and normal to high subcooling. In cases like this we know it is not simply “low on charge” because of the subcooling reading, and we also know it isn’t just a an evaporator airflow issue because of the high superheat. This leaves us in the realm of restriction.  Like anything else, some common sense, a look at the system history and a visual inspection can find many restrictions without any fancy diagnosis, but sometimes you have to put on your thinking cap, grab a pipe or a cigar, and go to work.

In a perfect world we could just connect a gauge anywhere in the system and we could find the pressure drop, in the real world we only have two or maybe three points on connection and they are not sufficient for us to pinpoint a restriction. Luckily we have temperature drop as a proxy for pressure drop, whenever the pressure drops there will also be a temperature drop. The trouble is, by the time the temperature drops enough for us to reliably measure it with a thermometer it is usually pretty bad, making minor restrictions hard to find.  It can also be challenging when the metering device itself is a suspect (and it often is), because the metering device is a DESIGNED RESTRICTION. This means that a pressure drop is it’s very purpose, but is it restricting too much?

So to actually FIND a restriction you are left with a few tools in your arsenal.

Common Sense

Get acquainted with the history of the system. How old is it? What has been done on it recently? Has the refrigerant circuit been open to atmosphere?  If you recently had a burnout compressor then it is very likely that suction and liquid line driers could be restricted. If the system has been running just fine for 7 years it is more likely that that TXV element tube rubbed out and now the TXV is slammed down. If the distributor just a leak repaired on it, it is very possible that they accidentally filled one of the feeder tubes with solder when they made the repair. A little common sense can save a lot of random hypothesis. Any experienced technician will agree with the problem solving principle called Occam’s Razor that states

“With all things being equal, simpler explanations are generally better than more complex ones”

This certainly hold true when looking for restrictions.

Temperature Drop 

Grab your most accurate line temperature clamp and start making measurements across possible restrictions like line filter driers and the liquid line itself. If you find any confirmable temperature drop across a line drier than you can knows it’s restricted, just make sure to double check. Across a typical liquid line you will generally only see a few degrees of temperature drop but it does depend on the ambient temperature, condensing temperature and the line length.

Freeze Test

Sometime the exact point of temperature change can be tough to locate. In these cases when the metering device, distributor, feeder tubes, inlet screen or evaporator are all suspects you can do the freeze test. Disconnect the blower and watch the frost patterns. On a properly functioning system the ice will start right at the outlet of the metering device and extend forward though the feeder tubes and work its way fairly evenly through the coil on the coil piping route. Look for inconsistencies in the pattern and you can often find a restriction.

If for example, you see that the frost is starting BEFORE the metering device instead of after, you can bet the restriction is an inlet screen. This test is finicky and requires a trained eye to track the tubing patterns, otherwise you might think a coil is restricted when it’s just the way it’s piped. Also be aware that the designed pressure drop of metering devices that also contain a distributer and feeder tubes is cumulative across all of those restriction points. This means that in some cases you may get more frost after the distributer than you do between the metering device and the distributer, this is to be expected.

Photo by Ulises Palacios

Thermal Imaging The holy grail of finding restrictions is the thermal imaging camera. You are able to see restrictions in real time and pinpoint the exact location where the temperature change begins. Thermal imaging can even be used to find illusive restrictions like discharge line restrictions caused by poor brazing practices, condenser feeding issues, evaporator restriction Photo by Ulises Palacios

So the process for finding restrictions is –

  1. Prove you have one by looking carefully at your readings
  2. Use some common sense and perform a visual inspection
  3. Take lots of temperature measurements until you find it
  4. Whip out the fancy pants thermal imaging camera and spot that sucker in no time flat and be the hero with throngs of adoring fans

Keep in mind it get’s even trickier to diagnose when you are working on a system with a receiver, because the receiver can usually hold a lot of excess refrigerant, often making a liquid line restriction appear more like a low charge in the readings. Also, minor suction line restrictions like a kinked suction line can be very difficult to find because the temperature drop will usually be unmeasurably low.

This is why taking all the system readings in conjunction with some common sense and knowledge of the systems history are your best allies. And when in doubt… get a thermal imager from TruTech tools .

I told you it wasn’t easy

— Bryan

Here is a great article addressing restrictions in refrigeration systems – Diagnosing A Restricted Liquid Line Can Be Tricky

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