Month: July 2018

Often in commercial HVAC and refrigeration, you will either find or install sight glass/moisture indicators. The sight glass portion is simple, it’s just there to show if the liquid line has a full line of liquid or if it has bubbles which shows it’s a liquid/vapor mix.

A clear glass on a running system generally means a full line of liquid (or totally flat but you would know that already if you have gauges attached). Reading subcooling essentially does the same thing as a sight glass, it simply proves that the system has a full line of liquid. In HVAC Subcooling actually gives you more data that a full sight glass in that it tells you the actual amount of heat that the refrigerant has lost past the condensing temperature.

In refrigeration systems with receivers, a sight glass is an excellent tool and can be relied upon as an indicator of liquid refrigerant to the metering device.

The moisture indicator shows you if the system is dry or if it has moisture content.

First, make sure you are aware that older sight glasses may not be sensitive enough to pick up wet conditions with HFC refrigerants that contain POE oil.

Second, when installing a sight glass keep it sealed as long as possible before installing. If you open the indicator to air prematurely it may change color due to moisture in the air. If that does happen most indicators will change back after being installed, a proper vacuum pulled and the system run for several hours. If it still reads wet after that time the system likely is wet and new line driers should be installed and deep vacuum pulled.

You best defense against a wet system is fresh line driers, good installation practices that prevent moisture entry and proper evacuation confirmed by an accurate micron gauge.

— Bryan

Tech Brandon Livingston posted about fire dampers and took some photos shown here. He gave me permission to share this here and his original post inspired this tip. Thanks Brandon.

Before the damper was opened is on the left. After it was opened and new link installed on the right.

A fire damper is an important part of commercial fire safety preventing the spread of flame and smoke but it can lead to a loss of airflow in a building when they close that can lead to a service call.

A fire damper is designed to remain open during normal conditions and slam shut to prevent the spread of flame, heat and smoke during a fire. When the link in the fire damper reaches the rated temperature the link will break and the damper will slam shut. Sometimes this can happen as the links age and become brittle and/or due to vibration over time. Fusible link temperature set points are usually 165°F, 212°F, or 286°F with 165°F being the most common.

Generally, you will find fire dampers where ducts pass through partition walls and /or floors in commercial applications.

Fire dampers commonly come in 1.5 and 3-hour fire ratings. The hour ratings for fire dampers must be 75% of the hour rating for the wall, floor or partition. That is why a fire damper rated for 1.5 hours can be used in a fire barrier rated for up to 2 hours and a fire damper rated for 3 hours can be used in a fire barrier rated up to four hours.

It is a good practice when installing any new system to measure and mark the normal static pressure on the supply and return ductwork once the air balance has been completed. On commercial buildings that you maintain or service regularly it is a good idea to do it once you take over the building to make future service easier. This way whenever a damper shuts you will know very quickly by comparing the current static to the baseline you have established. You can easily check duct static pressure using a quality manometer or magnahelic gauge. Keep in mind that on high air velocity systems you will need a pitot tube adapter to get an accurate reading.

If a fire damper is shut they can very difficult to get open by hand. While it is possible, a tool like the FiDO Fire Damper Opener will come in very handy.

— Bryan


To answer the question in the title, it is a measurement of pressure, but REALLY it is a measurement of distance.

First, any scale CAN be used to measure vacuum (negative pressure) as well as positive pressure. The trick is knowing which is best suited for which and the size of the scale. Larger units of measure are better suited for higher pressure and greater differentials, smaller units of measure are better suited for lower pressures or smaller more critical differentials.

A Micron of Mercury (or micron) is a very small/fine unit of measure related to the displacement of a mercury column by atmospheric pressure thus the distance part. In fact, a micron is one-millionth of a meter of mercury displacement. That’s a tiny amount of pressure.

Inches of Mercury is a more rough measure of pressure, usually vacuum or even barometric pressure or altitude. Inches of Mercury is represented by the abbreviation HG

1 HG is equal to .491 PSI or roughly 1/2 of a PSI.

The force of the atmosphere around us is equal to 29.92 inches of mercury or hg or 14.7 PSIA Therefore a perfect vacuum can be thought of as 0 hg although a “perfect” vacuum can never be achieved.

When we read pressure as a tech with a gauge we read it in PSIG which means it is already set to zero at 14.7 PSIA and 29.92 hg.

So in the case of the suction/compound / blue gauge when it goes into a vacuum it reads in the “negative” hg scale down to -29.92 because it is PSIG, not PSIA.

1 inch of mercury (HG)  is equal to 25,400 microns (of mercury)

In the micron vacuum scale, we start at 760,000 microns at sea level atmospheric pressure and work down towards a perfect vacuum of 0 microns or 0 hg. This is why a lower # in the micron vacuum scale equals a better / deeper vacuum, a higher number equals a worse / less deep vacuum.

This shows why pulling a deep vacuum is done in microns, it is a very fine measurement that provides very detailed results. This is why very small changes can make such a huge difference in the micron reading on a micron gauge.

It also shows why micron gauges can seem finicky. They are really precise instruments.

— Bryan

Can a single phase motor run backward when start and run are swapped? The answer is (generally) yes. Is the motor designed to run backward by simply swapping run and start? The answer is (generally) no with a few notable exceptions.

Before we jump in, this article has two purposes. #1 – It helps you understand a compressor design you may find in the field and #2 – It will help new techs with reading and understanding wiring schematics and diagrams.

If it gets too technical for you, jump down to the bottom and just watch the videos before you get fed up and move on.


In modern residential air conditioning, we see this design where the motor can run forward and backward depending on the wiring of start and run in the two-stage compressors made by Bristol shown in the USPTO drawing above which activates the full stroke of both pistons in one direction and only one piston in the other direction. This design allows two distinct capacities from a single compressor with no special unloaders, speed changes or bypass.

This is an extension of an earlier design by Westinghouse shown in the image above. The diagram on this one is pretty vague, but the general idea is a swapping of the phases to the compressor motor R & S to reverse the rotation. Now you may be thinking-

On single phase 240v power the two phases are the same and swapping them makes no difference

You would be totally correct in this assertion other than the purpose of the start (aux) winding is to have a force at play on the motor that is out of phase to an extent to provide the necessary starting torque as well as the improved efficiency and power quality that comes along with the constant phase shift provided by the run capacitor.

In layman’s terms –

We are trying to make single phase motors as close as we can to 3-phase motors and capacitors are our best tool to try and get close

Single phase motors are like a two-handed juggler trying to compete with a three-handed juggler by optimizing our toss and catching angles. I’m running out of metaphors here so I hope you’re getting it…

In order to make a motor that works in either direction the run and start windings need to both be designed to carry the continuous amperage that is usually reserved for run. You may think that the start winding draws higher amperage than start because START sounds like it would take the bulk of the amps during start. Actually, the start winding is generally a smaller, higher resistance winding and its amperage is limited by the connected capacitor. In order for a compressor like the one shown below to work, it needs to have a start winding engineered to function as a run winding and vice versa.

This diagram from Bristol really simplifies how they initially envisioned it, I also like how they give directional arrows so you can follow the circuits in both high and low modes. Obviously, it is alternating current so it doesn’t travel in only one direction but it helps you see how the capacitor is connected to Start in High on top and the Run winding on the bottom.

Here is a diagram from a Carrier 38YDB that used this compressor in the early 2000’s and this diagram shows it in the usual schematic form with the addition of a start capacitor and a potential relay.

Look at the left side, CH is the “compressor high” contact and CL is “compressor low”. When CH is Closed, CL needs to be open and the unit will be in high-speed. When CL is closed CH needs to be open and it will be in low-speed. If you trace it out you will see that in low-speed L1 is connected directly to start and in high-speed, L1 is connected directly to run. From there the opposite side is then only capacitively coupled to L1 through the run and start capacitors. This swap in phase is what causes the motor to run in one direction in high which grabs both pistons and the other in low which only pumps one.

Here are two videos that I did recently. One of a teardown of this compressor and another going through the schematic shown above.

— 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

My friend Ami Slavin requested that I write about this important topic in response to the horrifying videos that are showing up online of death and gore associated with compressors exploding. He pointed out that the “diesel effect” can be the cause, so let’s explore what may be, and likely is leading to these horrible incidents.

In a diesel engine, there are no “spark plugs” like a traditional gas engine. Instead, the engine allows air and atomized fuel to enter and then it compresses it to a much higher level than a gas engine, so much so that the air temperature increases to the point that the fuel spontaneously combusts (autoignition) causing an explosion that drives the piston. Like all combustion this process requires –

Fuel + Oxygen + Heat

The fuel is diesel, oxygen is in the air and the heat or temperature increase is created by the high levels of compression.

If any of this sounds familiar it is much like an A/C or refrigeration just without the fuel and air parts of the equation… at least that’s what we think.

This study shows that the “diesel effect” can occur inside an HVAC/R system even if it is exceptionally rare and can be easily prevented.

Inside of many modern systems you have oil, which is combustible at very high temperatures. Mineral oil, for example, has a flash point of about 355 degrees F  which is unlikely in an air conditioning system and for it to burn at flashpoint it requires an ignition source like a spark. Autoignition is the spontaneous combustion of an oil or other flammable substance at a given temperature and is a much higher temperature than the flash point.

As we see more and more flammable refrigerants (Propane, Isobutane) and slightly flammable refrigerants (R-32) being used widely overseas and in smaller instances in the US. We even see 30lb jugs of “drop-in” refrigerant for R22 on eBay and elsewhere that turns out to be largely R290 (propane). The study above and one performed by Purdue shows that even non-flammable refrigerants will burn in addition to the oil if the temperatures rise high enough and enough oxygen is present.

With all of these factors let’s look quickly at what we can do to prevent this exceptionally rare issue.

Evacuate Properly 

A system that is properly evacuated will have little to no oxygen mixed with the refrigerant and oil. No oxygen means no combustion. This is likely one reason that many of the explosions are happening in countries with poor installation and evacuation practices.

Pressure Switches are Important 

A low pressure or loss of charge switch will shut the system off before the low side runs in a vacuum, this will help prevent drawing air into a leaking system and will also prevent a compressor from running with no cooling provided by returning refrigerant. A high pressure switch is an obvious preventative becasue it will help reduce the possibility that a compressor could get to high enough pressure for autoignition to occur.

Proper Overload Protection

So long as a motor has an overload that is properly designed, sized and installed it will shut the compressor off from overtemperature far before the motor gets hot enough for combustion of non-flammable refrigerants.

Combustible Refrigerants

In the case of combustible refrigerants, there is always a danger of combustion in cases of brazing and soldering or during terminal venting. It is critical that proper precautions be taken to purge the system complete with an inert gas and ventilate the area before it is exposed to a torch or sparks. Because you cannot always be aware if someone may have “dropped in” a flammable refrigerant these are good practices even if you think the refrigerant is non-flammable. This is another reason to cut out old components rather than unsweating them when making a repair.

Pump Down With Care

Before you pump down a system consider that the unit may be severely overcharged or have a microchannel condenser. If you attempt to pump down a unit in either of these cases you can build up huge pressure and temperatures very quickly increase the chances of a dangerous explosion due to high pressure even if not from combustion.

Combustible Recovery 

If you are working with a known combustible refrigerant make sure your recovery machine is rated for it before performing a recovery.

Be Careful With What You Work On

Sometimes products can make it into the marketplace that isn’t properly rated and tested up to US and European safety standards. If you see a system that looks suspect, especially in the ductless and self-contained refrigeration market it may be a good idea to stop and do some research.

In Closing –

  • Keep your eyes open
  • Evacuate Well (pull a proper vacuum)
  • Purge and flow nitrogen to keep oxygen and residual refrigerant away from your torch work
  • Keep proper safeties in place and add them where appropriate
  • Think before recovering or pumping down

These tragic events are very few and far between so I’m not wanting to be alarmist, just stay informed and use good practices and all will be well.

— Bryan

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