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I feel like that needed to be said before I state these seemingly obvious facts,  but these very obvious facts are so commonly mistaken that it needs to be covered.

Take a look at the two sides of the same hose shown above. The one on the left has a core depressor in the center that is designed to push on the center pin of a schrader core to open the valve and allow flow.

This is why you can take the caps off of refrigerant ports and the refrigerant doesn’t leak out and why the caps, while they are important for keeping dirt out, aren’t there to hold back refrigerant.

I see many new techs remove the wrong side of the hose and connect to the schrader which will NOT ALLOW FLOW.

Some hoses have a core depressor on only one side and some hoses, especially those designed for vacuum and large diameter refrigerant transfer hoses have no core depressor built into the hose ends at all.

In these cases where the hose has no core depressor you must either use a core remover tool and remove the cores to allow flow or to use a core depressor tool like the one from Accutools shown below.  This tool works easily and simply by threading it on the port with the top knob turned all the way out (counter-clockwise). Once you are ready for refrigerant to flow you turn it in clockwise until you have desired flow. These are also handy because they help reduce refrigerant loss and they give you additional charging control to prevent liquid flooding while charging.

Many times this simple mistake of putting the hose on backwards happens when a tech uses a new set of gauges and fails to notice which part of the gauge is the open “port” and which is simply a threaded “park” to hold the hose. The Schrader depressor side should go on the part and the open side on the park.

Automotive A/C is similar with a quick disconnect that should be taken on and off with the knob in the out, back-seated, counter-clockwise position and then turned in or clockwise to open the valve and allow flow. This is a little counter-intuitive to turn a valve in to open a valve but pressing on that schrader pin is what opens the seal and allows refrigerant to flow.

When your readings are off or when you are measuring nothing at all, first check to ensure you are pushing in the schrader before you waste time on more complicated theories.

Keep in mind, there are valves that have no schrader and are opened by turning the stem clockwise. Here is Tech tip and video on that type.

— Bryan


When I started in the trade I would see these devices (above) in the field and I would hear guys call them a PTC or a “Soft Start” and I just accepted it and moved on. In fact, I’ve been calling PTCR (Postive Temperature Coefficient Resistors) a “soft start” for most of my career.

Turns out I was wrong

A PTCR is just a resistor (thermistor) that increases in resistance as it heats up. It connects from run to start in parallel with the run capacitor and allows a surge of current the start winding of a motor when it starts. When the PTCR heats up, the resistance inside increases and it essentially takes itself out of the circuit.


A PTCR by itself does allow a spike of current to the start winding but it does not create a phase shift. This is why some devices add a start capacitor and just use the PTCR as the “relay” to take it out of the circuit like the products shown above. It is a start device… but there is nothing “soft” about it.


Then there are the more traditional hard start kits that use the tried and true start capacitor and potential relay instead of a PTCR. This serves the same basic purpose, increase current and phase shift to the start winding for a fraction of a second and then remove it from the circuit. It just does the “removal” part of the equation in a more precise manner.

All of these technologies serve to increase current to the start winding quickly then drop out

The idea is to get the motor to 75% to 100% of running speed as QUICKLY as possible by increasing start winding current and phase shift.

There is a good reason for this. When a motor is stationary or running at low speed, it’s windings act as low resistance resistive loads, essentially really high amperage heaters. The longer the motor spends trying to start at full voltage the higher the current it will draw and the hotter the windings get.

Hard start devices can do nothing to actually reduce the current the motor draws when it is at locked rotor (stalled), the hard start device simply gets it out of that stalled / low RPM as quickly as possible.

Many of you may note that when you measure inrush current with a hard start in a place that it will show lower than when it is not in place. This is simply because a hard start shortens the time the motor remains in the locked rotor, not because it actually reduces the starting amps.

There are also some concerns about the added torque that a hard start provides over such a short time period and the side effects of that “torque shock” to the internal components of the compressor as well as the connecting copper lines.


Soft Start

Different methods of “soft starting” have been in use in large 3 phase motors for a long time. The purpose is the start a motor more slowly and therefore reducing the current inrush.

The devices shown above are single phase soft start devices. They reduce the voltage during starting to reduce the current associated with the start. These devices carefully control the voltage and therefore the current applied to the start and run windings to provide a lower initial current during start and slowly increase up toward full speed. These devices require advanced algorithms to do this which makes them significantly more expensive than the traditional hard start technologies and they are not capable of producing a large shifted / current boost to a start winding like a hard start.

This means that while a soft start is a great device to help reduce light flicker, decrease start amps and increase compressor life, it is unlikely that it will start that old, stuck compressor.

Soft starts and hard starts both serve a purpose but what they do and how they do it couldn’t be more different.

Have an old, locked compressor? Hard start is likely the best bet. If you have a complaint that lights are dimming on compressor start? A soft start will give a better result.

— Bryan

Here is a pretty dramatic demonstration of hard start and soft start HERE

And a great application guide on soft start HERE

Travel back with me back in time to 2001. I was a young tech, proving my salt out there in the big world of commercial RTU maintenance. One of the steps in the Fall PM list was “Test aux heat” and by golly… that’s what I was going to do!

I was on a roof of a bank and I began testing the heat strips one unit at a time… and sure, I smelled a little something.. but that’s normal right?

I turned around to see the bank manager frantically scrambling up my ladder onto the roof.

Now I have seen many bank managers in my day and this sort of athletic feat was not usual. THERE IS SMOKE EVERYWHERE AND THE ALARM IS GOING OFF! he gasped.

So the fire trucks arrived and my Father also chose to visit that same branch at that exact moment. He looked around and asked “what’s going on here?”….

Oh nothing much Dad… I wouldn’t go in he bank though… it’s a little stinky in there.

So I learned my lesson. Sometimes just “burning off” the heat isn’t the best bet. If the heat hasn’t run much it would be wiser to remove the strips and blow them off with nitrogen or compressed air before you turn them on.

Unless you like watching bank managers do gymnastics.

— Bryan

If you work in large commercial office or mixed use buildings you almost certainly work with VAV systems on a regular basis. If you aren’t familiar with VAV this is a quick intro to the basics so that they won’t seem so overwhelming.

VAV stands for Variable Air Volume, this means that the AHU (Air Handling Unit) can vary amount of air output to suit the amount of air needed for the areas served.

A typical configuration for VAV would be to set the blower to  hit a fixed static pressure target (say 1 to 1.2 inches of water column pressure) and the cooling capacity varies to maintain around a 55° supply air temperature.

Each zone is served by at least one VAV box or more accurately a VAV terminal. The terminal is controlled by a temperature (Generally the BAS system) to open and close to provide more or less air to the zone. As VAV terminals open the static pressure in the supply duct decreases and the blower increases air volume to maintain the static. As the air volume is increased the system cooling capacity also needs to increase to maintain the supply air temperature.

This constant modulation requires variable refrigerant capacity in the form of multi stage or variable refrigerant compression and generally a blower fitted with a VFD (Variable Frequency Drive) although older VAV systems often used a adjustable vane at the blower inlet to vary the air volume.

One of my favorite YouTube channels is THE ENGINEERING MINDSET and the Video below covers VAV in more detail. You can click this LINK to watch it direct on YouTube

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