Category: Tech Tips


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

Have you ever noticed that the more you’re required to speed up to get all your work done in a day, the more the cleanliness of your work vehicle suffers?

Some techs won’t clean their vans no matter how slow or busy the schedule gets, but for most of us, we prefer a clean and organized vehicle but in the really busy season it can slip a bit.

This is entropy at work

One way to think of entropy is the tendency for energy to be “wasted” or for energy to become more disorganized as it is changed from one form to another. In practical terms the higher the entropy the greater the waste in moving energy around and the lower the entropy the less energy is wasted or unusable.

When your van is clean you waste less energy trying to find what you need but that is easier to do when you aren’t “rushed”.

Don’t confuse entropy with enthalpy. Entropy is the measure of the “availability” of energy to do useful work, enthalpy is a measure of total heat content.

Another way to think of entropy is simply the tendency for everything to go from more organized to less organized over time.

When we talk about entropy in classroom settings when analyzing a pressure-enthalpy diagram of a refrigeration circuit like the one shown above. You will notice the red shape is nearly a rectangle other than the slanted line on the right that shows what happens when the refrigerant is compressed. Rather than going straight up and down it curves to the right to show that additional heat is added to the refrigerant (enthalpy) as it is compressed. This increase in enthalpy follows something called lines of constant entropy, in other words, as more energy is added to a system the faster the molecules move and the less organized they become.

As pressure and temperature increase, so (generally) does entropy, just like when it get’s hot and the dispatcher starts putting the pressure on you your van entropy also increases.

In a refrigeration circuit, the system will work more efficiently when we achieve the desired movement of BTUs with a minimal amount of entropy. Practically speaking this means using lower pressures and temperatures to move heat from one place to another when possible, but we still need to get the job done of moving heat from one place to another and in that process, entropy WILL occur.

We all know the guy who has the PERFECT van, the spotless tools and the clean uniform. His van stays that way because he hardly does anything and then he takes 20 minutes after every call wiping down and oiling his tools. He has very little entropy in his van because there is very little WORK being done.

Absolute zero is the temperature at which no molecular motion exists, in that state there is also no entropy. No heat, no work, no disorganization. More heat, more work, more disorganization (entropy).

So don’t use me as an excuse for a messy van, but if your boss hassles you too much on a busy day you can remind him. Heat and pressure result entropy and the back of that van… that’s entropy baby.

— Bryan

P.S. – For those of you engineering types I know that this article took some liberty with definitions. It’s an analogy, not a doctoral physics thesis.

Most techs know that you shouldn’t fill a recovery tank more than 80% with liquid based. Many know that the WC rating stands for “water capacity” and that you need to adjust for the density of the actual refrigerant rather than just using 80% of WC.

I hope most of you know that the TW marking stands for “Tare Weight” and tells you empty weight of the tank.

But do you know the service pressure your tank is designed for? 

Because R-410a is one of the higher pressure refrigerants in common use today most modern recovery tanks are built to handle R-410a pressures. The standard they are built to is called DOT-4BA400 and will be stamped on the tank collar. The DOT (department of transportation) is the governing body in the USA that has the rule making and enforcement authority on tanks, tank handling and tank shipment.

Tanks listed as DOT-4BA400 are designed for a service pressure of 400 PSIG with a test pressure of 800 PSIG. This means that R410a is safe in the tank at a tank temperature of up to 116°F which equates to 400 PSIG for typical use but that the tank is tested not to fail until over 800 PSIG. 

If a tank has any physical signs of damage it is to be taken out of service and tanks must be re-certified via hydro-static test and visual inspection forma certified facility every 5-years.

— Bryan

 

 

I was about 13 years old the first time I bent EMT with my uncle. We were doing a renovation at a church, and watching him bend EMT and then getting to do it MYSELF was a truly religious experience.

There are a few things in the trade where workmanship really comes into play such as copper pipework, making up a panel or fabricating ductwork… bending EMT belongs on that list. While most commercial electricians do it every day, HVAC techs and installers only run into an application where we do it on a rare occasion. When that does happen its good to have a basic understanding of how it works. In this video Juan from the The Air Conditioning Guy channel goes over some quick basics on bending EMT

For more info you can read this great guide on bending from Klein

— Bryan

An important rating on motors is the AMBIENT  temperature rating that the motor can operate at.  This rating means the temperature of the air around the motor, not the temperature of the motor itself or even the temperature of the outdoor air since the motor is often in a condenser air stream that is higher temp than outdoors. In HVAC/R we will commonly see condensing fan motors at 60°c (140°f), 70°c (158°f) and 80°c (176°f) and blower motors will often be rated at40°c (104°).

In residential and light commercial HVAC it is fairly common for condensing fan motors that are experiencing issues to overheat and go out on internal thermal overload during the heat of the day which then drives up the head pressure until the compressor goes off on thermal or on a high-pressure fault. In some cases, the system will cool off overnight and run again once the tech arrives, or if the customer shuts it off and it cools off the issue may not show up again right away causing a nuisance intermittent callback.

The temperature of the motor shell itself will vary motor to motor but commonly will be 30° – 60° warmer than the outdoor temperature during normal operation depending on factors such as if the sun is shining on the top, the efficiency of the motor and how long they have been running.

In many cases, you may be able to compare a motor you suspect to be overheating against other units nearby with the same motor operating in nearly the same conditions. Look at the photo below compared to the one above. Both of these are similar motor taken a few minutes apart but you can see that one motor is running quite a bit warmer than the other. Sure enough, the hotter motor is noisier and has more side to side play in the bearings.

If you have reason to believe a motor is running hotter than it should there are a few things that can cause the issue to watch out for.

  • High condensing temperature – If the air around the motor is hotter the motor will also be hotter. Watch for dirty condensers and overcharge.
  • Direct Sun – Pretty obvious but if the radiant heat from the sun is right on the motor it will run hotter.
  • Voltage – Check and make sure the motor voltage is in the proper range while it is running (under load).
  • Capacitor – Make sure the capacitor is the correct size for the motor, both weak and oversize capacitors can cause overheating.
  • Bearing Issues – When bearings start to fail there may be increased noise or side to side shaft play (but not always)

A thermal camera can produce a great look at the temperature of the motor but keep in mind that depending on the motor surface there will be some inaccuracy of the temperature reading due to varying emissivity so it’s best to use it to compare motors rather than trusting a single reading as a pass / fail test.

— Bryan

P.S. – These images were taken with a FLIR One Pro that I got from TruTech Tools

 


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

Some of the scariest practices that occur in the field surround brazing practices and tank and regulator handling. A few obvious tips are….

  1. Store tanks completely secure and upright with nothing nearby that can easily open or damage the tank valve
  2. NEVER store tanks in a torch kit with it off only at the torch handle. always turn off the main valves and purge out the gas and oxygen when done.
  3. Remove regulators from tanks when not in use.

And now to the point of this article that you may not be aware of –

Do not use oil or grease anywhere on oxygen regulators unless it is a product specifically designed for that use

Many techs incorrectly believe that an open flame is required for combustion and this is simply not true. If you have ever seen or heard of a fire starting in a pile or barrel of greasy rags you know that heat can build when fuel sources oxidize until the the temperature increases to the combustion point. Pressurized oxygen can cause “adiabatic” combustion when a fuel source is present and can be a very real risk.

So keep your regulator threads and adjustment screw nice and clean and don’t put grease or oil on them unless you know 100% the product you are using is for that purpose.

Stay safe out there and treat all pressurized gas containers like you would a loaded gun, with great caution and attentiveness.

— Bryan

Air changes per hr (ACH / ACPH) simply describes how many times the quantity of air in a room (or structure) is completely replaced per hour.

If you have a 10’x10’x10′ room, the room contains 1000 cubic feet of air If the supply and return to the room is supplying a balanced 100 cfm (cubic feet per minute) of air to the room you would have 100 cfm x 60 minutes = 6,000 cfh (cubic feet per hour) which would equal 6 air changes per hour (6,000/1000 = 6).

Different spaces have different ventilation requirements based on occupancy level (how many people are in the room) and use type. This is a fairly simple thing to calculate based on the cubic feet of air in a room, this is NOT the heat gain /loss of the space, it is only the ventilation rate of the space. Generally, ACH is used as a guideline or reference for design and not as a basis for the design, a sort of solid rule of thumb used to check a ventilation design against the real world.

This ventilation rate also shouldn’t be confused with the outdoor air requirements which is a different, but related consideration and is based on ASHRAE 62.1 and 62.2.

In some cases when discussing ventilation for an entire structure ACH will be used as a means to describe “tightness” and is referenced in many codes as a standard for the structure. For example, when performing a blower door test a building test professional will use an air changes number to demonstrate whether a home or building is built to a tightness standard and how much supplemental outdoor air needs to be added to the space per hour.

Even more specifically a standard called ACH50 is used with a blower door to calculate the leak rate of a structure when the space is pulled into negative 50 pascals of pressure by the blower door and the leak rate calculated.

In most cases, a room/zone will be sized for airflow according to the heat gain/loss to include outdoor air requirements and then the ventilation requirements will dictate the need for additional mechanical ventilation and/or fan on position to provide the required air changes per hour.

The main point being, don’t simply provide a room with system airflow that is consistent with air change per hour rule of thumb and assume it will be properly conditioned and balanced… like with most things it isn’t that simple.

— Bryan

 

 

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