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When I started in the trade in 1999 there were still a lot of oilable blower motors in service. As part of the maintenance, we would remove the housing, oil the motor plus vacuum / wipe it down.

As oilable motors have become extinct I see fewer and fewer techs pulling the blower housing. Here are some reasons you may want to consider doing it more often.

  • Cleaning the motor itself can help it run cooler and last longer. A hot motor not only is more susceptible to winding breakdown but also to bearing/lubricant failure. Grab a vacuum, soft bristle brush, and a rag and get the dust buildup off the motor. If you have any dust that gets stuck inside, use some low-pressure nitrogen or compressed air to get it clean.
  • Get in there and look carefully at the wheel. A wheel that is even slightly dirty can have a significant effect on air output. If it’s dirty,  recommend cleaning.
  • Check the blower bearings, it’s easier to do when it’s out
  • On high-efficiency furnaces pulling the blower is a good way to check the secondary heat exchanger. On 80% furnaces, you can check parts of the primary exchanger and even the evaporator coil with a mirror or inspection scope.
  • Pulling the blower gives you the ability to wipe down the inside of the furnace or Fan coil.
  • You can check blower mounting bolts and set screws as well as blower alignment and balance more easily.

Obviously, when and why you pull the housing will vary from contractor to contractor but I advocate it being done more often than it is now.

What say you?

— Bryan

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First off we need to clarify that very few unitary manufacturers use flares anymore. You will most often find flares on ductless and VRF / VRV systems and in refrigeration. A flare uses a flared female cone formed into tubing (usually copper) that is pressed onto a male cone (usually brass) by a threaded flare nut. A flare shouldn’t be confused with a chatleff fitting that uses a threaded nut and seals with teflon or nylon seal.

This is not a full lesson on how to make a flare, this will give you some best practices to make a flare that doesn’t leak.

  • Use proper depth, the old school method is to bring the copper up a dimes width above the block but modern flaring blocks usually have built in gauges that work well.
  • Don’t trust factory flares. In many cases factory line-set flares are made poorly, often it’s better to just cut them off and start over
  • Ream the copper before flaring to remove the burr but don’t OVEREAM and thin out the copper edge.
  • Use a good, modern flaring tool designed for refrigeration. This is a great one
  • When making the flare use a bit of refrigerant oil, or even a better a bit of Nylog. You only need a drop or two, one drop on the flare while making it to prevent binding and create a smoother flare surface with a bit on the back of the flare as well to allow the nut to slide easily. I also like one small drop on the threads and spread to the mating surfaces. Some manufacturers disagree with this due to the effect it has on torque specs so always follow their recommendations when in doubt. In my experience a bit of assembly lubricant really helps.
  • Use a flare wrench instead of an adjustable wrench and tighten with a torque wrench.  I understand that very few techs do this… but it is a great practice if you want to get it right the first time with no leaks and no damage. This can be done easily be done with a set of SAE crowsfoot flare nut wrenches and a 3/8″ torque wrench. As always use manufacturers torque specs if available. If not you may use the chart below. Make sure to keep the crowsfoot at 90 degrees to the wrench (perpendicular) and place your hand on the end grip of the wrench. If you have lubricant on the threads stay on the low side of the torque rating.

Some things NOT to do that I’ve seen –

  • Don’t use leak lock or teflon tape on flares
  • Don’t Over Tighten flares to try and get them to stop leaking. If they are properly torqued and still leak they are made wrong
  • Don’t use too much oil or nylog, a drop or two will do
  • Don’t try and jam a teflon seal from a chatleff on a flare

Using these practices we have VERY FEW leaks on flare fittings.

Some other things to note –

There is a company called Spin that uses a flaring tool that goes on a drill. Their tool actually heats and anneals the copper. They claim they don’t need to get the flare to 45 degrees because the annealing makes the copper soft enough that the nut itself with finish the flare. We have used it a few times with good results.

There are now companies that make nylon / teflon (I’m actually not sure what they are made of) gasket inserts that go into a flare. Some techs swear by them, I really don’t see the necessity but I don’t have any experience with them.

Finally, make sure when your system has flares to pressure test to the rated test pressure and bubble test the joints. Then perform a vacuum to below 500 microns and decay test. This will help ensure that you got it right. If it leaks, cut it off and remake it.

  • Use a good tool
  • Get depth correct
  • Ream properly
  • Use a good assembly lubricant
  • Torque properly
  • Pressure test to 300+ PSIG (in most cases) and bubble test carefully

— Bryan

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Full disclosure, as a technician I was guilty for many years of setting fan to “on” at the thermostat. I never really thought of any of the negative impacts that could happen.
I wanted to circulate the air, and to keep air moving through the high efficiency air filter that most of our houses had. Later I learned that in many scenarios fan “on” is not a good idea.

For this discussion I will be talking about the cooling season in a humid climate. Many adverse impacts may occur in the heating season, depending on the region.

Things to be aware of when running the fan “on”.

Condensate on the coil after a cooling call with the fan running will evaporate back into the living space. Some thermostats combat this by having a fan off period at the end of a cooling call to let the coil drain.

If the ducts are in unconditioned spaces, outside the thermal envelope of the house, the sensible heat will be added back to the space. If the ducts are warmer than the air traveling through them there will be a transfer of heat. If there is any duct leakage latent heat (moisture) will be added as well. Latent heat gains do not only apply to return duct leaks. Supply air leakage can also contribute to this.

It is common that the HVAC system can cause the house to go into a negative pressure. When this happens sensible and latent heat will be added. A common cause of the pressure imbalance is when the duct system is in an attic or crawl space, and the return duct has fewer connections than the supply ducts.

Since the supply has more connections than the return there is more of a potential to leak air. If the supply air leaks into the attic or crawl space this can cause the living space to go into a negative pressure. The leaked air is replaced with either attic, crawl space, or outside air. One CFM(M3/h) in = one CFM (M3/h) out.

Ducts in conditioned spaces with panned returns can add latent and sensible heat as well. This happens when the panned joists and studs are not sealed by the HVAC contractor on all six sides. Joists and studs are part of the building network that when not air sealed during construction, by the builder, they will communicate with air outside the building envelope. With blower door testing, and air changes per hour requirements now code in many jurisdictions houses are being built much tighter. In an older home, with panned returns, expect to be bringing in some outside air.

Even if the ducts are sealed, and 100% in the conditioned space it still costs money to run the fan. Running a PSC motor 24/7 can be costly. (ECM motors on a property sized duct system do have considerably lower operating cost when compared to PSC motors.)

The duct leakages and pressure imbalances mentioned above will also occur during a cooling call. Most of the time these issues can go unnoticed because of the ability of the HVAC system to overcome or mask them.
The goal is to get maximum customer comfort with minimum power usage and maximum system longevity. In many cases the fan being left in the ON position detracts from these goals.

Hopefully this Tech Tip will make you think twice about running the fan “on”. Every situation is different. I encourage you to think outside the box, if you are not already.

— Neil Comparetto

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I had an old timer tell me that you can never connect two transformers together because they will “fight one another”.

If you are anything like me (and heaven help you if you are), whenever someone says something like that, a cartoon in your head starts playing.

In this case, I imagine two transformers with boxing gloves on duking it out to see which one “wins”.

The truth is you can connect two transformers together so long as you are careful, but you need to know why you’re doing it and then do it properly.

Transformers have a VA (Volt-amps) rating that dictates how many volt-amps (volts x amps, which is watts simplistically but there is a more complicated reason it is called VA in transformers that we won’t get into here) the transformer can handle on the secondary.

Above we show two 75VA transformers with 24V secondary windings.

75VA÷24V=3.125A

So with a 75VA transformer, you can run a maximum of 3.125 Amps, if you needed more power you would need to either go get a larger, more expensive transformer or…. you could connect another identical one in parallel. If you connected two 75VA transformers in parallel you would then have 150VA of secondary capacity which can be necessary in some cases with multistage commercial units or some large accessories.

In this case, parallel simply means connecting the two primary and secondary windings together in the exact same way like we show above… Pretty easy

It is SUPER important to get the polarity exactly the same and use two transformers with identical winding turns in the primary and secondary and identical secondary coil impedance (resistance).

In fact, it is so important that I advise that you only do this if you have two identical model transformers.

To be even safer, connect the primary windings first and check the secondary’s against one another with a voltmeter before actually connecting them to the system. For a typical 24v secondary you can connect the two common wires to ground to act as a stable reference first then check the two R or Hot side leads to one another and then to common. They should read 0v to one another and 24v to common. If you get anything other than 0v from hot to hot then you want to recheck your primary wiring and ensure that they are exactly the same.

— Bryan

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First I want to give credit where credit is due. This post is made possible by the fantastic demonstration video by Neil Comparetto that I embedded below.

Before you get bored and stop reading I want to give some conclusions. Ice can form in a vacuum, but I still advise pulling a fast, deep vacuum. Now… keep reading to see why.

The statement that is often made by techs is that pulling a vacuum “too quickly” can result in freezing of the moisture inside the system and reduction of evacuation speed. This conversation usually occurs when another tech is demonstrating a SUPER FAST evacuation, or by a tech who is advocating for the consistent use of triple evacuation.

Neil’s video proved that pulling a deep vacuum quickly can result in freezing with even a small amount of liquid water present. He also demonstrated that this is possible with a typical HVAC hookup and that it can pull down to 500 microns with substantial ice present. This was done in 50 degree shop, with a glass jar (insulator), with relatively low internal volume, pulled down using a vacuum pump direct connected with large hoses.

So he proved that under certain circumstances, ice can form and cause a real problem with evacuation speed and even trick a junior tech into thinking they pulled a proper vacuum when they did not.

Now before you get too excited

In a typical system with larger internal volume than a jar this experiment doesn’t replicate the same results unless of course the ambient temperature is already near or below freezing.

So what causes this (Water freezing under vacuum) and how do we prevent it?

Water, like all substances, changes state due to the molecular density and configuration based on the pressure surrounding the molecules and the temperature of the molecules (average molecular velocity). When we pull a vacuum on water there are two opposing forces, on one hand we are DECREASING the pressure which leads to evaporation and then boiling, but as the water boils it begins to lose heat because that molecular energy of the highest velocity molecules are being evaporated and removed by the vacuum pump, thus reducing the average molecule velocity (temperature). The key reason why it can freeze in these experiments is because the heat rejected through evaporation / boiling is significantly higher than the heat ADDED through the sides of the jar which is why it starts by flashing, then boiling, then back to liquid then freezing. You are WATCHING the change in energy state in real time as the available heat in the jar and added through the walls is overcome by the heat REJECTED from the boiling liquid and out the pump.

Now, if the pump was left on the ice long enough, the ice would eventually all SUBLIMATE (change directly from solid to vapor) but the rate at which that would occur is based solely on the amount of heat being added to that ice through the walls of the jar. This addition of heat is equal to the differential in temperature between the ice in the jar and the temperature around the jar.

The deeper the vacuum pulls the colder that ice will get, which will increase the differential between the ambient around the jar and the ice temperature inside.

If we hit the jar with a heat gun, the ice would melt quickly because we are ADDING a huge amount of heat very quickly. In the same way, if the ambient temperature in Neil’s shop was higher or if the vessel was made of copper (conductor) instead of glass (insulator) it would be less likely that ice would form and if it did form it would sublimate more quickly.

Here are my current conclusions based on this video (and many others), good science and practical field experience

It becomes increasingly more important use a heat gun on components and sweep nitrogen as ambient temperatures drop

  • Liquid water inside a system should be exceptionally rare, follow good copper handling practices and don’t work with open copper in the rain.
  • When the internal volume of the system you are evacuating is very small it would be easier to create ice (ice machines, ductless linsets etc…) use more caution in these circumstances by employing triple evacuation / breaking the vacuum with nitrogen / sweeping with nitrogen.
  • Most important is to valve of the micron gauge from the pump and watch for rapid increases. If you have ice you WILL see and quick increase when the micron gauge is isolated from the pump in the system.
  • Use a quality micron gauge that can show you a decay / leak rate so you can easily be aware when there is an issue.
  • When you are pulling a vacuum on most systems, during warm ambient conditions you are RARELY if ever going to make ice in a system under vacuum.
  • The conclusion is NOT to pull a SLOWER vacuum, it is simply to use heat and breaking the vacuum with nitrogen to get the moisture removed.

Here is the video-

— Bryan

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First off, if you’ve never heard the term “beer can cold” you are either not in the trade, or you have been living a pretty sheltered existence. I started as a tech apprentice when I was 17 years old and on my first day in the truck my trainer grabbed the suction line of a running split system and said “She’s running good! beer can cold”. Now before you freak out, my trainers were primarily a couple of guys named Jimmy Wells and Dave Barefoot and these old school techs would JOKE about beer can cold and then they would proceed to connect their gauges and properly check superheat and subcooling.

There are two things to know about old sayings like “beer can cold” or listening to your vacuum pump or feeling the air velocity out of a register.

#1 – They Can Be Useful Tools of An Experienced Tech

When I was in trade school my instructor taught me to “feel my way” through the refrigerant circuit to identify the liquid line, suction line and discharge line by touch. This resulted in some minor burns and a perspective on the “qualitative” or intuitive understanding of the refrigerant circuit.

Using your senses to hear, feel and smell the system are really important tools an efficient and effective tech builds over time to alert themselves of slipping belts, a vacuum pump that isn’t operating properly, a burned board or transformer, a bad bearing or even… an underfeeding or overfeeding evaporator. This is where “Beer can cold” (grabbing the suction line to get an approximate temperature) isn’t always a bad thing… but only when used as an initial qualitative test.

#2 – Senses Should Lead to Measurement

A good diagnostic technician finds THE problem first, whatever is primarily causing the problem is the first order of business. Once that primary problem is identified THEN a good tech moves on to inspecting the entire system and making more measurements as possible to identify additional issues.  Once the initial set of know issues have been rectified then a good technician will always verify proper system performance using real measurements that PROVE that the system is operating properly.

So let’s be 100% clear

You cannot charge a system by “Beer Can Cold“. It is nothing more than a long running inside joke that refers to grabbing the suction line and it feeling cold like a beer on a functioning A/C system.

But….. (Warning, I’m about to take this WAY TOO FAR) 

Depending on the type of beer and the preference of the drinker, beer can be anywhere from 36°F(2.22°C) for a good old can of American Lager all the way to about 55°F(12.77°C) for a British stout kept at cellar temperature. Craft Beer enthusiasts will tell you that about 45°F(7.22°C) is a good compromise between flavor and temperature.

On average your evaporator temperature will have a 35°F(19.25°K)DTD (Design Temperature Difference) which means the coil temperature will be about 35°F(19.25°K)) colder than the return air DB temperature. This means if it’s 75°F(23.88°C) in the return the evaporator will be at about 40°F(4.44°C). We then need to add in superheat which will vary quite a bit on a fixed orifice system. On a TXV or EEV system it will be between 5°F and 15°F(2.75°K – 8.25°K) on a properly functioning system. This means that the suction line indoors could range from 45°F to 55°F(24.75°K – 30.25°K) by the time you account for the TXV superheat range, the uncertainty of the temperature measurement and the variability in DTD. If you are grabbing the suction line outside you will also need to account for anywhere from a 1°F to a 8°F(.55°K –  4.4°K) rise in temperature on the suction line by the time it get’s from the coil to the outdoor unit where “Beer Can Cold” is taken. Now the range is all the way from a acceptable beer temp of 46°F(7.77°C) all the way up to a putrid 63°F(17.22°C) that even the British would find unacceptably warm.

All of this, just at 75°F(23.88°C) return temperature WITH a TXV

I don’t know about you, but my hand is only calibrated to within +/- 4°F(2.2°K), when you add that to the mix I find that using my hand to feel the suction line gives me only the roughest estimation of what is going on and if 50°F(10°C) is the average… that is too warm for my taste in beer anyway.

Beer can cold, like most “rules of thumb” is far too inaccurate to be useful (at the risk of overstating the obvious).

What I do recommend, is becoming fully familiar with….

  • The Design CFM of the System and the sensible / latent requirements of your area
  • The efficiency of the equipment you are working on (to help anticipate condensing temperature)
  • Type of metering device (To understand target superheat)
  • Evaporator Coil Design Temperature Difference (DTD)
  • Condensing Temperature Over Ambient  (CTOA)
  • Superheat
  • Subcooling
  • Delta T
  • Static pressure

Add a good understanding of all of these readings and when and where to take then in the mix and THEN and ONLY THEN have you earned the right to make jokes about beer can cold. If you have not yet understood the concepts I would advise starting by reading THIS and then listening to THIS

— Bryan

P.S. – Just for fun we created some “beer can cold” and other inside joke gear that you can find at HVACSchoolshop.com

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Tunnel Vision and How To Avoid It

How many times has this happened to you: You’re on your way to that final service call. While you’re listening to the customer explain their complaints over the phone, there’s this precise moment where you’ve thought: “I know what it is already. This will be a quick one.”

Sometimes intuition proves to be a useful tool for an efficient service technician, but that same intuition can bite back fiercely if it leads to ignoring the whole picture.

Let’s take a simple example. The customer reports that their unit isn’t cooling very well and it seems like it’s running longer than normal. The immediate thought may be that the refrigerant charge is low. Reading the pressures on site, it’s discovered that the unit has lost most of its charge. While it can be tempting to restore the refrigerant charge, find the leak, and write up the repair to keep moving, it’s vital to evaluate the rest of the system before proceeding.

Failing this can lead to upset customers who, after paying for the initial service, can face the prospect of additional repairs. Here are a few simple steps to avoid this pitfall:

  1. Listen closely to the primary complaint and address this problem first.

  2. Take care to note any contributing factors to the primary complaint. Ask yourself questions like: “What caused this problem in the first place? Could this happen again if these conditions persist?”

  3. Watch out for any other potential failure points unrelated to the primary complaint.

  4. Document all findings in detail and take the time to explain why each concern is valuable to your customer.

Taking these few extra minutes on the initial visit can save you and your customer precious time and frustration. Resisting the temptation to only solve the first problem will often lead to a more fruitful service call. Nothing beats the peace of mind that comes with a thorough diagnosis.

– Zach S.

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