Month: June 2017

I live in Central Florida, and while it can get pretty hot in the Summer we also tend to get afternoon thunderstorms that come and go in a flash. I have been connecting gauges, checking charges and even pulling vacuums in the rain as well as under umbrellas or cardboard boxes most of my career and only recently did I stop to think if this was a good idea.

Moisture in the System 

I am going to go ahead and make the blanket statement that opening the system or connecting gauges while it is actively raining is just a bad idea. Not because you are “made of sugar” like the old timers might claim, but rather because even a drop of water in the system can do a lot of damage in the age of POE oil. If you have a good shelter or large umbrella you might be OK, but in Florida we get these bursts of crazy weather that you probably aren’t going to keep out of an open suction line.

This isn’t to say you don’t start a compressor just because it looks like it “might rain”, but I would suggest being prepared with caps or plugs to seal it up quickly if it does start to pour.

If it is actively raining I would also advise against connecting gauges or opening the panels and testing electrical components unless you have a good umbrella or shelter in place. Electrical testing can damage the components as well as be unsafe, and connecting in the rain can lead to moisture contamination.

Wet Condenser Coil

You will not be able to test or set a charge with any level of accuracy when the condenser coil is wet. The system pressures will be low and the subcool will be high due to the evaporative effect and the superior heat transfer of water over air. If you want to simply confirm that the unit is functional you can take an evaporator delta T and measure the liquid line and suction line temps at the evaporator to approximate proper operation. You will not be able to “set the charge” until the condenser coil has been allowed to dry completely.

The liquid line will generally be around the outdoor temperature or maybe even a bit lower depending on the SEER of the unit and how wet the coil is (Wetter / Higher SEER = Cooler Liquid Line)

The suction line will be approximately the return temp minus 40°F(4.44°C) + the desired superheat +/- 5°F(2.75°K), It will tend to be on the lower side of the evaporator temperature scale because of the lower liquid pressure.

My Opinion

If you read our articles you know that we are huge advocates of taking accurate measurements and not just walking away from a system without doing appropriate testing. However, if it is raining you are just not going to get good readings and you also risk doing more harm than good to the system by taking them. Sometimes taking fewer readings can be the best call. When it’s raining I would rather have my techs note the delta T and indoor liquid line and suction line temps and note”raining” than to risk an issue by connecting in the rain.

In cases where the charge must be set, we will need to go back once it is dry to set it. In cases where we did a simple drain cleaning, replaced a blower wheel or thermostat or a capacitor those indoor readings will suffice. Are they conclusive? NO! would I rather contaminate a system? Nope. Should we return to every system the next day just because it was drizzling to check the charge with gauges? I say no to this as well.

You may say (as many do) that connecting while raining has never caused issues for you before. To that I would say, How do you know?

It’s not like introducing moisture causes the compressor to instantly explode.

It was also less of an issue when mineral oil was the prevalent oil in use.

Am I saying that you can never check a charge even in a light sprinkle? No

Just use common sense, don’t be a robot that ALWAYS connects gauges even when it will likely do more harm than good.

— Bryan



I get emails from time to time with questions that stem from the articles or the podcast. This was a great question, but I was not the best person to answer it.

I reached out to Jeff Neiman, our resident HVAC School chiller tech and he answered it. Here is the question

Hello Bryan,

Thanks for all the good material you provide. I mostly work on the commerical building side of HVAC where chilled water is used as cooling medium and cooling towers provide condenser water. We have chillers as well as heat pump and air cool splits throughout facilities. Most of your diagnostics and troubleshooting methods are for air cooled units. Can they be applied to water cooled evaporators and water cooled condensers? My thinking is yes and no, because with cooling tower 85 supply and return 95 is maintained and 45 supply and 55 return chilled water is provided. Since there is not much change in these temps as opposed to outdoor ambient temperature there won’t be much pressure change in condenser. And as long water is regulated at proper flow to evaporators and condenser then all should hold steady. Do you have any input on this? I’m in NYC. Went to 2 year hvac school and worked almost 3 years in field starting out as a helper in service van as experience and learned as much then got into the building side for about 8 years now. I like listening to your podcast and reading your material as it keeps me refresh with field work as the building side is a little different but the basics and fundamentals are the same. Thanks!

Hey Anand,

The answer is yes.

Some of the measurements can be applied to chillers as well. Just some of the verbiage is different and the values differ.
The numbers for chilled water (44 out 54 in) and condenser water (85 out 95 in) are industry standard values at full load conditions. Most chillers regardless of manufacturer will have a 10*F delta T on the cond and evap. Machines that operate outside of those ranges are chillers that were ordered specifically to provide a lower temp or larger delta T.

Many people look at the compressor motor RLA% as the chiller capacity, which is not accurate. Chiller capacity is measured by the evap delta T. If the chiller is designed for 10°F(5.5°K) delta, and is currently providing 44°F(6.66°C)) water and the return water is at 49°F(9.44°C), the delta T is 5°F(2.75°K). So that chiller is currently running at 50% of its total capacity.

Subcooling is still measured the same, although the reading that you get will change as chiller capacity changes. At low loads your subcooling will be lower and will increase as capacity increases.

Suction superheat is a value that I really don’t look at because the reading on a flooded type of system will usually be very low or even 0. Rather discharge superheat (discharge temp – cond sat temp) is a more accurate reading and will be a direct result of you suction superheat. High suct SH, there will be high dis SH and vice versa. Again, this value will change as chiller capacity changes.
However if the chiller is a DX type, the suction superheat is just as valid as on a residential system

One of the values that was described in the podcast was temperature difference (supply air temp – coil temp).
In regards to air handlers with chilled water coils you can do the same thing. Measure your supply air temp minus the coil leaving water temp. This will tell you how well the heat is transferring to the water from the air going across the coil.

In chiller lingo this measurement is called approach

There are two different approach temps that i look at on a chiller:

Condenser approach (cond sat temp – lvg cond water temp)
Evaporator approach (lvg water temp – evap sat temp)

Approach values should range in 0 – 3°f(0°K – 1.65°K), given that your flows are correct.
Just like on air cooled units where proper airflow is needed across the evaporator and condenser, you need to verify that you have proper water flows.

In air to air applications you are measuring static to identify airflow issues. In water applications, I’m measuring pressure differential across each barrel. If I know my design pressure drop on the evap and cond, I can compare to my actual to know if my flows are proper. Keep in mind though that most chiller manufacturers will give the the design pressure drop in ft/hd. You will need to convert your real time reading to ft/hd to have an accurate comparison if you are using a gauge with a psi scale.

Even if your water temps stay pretty constant while in operation, your pressures will veer off as problems arise and your approach values will increase.
The chiller will always try to maintain at 44°f(6.66°C) chilled water out (or whatever the setpoint is) as long as it can do so.

The refrigeration cycle doesn’t change, stick to the basics and don’t over think it

When running building, try to get your condenser water as low as possible when running. But stay above 65°F(18.33°C).

Anytime you can provide condenser water lower than the design of 85°F(29.44°C) you will lower your condenser pressure and lower the lift (cond pressure – evap pressure). This will result in less work the compressor has to do and lower KW. This is a common method called condenser relief.

— Jeff Neiman

First off I want to thank Ulises Palacios for taking these photos. He is in the habit of cutting open the compressors he replaces to see why they failed (when possible). I think that’s pretty boss.

So why would the compressor have copper plating on the inside? They certainly aren’t manufactured that way.

The short answer is the acid inside the system eats away at the copper and brass components in the system. The copper is then deposited in the high pressure/temperature environment of the compressor.

Why does this happen?

The presence of any acids in the system can cause this to occur but the most likely causes are the combination of air and moisture reacting with the refrigerant oil (most prevalently POE) to create an environment in which the copper is dissolved internally and redeposited on the steel in the compressor.

The result inside the compressor is reduced clearances and ultimately locking, overheating and even short circuits if the mechanical failure results in winding damage as is fairly common.

So for a technician, what we can do in ensure we are properly evacuating the system and installing appropriate filter driers to reduce or eliminate the presence of air and moisture.

— Bryan

P.S. – For an in-depth analysis of a study on copper plating in compressors you can read here

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

This tip was created by Jason Pinzak and originally posted on the HVAC Technician’s Facebook group. It is reposted here with permission from Jason. Thanks!

Contactors are useful in commercial and industrial applications, particularly for controlling large lighting loads and motors. One of their hallmarks is reliability. However, like any other device, they are not infallible. In most cases, the contactor does not simply wear out from normal use. Usually, the reason for contactor failure is misapplication. That’s why you need to understand the basics of contactors.

When someone uses a lighting contactor in a motor application, that’s a misapplication. The same is true when someone uses a “normal operation” motor contactor for motor jogging duty. Contactors have specific designs for specific purposes.

When selecting contactors, you’ll use one of two common standards: NEMA or IEC. Both match a contactor with the job it has to do, but they do so in different ways.
The NEMA selection process always results in a choice of a contactor you can use over a broad range of operating conditions. For example, you could use a NEMA Size 5 contactor to run a 50-hp motor operating at 230V or a 200-hp motor at 460V.

Using IEC standards, however, you can size contactors very close to their ultimate capabilities. In many cases, this precision allows you to predict how long they’ll last. For example, an IEC-rated contactor may run a motor that draws 40A at full load. In that duty, it should last for more than two million operations. But, if you used it for consistent jogging and plugging, you’d have to replace it after just a few thousand operations.

Since a contactor should last for years, don’t automatically replace one that fails with an identical unit. Instead, take a few moments to see if there is an obvious problem. A contactor really has only two basic parts: the contacts and the coil. The coil energizes the contactor, moving the contacts into position. The contacts transmit the current from the source to the load. Heat can destroy either of them, so take a good look at both.

Contacts will overheat if they transmit too much current, if they do not close quickly and firmly, or if they open too frequently. Any of these situations will cause significant deterioration of the contact surface and the shape of that surface. Erratic operation and failure will be quick. To check the contacts, just look at them. Some minor pitting (see photos) as well as a black oxide coating is normal, but severe pitting or any melting or deforming of the contact surface is a sure sign of misapplication. Replace contacts with such symptoms.

Coils can overheat if operating voltages are too low or too high; if the contacts fail to open or close because of dirt or misalignment; or if they have suffered physical damage or experienced an electrical short. Coil insulation degrades quickly when it gets too hot. When it degrades, it will short out (and blow a fuse) or just open and stop operating.

To check a coil, measure the ohms across the contactor coil. Infinite resistance means the coil is open. A shorted coil will still often register significant resistance and can be confused with a good coil . If you happen to have a matching contactor nearby, compare the two coils. The shorted coil will usually have significantly lower resistance than the good one but a compromised coil can alos have a higher resistance. If the difference is significant, replace it. Replacing the contacts or coil often means replacing the whole contactor. But no matter what you replace, compare the NEMA or IEC rating with the job the contactor will be doing. If you match it to the application, it should last a long time.

— Jason Pinzak

P.S. – Here is another good article on the difference between IEC and NEMA rated contactors

See the photo above? This is a unit we (my company) recently serviced for a commercial customer. 

It doesn’t matter if we aligned the belt, dialed in the charge, cleaned the condenser and got the drain pan clean. We look like dummies because the panel fell off.

It doesn’t matter that we’ve had some crazy storms or that some of the screws were stripped out long before we got there. What matters is that we serviced it and the panel fell off.

Some of you will roll your eyes that this is even a tech tip. But if you are honest, how many units have you left that didn’t have ALL the screws properly in place. How many times have you left a unit where one of them are so stripped out that the screw was doing nothing?

So, the primary message is

Don’t leave unless all designed fastening points are secured

This occasionally means tapping in a new screw, sometimes in a new location. Sometimes it may mean running to hardware store to get a fastener thats lost. Just take care of it properly and take pride in the finished product of your service. 

While on the topic keep in mind that screws left on a roof or in the grass can cause roof damage or get picked up by a lawn mower and thrown into a car or another person. It isn’t just the panel that comes off that lead to property damage and a safety hazard, it is also the screw itself.

Now… there is something else to consider. The use of impact drivers and drills with no clutch or the clutch set too high has resulted in a big increase in stripped out fasteners. 

An impact driver (like shown above) is meant to DRIVE screws either in repetitive or high torque applications. Impact drivers are designed with a “percussion” action that drives screws quickly and forcefully into the base material. That high torque action also does a great job of stripping out screws.

A driver like the one shown above does not have the TORQUE of an impact but it turns screws and fasteners with a smooth motion without the percussion of an impact. It also has a clutch that should be set as low as possible to get a snug fastener without the risk of striping out.

For the average HVAC/R technician I would advising using a clutched driver as your primary “go bag” tool and only reach for an impact or larger drill if you are driving screws repeatedly into new material.

Using the right tool consistently can make keeping panels firmly in place an easier task and avoid embarrassing situations like the one at the top.

— Bryan

P.S. – you can get a great discount on the Milwaukee driver shown above by clicking HERE and using the offer code getschooled at checkout 

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 I created some “Beer Can Cold” T-shirts that you can buy for the next 5 days by going HERE but ONLY if you solemnly swear not to actually charge a system that way.



A while back I had a tech who was having some trouble finding a 35 PSI(2.41 bar) make on fall pressure switch. In the catalog, one adjustable switch said (SPDT) but he didn’t quite understand what that meant. In that case, it means single pole, double throw, and the “double throw” part means that the switch has terminals in both the close on rise and close on fall directions. Another common example of this sort of switch is a “3-way” light switch.switches

A single pole, single throw (SPST) switch is like a typical light switch. It only has one path (pole) and it is only closed or open.

A double pole singe throw (DPST) switch that is quite common is a 2-pole contactor. It has two switches but they only open and close in one direction.

A common double pole, double throw (DPDT) switch in HVACR is the 90-340 relay (and many other relays), where it has two circuits and they alternate between closed and open terminals.

— Bryan




We do this exercise when I teach electrical basics where we sit down and connect a 10 watt bulb to a power supply and through a switch. A SUPER SIMPLE circuit, the kind you might have learned about in high school science class.

But then I grab another 10 watt bulb and tell them to connect it in line with the other 10 Watt bulb (series circuit) and BEFORE they can turn the switch on I ask them a series of questions.

  • Will the two lights be twice as bright as the one? the same? or half as bright?
  • Will the circuit draw twice the amps as before? The Same? or half the amps?

Before we move on, I want you to make your choice.

So everyone makes their choice.. we turn on the switch…


The bulbs combined are half as bright, using half the amps and thus half the watts. On my quizzes this is an area where experienced techs and electricians will even get frustrated “If you have X2 10 Watt bulbs that is 20 watts” they will say.

The science is actually really simple. In a light bulb, they may be stamped with a rating wattage but that wattage is just a rated wattage when the full rated voltage is applied. The constant in a light bulb is the resistance in Ohms, not the wattage. When you double the resistance of a circuit by adding in another 10 watt bulb in series you are cutting the amperage in half and therefore also cutting the wattage of the circuit in half.

an electrical circuit is a path between two points that have a difference in electrical potential (Voltage) the amperage (and by extension the wattage) is a function of the total resistance of that circuit between those points. If the resistance goes up, the amperage goes down and vice versa. It doesn’t matter if that resistance is added by a bulb, resistor, thermistor, pitted contactor points, motors etc…

Now when we mix in inductive reactance in motors and other inductive loads that resistance is bit less cut and dry to understand… but we will save that for another tip.

— Bryan

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