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 subcooling 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



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 is 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 a 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 lawnmower 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 advise 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

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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