Month: December 2016

This quiz was written by Benoît Mongeau


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It depends where you live how much pressure you’re under. I live in Florida so I’m very close to sea level, therefore the weight of the atmosphere is exerting right at 14.7 pounds per square inch (PSIA) of pressure on every surface in every direction.

When I hook up to a piece of equipment with my gauges, my gauges read zero, this is because I zero out my gauges so they compensate for the atmospheric pressure.

This is known as PSIG or pounds per square inch gauge, which is the pressure within the system minus 14.7 to compensate for the atmospheric pressure (zero out at the atmospheric pressure)

The issue is, not all locations have 14.7 PSIA.

In the chart above, the far left shows altitude (distance) above sea level and the far right column shows PSIA.

You can see if you are 6000 ft above sea level that there is about 3psi less pressure being exerted on you than where I live in Florida. This means that when you zero out you gauges the actual pressure in the system is 3 psi lower than your low lying brethren.

The problem is that most PT charts and gauges refer to PSIG with the 14.7 already added in. This means that calculating saturation temperatures to calculate evaporator temperature, superheat and subcool will be a real pain.

The easiest way to compensate (if you are at 6000 ft for example) would be to zero your gauge out at 3psi instead of 0, that way the compensation is already built in.

I know it doesn’t seem like much, but a 3 psi difference when taking an accurate superheat and subcool is quite significant, so this is more than just an intellectual exercise.

Some other consequences of altitude is that water boils at a lower temperature, making it more difficult to boil potatoes, but easier to pull a vacuum.

NOW – if you want to get really nerdy. Weather conditions can also alter PSIA based on barometric pressure… but seriously.

You can find a full calculator here just make sure to change the units to PSI and Ft. If you want to find the altitude of your location you can find that here

— Bryan

Electronic leak detection is a critical part of any HVAC technicians common practice. Unfortunately, it is also one of the most common sources of misdiagnosis. Here are my tips to make your leak detection more successful.

Use Your Detector Second 

Before starting to use your detector STOP! look for signs of leaks and corrosion throughout the entire system. I see so many techs who use an electronic leak detector with a very large leak when they would have been better served pressurizing and pinpointing the leak with soap bubbles.

Get a Good One

Use a good quality leak detector. Hint: If it costs less than $300 it probably isn’t great. I am fan of the H10G and the H10Pro although we are testing the Tifzx-1 as a possible option on the recommendation of a few good techs I trust.

Test Your Tools

Check your detector and make sure it actually works EVERY TIME. The H10G has a reference bottle for testing.. USE IT

Let it Warm Up

Many leak detectors require a warm up time for the sensor. With the H10G I allow it to run for at least 5 minutes before I start to use it. 

Start at The Top

Most refrigerants are heavier than air, starting at the top and working your way down will help keep you from picking up a leak below the actual point of origin.

Don’t Rush

Move really slow and when you do get a hit, remove the wand, let it clear and go back to the same point a few times before calling it a leak. Once you think you found a leak, attempt to use bubbles to fully confirm.

Use Common Sense

No matter what leak detector manufacturers tell you.. there ARE other substances that can trigger your detector and refrigerant can move from one place to another due to drafts. I have seen several cases where chemicals in a garage are triggering the detector or where a tech has misdiagnosed an evaporator coil because of a chase leak where the refrigerant is being pulled from beneath the unit into the return. Look around and make sure there is nothing causing interference.

Be Sure

Before you condemn that coil BE SURE. Use all of your resources to positively confirm the exact location of the leak. A little patience goes a long way.


— Bryan

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