This quiz was written by Benoît Mongeau
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In this unedited episode of HVAC School Bryan and Nathan talks about some basic rules for circuit board diagnosis including –
– Isolation Diagnosis
– Open Circuits
– Short Circuits
and many other best practices. ..
I started working as a tech when I was 17 years old, fresh out of tech school. My first winter out on my own I went to a service call in an older part of Orlando, a part of town I had never worked on before. It was an especially cold Winter that year, and the service call was for insufficient heat.
When I arrived, I found the system was a really old GE straight cool system. After testing the system, I found the system had a 10kw heater, but only 5kw was working. After a closer look it was discovered that 5 KW of the heat was disconnected. This was no problem for me; wiring was always my specialty! I grabbed some #12 stranded and had that puppy heating in no time.
#1 – It smoked like a chimney and set off every alarm in the house
#2 – Once I got the doors and windows open and the smell cleared out as best I could it got me thinking… How long has it been since that second 5kw was connected?
When I looked closer I saw that the feed wire going to the air handler was only #10… then it dawned on me.
The REASON they had one-half of the heat disconnected was because the breaker and wire size were only rated for 5kw. Why did they a 10kw you might ask? Likely it’s what they had on the truck and they figured if they disconnected one-half it would be safe.
Lessons to learn –
#1 – Never assume that a system was installed properly, to begin with and keep an eye out for proper feed wire size.
#2 – Don’t use improperly rated heat strips or other rated parts and simply make an “alteration”. When the next technician arrives he likely won’t understand what you did. At best you confuse him, at worst you kill him.
P.S. – We released a new podcast on circuit boards today, you can listen here
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.
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.
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.
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.
Depending on what segment of the business you are in and what area of the country you work, you either work on pump down solenoid systems all the time or YOU HAVE NO CLUE what they are.
A liquid line solenoid is just a valve that opens and closes, it has a magnetic coil and depending on whether the valve is normally open or normally closed it opens or closes when the coil is energized.
If you work on refrigeration or straight cool units up north, you are likely very well acquainted with “pump down” solenoids. If you do residential HVAC in the south you may have never seen one.
You know that you pump down a system by closing the liquid line? That’s all a pump down solenoid does. It closes when the system is running causing the system to pump all of the refrigerant into the condenser and receiver (if there is one).
The trick is that in order for it pump down the compressor needs to be running and then it needs to SHUT OFF once it is done pumping down. This means to need a good, quality, properly set low-pressure switch near the compressor to shut it off when the suction pressure gets low enough.
There are a few benefits of a pump down solenoid. First, it helps prevent liquid refrigerant migration down the suction line into the compressor. When liquid refrigerant migrates into the compressor it dilutes the oil and can cause a “flooded” start.
The other cool thing is you don’t need any Low voltage controls between the indoor and outdoor unit. The solenoid is in the liquid line near the air handler inside, so by opening the valve the suction pressure increases and the compressor turns on and when it closes the compressor pumps down and shuts off.
Obviously, this would not work on a heat pump system because in heat mode it would attempt to pump down into the indoor coil which would not work. They also won’t work in most cases when you have complex or proprietary controls.
In some cases the liquid line solenoid is not used to “pump down”, it simply closes during the off cycle preventing refrigerant flow and migration in that way.
So… there are places where a liquid line solenoids make sense and applications where they don’t but they are fairly simple and easy to understand.