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
This article is part 5 of a 5 part series on troubleshooting by Senior Refrigeration and HVAC tech Jeremy Smith
This might be the most challenging part of troubleshooting. We’ve got a “Most Likely candidate” for the trouble, but we don’t know for certain that’s what is wrong.
So, we have to combine our customer skills, our experience, and our troubleshooting skills.
Let’s correct that “Most likely” problem that we’ve identified. Clean a dirty evaporator or
condenser coil, replace the plugged filter drier, repair the leak and recharge the unit to
You’re done, right?
Not so fast…
This is where things can get interesting. Looking at our flowchart, we’ve got a decision loop
here. Make the repair or correction to system operation, then reevaluate system performance. In
reality, this puts us back to the gathering data phase of the process, but we don’t have to
Necessarily gather the same data twice. If we replaced an air filter or a belt or we cleaned a
coil or replaced a capacitor, we can ignore that on our second (and maybe
We’re now looking at system performance. Most manufacturers publish methods to evaluate
their systems. If those fail, we can always resort back to the ‘rules of thumb’ and check to see if
our system operations data now falls into line with accepted industry norms.
If the unit doesn’t match up with manufacturers specifications or industry standards after making
the initial repair, continue the data gathering, data evaluation and repairing the next most likely
problem the data points to.
Be very careful here not to focus on a single aspect of the system. Let’s say you had a high-pressure
trip due to a dirty condenser. So, you clean that coil and reset the pressure switch.
Don’t key in exclusively on the high side readings and miss a low superheat issue. Monitor
ALL of the system conditions and only when everything is within industry norms (or the
customer refuses the work, of course) do you move to the final part of the flowchart and
terminate the troubleshooting process.
Now go out and fix some stuff right the first time.
This article is part 4 of a 5 part series on troubleshooting by Senior Refrigeration and HVAC tech Jeremy Smith
Ok, so we’ve got our data scribbled and scratched out on paper. Maybe a bit of grease, dirt and oil, too, if you’re doing things right and blood if you’re doing it wrong.
Now, time to take a short break and congratulate ourselves on doing it right while sitting and thinking. Have a coffee and look over your data. Now you have some decisions to make.
Much has already been published on analyzing data on a refrigeration system, so I don’t think I need to reinvent the wheel here and review various combinations of pressures, superheat and subcooling and airflows. If you haven’t yet internalized this information, don’t be afraid to have a nice laminated copy of the printout on your truck until you do.
The thing to remember here is that the more data you have and the more accurate that data is, the easier troubleshooting will be for you.
As an example, if you’ve got a unit with a TEV running a 10° subcooling and your low side shows a lower than expected suction pressure and superheat, do you have an airflow problem, a low load problem or a sizing issue? Without collecting good data, it can be difficult to distinguish between the problems but, if you’ve taken TESP readings, return and supply dry and wet bulb temps and have the unit model/serial info on hand when you sit down to analyze the data, the problem should be more apparent.
Evaluate the patterns in the data. Look broadly at all the data and see the patterns. If you have a good data set and a good understanding of the operation of this equipment, a “Most Likely” candidate for problem is going to emerge.
The final step is coming tomorrow.
This article is part 3 in a 5 part series by Senior Refrigeration and HVAC Technician Jeremy Smith
Let’s start with Step#1 in the flowchart.
This is why we spend money on those fancy digital manifolds, shiny electrical meters and other gadgets, widgets and doodads. It isn’t to brag about them on Facebook, it’s to find problems better and faster than someone else.
So, before you start trying to change things, start by gathering and recording data. Inspect filters, inspect coils. Look over the wiring. Check your voltages, resistances, airflow, pressure readings, temperature readings. Locate any open switches in the control circuit and try to determine WHY that switch is open. A pocket notebook is nice but, for larger problems, I’ve taken to carrying a full sized college type notebook. This gives me more room on the page to write my notes, draw pictures, scribble thoughts and observations about the equipment I’m working on.
Write down every measurement and reading. EVERYTHING. Even if you find that capacitor blown up and you “just know” that’s the problem, take your time and keep looking.
Before we leave the Data Gathering step, we do need to take whatever steps are necessary to get the equipment running if it isn’t already and gather another set of data
Once you have all this data together, we can proceed to Step #2. Analysis.
This article is the second in a 5 part series by Senior refrigeration and HVAC tech Jeremy Smith
The Ground rules
I’ve spent some time thinking about troubleshooting and the processes and procedures that
I use to find problems. Not the “why isn’t my air-conditioner running?” problems but the “Things
just aren’t quite right.” type problems. The really difficult ones.
I’ve boiled it down to a sort of flowchart to simplify things and we’ll take the flowchart
step-by-step, explaining each step as we go along.
Something to keep in mind as you read this. There is no step by step, color by numbers guide
to troubleshooting. I’m not trying to give you a magic wand to wave at broken air conditioners
because such a thing doesn’t exist. Troubleshooting is more of a “can do” attitude combined
with experience and some applied critical thinking.
First thing, let’s start with a couple of “Don’ts” when troubleshooting.
#1. Don’t rush
Yes, I know that many of us get piled up under a load of calls and can
be pressured to rush through them to get home to the family. Yes, I know the boss or dispatcher (or both) are calling
you every 10 minutes asking if you’re done and ready to move. Yea, I know the customer is
breathing down your neck to get the machine running. This is probably the hardest part of
troubleshooting. You NEED TO block that stuff out. You need to take your time and work
through the problem methodically.
#2. Don’t assume
Follow your troubleshooting procedure through to the end. Taking
shortcuts is almost as bad as allowing yourself to be distracted.
Over the course of a couple of articles, I’m going to share my troubleshooting processes and
procedures and hopefully give you some tips to build a process that will help you to be better.
Part 3 is coming tomorrow
Often in commercial HVAC and refrigeration you will either find or install sight glass / moisture indicators. The sight glass portion is simple, it’s just there to show if the liquid line has a full line of liquid or if it has bubbles which shows it’s a liquid / vapor mix.
A clear glass on a running system generally means a full line of liquid (or totally flat but you would know that already if you have gauges attached). Reading subcool essentially does the same thing as a sight glass, it simply proves that the system has a full line of liquid. Subcool actually gives you more data that a full sight glass in that it tells you the actual amount of heat that the refrigerant has lost past the condensing temperature.
The moisture indicator shows you if the system is dry or if it has moisture content. First make sure you are aware that older sight glasses may not be sensitive enough to pick up wet conditions with HFC refrigerants that contain POE oil. Second, when installing a sight glass keep it sealed as long as possible before installing. If you open the indicator to air prematurely it may change color due to moisture in the air. If that does happen most indicators will change back after being installed, a proper vacuum pulled and the system run for several hours. If it still reads wet after that time the system likely is wet and new line driers should be installed and deep vacuum pulled.
You best defense against a wet system is fresh line driers, good installation practices that prevent moisture entry and proper evacuation confirmed by an accurate micron gauge.
Tech Brandon Livingston from the HVAC Pro Talk Facebook group posted about fire dampers and took some photos shown here. He gave me permission to share this, inspired by his tip. Thanks Brandon.
Before the damper was opened is on the left. After it was opened and new link installed on the right.
A fire damper is an important part of commercial fire safety. A fire damper is designed to remain open during normal conditions and slam shut to prevent the spread of flame, heat and smoke during a fire. When the link in the fire damper reaches the rated temperature the link will break and the damper will slam shut. Sometimes this can happen as the links age and become brittle and/or due to vibration over time. Fusible link temperature set points are usually 165°F, 212°F, or 286°F with 165°F being the most common.
Generally you will find fire dampers where ducts pass through partition walls and /or floors.
Fire dampers commonly come in 1.5 and 3 hour fire ratings. The hour ratings for fire dampers must be 75% of the hour rating for the wall, floor or partition. That is why a fire damper rated for 1.5 hours can be used in a fire barrier rated for up to 2 hours and a fire damper rated for 3 hours can be used in a fire barrier rated up to four hours.
It is a good practice when installing any new system to measure and mark the normal static pressure on the supply and return ductwork once the air balance has been completed. On commercial buildings that you maintain or service regularly it is a good idea to do it once you take over the building to make future service easier. This way whenever a damper shuts you will know very quickly by comparing the current static to the baseline ou have established. You can easily check duct static pressure using a quality manometer or magnahelic gauge. Keep in mind that on high air velocity systems you will need a pitot tube adapter to get an accurate reading.
If a fire damper is shut they can very difficult to get open by hand. While it is possible, a tool like the FiDO Fire Damper Opener will come in very handy.