This quiz was written by Benoît Mongeau
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This article was written by HVAC / Furnace technician Benoît Mongeau. Thank you Ben.
High efficiency (or 90%, or condensing) furnaces use a set of two heat exchangers in order to retrieve more heat from the combustion products than their mid-efficiency counterparts. Because of this, they generate flue gases much colder than those of a mid-efficiency or natural draft unit. This not only completely changes the way the furnace has to be vented (I will talk about venting specifically in a later tip) but also, and it’s what we’ll focus on, a lot of condensate is generated. This water comes from two sources: moisture which was already present in the combustion air, and the combustion process itself, as the hydrogen atoms from the natural gas molecules (methane, CH4) combine with oxygen to form water. Now as technicians you don’t need to know this part but if you’re a bit into chemistry, here’s the basic chemical equation:
CH4 + 2 O2 + heat = CO2 + 2 H2O
This means that in perfect combustion, for every molecule of CO2 you produce, there is also 2 water molecules produced. This adds up to a lot of water vapor.
In order for the furnace to work properly, that condensation needs to be drained out or else it would accumulate inside the heat exchanger, inducer and venting, impeding proper gas/combustion product flow. Most furnaces will have at least 2 internal drains, typically one for the heat exchanger and one for the vent, usually at the inducer outlet or on the inducer housing.
The secondary heat exchanger outlet is sealed inside a plastic part called the collector box, which is designed to collect the condensate and drain it out.
All condensate drains go into a trap. The condensate trap is absolutely mandatory for a high efficiency gas furnace. Since the drain taps into the exhaust system, leaving it open to the air would allow for a potential exhaust/flue gas leak in the living space, which is a big no-no. Additionally, the inducer motor would suck air through the drain if it weren’t trapped, which could affect combustion, and would prevent proper drainage. Keep that in mind, because if you ever add an extra drain (off a tee on the venting, for example), you will need to TRAP it, always.
The only downside to the trap is potential for blockage. The trap needs to be cleaned out regularly, and that should be done every maintenance. Rinse it out, make sure water flows through the trap properly from all its ports. If there’s any poor flow, fill it up and blow through it a few times to get the dirt out. Hotter water helps for stubborn blockages. The need for regular cleaning also means that drains should be installed as much as possible in a way that allows for the trap to be easily removed. I highly recommend using clamped flexible hoses for the drain, as close as possible to the trap. Avoid hard-piping the whole drain, as it will be impossible to remove and clean out the trap.
To ensure proper drainage, here are the proper practices:
-Make sure every component that produces condensate is sloped towards the drain. That means slope the venting down towards the furnace (typically a ¼’’ slope per foot of length, minimum), and also, slope the furnace itself! Look in your install manual, most manufacturers will call for the furnace to be installed with a slight forward pitch to allow condensate to drain from the heat exchanger.
-Slope the drain line itself, obviously. Avoid double trapping and vent the drain after the trap to prevent airlocks
-Avoid running the drain in an area where it could freeze. That includes running it under the natural fresh air inlet if there is one.
Finally, note that furnace condensate is acidic, and some states/provinces/countries may require the condensate to be neutralized prior to draining.
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.