This podcast is a class that Bryan gave to some of the Kalos apprentices. The cover –
and Much More…
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There are a few important things that I suggest checking on every service call to reduce callbacks and increase customer satisfaction. One of them that often gets missed is preventing wire rub outs.
One of my area managers and experienced tech Jesse Claerbout shot a video showing the simple step he takes to prevent major damage.
We also just release a new podcast episode today that you can hear in any podcast app or by listening HERE
Cheers!
–Bryan
In order to maintain combustion (burning) you need three things, fuel, heat and oxygen. If you have all three in the proper proportion you can maintain a continuous state of combustion.
Remove one (or reduce one sufficiently) and the triangle of combustion can collapse.
In a common NG gas furnace the heat is the igniter, the fuel is Natural Gas and the oxygen is provided by combustion air.
Combustion air is literally just the air needed to provide a continuous supply of air for proper combustion (burning).
All gas fired appliances must have both a flue / chimney to exhaust the leftover products of combustion (outlet) as well as combustion air to provide the oxygen for burning (inlet).
In high efficiency furnaces the combustion air is generally piped in, directly from the outside straight into the combustion chamber. This creates a dedicated source of oxygen and also a cleaner install as no other provisions need to be make for combustion air.
In 80% furnaces the burners usually have “open” combustion and they rely on air being drawn into louvers on the furnace cabinet. In this design the space on which the furnace resides must have open communication to the outdoors or other “uncontained” space.
Not to get into the specifics of code, but you must have a dedicated method to get significant air to the furnace . If you do not, the real possibility exists that the furnace could begin burning improperly creating an unsafe condition for the occupants due to Carbonmonoxide.
Different parts of the country provide combustion air in differnent ways, but you MUST have some method of providing unlimited fresh air to a furnace or to the room in which the furnace is located. This means when a furnace is in a tight space, ensure you have some sort of significant combustion air.
— Bryan
Proving flame is an important part of the gas firing sequence. Without proof of flame you risk dumping unspent gas into the heat exchanger resulting in an explosion.
There are many ways to “prove flame” we are focusing on the flame sensing rod method here.
Here are the facts-
Flame sensing rods, also know as flame rectifier rods or flame rectification rods are common place in modern hot surface and ISI (intermittent spark ignition) gas fired appliances.
Flame sensing rods stick out into the flame and connect back to the furnace board. Once the board sends a call to the gas valve to open, it monitors current flow on the flame sensing rod. It does this by generating a potential (voltage) at the flame sensing terminal, this terminal is connected to the sensor with a conductor. When no flame is present there will be potential at the rod and no current, when a flame is present a small microamp DC current will be present as a path is made between the rod and the ions in the flame. This small DC current signals the board that flame exists and all is well with the world. If it does not sense this microamp DC current within a few seconds it will shut off the gas valve and try again.
The board outputs this potential (voltage) on the flame sensing terminal right at the beginning of the sequence to confirm that the path is “open” with no flame. This ensures against false positives (sensing flame / current when there should be none) andonce it goes from 0 current to the rated micoamp current the board “knows” that flame is present.
These flame sensing rods are “dumb” devices. They do not generate potential (volts) or current (amps), their predecessor the thermocouple (seen in standing pilot systems) does generate a potential itself which is often the source of the confusion.
A flame sensing rod is a piece of metal with a ceramic insulator that keeps it from grounding out. That is all. However because it is conducting in the Millionths of an amp (microamp) a lot can go wrong with it that a normal electrical component wouldn’t have any issue with. Tolerances are tight so small factors make a big difference.
Flame sensors fail when:
Before I go any further I want to address a common question. Do flame sensors have a special coating that can be rubbed off with improper cleaning?
Well… If we are talking about a thermocouple or a thermopile then yes.. absolutely, but we aren’t discussing standing pilot systems here.
I have seen a lot of flame sensing rods, and I have done a good deal of research and I have found no evidence that most flame sensing rods have a special coating on them that can be rubbed off. Now, if you have real, quantifiable proof from an manufacturer that says otherwise.. PLEASE provide it to me so I can retract this statement.
Here are the steps to test a flame sensor –
If you do not have a proper microamp reading you can confirm the following
Once you have established all of the above and you are still not getting the required microamps then you are left replacing the board.
Word of warning –
Test your tools regularly. If you are trusting your meter and you aren’t 100% sure your meter is working and set up properly you may end up with a misdiagnosis. Test and calibrate your tools regularly.
Do every possible test before replacing a board. Many techs advocate just replacing a flame senor if they suspect it isn’t conducting well. I am cool with that so long as
Or.. you just install a new one long enough to test. That is all fine and good if you have extra flame rods in your truck. Many techs do not have that luxury.
Finally…
If flame rods are getting dirty / coated often, you will want to find out why. There is something in the environment or the combustion that is causing it.
In Summary flame rods should be
Now is the part where the furnace techs from all over the world tear me apart.
— Bryan
One of the most common questions we get from techs is about using a volt meter to diagnose a high voltage circuit. It’s especially tricky when a tech is used to working on Low voltage or 120V circuit where there is a clear “hot” side of the circuit and a clear “grounded” side of the circuit. In 120V you have one hot leg and the other side is neutral which is actually connected to ground back in the panel. Most (but not all) 24v transformers have one hot leg and the other leg is grounded. A car has one 12VDC Hot and the other side is grounded to the chassis.
All of these cases cause techs to get used to putting one meter lead to ground and “walking” the other lead through the circuit, looking for where the voltage is lost. While this is still not the best idea even on these circuits, it usually works.
However…
in 240v or 3 phase diagnosis it doesn’t work. Here is why –
The other “side” of the completed circuit is not grounded at all. So when you check to ground, you are checking to a point that has literally NOTHING to do with the completed circuit you are diagnosing. Even more important is the fact that you will often read “120v to ground” even when the leg you of power you are attempting to diagnose is open.
Here’s an example
Let’s say you are trying to see if the IFR contact is open. So you put your meter from L1 to ground. Good news you have 120v. So now you are feeling confident and you read from IFR terminal 2 to ground and you still have 120v. So now you think, “The IFR terminals are closed because I have 120v on each side”…
WRONG!
You will have 120V to ground on IFR terminal 2 regardless of whether the contacts are open or closed. If they are open you will be reading 120v backfed through the motor from L2, if they are closed then you will read L1.
In other words it’s a pointless test.
Take a deep breath…
This next part is gonna take some focus to understand. If you don’t intend to pay careful attention to these next paragraphs you won’t benefit.
Instead read from L1 to L2 and confirm 240V then read from IFR1 to L2 then from IFR2 to L2. If you have 240v on IFR1 and not IFR2 then you know IFR is open…
An alternate method if you are DEAD SET on reading to ground is to check IFR1 to ground. If you have 120V then check from IFR1 to IFR2. If you read anything across the contacts you would then know they were open.
Remember..
You will read potential (voltage) so long as a path and difference in charges exists, across a load and across an open switch. You will not read potential (voltage) across a closed switch because a closed switch has no potential difference across it.
Final notes –
You are encouraged to check both legs to ground for safety purposes to confirm. disconnect is actually off and open.
Checking to ground can be a way to check the ground itself, although in that case a de-engergized ohm or megaohm test can often be a better test.
–Bryan
In this episode of HVAC School Bryan talks to Leslie about the illusive TXV. They talk about:
And Much More….
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In this episode of HVAC School we talk about thermodynamic basics, including:
And Much More…
As always if you have an iPhone subscribe HEREand if you have an Android phone subscribe HERE
I walked in to my first real job interview in the A/C business. The manager was a guy named Ernie and he walked me out to the warehouse.
Quick warning.. guys named Ernie are tough. Don’t mess with dude named Ernie.
Anyway..
He walked up to a box, snatched a pen out of his shirt pocket and scribbled a circle, 3 dots and three numbers on it while grunting “which is common, start and run”
I was in luck….
While I may have had almost zero practical knowledge of air conditioning, this was one thing I HAD actually learned in school.
I marked the terminals and I got the job.
Before you say that this information is useless let me stop you.
It isn’t useless. It may not be something you use every day, but I have needed to ohm out a motor or compressor a handful of times and it got me out of a pinch.
So here it goes
The lowest ohm reading is between Common and Run
The middle ohm reading is between Common and start
The highest ohm reading is between start and run
Common is just a point between start and run and therefore the common to start and run to start readings will add up to the run to start reading.
Here is how I remember this (let the mockery begin)
Starting is hard… so it has the highest resistance
Running is hard also… but not as hard as starting, so it has a resistance less than start.
Common is easy… being common requires the lowest resistance
So common to run is the least and start to run is the most.
Understanding common, run and start is uncommon… so it requires a lot of resistance… so start… knowing it
OK I’m done.
Happy Thanksgiving ?
— Bryan
In this episode of HVAC School I talk with my brother Nathan… and he just whines a lot about cool tools I like…
That’s basically all.
And we talk about solder rings and thermal imaging cameras and digital gauges.
–Bryan
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In this episode of HVAC School we discuss the entire basic refrigerant / Compression refrigeration cicuit.
And Much More…
As always if you have an iPhone subscribe HEREand if you have an Android phone subscribe HERE
