Welcome to the Windshield Chronicles, a mental sequence of operation.
Always refer to your OEM installation and service manuals, but this is a guide on how
5 volt DC communicating systems function and how to troubleshoot them in Daikin equipment
All right, everyone, thank you for joining us on Did You Know?, the ESCO HVAC podcast.
So as part of our Windshield Chronicles, we're going to do a mental sequence of operation
of a communicating system.
In particularly, we're going to talk about the Daikin communicating system that's used
in the residential unitary products.
So when we're looking at a Daikin, this actually falls into play for some of the earlier Goodman
& Manna communicating systems before they went to Comfort Bridge, but it also applies
to the communicating that is going from the indoor to the outdoor, even on the Comfort
Bridge system.
So when we look at the Daikin, the Daikin is using the 5 volt DC communicating platform
that is very common in the industry.
There's other manufacturers that use this same protocol, but we're going to focus on
this Daikin one in particularly, just so you can understand how it's functioning.
So with that communicating system, so say we're working on like a Daikin Fit or any of
the Daikin communicating systems, you're going to have four wires going between the thermostat
and the indoor unit, whether it's air handler or gas furnace.
We're going to have two wires that are going to be a part of the communication network,
and that is number one and number two.
But we also have R and C. Sound familiar, Ty?
Very familiar.
I mean, what would you assume would be on a R and C connection, even if it wasn't a
communicating system?
That's our 24 volt R and C. It's alternating current, 24 volt low voltage back and forth.
Exactly.
And so on this four wire connection, we actually have AC voltage as well as DC voltage.
So from our thermostat, the Daikin communicating thermostat to the indoor unit and from the
indoor unit to the outdoor unit, we are typically only going to use the two wire going outside,
but I have seen applications that use those four.
So if we think about this, let's say that I have my communicating indoor unit, my communicating
thermostat and a communicating outdoor, and I only have two wires going to the outdoor
unit.
And because it would have been on a communicating system, that would have been at least a two
stage communicating, it's probably going to be an inverter.
Do I need 24 volts AC for the control board outside, Ty?
Well, that would be depending on what the manufacturer wanted.
The manufacturer wants that then absolutely.
And if they didn't want that, then I figured those engineers have designed it to where
they are already sealing the high voltage to get that, get that dropped down for their
controls as needed.
Exactly.
So what they typically will do if I only have my two wires going to the outdoor unit, that
is my DC communicating number one and number two.
And so if it was like a two stage unit, they simply have you put the factory transformer
or it's already supplied with a factory transformer on the outdoor unit.
And so we do not want to connect the R and C from the outdoor unit to the indoor unit
if I have a transformer out there, because then our AC voltages would be doing what?
They'll be fighting each other.
They'll be out of phase.
Exactly.
So what we do is if we have a transformer outdoor, that is supplying power to the outdoor
unit.
So we do not need AC voltage outside.
And if it's an inverter, it's doing that in the control board itself.
So the power supply of the board, the inverter board is managing the entire outdoor unit.
So all it needs to do is communicate what are we doing?
Are we running in heating?
Are we running in cooling?
And so how does it do that?
Well, it's actually a very common process, not just in our industry, but in many communicating
networks.
So it's part of the RS-485 communicating system that's been around for decades.
We use it for telephones.
We use it for computer communications.
We use it for a lot of things.
And so what we do is we create a positive and a negative pathway in reference to ground.
And this is where a lot of technicians, this is where a lot of people get caught up when
we're thinking about the sequence of operation of a communicating system.
If I have a positive and I have a negative, but it's in reference to ground, well, what
do I normally do with the common side of an AC transformer tie?
Well, half of the industry grounds that and the other half does not.
Exactly.
So with this particular system, you're going to find all of these transformers are going
to be grounded so that common is a reference to ground.
And so with that DC voltage, what we actually do is we separate out the signals in reference
to ground.
So we reduce the voltage down to five volts and then we put it in reference to ground.
And so by using diodes and by using resistors, we can actually create a five volt DC pathway.
So that's the bandwidth.
And because we are now putting it into reference to ground, we can shift that ground from center
just a little bit.
And so that we have more voltage going to the positive side than what we have going
to the negative side of our bandwidth.
And so as technicians, it's a little bit different to think about this DC voltage.
But when we think about it in reference to ground, it starts simplifying it.
So with that DC, remember, most of our power supply is starting at that indoor unit.
If I disconnect the wires at the indoor unit, whether it's a gas furnace or an air handler,
when I put my meter on volts DC, I can now look for my five volts, but because it is
in reference to ground and we have a negative voltage and we have a positive voltage, it's
a little bit different of a thought process.
So Ty, if I was thinking five volts DC, what would my brain immediately think of to look
for between the positive terminal and the negative terminal?
Oh, you would think five volts.
Yeah, you would.
That doesn't actually happen.
And here's why.
When we add the reference to ground into that DC circuit, we put that reference to ground,
remember, I said just below its neutral point.
And so we actually have to find ground to be able to check these DC voltages.
And so what did we do with our transformer?
We grounded one side.
Yes, to common.
So on the DC communicating Daikin platform, we simply put our meter on the volts DC.
We use ground or common as our reference point, and we can go from ground to number one, and
we're going to see our DC voltage.
And it's actually going to typically will be around 2.8 volts DC on the gas furnace.
Air handler is a little bit different, but on the gas furnace, we're going to see about
a 2.8 volts DC from common or ground to that positive terminal.
And then if we keep our one terminal on ground and we go to that number two, we actually
read a negative DC voltage because it is below ground.
Kind of makes sense?
It does make sense.
And between the two, we actually have five volts DC.
But because one of them is going above ground 2.8 volts and one of them is going below ground
2.2 volts, it actually calculates out to five volt DC.
So if I was to take my meter, Ty, and I went between number one and number two, and I was
expecting to read five volts DC, but I didn't read five volts DC, what do you think we might
see between number one and number two?
Oh, you got me, Cliff.
I don't know.
I know.
This got me stuck from the beginning.
So think about that.
As we're driving down the road, we're doing a sequence of operation, we're preparing to
go to this Daikin communicating system, and we want to check our communicating voltages.
If we were thinking about it as a five volt DC system, and I was expecting to see five
volts DC and I didn't, I might think that there was a problem.
But because we went above the ground reference 2.8 volts for one point, and we went below
the ground 2.2, a negative 2.2 volts DC, on the other point, when we measure from those
two points, we actually add them together.
A positive 2.8 minus the 2.2, that negative 2.2.
So what we end up with on the number one to number two terminal should be approximately
0.6 volts DC.
Right, that makes sense.
Yeah.
So we have to change our thought process.
AC, we always think about AC as being positive.
Even though it's really, when we look at the sine wave, it's got a positive and negative
reference.
We're typically looking for an RMS voltages in that.
But in DC, we truly can have voltages above the ground reference and voltages below the
ground reference.
So on that DICAN communicating system, when I go between number one and number two, I'm
actually looking for 0.6 volts DC or greater.
And the reason being is that 0.6 volts DC is our bandwidth for our signals to communicate
on.
And if I have less than 0.6 volts DC, that means that my positive and my negative signals
have been affected by something.
I've got a break in the wire.
I've got a bad connection.
I've got a control wire that I reuse that was chewed up by a mouse and got corroded
and worked fine on 24 volts AC but doesn't like 5 volts DC.
It's actually some of the most common things that we see for the communication.
So what we do when we're troubleshooting a DC communicating system on these, particularly
the DICAN, is we disconnect everything.
We start at our indoor unit.
And if it is the gas furnace, we're going to see a 2.8 and a minus 2.2 from ground to
number one, ground to number two.
If it's an air handler, it'll sometimes be less and be like 1.9 and 1.3.
They use more of a 3 volt DC versus that 5 volt.
But the most important part is that we have a 0.6 volt bias or greater between number
one and number two.
If I don't, I've got a problem.
Now Ty, let's think about this.
If I don't have my proper voltages in reference to common, is there any place that I could
possibly look to find a problem with why I don't have a good common reference?
I would start looking at the connections first.
In particularly the connections to ground.
Yes.
So here's one of the things that we see out in the field, especially here in the Midwest,
where we have old wiring and a lot of furnaces were only wired with a hot and a neutral and
didn't have a proper ground.
I mean, we'd see that and go, oh my gosh, does that really happen?
Yes, it absolutely happens.
And so what we have is we have the potential for a bad ground to that indoor unit to throw
off those DC voltages.
And so unfortunately, a lot of technicians, if they were learning how to check the voltages
and they measured and they found that they had no voltage or very, very low voltage,
they would immediately go to the indoor board to replace it.
In actuality, we need to verify the ground to the indoor unit.
And Ty, what are some good ways we can check ground coming into the power supply of our
indoor unit?
The ground lug, the ground terminal?
Absolutely.
So what we end up finding a lot of times on that ground is if it has a bad connection,
we can lose that potential.
So what I always like to do is to be able to check the voltages from ground to our hot,
make sure that we have, if it's a 120 volt system, we've got 120 volt from ground to
hot and we have the exact same voltage from neutral to hot.
And if there's any difference in those voltages, we know that we've had some kind of a bad
connection on that ground lug or a poor connection of ground somewhere or no ground.
And so if there's a difference in those voltages from ground to hot and neutral to hot, we
know that we have either on that neutral or that ground, we have some kind of a failure,
some kind of an interruption.
So if we verify that we have good ground, we've started our indoor board and we have
0.6 volts or greater, we know that we have started with a good DC voltage.
And now we would connect the indoor communicating thermostat.
So we connect those four wires.
And so from common to R, I got to put my meter on, how about volts AC?
Make sure that I got my 24 volts AC.
But then from common to one and common to two, that's where I would have my DC voltages.
And so I connect the indoor thermostat and I check my voltages between number one and
number two at the thermostat.
And it should be the same as what I had started with my indoor unit because that thermostat
doesn't have much of a load on it.
It's just communicating.
Now if I connect my outdoor unit and my voltages, my DC voltages get janky, now I've actually
determined a place to look for my problems.
And here's where the most common failure on a communicating system on the outdoor unit
comes from.
So Ty, as a technician, and we were working on a AC powered outdoor unit.
So whether a single stage, two stage, whatever it was, if it was just a plain Jane, I got
a compressor, a contactor, and a motor and a capacitor.
Did it really care about ground?
Nope, not at all.
Because it would still run on common and hot.
It only used ground as a reference point if it had a failure and it had current being
able to make its way back to the panel through the ground circuit.
So with our traditional legacy systems, ground was not really important for the outdoor unit
except for safety.
Okay?
So let's think about this.
Because my communicating system relies on the reference to ground and because my new
unit has power from its own power supply outside, where does it get common or the ground reference
for number one and number two?
This is where I sit in class and go, do, do, do, do, do, do, do.
Where my mind is running is if the grounding rod outside is corroded, which often happens,
then that could also lead to an issue with your communication system because it's not
having a reference to ground.
Bingo.
The most common misdiagnosed portion of a failed communicating system when it's referenced
to the outdoor unit is the ground connection back to the electrical panel or through the
grounded of the circuit.
Because when it was engineered, they assumed that all electrical systems were properly
grounded, including the outdoor unit.
And so when that 24 volt common and 24 volt hot was removed from the circuit going to
the outdoor unit and it only had number one and number two, those DC voltages that were
referencing or signaling to ground, the assumption was that they would get that path from ground
of the outdoor unit.
And so what often gets overlooked is the ground from the outdoor unit to the disconnect, to
the ground rod, or to the back to the ground of the indoor panel, the electrical panel.
And so how we find that is we do that same DC voltage check.
So we check from common to number one and then common to number two to look for that
0.6 volt DC difference at the indoor unit and at the thermostat.
When we go to the outdoor unit, since we don't have common, we now use ground because it
is our common.
And if we don't have those same signals and it's an immediate indicator that we have a
poor ground between the outdoor unit and the disconnect and the electrical panel.
And a lot of technicians don't realize what that is.
They assume that those two wires that are going to the outdoor unit, all of the communication
is happening on those two wires alone.
And that is somewhat true, but the circuit relies on ground for the reference point for
those DC voltages.
And if we have a bad ground, if we have a corroded lug, if we have a ground wire that
is rotten, is not making good connection, if we have what I've also seen in some of
the older disconnects, they used aluminum lug screws for the ground wire.
And those aluminum lug screws when connected to copper oxidize and corrode really bad and
they actually lose their conductivity.
So I've had troubleshooting calls that were non-communicating outdoor units that the fix
was as simple as loosening the screws, cleaning off the corrosion, putting some dielectric
grease and cranking them back down to restore our communication to ground and our communicating
systems start talking.
That's interesting.
Isn't that crazy?
So when we start thinking about DC communicating systems, our sequence of operation is we begin
at the indoor unit.
We verify our voltages from ground or common to number one and to number two.
We check the DC voltages between number one and number two to make sure we have 0.6 volts
DC bias or greater.
We check our indoor unit, we add the thermostat to the indoor unit, we check our thermostat.
And then when those are the same, we add the outdoor unit and make sure that nothing changed.
In the instance that we have an extremely long control wire from the indoor unit to
the outdoor unit or we have poor conductors, it's actually going to show itself.
And in some cases, we have to increase that bias voltage.
And on those DICON units, they actually have a switch on the outdoor unit that boost that
bias voltage.
It opens up that bandwidth a little bit.
And so on some systems, when our bias is lower than 0.6 volts DC, we have to engage that
to be able to make sure that we have above 0.6 volts DC.
Because that 0.6 volts DC might be good right now, but because it's in reference to ground,
it could change the next time that it rains or the next time that it gets really dry and
arid outside and affects ground.
And that 0.6 volt DC bias can drop.
So moral of the story, on a DICON in particularly, 5 volt DC communicating system, we must check
from common to one, common to two, and the difference between one and two shall be 0.6
volts DC or greater, or you're coming back.
That's interesting.
I did not know that.
I love that.
All right, Ty, thanks for joining us and we appreciate having a conversation on DC voltages
and we'll catch you next time on Did You Know?
The Esco HVAC Podcast.