Carrier/Bryant circuit board question - Any HVAC techs able to help?

[ScaldedDog]

I'll cut to the question, then fill in the background:

Are Carrier circuit boards HK42FZ0723313 and HK42FZ0723715 functionally equivalent, or are there differences that make the latter an inappropriate replacement for the former?

My furnace (Bryant CNPVP4821ALAAAAA) is not kicking on the blower reliably, and it goes into a lockout mode whenever that happens. Local HVAC company *thinks* the board is bad, and they want $1700 to replace it. The -3715 board is available from repairclinic.com for $480, so I'm trying to figure out if it is a real replacement for what I have, or not.

Might any of you be able to tell me the significance of Carrier part number differences?

Thanks!!

Mark

 

[Buckaroo5]

Might call this place and discuss.....

Shortys HVAC Supplies | Short on Price, Long on Quality! | Indianapolis, IN

Shortys HVAC Supplies - full supplier of HVAC parts & equipment in Indianapolis. We supply parts for Heating, Ventilation, & Air Conditioning repair needs.

shortyshvac.com

 

[fitter30]

Could run the fan in the on position it will run on high speed if you have ac. That will take the fan out of the picture.
That.model.of furnace looks.like it's the evaporator coil. Furne numbers in burner compartment might be on either side.

 

[danski0224]

My furnace (Bryant CNPVP4821ALAAAAA) is not kicking on the blower reliably, and it goes into a lockout mode whenever that happens.

Is the fan operation connected to a call for heat?

If you can turn on the fan independently from the thermostat (fan on) without a call for heat or cooling (turn those off), then the problem is elsewhere.

Don't know on the board interchange question.

Assuming "blower" = duct fan.

 

[Bert_]

Which blower? Draft inducer or the indoor air blower?

Are you getting voltage on the board at the blower terminals?

 

[ScaldedDog]

Is the fan operation connected to a call for heat?

If you can turn on the fan independently from the thermostat (fan on) without a call for heat or cooling (turn those off), then the problem is elsewhere.

Don't know on the board interchange question.

Assuming "blower" = duct fan.

Which blower? Draft inducer or the indoor air blower?

Are you getting voltage on the board at the blower terminals?

EDIT: The error code is 33, not 34. That still references a high limit error, but I'd remembered the code incorrectly when I wrote this post.

The symptom is the blower fan fails to start after a call for heat. The inducer motor starts, then the burners light, but the blower fan doesn't start. When this is detected, the burners shut off and a code 34 is thrown. After displaying that for a few minutes, the code changes to 13, which is the code for a lockout. This may happen 3-5 times, then it'll start normally one to several times, then fail to start again.

The local Carrier guys have looked at it twice, without being able to pinpoint the root cause, and have just said, "the board may be going bad." Simple stuff can cause this - like a blocked vent for clogged filter - but I've verified the roof vent is clear, and it'll fail to start with no filter installed.

If either of you have ideas, I'd love to hear them, and really appreciate the help!!

Mark

 

[Bert_]

The symptom is the blower fan fails to start after a call for heat. The inducer motor starts, then the burners light, but the blower fan doesn't start. When this is detected, the burners shut off and a code 34 is thrown. After displaying that for a few minutes, the code changes to 13, which is the code for a lockout. This may happen 3-5 times, then it'll start normally one to several times, then fail to start again.

The local Carrier guys have looked at it twice, without being able to pinpoint the root cause, and have just said, "the board may be going bad." Simple stuff can cause this - like a blocked vent for clogged filter - but I've verified the roof vent is clear, and it'll fail to start with no filter installed.

If either of you have ideas, I'd love to hear them, and really appreciate the help!!

Mark

Indoor blower then.

Are you getting voltage to the motor?

I can't tell what a code 34 is from all the way over here at my house. You have to look at the chart on your furnace to see what that means. Guessing high limit.

 

[mm08822]

Indoor blower then.

Are you getting voltage to the motor?

I can't tell what a code 34 is from all the way over here at my house. You have to look at the chart on your furnace to see what that means. Guessing high limit.

Or is it even sensing a temp rise to run fan?

 

[ScaldedDog]

Indoor blower then.

Are you getting voltage to the motor?

I can't tell what a code 34 is from all the way over here at my house. You have to look at the chart on your furnace to see what that means. Guessing high limit.

Yes, it's a high limit error. The motor runs fine under AC, or when manually commanded through the thermostat, so it is getting voltage under those conditions.

@mm08822 , do you mean temp rise in the combustion chamber (or whatever it's called in a furnace)? If so, I believe so, as it is sensing that the burners are on and the fan is not, and so shuts down. At least that's how the HVAC guys explained it.

If you mean a call for heat from the thermostat, then yes, it is.

Mark

 

[mm08822]

Yes, I meant temp rise.

I'll ask the obvious q.....filters are clean and blower is moving air?

 

[Bert_]

It doesn't sense if the fan is running. The only feedback is the high limit that trips if things get too hot. Unless it has a variable speed ecm motor. Those usually communicate with the control board.

If you don't have voltage at the blower in about 30 seconds after the flame is established then the board has failed. Get a meter out and test it, No guessing about it.

Most furnaces in the last 20 years have the blower and flame control built into the same board.

 

[Bert_]

Or is it even sensing a temp rise to run fan?

If it's old enough to have a fan/limit that could cause the problem.

Most anything made in the last 30 years has a simple time delay

 

[ScaldedDog]

Yes, I meant temp rise.

I'll ask the obvious q.....filters are clean and blower is moving air?

Yes, and it's not as obvious as it would seem. I didn't see the HVAC guys check it, and I know they didn't check the exhaust on the roof. I did both after they left, and the problem is present even with the filter not installed.

It doesn't sense if the fan is running. The only feedback is the high limit that trips if things get too hot. Unless it has a variable speed ecm motor. Those usually communicate with the control board.

If you don't have voltage at the blower in about 30 seconds after the flame is established then the board has failed. Get a meter out and test it, No guessing about it.

Most furnaces in the last 20 years have the blower and flame control built into the same board.

Cool. I'll report back this week when the new board arrives. Thanks!!

Mark

 

[BurtEggley]

mine was doing that same sequence, and as I recall it was one of two repairs they made. I forget which one of these fixed it.

A) the repair required a sensor in the supply line to read temp. The tech called Carrier, they told him to install a sensor in the return line a foot or two above the coil. The model required it.

B) One of the two pressure sensors was out of limits and had to be replaced.

 

[ScaldedDog]

Small update before the new board arrives on Friday: I tried turning on the blower fan from the Nest thermostat, thinking if the fan was already running when a call for heat was made, it would continue to run and the furnace would stay on. However, the fan will not start when commanded by the thermostat. So:

Manually commanded by thermostat: Blower fan does not start
Commanded during furnace start: Blower fan does not start
Commanded during AC operation: Blower fan starts

Does this still likely point to a board failure, or somewhere else I should look in the meantime?

Thanks!!

Mark

 

[fitter30]

Small update before the new board arrives on Friday: I tried turning on the blower fan from the Nest thermostat, thinking if the fan was already running when a call for heat was made, it would continue to run and the furnace would stay on. However, the fan will not start when commanded by the thermostat. So:

Manually commanded by thermostat: Blower fan does not start
Commanded during furnace start: Blower fan does not start
Commanded during AC operation: Blower fan starts

Does this still likely point to a board failure, or somewhere else I should look in the meantime?

Thanks!!

Mark

Blower door should have a wiring diagram inside take a pic. Model of furnace burnrer compartment. Q

 

[Bert_]

Small update before the new board arrives on Friday: I tried turning on the blower fan from the Nest thermostat, thinking if the fan was already running when a call for heat was made, it would continue to run and the furnace would stay on. However, the fan will not start when commanded by the thermostat. So:

Manually commanded by thermostat: Blower fan does not start
Commanded during furnace start: Blower fan does not start
Commanded during AC operation: Blower fan starts

Does this still likely point to a board failure, or somewhere else I should look in the meantime?

Thanks!!

Mark

Assuming you have a regular old split capacitor motor...

Heat and cool are different fan speeds. Each speed will have its own relay on the circuit board.

You seem really resistant to using a meter to test voltage. You could positively identify the problem instead of just guessing.

 

[BurtEggley]

when was the Nest thermometer put in?

 

[ScaldedDog]

when was the Nest thermometer put in?

A couple of years ago. The problem started last winter, about this time.

Mark

 

[ScaldedDog]

You seem really resistant to using a meter to test voltage. You could positively identify the problem instead of just guessing.

Not a problem, actually, but where? The heat output terminal on the circuit board (assuming I can identify it)?

Mark

 

[Bert_]

@ScaldedDog

There is a handful of wires that go from the board to the blower motor. One is a neutral, the rest are different speeds. Anywhere from 2-4 speeds most common. Where each wire connects will depend on what fan speed the installer needed for heating and cooling.

On the board one of the wires will connect to a terminal labeled "heat". That terminal should get power 30sec or so after the burner lights.

You want to connect a meter to the heat terminal and the neutral terminal.

 

[ScaldedDog]

@ScaldedDog

There is a handful of wires that go from the board to the blower motor. One is a neutral, the rest are different speeds. Anywhere from 2-4 speeds most common. Where each wire connects will depend on what fan speed the installer needed for heating and cooling.

On the board one of the wires will connect to a terminal labeled "heat". That terminal should get power 30sec or so after the burner lights.

You want to connect a meter to the heat terminal and the neutral terminal.

Good call, man. In addition to Common, there are Heat, Cool and Fan terminals, in addition to two labeled Spare. Here's what the voltage measures across those to Common:

Heat - 4v all the time, 26v when a call for heat is made, and the burners have been on for roughly 30 seconds. The fan does not start.
Cool - 3.5v all the time, 27.5v when a call for cooling is made, and the fan does run
Fan - 4v all the time. When Fan is selected from the thermostat i hear a click from inside the furnace, but this voltage does not change, and the fan does not start. The fan does start when a call for cooling is made, but the voltage on this terminal doesn't change.

What does this tell you?
Why is there 3.5-4v all the time, even when no calls for service are being made?
Should I expect the fan to run when the heat terminal is energized to >24v? Or does the fact that the Fan terminal never is the root of the problem?

Thanks again!! I really appreciate your help.

Mark

 

[Bert_]

Good call, man. In addition to Common, there are Heat, Cool and Fan terminals, in addition to two labeled Spare. Here's what the voltage measures across those to Common:

Heat - 4v all the time, 26v when a call for heat is made, and the burners have been on for roughly 30 seconds. The fan does not start.
Cool - 3.5v all the time, 27.5v when a call for cooling is made, and the fan does run
Fan - 4v all the time. When Fan is selected from the thermostat i hear a click from inside the furnace, but this voltage does not change, and the fan does not start. The fan does start when a call for cooling is made, but the voltage on this terminal doesn't change.

What does this tell you?
Why is there 3.5-4v all the time, even when no calls for service are being made?
Should I expect the fan to run when the heat terminal is energized to >24v? Or does the fact that the Fan terminal never is the root of the problem?

Thanks again!! I really appreciate your help.

Mark

You must have an ECM motor. The 3-4 volts doesn't matter. You get that reading just because the wires are all ran close to each other.

That testing sure looks like a bad fan motor.

Just to confirm you could swap the wires from the cool terminal and the heat terminal. Then if the fan runs on heat but not on cool you know the problem is in the motor.

 

[dscheidt]

Also verify the voltage is getting to the fan, as wires can get broken, munched on, or just fall out of terminals.

 

[ScaldedDog]

Thanks guys! I'll have another look in the AM.

Mark

 

[Bert_]

The fact that the fan terminal never gets power is interesting. I think that terminal should be hot when you switch the thermostat to constant fan. I wouldn't get too far off in the weeds with it though. The fact that you have 24 volts at the heat terminal means the fan should be running.

 

[fitter30]

Ask for a pic of the wiring diagram? Model number of furnace ask for that.

 

[ScaldedDog]

I tested this morning, with unexpected results:

With the Cool lead from the fan attached to the Heat terminal on the board, the terminal gets 24+v, but the fan still does not start. I attached one of the Spare leads from the fan to the same terminal, and the fan didn't start then, either.

What are the chances the relay on the board has failed in such a way that it is allowing voltage to the terminal, but with not enough amps to start the fan?

Mark

 

[ScaldedDog]

The other thing I tested was resistance between the speed leads on the fan to its common lead. I got 6ohms on the cooling lead (the speed that works), but open on heat and spare 1. I expected some positive number on at least the heat lead. What story does that tell, particularly in light of the test results from my previous post?

Mark

 

[danski0224]

Coming up on a full week, and it is still broken....

Guess maybe it isn't so easy to fix...

 

[ScaldedDog]

Coming up on a full week, and it is still broken....

Guess maybe it isn't so easy to fix...

It actually started failing last winter, this is just the week I'm trying to figure it out. Happy to hire a pro, but so far that's been four guys standing around shrugging, then presenting me with wildly overpriced things that "might fix it". Then a follow-up with a sales guy offering a $15k system replacement. I've gotten far, far more professional help from a few guys in this thread.

Mark

 

[PCustoms]

It actually started failing last winter

Summer would have been really convenient timing to fix this...

I may be wrong, but can someone explain to me what 24 volts coming off the boars is telling us?

Seems to me the fan probably runs on something much higher than that, and that the Opie needs to track down what that 24 volts should be actuating. I suspect that there's a relay somewhere else in the wiring

 

[ScaldedDog]

Summer would have been really convenient timing to fix this...

I may be wrong, but can someone explain to me what 24 volts coming off the boars is telling us?

Seems to me the fan probably runs on something much higher than that, and that the Opie needs to track down what that 24 volts should be actuating. I suspect that there's a relay somewhere else in the wiring

I live in CO in the summer, and just got back to FL the other day.

I wondered the same thing about the 24v, and whether the fan really runs on that. If there is a relay it's inside the fan motor. The leads from the motor to the circuit board are hard-wired.

Someone asked for a schematic:



Mark

 

[danski0224]

That's an ECM motor.

There's 24v control (signal) leads and line voltage leads.

You have already determined that the motor works in cooling mode, so the 120v part works in the ECM motor.

There is a relay on the 120v side, so is there 120v there during a call for heat?

There are ECM motor testers.

You want to be certain, because that motor will likely be expensive. Last one I priced was almost $2k wholesale.

My money is on the motor controller, which may or may not be replaceable.

Restrictive ductwork and/or ****** and restrictive air filters take out ECM motors.

 

[ScaldedDog]

That's an ECM motor.

There's 24v control (signal) leads and line voltage leads.

You have already determined that the motor works in cooling mode, so the 120v part works in the ECM motor.

There is a relay on the 120v side, so is there 120v there during a call for heat?

There are ECM motor testers.

You want to be certain, because that motor will likely be expensive. Last one I priced was almost $2k wholesale.

My money is on the motor controller, which may or may not be replaceable.

Restrictive ductwork and/or ****** and restrictive air filters take out ECM motors.

Thanks!!

Can I assume the motor controller (and relay?) are integral to the motor? I know the wires from the PCB go into the motor, and there are no components between them.

It looks like the replacement for the motor I have is Carrier P/N HD 52MQ 131, which I can order for just over $1000. Hard to imagine what an HVAC outfit would want.

Mark

 

[danski0224]

The relay is not in the controller.

Look at the wiring diagram and figure it out.

Or buy the board and the motor and flip a coin.

Sorry (not) to be a ****, but if you want to DIY, then DIY. The information is right there. I know there's videos on YouTube about testing an ECM motor.

Unfortunately, the "techs" that looked at it just wanted to sell new boxes and maybe they didn't know either.

And maybe you have an unusual problem.

I know that the motor can be tested relatively easily, and I would for sure do my due diligence. I'd surely spend a hundred on a tester before spending a grand on a non returnable motor.

There's also a "Test" pin on the schematic, and I bet that the equipment will test itself. All you need is the manual, and then read it. Hopefully the failure mode is conclusive.

 

[BurtEggley]

I looked up the air handler / furnace model you posted: Bryant CNPVP4821ALAAAAA. That comes back to an evaporator coil. https://webstore.habeggercorp.com/3380998/p/n/carrierbryantpayne

I'm out

 

[Bert_]

--Grey wire (cool speed) worked when connected to the cool terminal. When you moved it to the heat terminal it did not work, but you also measured 24 volts at the heat terminal.--

I would verify you have 120v on the black and white wires at the fan motor. I'm pretty sure that you should have 120v at all times. I'm not seeing any relay in the diagram like others have mentioned.

Unless you are missing 120v some of the time, I don't see how your readings are possible.

 

[danski0224]

I'm not seeing any relay in the diagram like others have mentioned.

The diagram on the left is the connection diagram.

The diagram on the right is the schematic, which is cropped or partially out of frame. However, blower motor relays are clearly visible. There are relays on the low voltage signal side.

The motor has to have a 120V feed to operate, which may be "on" all of the time.

There is no easy path to troubleshoot this. AFAIK, there are no commercially available ECM testers that allow each "speed" to be engaged individually. * But, there are videos that show how to test the ECM motor. And there are at least 3 different ECM motor types, for furnace blower motors. The motor is almost never bad, it is the controller. It is not always possible to buy the controller, and the OEM's do not always recommend just replacing the controller. If one is doing this on a customer piece of equipment, it is best to replace the assembly.

It is certainly possible that there is a faulty signal relay on the circuit board.

It is also possible that one of the Molex plugs has a partially pushed out pin or socket.

Looking very closely at the Molex plugs is by far the easiest.

Running the built in function check is also super easy. Could also verify output at the motor signal points for each thermostat setting.

Next would be testing the blower motor. It might have to be taken out, and some wiring fabbed up. Considering that a new OEM one is a grand... or buy a new one and hope. Buying a tester would be the easiest method, but it can be done without one.

Last would be the circuit board, unless it fails the self test.

No easy path to actually troubleshoot, unless one has "known good components".

*: Zebra has a tester that allows each speed to be tested. These are more expensive than the go/no go testers that can be had for under $100.00.

As a side note, Bosch furnaces for R454B have the ECM controller separate from the blower motor, it's a separate circuit board in the blower compartment.

 

[Bert_]

The diagram on the left is the connection diagram.

The diagram on the right is the schematic, which is cropped or partially out of frame. However, blower motor relays are clearly visible. There are relays on the low voltage signal side.

The motor has to have a 120V feed to operate, which may be "on" all of the time.

There is no easy path to troubleshoot this. AFAIK, there are no commercially available ECM testers that allow each "speed" to be engaged individually. * But, there are videos that show how to test the ECM motor. And there are at least 3 different ECM motor types, for furnace blower motors. The motor is almost never bad, it is the controller. It is not always possible to buy the controller, and the OEM's do not always recommend just replacing the controller. If one is doing this on a customer piece of equipment, it is best to replace the assembly.

It is certainly possible that there is a faulty signal relay on the circuit board.

It is also possible that one of the Molex plugs has a partially pushed out pin or socket.

Looking very closely at the Molex plugs is by far the easiest.

Running the built in function check is also super easy. Could also verify output at the motor signal points for each thermostat setting.

Next would be testing the blower motor. It might have to be taken out, and some wiring fabbed up. Considering that a new OEM one is a grand... or buy a new one and hope. Buying a tester would be the easiest method, but it can be done without one.

Last would be the circuit board, unless it fails the self test.

No easy path to actually troubleshoot, unless one has "known good components".

*: Zebra has a tester that allows each speed to be tested. These are more expensive than the go/no go testers that can be had for under $100.00.

As a side note, Bosch furnaces for R454B have the ECM controller separate from the blower motor, it's a separate circuit board in the blower compartment.

Ok, you were talking about the relays controlling the 24v side. For some reason I read one of those posts as if you were talking about a relay in the 120v supply to the motor.


This is just a multi-speed ECM motor. Sure a tester would be convenient but there's really no need to buy one. An alligator clip lead can be used to jump from 24v power to each fan speed individually.