Furnace Blower Fan Cycles On/Off During the off Cycle

[capwizard]

Re: Furnace Blower Fan Cycles On/Off During the off Cycle

SCM3 Carbon Monoxide Detector, Scams, $6000.

[capwizard]

Re: Furnace Blower Fan Cycles On/Off During the off Cycle

Woo.....Woo... I have no face to live anymore, but i'm too old to suicide can i castrate myself instead.......

redwire: For me 30 years, first time, I touch JFETs, Never stop learning because life never stops teaching.

PS: I will make A furnace PCB test JIG, and try to read the microcontroller program then Zip files for Badcaps veterans.

Q1 J176 P-channel silicon field-effect transistors, P-JFET I=2.1ma Vg=1.5v pinout S G D


Q2 j112 N−Channel — Depletion JFET Chopper Transistors N-JFET I=3.8ma Vg=2.6v pinout D S G
https://cdn.badcaps-static.com/pdfs/...7dd499420c.pdf

PS: Isn't that two wrongs make a right?....................

[srhofmann]

Re: Furnace Blower Fan Cycles On/Off During the off Cycle

That is true for that function and the control switch is on and the furnace was running but goes into over temp, but the gas Valve is also has power fed through the thermal sw incase the controller fail to turn off the relay or the relay contact stuck closed, it is double protection in case the relay contact welded it self closed or faulty control board.
I can see it will turn on the blower too cool down due over temp occur and control panel is in the ON position (I can also see the furnace will be in the loop, so when the thermal sw cool down the furnace will start back up and heat up again until over temp is reach, I would think it should be in the latched shutdown mode until the cause of over temp is identified), but if the control panel switch is in the Off position and the furnace has been off for day so if the thermal sw goes open circuit for some reason or the wire broke off, will the blower still come on to cool down anyway even if the control panel is in OFF position, I would think it should not, I would think nothing should be on when the panel switch is in the OFF position.

I don't remember off the top of my head what happens if the thermostat is in the off position. It would make sense, to me, if the furnace saw an overtemp condition in any form, thermostat on or off, it would turn on the blowers. This is just a cover your ass manufacturer liability issue, we did everything possible when we designed the board to make things safe. These aren't really intelligent boards, they don't know how long the furnace has been off, they just know something is saying "I'm overtemp" Also, think about what happens if you shut the thermostat off while the furnace is running, the blower will continue to run until the heat exchanger cools down.

I'd love to try it and see what happens but neither of my furnaces have inducer motors and I don't think my neighbors would appreciate me experimenting with their furnaces......

[redwire]

Re: Furnace Blower Fan Cycles On/Off During the off Cycle

I always see a vane type airflow switch on the blower fan in LPG heaters, like in RV's, motorhomes, caravans as the Aussies call them.

You would have to look at the fan opto sensors and see why they are not generating pulses (instead the battery charger pulsed DC fools the controller).

IF the blower stops while the burner is lit... the heat exchanger overheats and the high limit switch will shut off the burner. Not such a hazard, but the LPG blower fans I've seen are dual impeller - the fan also forces exhaust gas out the side vent! Without this you have a disaster, flame everywhere

[petehall347]

Re: Furnace Blower Fan Cycles On/Off During the off Cycle

my lpg blow heater is a bit strange . it wont start up unless a dirty power battery charger is connected . i know the fault is the air flow sensor .. simple fan with guts missing and black and white blades and a sensor to watch it . the 50hz or maybe 100hz from charger tricks the circuits the fan is turning and air flow is correct . the company wants me to spend lots more money to do the upgrade to fix it ... its a design fault ..
i know the air flow is correct but the circuit boards thinks different unless i connect a battery charger that puts out dirty dc .

[redwire]

Re: Furnace Blower Fan Cycles On/Off During the off Cycle

Flame detect circuits are very high impedance. A pilot flame looks like a weak rectifier diode with 10-50MEG ohms resistance conducting.

The most common problem is a dirty flame-rod. This causes high resistance to the flame (rectification), which gives intermittent flame-out signal to the MCU. No flame seen= flame-out and the burner shuts off the gas ASAP before things go boom.
Second problem is leakage current to GND on the flame-rod. Any soot, carbon, moisture on the flame-rod porcelain or wire, or on the PCB around the sensing transistor will also cause an error.

Honeywell uses J-Fet's (J176) as in US Patent 5472336.
Fenwall uses a Darlington like MPSA27 for flame sense as in US patent 6985080.

A typical flame sense circuit in a home furnace is basically a 1/2 wave rectifier with fail-safe modes.

[petehall347]

Re: Furnace Blower Fan Cycles On/Off During the off Cycle

jfets would maybe turn on from tracking more than darlingtons would do .. i wondered why darlingtons were there and thought about this .. i dont have a schematic so i can only guess from what you are saying ..

[capwizard]

Re: Furnace Blower Fan Cycles On/Off During the off Cycle

You are A PRO!

Introduction to Field Effect Transistors JFET MOSFET
https://www.youtube.com/watch?v=oambDFa0Pr8


PS: It's probably my fault, I will double check see what Q1 and Q2 are?

_________________________
Inauguration.... I bet ju.............the First Lady gonna wears white dress and Trump will dresses black !

[redwire]

Re: Furnace Blower Fan Cycles On/Off During the off Cycle

Maybe Relay control circuit, on PCB has two Trs, as I tested they are Darlington Transistors, daytime maybe trigger the high gain Darlington Transistor to turn on because the dirt from water mark.

PS: cant find any information about this two Trs?

The J176 is a J-FET for the flame detect circuit

[budm]

Re: Furnace Blower Fan Cycles On/Off During the off Cycle

Not always. I've worked on furnaces when a thermal safety switch opened the control board turned on both the inducer and blower fans and turned off the gas valve. This is actually very good because it's the furnace attempting to cool itself off.

That is true for that function and the control switch is on and the furnace was running but goes into over temp, but the gas Valve is also has power fed through the thermal sw incase the controller fail to turn off the relay or the relay contact stuck closed, it is double protection in case the relay contact welded it self closed or faulty control board.
I can see it will turn on the blower too cool down due over temp occur and control panel is in the ON position (I can also see the furnace will be in the loop, so when the thermal sw cool down the furnace will start back up and heat up again until over temp is reach, I would think it should be in the latched shutdown mode until the cause of over temp is identified), but if the control panel switch is in the Off position and the furnace has been off for day so if the thermal sw goes open circuit for some reason or the wire broke off, will the blower still come on to cool down anyway even if the control panel is in OFF position, I would think it should not, I would think nothing should be on when the panel switch is in the OFF position.

[wakedoc]

Re: Furnace Blower Fan Cycles On/Off During the off Cycle

So if I wanted to test the limit switch B13701-55 I run a wire/jumper from one terminal to the other, basically connecting them? If issues is resolved replace switch if not replace board?

[capwizard]

Re: Furnace Blower Fan Cycles On/Off During the off Cycle

Then srhofmann is right, the furnace is attempting to cool itself off.

______________________________________________

Inauguration.... I bet ju.............the First Lady gonna wears white pants!............

[srhofmann]

Re: Furnace Blower Fan Cycles On/Off During the off Cycle

Since the thermal sw is normally closed, if it goes open circuit then what is connected to it will not function and NOT be able to come on, so for something to turn itself ON when the panel control switch is set to OFF then it cannot be caused by the thermal switch going open circuit, if thermal sw goes open circuit then the furnace should not come on and that is the purpose of the thermal sw to stop the furnace from coming on if there is problem due to over temp.

Not always. I've worked on furnaces when a thermal safety switch opened the control board turned on both the inducer and blower fans and turned off the gas valve. This is actually very good because it's the furnace attempting to cool itself off.

[capwizard]

Re: Furnace Blower Fan Cycles On/Off During the off Cycle

Budm is right, It is driver or relay on PCB has problem then you will need B1809913S PCB, cheaper on Ebay, or try to clean the legs of Trs (Q1 and Q2) with small watercolor brush and some vodka or rubbing alcohol.


https://www.repairclinic.com/PartDet...7d1443052&mr=1

______________________________________________

Inauguration.... I bet ju.............the First Lady gonna wears white dresses!............

[budm]

Re: Furnace Blower Fan Cycles On/Off During the off Cycle

I have the same basic issue but, it is a GMP075-3 unit. Had a tech look at it and was told most likely it is the B13701-55 Limit switch that is causing the issue. Any suggestions, should I start there or start with the PC board. Note: the unit is in saltwater coastal environment and is what lead me to lean towards the limit switch because of possible corrosion. The turn on and offs of the fan are completely random, two minutes one time shut down and start up the next minute and might be 30 minutes and then run for 15, this is all while stat is in the off position.

Since the thermal sw is normally closed, if it goes open circuit then what is connected to it will not function and NOT be able to come on, so for something to turn itself ON when the panel control switch is set to OFF then it cannot be caused by the thermal switch going open circuit, if thermal sw goes open circuit then the furnace should not come on and that is the purpose of the thermal sw to stop the furnace from coming on if there is problem due to over temp.

[capwizard]

Re: Furnace Blower Fan Cycles On/Off During the off Cycle

Very easy, just bypass the switch ( all switches are normally closed on this Goodman furnace if its intermittently open will give you problems) (B13701-55 Goodman Limit Switch) with A metal wire , see what's happening.

The auxiliary limit switch (B13701-55) is a safety control
that senses high temperature within the furnace or duct work.
This control is preset to open at 120°F and close at 90°F. It
is not adjustable and automatically resets. The control is
located in the blower compartment of the furnace on the
blower housing. Auxiliary limits may be on the side or front
of the blower housing.

Service manual for you.
https://cdn.badcaps-static.com/pdfs/...e2ee1acf8b.pdf
________________________________________________________________

Inauguration.... I bet ju.............the First Lady gonna wears white dress !


.

[wakedoc]

Re: Furnace Blower Fan Cycles On/Off During the off Cycle

I have the same basic issue but, it is a GMP075-3 unit. Had a tech look at it and was told most likely it is the B13701-55 Limit switch that is causing the issue. Any suggestions, should I start there or start with the PC board. Note: the unit is in saltwater coastal environment and is what lead me to lean towards the limit switch because of possible corrosion. The turn on and offs of the fan are completely random, two minutes one time shut down and start up the next minute and might be 30 minutes and then run for 15, this is all while stat is in the off position.

[capwizard]

Re: Furnace Blower Fan Cycles On/Off During the off Cycle

Old PCB board has a protective coating of transparent primer, it limited problem only to the legs of Darlington transistors, which I did clearing those two Tr but not over the bottom leg part of the transistors.

[keeney123]

Re: Furnace Blower Fan Cycles On/Off During the off Cycle

I am the Sherlock Holmes...........

pic1 Put cooler condenser on the top of the furnace uses same blower fan, like you put an ice cube bag on your head!

pic7 Water leaks everywhere!!

Sounds like you need to unplug drain pipe.


Old board does not have many components perhaps to could trouble shoot by looking up IC Data sheets.

[capwizard]

Re: Furnace Blower Fan Cycles On/Off During the off Cycle

I am the Sherlock Holmes...........

pic1 Put cooler condenser on the top of the furnace uses same blower fan, like you put an ice cube bag on your head!

pic7 Water leaks everywhere!!