# need furnace expert - Goodman furnace



## DMC

I have a Goodman GMP075-3, 80% furnace. The vent circulator comes on, ignitor comes on, burner ignites, then the burner usually only stays on for 5-10 sec. and goes out. The furnace then cycles again igniting the burner. Sometimes (75% of the time) the burner takes 2 or 3 times before it stays lit. But the other times everything works fine the first time. A couple times the furnace went into lockout and flashed the LED 1 flash. I checked the voltage to the gas valve and it is 25V when the gas is burning and the voltage does go away at the same time that the gas shuts off. So I guess the gas valve is good. The vent blower is working and pulls vacuum switch closed. I had a furance guy come out who swapped out the flame sensor and left, but the problem is the same. I called him back out and now he thinks it is the fact that I don't have a chimney liner. Is that possible?? He looked up the chimney and it is clear. I think it is an intermittent control board problem, any guesses???

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## NickTech

if there was a liner problem your pressure switch would not make. 
it sounds like a rectification problem. this is the system where you furnace proves ignition by sending dc amps (microamps) thru the sensor back to the module telling it that there is combustion. proper grounding to the module itself as well as the ignitor assembly is critical. poor grounding will result in symptoms like the one you are experiencing. check to see if you are properly grounded and check to see if the sensor lead wire isn't to long. approx. 24 " or so.


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## rakes9720

I agree that grounding issues would cause this.


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## DMC

I ran a new ground from the electrical box to the furnace ground wire which is screwed to the furnace chasis. No change. Any chance that the control board is bad. I found that the furnace had a new board installed back in 2000. The furnace is a 1993.


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## tinner73

DMC said:


> I ran a new ground from the electrical box to the furnace ground wire which is screwed to the furnace chasis. No change. Any chance that the control board is bad. I found that the furnace had a new board installed back in 2000. The furnace is a 1993.




check your heat exchanger. they were bad back then. real bad.


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## DMC

Why would a cracked heat exchanger cause the symptoms that I'm getting, ie burner shutting down first couple attempts and then fine after that. Once the furnace gets past the first 10 seconds it will run fine.


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## tinner73

DMC said:


> Why would a cracked heat exchanger cause the symptoms that I'm getting, ie burner shutting down first couple attempts and then fine after that. Once the furnace gets past the first 10 seconds it will run fine.



i didn't say it would cause your symptoms. i said check your heat exchanger. would it make sense to repair a furnace with a split heat exchanger? those are cheap ********************ty furnaces, period.


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## NickTech

make sure the module and the ignitor are grounded as well. this type of system proves the flame by sending a small current to the sensor using the flame as a conductor. if the burner and the sensor are not in circuit with the flame then this may shut your system down. check the sensor alignment as well as the manifold gas pressure.


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## DMC

NickTech said:


> make sure the module and the ignitor are grounded as well. this type of system proves the flame by sending a small current to the sensor using the flame as a conductor. if the burner and the sensor are not in circuit with the flame then this may shut your system down. check the sensor alignment as well as the manifold gas pressure.


So the control board sends a small current by wire through flame sensor which then if there is conbustion will continue through the flame to the burner to ground. I will check that the burner, control board ground, and furnace chassis have continuity with an ohm meter. There is no adjustment on the flame sensor but it is right in the middle of the flame. I don't have a means to measure gas pressure, but visually it appears as a good flame. What is confusing is, why does the furnace always continue to burn once it makes it past the first 10 seconds. You can raise the thermostat 10 degrees so that the furnace has to run for 30 minutes and if it makes it past 10 sec. it will run the entire 30 minutes without any problems. I unplugged the flame sensor during a period when the furnace is running and it will shut right down. So this is something that is always monitored. It seems that if there was a grounding problem that it would always be there and would at least once shut the furnace down after the first 10 seconds.


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## NickTech

well using the process of elimination, one of the last things that can be checked is the microamps produced by the flame itself thru what is called rectification. the a/c voltage is turned into d/c current (amperage) this is actually what the flame sensor is sensing. the actual microamperage varies between units, but it is in the range of .8 to 2.0 microamps (millionths). a microamp meter placed in series between the sensor and the sensor terminal is needed to check what the micramps. a manometer is used to check gas pressure to the burner(3.5" of water column). heres a thought. a common problem is 2 or more gas appliances are piped together in supply line that is to small. there is not enough volume of gas to support both burners. sometime the furnace comes on while no other gas appliance is on. therefore working right. however if the furnace is running when you hot water heater comes on (depending that you have gas hot water) then the HWH steals some of that gas allowing for less gas pressure and resulting in a smaller flame. this smaller flame would result in the microamop circuit to open shutting down the burner. i've have this happen on several occasions. check it out :Thumbs:


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## NickTech

well now that i know there is construction going on the in the house, i need ask you, was the furnace acting the same way before you began construction? if not, then i can guess you might have dirty nozzles, or burner tubes. this can interfere with the distribution of gas. sheet rock dust and or dirt in the air will make its way toward to burner via the inducer motor.


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## DMC

Yes it has been acting this way since I bought the house. The rehab has been pretty minor, mostly removing kitchen cupboards and new bathroom, paint, carpet etc... 
Today I removed the gas line and there is no strainer on the input to the furnace. Also, there is the extra vertical stub pipe on the gas line just before the furance. 
I adjusted the gas screw inward 1/2 turn and the flame increased in size and appears to envelope the flame sensor more. The furnace only double cycled once all day. I'm going to get someone to measure the gas pressure tommarrow. 
It appears to be either gas pressure or gas flow so I think were making progress. Thanks very much for all the assistance with this problem. I never would have figured out that the flame can be a conductor. :Thumbs:


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## NickTech

no problem i'm glad to help, it's all about process of elimination :Thumbs:


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## thouger1

I had the same problem...finally I got fed up and brought my shopvac/blower and blew the burners the inside of the furnace and vent and suprisingly now it fires up on the first or second try every time! I must have blown something clear. Mine was doing the same thing...only the first burner would ignite while 2 and 3 would sputter. Try blowing the entire furnace clean and see if that helps you as well.

Good Luck!


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## hivolt

having similar trouble,,with goodman new hi eff... replaced inducer motor,,,now hear excessive water in motor,,think its effecting pressure sw,,,causing to cycle every 10-15 seconds,,motor installed correctly?


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## Dr Heat

hivolt said:


> having similar trouble,,with goodman new hi eff... replaced inducer motor,,,now hear excessive water in motor,,think its effecting pressure sw,,,causing to cycle every 10-15 seconds,,motor installed correctly?


I had the same trouble with a Nordine had to replace the board. But yours sounds more like gas presure.

As far as water in the venter motor its a condensate issue or a cracked heat exchanger.


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## hvaclover

You have bad burners.


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## flashheatingand

Dr Heat said:


> I had the same trouble with a Nordine had to replace the board. But yours sounds more like gas presure.
> 
> As far as water in the venter motor its a condensate issue or a cracked heat exchanger.


How come you say that it's possibly a cracked h.e.? I can understand the condensate drainage issue, but the heat exchanger? If that were the case, I would think the limit switch was tripping regularly. Not the pressure switch


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## simplemen

you said already changed flame sensor and confirmed presure switch, then might be bad ground or fan timer board


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## MechAcc

flashheatingand said:


> How come you say that it's possibly a cracked h.e.? I can understand the condensate drainage issue, but the heat exchanger? If that were the case, I would think the limit switch was tripping regularly. Not the pressure switch


Depends. If the crack is in the area next to the limit it would add extra heat to the limit.


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