Author Topic: Intermittent Failure to Heat  (Read 8918 times)

Offline SnakeDoctor

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Intermittent Failure to Heat
« on: November 17, 2008, 01:59:45 PM »
I have a Goodman gas fired furnace which will occasionally (only 2 or 3 times per month) fail to start heating.  The failure USUALLY occurs in the morning when the programmable thermostat raises the setpoint.  I have ALWAYS been able to fix the failure by lowering the setpoint below the room temperature and then raising it back up again.  Presumably this causes the system to try again.  It has always succeeded to start on this second attempt and it will then continue to work for days / weeks before happening again.  Any ideas as to what is causing this infrequent failure and what I can do to fix it?

Offline Admin

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Re: Intermittent Failure to Heat
« Reply #1 on: November 17, 2008, 02:07:11 PM »
Next time it happens don't reset the system.  Look for the flashing diagnostic light on the board.  Count the flashes and let me know.

Offline SnakeDoctor

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Re: Intermittent Failure to Heat
« Reply #2 on: November 17, 2008, 02:11:26 PM »
Sorry for not including this originally - it is a single flash.

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Re: Intermittent Failure to Heat
« Reply #3 on: November 17, 2008, 02:31:02 PM »
Model #?

1 flash = ignition failure

Remove and clean the flame sensor with steel wool.
It sits I'm front of a burner with a porcelain base and a metal rod connected to one wire.


Offline SnakeDoctor

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Re: Intermittent Failure to Heat
« Reply #4 on: November 17, 2008, 02:49:17 PM »
I did that last year and it seemed to remedy the problem for the rest of the heating season but it has started occurring again this heating season.  I have two furnaces (upstairs / downstairs) and I have never had this problem with the upstairs unit so I thought maybe it was something else.  Is it normal to have to periodically clean the flame sensor?

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Re: Intermittent Failure to Heat
« Reply #5 on: November 17, 2008, 03:21:46 PM »
You probably use the downstairs furnace more often.  The furnace and sensor should be cleaned yearly.


Offline SnakeDoctor

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Re: Intermittent Failure to Heat
« Reply #6 on: November 17, 2008, 03:28:58 PM »
Good to know.  Thanks for all of your help.

Offline SnakeDoctor

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Re: Intermittent Failure to Heat
« Reply #7 on: November 18, 2008, 09:12:03 AM »
Just a follow up question...

Last night I removed and cleaned the flame sensor with steel wool.  Prior to cleaning it, I examined it and there was no visible build-up - just a slight discoloration in the middle part of the rod which was gone after I cleaned it.  Is this typical of a "dirty" flame sensor or would you have expected visible build-up?

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Re: Intermittent Failure to Heat
« Reply #8 on: November 18, 2008, 11:53:45 AM »
It sounds like a dirty sensor.  Did it solve the problem?

I use a meter to measure the micro amperage in series with the sensor.

Offline SnakeDoctor

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Re: Intermittent Failure to Heat
« Reply #9 on: November 18, 2008, 01:03:07 PM »
Time will tell - so far, so good.

I have the equipment to measure uA.  What would be considered a "normal" reading?

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Re: Intermittent Failure to Heat
« Reply #10 on: November 18, 2008, 02:47:52 PM »
Usually anything below 1 uA can cause lockouts.

Offline SnakeDoctor

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Re: Intermittent Failure to Heat
« Reply #11 on: November 18, 2008, 03:26:20 PM »
Thanks for the info.  I'll check it out.

Offline SnakeDoctor

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Re: Intermittent Failure to Heat
« Reply #12 on: November 26, 2008, 10:27:49 AM »
OK, here's the latest...

Yesterday evening I checked the current output of the flame sensor and got a reading of about 4uA.  The heat ran fine the rest of the evening.  This morning it ran fine until about 10:00 when I noticed the house getting cold.  Same failure as before - 1 flash on the mainboard LED.  I once again lowered the thermostat, raised it back up and now its working fine again.  Any suggestions as to how to further diagnose this problem?

Goodman Model GMNT080-4

Offline SnakeDoctor

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Re: Intermittent Failure to Heat
« Reply #13 on: December 03, 2008, 01:50:51 PM »
A second request for any ideas on how to further troubleshoot the problems discussed above.

Thanks in advance.

Bill

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Re: Intermittent Failure to Heat
« Reply #14 on: December 03, 2008, 08:21:28 PM »
Sorry missed your last post.

4uA is a great flame signal, looks like the flame sensor is fine.  There are no service bulletins regarding your problem with the GMNT model.
I have read other bulletins for different models in the past that had problems like yours.  There was a problem with the ignition timing on the board, so it had to be replaced.  Another problem was the gas valve sticking.  Check these 2 things,

Quote
1. Poor electrical ground to the equipment: All internal equipment ground wires should be checked for tight connections. Also, check for voltage between the “LINE N” terminal and the “C” Common 24 volt terminal on the ignition control board. If you measure 2 volts or less you have a good ground, more than 2 volts indicates a bad ground at the electrical panel or ground rod. The gas furnace must be on a dedicated electrical circuit going directly to a dedicated single-pole breaker in the breaker panel. The ground wire from the furnace should terminate to a ground rod.

2. Sticking Gas Valve: To check for this condition, disconnect low voltage wires from the gas valve, apply 24 volts “OFF/ON” several times to the gas valve directly. Replace the original low voltage wires to the gas valve and then start the furnace. Adjust the thermostat to call for heat and verify proper ignition.

Chances are you just have a bad board.  Intermittent problems like yours are usually caused by a bad board.  It could be that it's not always sending power to the igniter or the gas valve, causing the 1 flash lockouts.  Intermittent gas valve problems are rare.

Offline SnakeDoctor

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Re: Intermittent Failure to Heat
« Reply #15 on: December 04, 2008, 11:36:57 AM »
Thanks for the feedback.  The problem still occurs occasionally so I will try the suggestions you provided.

Is the control board something I can acquire and replace myself?  I feel I have the appropriate skill set to do it as long as it doesn't require any highly specialized tools or tearing the entire system apart to replace it.

As to the 4uA on the flame sensor, that was my best guess reading on an old analog VOM on the 0 to 60uA scale so that reading could be off a bit.

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Re: Intermittent Failure to Heat
« Reply #16 on: December 04, 2008, 05:04:11 PM »
I believe this is the board you need - Here.  Replacing the board is very easy.

And if the flame sensor is clean then it's unlikely the problem.

Offline Zzz

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Re: Intermittent Failure to Heat
« Reply #17 on: December 25, 2008, 09:25:29 AM »
I would take a ohm reading on the ignitor, also make sure you have 24+_ volts at the r and c terminals on the board. I have seen dehumitifiers rob a couple volts and cause this problem. Also if you have a mercury t-stat and smoke in the house nicotine can build up on the contact and cause greef. Another thing that is rare but happens is one of the terminal solder joints (in your case r or w) can crack at the edge of the solder(back side of the board) and cause this. I have resoldered them but told the customer no garantee. Intermiten problems can be tough. Does that unit have a smart valve on it? model # starts with SV.