Goodman GMP075-3 Flame "rolling out"


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Old 01-16-12, 09:33 AM
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Goodman GMP075-3 Flame "rolling out"

I'll start by saying I heve a decent electrical background. I have a lot of experience with oil fired units and limited experience with Nat gas/Propane units. So here is my story. We woke up Christmas morning with no heat. I checked my unit, flame would light but not prove and stay lit. I fixed that problem, finally. There were some melted wires between the lower and upper sections. Now to find out why I had burnt wiring. When given the call for heat unit procedes normally. Prepurge,HSI, gas valve opens, flame proven, time delay then indoor blower starts. To this point everything is normal. Except, I am getting a little flame backing up into the burner area. After a couple of minutes the flame is backing up worse. At that point I cut the gas to keep from damaging wires. I have checked for exhaust restrictions and the RPM on the blower motor(power venter). Pulled the heat exchanger and checked for restrictions. Nothing found. I had a heater tech friend go over everything and he left scratching his head too. Outside of a large crack in the heat exchanger what would let enough "house air" leak into the burner area? Has anyone else come across a similar problem?
 
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Old 01-16-12, 09:56 AM
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Say no more than "large crack in the heat exchanger". Unless it's under warranty that usually means a new unit to me.
 
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Old 01-16-12, 10:37 AM
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For openers, the pressure switch should stop the ignition sequence if there is inadequate negative pressure for the furnace to operate safely. So you shouldn't get flame rollout.

Did your tech friend measure the air pressure being applied to the pressure switch with a manometer? If so, what was it? If it hasn't been done, it's a test that needs to be done --- properly.

Have you checked for obstructions in the chimney or vent pipe that may be impairing air flow?

You are suggesting that you have checked everything. But there is a problem and perhaps you and your tech friend aren't recognizing the problem when you see it, which would not be unusual.

You could temporarily remove the vent pipe FOR PURPOSES OF TESTING ONLY, to see if the flame rollout condition continues or not. If it continues, it implies a problem in the furnace itself. If it stops, it implies a problem in the vent or chimney.

And yes, carefully inspecting the heat exchanger needs to be done as well.

These kinds of things are probably beyond beyond the ability of the typical DIYer, and beyond the skill of most tech friends unless they are skilled gas furnace repairman, which is what you probably need.

Unfortunately, a good many furnace repairman leave something to be desired as well.
 
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Old 01-16-12, 11:30 AM
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Sorry I am new to this. I should have replied to you directly. I think
 

Last edited by idahogary; 01-16-12 at 12:31 PM. Reason: Reposting
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Old 01-16-12, 12:01 PM
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I am sorry, I misread your original post. I'm glad you don't have a cracked heat exchanger.
 
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Old 01-16-12, 12:25 PM
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I was saying other than a large crack in the heat exchanger....I have carefully checked the heat exchanger and not found anything.
 
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Old 01-16-12, 12:28 PM
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>


That's correct.

Other defects in the fan assembly could include a fan plugged with debris, broken fan blades or defects in the motor such as not coming up to speed.

I'm supposing you would have checked for such defects.

Measuring the inches water column being measured by the pressure switch is an essential measurement. If the pressure is inadequate, the pressure switch shouldn't close --- or it's defective. A correct pressure and flame roll out is likely due to a defective heat exchanger.

>


The inside of the heat exchanger is where the combustion gasses belong. The outside of the heat exchange is where the room air circulates. Never the twin should meet. The heat exchanger needs to prevent any mixing of combustion gasses with room air. One tell tail effect of a bad heat exchanger is "fan interference" when the burner flames are being blown around or affected by the furnace fan that circulates air around the house.

You have fan interference, and almost certainly a defect in the heat exchanger causing these problems. It's quite common for it to be difficult to see such cracks, holes or other problems.

Observing fan interference is ample good reason to shut off and red tag a furnace by itself. Fan interference is likely to prevent the gas from burning properly, and can easily start producing carbon monoxide in the combustion products, which is then likely to be circulated into the dwelling space through the defect that allows the air from the fan in to the burner area.

Much better detail on that last post!
 
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Old 01-16-12, 12:41 PM
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Seattle Pioneer. Gee you're only about 8 hours west of me maybe you could swing by. This is the first time I have used DIY so I am not sure if you got my last reply.


For openers, the pressure switch should stop the ignition sequence if there is inadequate negative pressure for the furnace to operate safely. So you shouldn't get flame rollout.
Exactly why I am confused by this. My pressure switch closes when the inducer motor comes on. The RPM's on the blower are at specs but the flame still backs up. That makes me think air restriction or lack of air flow through the heat exchanger. I will reassemble the unit and start again. I am definitely overlooking something. I have disconnected the vent pipe..no change in operation. (unit is in my garage) Due to my location I have a very limited selection of HVAC techs. I am not impressed with what I have heard from friends regarding the local techs(?). I am a test don't guess type of guy. Did I mention I am stubborn? I hate to let a machine beat me.



Read more: http://www.doityourself.com/forum/ga...#ixzz1jehbCcQU
 
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Old 01-16-12, 12:50 PM
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I checked and cleaned the fan assembly. The RPM is correct.

I had thought of the possibility of a cracked heat exchanger. Perhaps a small crack that opens with the increased heat? Have you ever seen that?

PS I AM NOT using the furnace. Have no intention of filling house with CO I have been using an indoor approved portable propane heater.
 
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Old 01-16-12, 01:28 PM
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Looks like I will need to make a manometer. My friend, with the manometer, is out of town this week. So it's off to the hardware store for some plastic tubing. Do you know how many WC inches the inducer motor on my unit should pull?
 
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Old 01-16-12, 02:10 PM
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The pressure is too small to measure accurately with a conventional water filled manometer. You need to be able to measure down to about .25" WC, which requires a specialized measuring instrument.

You really need to have this checked out by a competent pro who has the measuring equipment needed and the experience and perhaps equipment to inspect the heat exchanger in detail.
 
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Old 01-16-12, 02:59 PM
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SeattlePioneer, Thank you for your help. Looks like this is one I need to back away from There comes a point where the amateur needs to admit he is in past his skill/experience level.
 
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Old 01-16-12, 07:44 PM
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That particular line of furnaces has a reputation for heat exchanger failures. In this thread there are some excellent pictures of the crimp ring failures this line of furnaces is noted for.
http://www.doityourself.com/forum/ga...ing-issue.html
 
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Old 01-17-12, 03:32 PM
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Grady, SeattlePioneer and all that helped me.......I set everything aside yesterday and slept on it.
Went back and took a look with fresh eyes today. There is a hole in the heat exchanger. I have no idea how I over looked it before.
 
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Old 01-17-12, 04:40 PM
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It's very easy to overlook heat exchanger failures. The symptoms you were having were shouting that there was a problem.

Glad you identified what can be a dangerous problem.
 
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Old 01-18-12, 12:52 PM
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The symptoms definitely were pointing to a cracked heat exchanger. I just refused to order one until I saw the crack. Overall I was lucky considering what had happened. The cracked heat exchanger was letting the indoor blower push some flame back past the burner manifold. There was enough heat to damage some wires but not enough heat to trip any of the high limit switches. The wire damage caused another problem that shut the furnace down. Now to the BIG lesson learned. I have 4 smoke detectors in my house. But no CO detectors! I am ordering combination smoke/CO detectors. Readers please check to see if you have CO detectors. If not get them! I had assumed my detectors were combo units. I could have been dead wrong! Once again thanks to all that helped me with my problem.
 
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Old 01-18-12, 01:10 PM
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Does the heat exchanger come as one entire assembly, or several units, one for each burner? You want to replace all the heat exchangers, not just one cell.

Can you get the heat exchangers under the furnace guarantee? If so, that can be worth doing.
 
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Old 01-18-12, 01:24 PM
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It is a 3 cell one piece unit. I didn't even check to see if warranty would cover the exchanger. The date of manufacture is 1993 so I assumed the warranty would be expired. From other threads I have read, Goodman wants a "certified" company to do the work. The part was covered but labor was not covered. I also took into consideration that we are in a winter storm pattern up here. It might take a while to get a tech to come to the house. Then, it is my nature to finish what I start. Sometimes not such a good trait. So I just bit the bullet and got everything underway this morning. I learned something from this and I hope I might help someone else with the CO detector suggestion.
 
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Old 01-18-12, 04:41 PM
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BEWARE the repalcement heat exchanger will smoke when first fire it! ALSO, the new heat exchanger will do the very same thing! Nothing has changed in their design.
 
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Old 01-18-12, 09:19 PM
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Thanks for the heads up regarding smoke. The origional exchanger lasted 15 years. Hope this one will last that long.
 
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Old 01-19-12, 03:44 AM
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Actually the original probably lasted less than that but the hole/crack finally got big enough to make the furnace not run. That furnace needs checked EVERY year!
 
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Old 01-19-12, 04:54 AM
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Boy did you get lucky to squeak in under the 20 year warranty. Did they tell you what the heat exchanger would have cost?
 
 

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