# Goodman GMH95 Intermittent problem



## mattyb (Oct 30, 2010)

Hello all,
I did a self install of the GMH95 furnace. Actually myself and another guy did it. I'm now running into an issue only after the furnace has run for about a half an hour. Here's what happens. During the half hour time period the unit might go through 4 calls for heat in which it performs the heating cycle exactly as described in the manual. It then hits a certain point which makes me think this is somehow temperature related. The t-stat will call for heat. The induced induction blower starts, the igniter switches on, valve opens and the burners light, it will then "flame out". It then gets stuck in this loop of trying to light to satisfy the call for heat. This same situation can happen even if the unit has been running. As I'm typing the furnace had been running for approximately 7 minutes, when I hear and feel the circulating blower cut out for a split second and burner flames have gone out. I've checked the rollout limit switches for continuity and they appear to be ok. The fuse on the control board is not blown and shows continuity as well. The furnace is level end to end and pitched slightly forward as indicated. Venting is not blocked and is per the manual. So I'm at a loss. Any ideas as to what this could be. Again, it only seems to happen after the unit warms up, which to me doesn't make any sense. It is hooked up to NG and both my dryer and water heater function perfectly.

Any help is greatly appreciated.


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## yuri (Nov 29, 2008)

Might not be draining the condensate properly/fast enough and building up in the secondary coil. Post some pics of it with the doors off and the condensate trap and drain line. May need a bit more elevation to the front.


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## mattyb (Oct 30, 2010)

I just raised the pitch of it a bit to see if that would help but same result. Here are the pics.


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## yuri (Nov 29, 2008)

When it stutters does the MAIN blower stutter or is it just the exhaust blower. Should not have 4 cycles in a half hour. 2 would be normal. Is the furnace properly grounded. Check from the electrical box inside the furnace to a copper water line with a cheater wire if necessary. Poor grounding can upset modern furnaces and their circuit boards. Try a new thermostat as it may be fluttering and causing that problem. That is a pretty good reliable furnace.


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## mattyb (Oct 30, 2010)

I had thought it was just the circulating blower that cuts out but it's both blowers. Both of the t-stats are brand new and it doesn't matter which one calls for heat, the furnace does the same thing. The reason for so many calls for heat in a short period of time is because my house is 107 years old and is brick with no insulation. The furnace will bring it up to temp. quickly, but I lose the heat pretty quickly as well. If the inside temp has been stable for a while the furnace kicks on far less.


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## mattyb (Oct 30, 2010)

Could this be a faulty valve? Should I shut the gas off and disconnect the line and check the valve to make sure that everything is clean inside?


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## yuri (Nov 29, 2008)

Did you check the grounding? Something is aggravating the circuit board and causing it to dropout. Make sure the furnace is grounded properly and you have at least 110 volts to it. Sounds like you got power supply problems to it and the board is getting low voltage and heating up. Is the wiring up to modern standards in your house. No extra loads on the furnace circuit. You may have a bad or floating neutral connection in the panel/house. My bud Beenthere knows about some issue with improper neutrals we discussed earlier I believe and should be around here later.


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## mattyb (Oct 30, 2010)

Grounding is good. When we installed it we put all new wiring in. And that's one thing I trust because the guy who helped me is an electrician. I just pulled the gas piping to check the valve and it looked good. The only other thing I see in the manual good be the primary limit switch. It might not be getting enough cool air over the heat exchanger and it keeps tripping the system.


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## hvac122 (Dec 10, 2007)

Is your temperature rise withen limits?


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## mattyb (Oct 30, 2010)

As of right now, yes. I was only able to run it for about 4 minutes before it started acting up again. For Celsius, the range is 19-36 degrees and it fell at 30 degrees. But, I don't know if I would be pushing close to the limit if it ran for the full 10 minutes like the manual states. I'm going to let the unit sit overnight and try it again tomorrow.


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## Marty S. (Oct 31, 2009)

Are the vents sloped and supported properly? Sounds like a pressure switch bouncing.


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## mattyb (Oct 30, 2010)

Yep. I had questioned whether or not the exhaust vent was so I trimmed the pipe that connects to the furnace which gave it a bigger slope. But all pipes or secured.


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## kenmac (Feb 26, 2009)

sounds like cycling off on high limit due to low air flow. Unit should stay on until t. stat is satisfied. Put a vm on the limit & see if it's turning off on limit


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## mattyb (Oct 30, 2010)

@kenmac
I did some more experimenting and found that there was not enough return air flow. After that was corrected, I ran into the issue of when only one zone called for heat, the furnace started to go through it's issue of tripping. The temperature rise during only one zone calling for heat was out of range. It looks like all I have to do is adjust the fan speed and I should be good.


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## Plumber101 (Feb 25, 2009)

Curious at to what you took out and what you installed BTU and effeciency

If you took out a 100K BTU 65% Did you install a 100K BTU 95% or a 80K BTU 95%. From the pics it looks like a 100 or 125K. Not many houses that I install take a furnace that big at 95% AFUE

Just corious to see if you are over sized, if you are it could be MAJOR duct resizing or change out the furnace again. 

I think you are tripping high limit


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## hvac122 (Dec 10, 2007)

What size is the by-pass for your zone system? What brand is it? What size is the furnace?


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## yuri (Nov 29, 2008)

High limits usually take several minutes to reset and not stutter/cut in quickly like his. Unless it is weak from overtripping.


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## mattyb (Oct 30, 2010)

I replaced a 80% 115K with a 100K 95% unit.


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## michael77 (Nov 7, 2010)

It looks like you have not connected the drain hose for the discharge side of the inducer blower correctly. you have it hooked up to the body of the blower. It should be hooked up to the small extension on the rubber elbow that connects from the inducer blower discharge to the flue pipe. See page 19 of the GMH95 installation instructions. You also need to replace the original plastic cap or you won't get a correct pressure to close the pressure switches.


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## FTAJ (Nov 8, 2010)

does your furnace have work properly when the cover is removed? what size is your exhaust pipe? also the drain hose on the exhaust side is connected to the wrong spot, but thats not your problem, check your manual for horizontal installation and get that changed though.


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## mattyb (Oct 30, 2010)

So a little update and answers. Now there are a few versions of the install manual out on the web. The one that came with the unit shows that I did not need to use the elbow with the drain spout. I had to cut a piece of pipe and then use a rubber coupling that came with it.

The furnace does not work properly with the cover off. I have to believe that this is a heat related issue though. It will only malfunction after it has been operating for a while. Once the unit has heated up, it then malfunctions. I just replaced the control board on the unit as it was diagnosed as bad by a service company and that didn't work. The exact same condition so they'll be back out again tomorrow to take yet another look. What doesn't make any sense to me is if there's an issue, why does the control board not flash a code?


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## mattyb (Oct 30, 2010)

Sorry I forgot to mention that both the intake and exhaust go right from 2" to 3". If you look on page 15 of the manual, that's where it shows the install I performed.


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## kenmac (Feb 26, 2009)

I would connect vm to ps & see if the ps is dropping out. If not connect vm to limit. Something is dropping 24v to make it shut down


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## FTAJ (Nov 8, 2010)

did you check your gas pressure? what is your elevation? I had some of these models do this to me before but dont be alarmed its not a bad furnace its just to powerfull, let me know if you have access to a manometer and I will be happy to tell you the procedure to fix the problem


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## mattyb (Oct 30, 2010)

I did not check gas pressure. I don't have a manometer. I had heard that about these units that overall they're good but can be a little temperamental. 
@kenmac - I'll try that when I get home today to see what happens. A service tech. is supposed to come out today anyway so we'll see what happens. At this point I just want consistent heat.


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## mattyb (Oct 30, 2010)

It's official. The problem was a pressure switch. The service guy had a replacement in his truck and installed it and the furnace works like a charm. The bad switch, when put on a multi-meter, would just show open/close randomly. 

I want to thank all of you for your help. Needless to say, I learned a heck of a lot more about my furnace through all of this.


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## yuri (Nov 29, 2008)

Yep, they will flutter quickly and the board needs time to recognize it. Glad it works. Make sure your bypass damper works properly and that you stay within the temp rise specs. Want to be 10 degF lower ideally. Long term slight overheating can wreck heat exchangers and collector boxes.


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## mikey101 (Oct 27, 2011)

*drain tube needs to be moved if furnace is set on side*



mattyb said:


> I just raised the pitch of it a bit to see if that would help but same result. Here are the pics.


 take hose ofinducer motor the one that drains to bottom go by longer hose you'll see plug in exiest from inducer motor take plug and put in hole on inducer motor drain water from inducer exiest to botom (your not drain condensation from pvc exiest properelly if you furnace is seting on it's left side waiter need to drain from pvc pipe down to pump if you dont under stand me i'm hard to keep up with I can send you a pic I've came acroos this problem about 15 times and it's always on goodman that are horizontal.and are not draining because they where installed incorect


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