Goodman GMH95 Intermittent problem

mattyb · 10-30-2010, 01:33 PM

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.

yuri · 10-30-2010, 01:51 PM

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.

mattyb · 10-30-2010, 02:07 PM

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

yuri · 10-30-2010, 02:14 PM

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.

mattyb · 10-30-2010, 02:38 PM

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.

mattyb · 10-30-2010, 02:40 PM

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?

yuri · 10-30-2010, 03:38 PM

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.

mattyb · 10-30-2010, 04:06 PM

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.

hvac122 · 10-30-2010, 04:27 PM

Is your temperature rise withen limits?

mattyb · 10-30-2010, 04:53 PM

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.

Marty S. · 10-30-2010, 05:30 PM

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

mattyb · 10-30-2010, 05:40 PM

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.

kenmac · 10-30-2010, 07:14 PM

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

mattyb · 10-31-2010, 07:03 PM

@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.

Plumber101 · 10-31-2010, 08:23 PM

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