[hammerlane]

My Goodman natural gas GMPN 100-4 furnace will fire up and run for about 10-15 minutes then the rollout or stack temp switch on top of the burner box will trip making the furnace shut down and go into lockdown until I reset the switch.

I notice the flame from the second burner on the right jumps around once the blower cycles on and is heating up the top of the burner box tripping this switch.

Is this a cracked heat exchanger?

1st pic shows a closeup of the switch on top the burner box

2nd pic shows the flame jumping/dancing from the burner 2nd from theright after the blower comes on

3rd pic is full view of furnace

 

[user_12345a]

Yup. Dump the turd.

100k btu input, likely oversized too. Go 90%+ efficiency and downsize if you can -> 100k is big and you'll be more comfortable with longer cycles. Can go a step further and go two stage/variable speed for even better comfort if you have the dough.

Check for heat exchanger warranty, may be able to get some credit for a new furnace through a goodman dealer. I know at least 10 years ago goodman was providing replacement furnaces in lieu of heat exchangers.

Change the heat exchanger and it will just fail again; pre-2006 ish goodman equipment has a really lousy rap.

 

[user_12345a]

if not logged in yet, not referring to this type of dough, linked to by the forums... :vs_bulb:

 

[beenthere]

Yep, heat exchanger is worn out, time for a new furnace.

 

[roughneck]

Shut it down and start shopping for a new one. Make sure a load calculation is done to properly size the new furnace. Bigger isn't better.

 

[yuri]

I find it STRANGE Mr.User etc that w/o posters giving us any details of where they live and size of house we can assume their furnace is TOO BIG.

Maybe he lives in Winnipeg and has a 2500 sq ft 1960s house. Sized perfectly. Maybe he has a large house in Ohio.

By the way manufacturers make up to 135,000 BTU furnaces for people with big houses.

Load calcs should be recommended not misleading info or statements IMO.

 

[hammerlane]

2800 sq ft home in northern Ohio. Built in 1997. I'm not getting a new furnace. No need. The heat exchanger is free. It's still under warranty. Just have to put it in

 

[user_12345a]

He's in ohio, not winterpeg.

100k+ furnaces, 3.5+ ton a/cs I will always suggest taking a close look at sizing, maybe getting a calc done and not allowing the contractor to just put the same size back in. I'll be the first to say that rules of thumb are terrible, but some things will always tip me off.

Only big and/or poorly insulated homes need the bigger equipment; the "poorly insulated" part is something that should be mitigated if the money's there before furnace replacement. Do it the other way around and you get stuck with the discomfort of an oversized furnace, short blasts of hot air, higher energy bills.

 

[user_12345a]

2800 sq ft home in northern Ohio. Built in 1997. I'm not getting a new furnace. No need. The heat exchanger is free. It's still under warranty. Just have to put it in

Unless you can do it yourself, which you can't to my knowledge (the manufacturer's distributors won't give it to you), you will pay through the nose for heat exchanger replacement (north of $600, could be up to $1000) and then get stuck with a furnace with old parts.

Also the GMP is known to be terrible design, and it will split/come apart again.

Putting a new heat exchanger in a 19 year old goodman is similar to giving a 85 year old a heart and lungs transplant.

------------
Edit: Now I see it's a high efficiency. 100k @ 90%+ is on the large side if the house is decent. (assuming detatched, insulated walls, reasonable leakage, ducts in basement) If you change your mind and end up replacing, likely can drop down to 80k input)

 

[hammerlane]

I'm replacing the exchanger. It's free. My Uncle already picked it up.

 

[beenthere]

If you get the replacement heat exchanger through a distributor that didn't sell the furnace. They can and do charge a handling fee. Plus they will charge for shipping. So find out what that will cost before just ordering it. The new heat exchange will have 1 year and some months warranty on it.

When your furnace runs, are the supplies or returns loud? If so, this is a sign of under sized duct work.

If you decide to go with a new furnace, you most likely don't need the size you have. And often, manufacturers give a credit toward a new furnace if you opt to go that way.

 

[user_12345a]

Good luck in that case - good idea to check temperature rise and clock the gas meter after it's done. Not likely, but overheating due to low airflow or being overfired may have contributed to this failure.

 

[beenthere]

I'm replacing the exchanger. It's free. My Uncle already picked it up.

Dang, I type too slow. Let us know how it goes.

Also, when you first run it. Open the windows in the house. The heat exchanger often has a thin oil coating to prevent rusting while stored in the warehouse, that sometimes smokes when the furnace is first fired up.

 

[roughneck]

Dang, I type too slow. Let us know how it goes.

Also, when you first run it. Open the windows in the house. The heat exchanger often has a thin oil coating to prevent rusting while stored in the warehouse, that sometimes smokes when the furnace is first fired up.

The last one I did, had liquid oil running out of it. Thing smoked like a coal locomotive when it first fired.

 

[hammerlane]

No loud noises from the supply or return vents. As far as I know it's not costing me anything for the new exchanger. We're getting the part tomorrow. He's coming over in a little bit to help take the old one out. I think we're gonna need to cut the top of the firmer to kinda hinge it out of the way to get the old on out. Also gonna need to cut and repair the 3" PVC vent to get it out the way.

And yes I know about the oil coating on the new part.

 

[user_12345a]

post pics of the old exchanger. (please?)

 

[hammerlane]

post pics of the old exchanger. (please?)

Will do. Hopefully will see cracks

 

[hammerlane]

Got the old heat exchanger out. Like i said we had to cut the top pary of the firnace to kinda hinge it out of the way. Also cut the 2" PVC from the ID blower cuz I have a 2" repair coupling on hand.

Here are pics of the disassembly:

 

[hammerlane]

The old heat exchanger was cracked on each side of the 2nd tube near the bottom. I put green arrows in the 2 bottom pics to point them out:

 

[user_12345a]

are the rivets entact? the cracks alone aren't that bad, but crack is a crack.

that thing looks like it has been badly overheated. check your fuel input and temperature rise.

-----------------
may be a good idea to remove the secondary and wash the air side of it out. over 18-19 years it gets dirty and vacuuming in place has it's limits.