Please help cracked heat exchr?????

[1stanner]

Hi, Wondering if anyone can help me I am a new apprentice and have a lot of responsibility with my job. I am stumped by this rooftop furnace. It is a keeprite PGB series furnace and what is happening is that the unit keeps kicking out because of a roll out switch. I have a fluke 16 digital meter which takes temperatures and put the lead in the unit next to the disc type switch with the door closed and the unit goes up to 420 degrees and kicks out. When the door is off the unit it will run forever. I have checked air supply, blockage of air into building, venter amp draw, plugged vent, replace gas valve, 3.6 w.c. Blower is working great, cheked filters, took them out with doors open to let extra air in. Cant figure it out. I called tech and they say to do a CO reading in the exhaust gas to see if it has a cracked heat exchr. The unit is about 10 years old and looks clean. I did not yet check the exchr. Looking for any advice. There is an exact same unit 15 feet away and it runs about 90 degrees near the limit switch with the door closed. Also there is slats in the front door where air can get in so its not that. If anyone has info It would be appreciated.

[pcscott]

Look for a hidden filter grille. Then grab a Ductulator and check return size. You may not be the first guy there.

[pcscott]

Look for a hidden filter grille. Then grab a Ductulator and check return size. You may not be the first guy there.Also check inducer.

[1stanner]

The duct dumps right into the building with not much restriction at all. It is the right size and there is definately no restrictions

[pcscott]

Then again the limit might be soft, if you get decent temps change limit before gas valve.

[1stanner]

It's 420 frikken degrees in there, definately not a bad limit sw. Plus it kicks out, and It was replaced already a week earlier.

[Carrene #2]

1stanner,
Is there any laws or codes in your area that require you to have a licence to do such work?
Actually, there is, and they are serious about that sort of thing.
Read nameplate, check pressures and go from there? You will probably have to check heat exchanger!
Turn 'stat up to 90° and see if flame reacts badly.

[1stanner]

thanks. I have a gas fitters lic + working with my boss so I should be covered. I know what you mean though

[BobbyBJr]

Check the heat exchanger. Almost 100% of the time that I have seen roll out switches kicking out a unit of that vintage, I find a bad heat exchanger.

Bobby

[pcscott]

Sorry ,Stanner,Just asking, Im not at the unit so I dont know what to expect.

[speer007]

I would say it is a cracked heat exchanger when you open the door that is a path of least resistance and the fan will blow out there. When the door is on the fan will blow air into the heat exchanger and blow the flames on the burners back, making it trip on roll out.

[1stanner]

thanks everyone for your feedback I am going to run some tests on the unit tomorrow
appreciate it

[wasp]

Gas valve pressure regulator may need to be backed off. I had one like that a few weeks ago.

[1stanner]

Hey everyone I was at the system today and pulled the back door off and some panels and found huge cracks in 5 out of the 7 heat exchanger tubes. Right at the bends. I could fit my flat head screwdriver into them. Thanks for everyones replys

[jaybee]

bad heat exchanger