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Unit: Goodman PGJ042075-1 It is a rooftop dual pack gas

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heat. Going on about 12 years...
Unit: Goodman PGJ042075-1 It is a rooftop dual pack gas heat.
Going on about 12 years now, and have never had a problem until recently. From inside the house, the stat called for heat, the blower comes on, and after a while you realize there is no heating going on. Upon inspecting it on the roof, the burners ignite, then after a minute or so the burners extinguish, then after a delay, the burners ignite, then extinguish after a minute or so. After the third time this process happens, the burners do not re-ignite. The blower continues to run as well as the inducer motor I think. This has happened 4 or 5 times this winter, and it just dawned on me that it only happens when it rains. Looking in and around the unit shows no obvious leaks. It has no humidistat that I know of. Any ideas?
Submitted: 2 years ago.Category: HVAC
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Answered in 2 hours by:
1/31/2016
HVAC Technician: gotoman for ac, HVAC Technician replied 2 years ago
gotoman for ac
gotoman for ac, HVAC Technician
Category: HVAC
Satisfied Customers: 5,358
Experience: 51 years of experience install design and repair of all makes and models of AC
Verified

This usually indicates a bad limit switch or rollout switch. Moisture on package units cause corrosion. Its hard to say which it will be. On a unit that old I would replace both limit and rollout switches. Easy change out. If you need source and service manual let me know. Russ

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Customer reply replied 2 years ago
Thanks Russ,
I've been pouring over the service manual(s) and my inclination was that it might be the two switches you noted and maybe the "end switch". The manual shows all three and mentions them in the 'Theory of Operation' booklet, but doesn't say exactly what operates each. That makes it a little hard to troubleshoot unless you know that information. Could you briefly explain what causes the rollout switch to operate? It looks like the limit switch is pretty major and nothing in the IIC will work if it is open, like its a cover open or something basic like that. The schematics show an "auxiliary limit switch" right before the rollout switch. Would that be something to consider? It's still raining, so I'm just getting prepared to go up on the roof when it stops. Can you recommend a supplier for parts? Thanks again, John.
HVAC Technician: gotoman for ac, HVAC Technician replied 2 years ago

Yes the aux limit would be something to consider. These units are located in the weather. The induce draft fan pulls are across the burners which also pulls air across the roll out switches. When raining it pulls the moist air across rollouts and causes corrosion. The roll out are designed to open when the flame rolls back. Causes are to much gas pressure stopped up heat exchanger,sooted heat exchanged. High winds. Russ

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