I have a GMPN080 horizontal gas furnace in attic of my home. What is the purpose/function of the drain trap assembly in furnace. Cooling coil is external to furnace?? Please advise, looks as if someone has previously modified/homeownered some of the parts. There is some condensate overflow into secondary pan and onto ceiling in home. Please advise. Thank you.

Check drain from bottom of condensing furnace, check that drain hoses are routed correctly. Hoses have to be rerouted if converted to horizontal flow.

This trap will fill itself up when the furnace runs for the first time. It does not need to be primed to make it work. If you have the drain end above another p trapped drain.

the switch is a service or emergency switch if the condensation hose / line is not a sealed connection and the trap is empty the sewer smell will be constant either make it a sealed connection or close off the trap and reroute the condensation line --during heat season not condensation so heat will cause water in to evaporate

Hello, it sounds like your condensate line is clogged and not be able to drain into the condensate pump or your condensate pan is cracked and leaking. Best thing to do is clean the condensate line with a shop vac and see if the water will start to drain, if not looks like the pan is cracked.

it has a thermister on it, but would recommend an attic fan will keep attic cooler and save money on cooling bills.

If you have a shop vac (wet vac) I suggest finding the drain outlet and sucking the restriction out. However if you don't then you need to open the air handler (coil) and pour about a cup of Bleach, not drain cleaner in to the coil drain pan. The restriction is likely in the trap ( U shaped piece of pvc). Good luck!

If the wheel is rubbing on the housing obviously that's no good. Yes that is one solid piece, it needs replaced. Not an extremely difficult job, toughest part likely getting the wheel off the shaft. Sand exposed shaft and use some lubricant. Before removing motor mark on the housing where the wires are located/ direction they point. Do not forget to tighten set screw upon reassembly. Good Luck!!

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This is not a lot to go on.

I assume since you called it a furnace, you have hot air heat and central air conditioning. If so, the only way a furnace/AC unit can generate water is by condensing moisture from the air. This is common to all air conditioning and removes humidity from your home. If so, this water, called condensate, must drain. The condensate is collected at the AC cooling coil that is typically located on top of a vertical upflow furnace or downstream of a horizontal furnace. If the drain is clogged, it will overflow and create a mess in and around the furnace. If this is your problem, cleaning and clearing debris may solve the issue. I suggest starting with the easiest accessable part of the drain path. You may need to open the panels enclosing the AC coil and clean the small pans at the bottom of the coil and clear openings from above that look similar to a laundry tub drain. If you get that far and find rust holes in the pan, repairs are needed. If you have an electric condensate pump that may also be the cuprit and require cleaning. Check and clear all drain lines from the coil connection to the final dicharge - sump pit or outside.