My Lennox furnace (at least 20 years old) was gulgling this morning, while the furnace cycled off and on every few seconds, not getting the house up to temperature. I drilled a hole in the trap, parallel to the "T" connected to the clear hose to the drain, to release the water, which allowed the furnace to run. Then cut off the "T" connector, and found it plugged. Question: can I just add a new "T" to the trap, or is there more to it than just the "T" connection? Thanks.

Give it a go and see what happened. You are in no mans territory when it comes to lennox furnace modifications but if you make it like the original it should work. At 20 years of age that lennox should have a heat exchanger test performed. They are trouble prone when they get older.