If you're going to post about fuel pressures, I'd be nice if you post CORRECT information!
Normal input pressure for Nat is 5-7" wc with a 3.5" wc burner pressure.
Normal input pressure for LP is 11-14" wc with a 10" wc burner pressure.
**Always check equipment labels for the exact specs, as all equipment is not the same.
BTW, 80% don't sweat, a low temperature rise causes the flu gases to condense. Condensation can also destroy a high efficient furnace, when the condensation happens in the primary heat exchanger. Temperature rise specs are there to protect your equipment.
i have ran these pressure for years.....and yes low pressure make low temps which will make them 80%'s sweat......seen it to many times with rotten out draft blowers and upper heat exchangers......
only time i have seen a heat exchanger rot from low temp was when a outside air damper was stuck open.........if your find them with condensation in the primary tell your customers to shut there windows!!!!!!!!!!