On the Lennox Pulse 21 the limit was factory set at 200 degrees F. If the unit runs hot the secondary heat exchanger fins are probably plugged with lint. Lennox recommends on “Service and Application Notes”, Guidelines for Pulse Service dated Dec. 14,1993 & re-released Nov. 3,2003 (H-93-15). “inspect heating system yearly”.
This should be formed by a Lennox Dealer. In the inspection a pressure test of the heat exchanger is performed, along with checking for items like carbon monoxide, temperature rise of unit, supply & manifold gas pressure. Inspect air diaphragm flaapper material for dirt or deterioration and replace every four years. Lennox recommends a pressure test of the heat exchanger every four years. Make sure this performed!
If heat exchanger has failed the pressure test of 4 psig of air within 10 minutes. Now here’s the good part, you get a NEW FURNACE from Lennox.
Heat exchangers for the Pulse are no longer made, so you get a new model G51MP-48C-110 that came of last summer 2003.
The replacement furnace will be no cost, but the labor, shipping & additional installation materials, taxes extra.
If I was you, I would go to my Lennox dealer or http://www.davelennox.com and enter your zip code or postal code (Canada) for dealer nearest you.
If your furnace passes the test and the service check-up then you will have peace of mind, and good for awhile.