My Laars LT 400 recently stopped working properly and I wanted to share my experience and theory about the cause, and see what others might have to say about it.
The heater fires and starts properly, but after a short amount of time (1 - 3 minutes) there is a small bang/poof and a rattle and the heater stops. After 3 cycles of this the AGS light comes on. I check the troubleshooting guide and it indicates that the cause could be poor water flow or flame rectification. The water flow is fine, so I began checking airflow and flame.
The heater is under several trees and leaves do get stuck at the bottom of the heater and the flue vent. I cleared out both. No improvement.
Next, I removed the access panel from the front of the heater and watched the entire process. Aha! The heater was backfiring: the small bang/poof was the flame reaching the gas orifices. The rattle only occurs with the access panel on and is essentially the access panel rattling against the heater body when the backfire occurs. I removed the flue vent, cleared it out, checked the blower cage, cleared it out, and verified that airflow was correct; it was. The blower works well and air is flowing well when the blower is running.
I did notice that one of the burner tubes is rusted. My theory (as of this morning) is that that burner tube was not well galvanized and has corroded entirely, clogging up the burners. Gas builds up at the orifice and eventually reaches the inside of the chamber and causes a backfire. The flame in the box goes out because of oxygen starvation (the backfire consumed it all) and the heater shuts down.
A quick check for part L0052300 (Burner) shows that it costs < $30. I just need to pull the burner tray and verify that this is the case. If so, then I can order one and replace it.
Has anyone else had this problem? Am I on the right track?
The heater is 5 1/2 years old and well out of warranty, so no hope there.
Yes, I will add pictures soon to show the corrosion on the tube.