I have a 21 year old Lennox WhisperHeat G20 NG furnace, and the RobertShaw SP735L ignition control module is failing (confirmed by a Lennox HVAC tech).

Lennox sells the 30W33 which includes a Honeywell S8670K ignition control module, a mounting plate adapter, a new flame sensor, and misc. parts used for mounting. Honeywell offers as a replacement their S8610U3009 Universal ignition control module that will also replace the SP735L. Except for some nomenclature differences on the terminal strip of each: For example, on the Lennox-sold S8670K module the 24V(GND) terminal is the same as the TR terminal on the RobertShaw, and the 24V terminal is the same as the TH terminal; and, on the S8610U the 24V(GND) terminal is the same as the TR on the RobertShaw, and the TH-W terminal is the same as TH terminal.

However, it appears the major difference is the Lennox-sold Honeywell S8670K module has a 30 sec. prepurge programmed in, whereas the S8610U allows various settings for prepurge, including 0 seconds. I don't believe the G20 has prepurge, thus it would be set at 0 seconds on the S8610U. However, since Lennox sells the S8670K in the 30W33 kit as a direct replacement for the RobertShaw SP735L, I would hope either module could be used as a replacement.

Am I correct with the above assessment? BTW... the heat exchanger has been checked by two techs - from two different local Lennox dealers - and, it's fine. Surprisingly, as I've read various forums where the G20 is notorious for heat exchanger failures.

Thanks.