For my application, a 7 year old Smith Gas Boiler. This thermocouple is perfect. Ive used longer, but 30 inches is suffice and not too long. Every year by October, before winter is here, i give my boiler a simple tuneup. A brand new thermocouple, a cleaning outside of boiler and ducts and after first ignition for the year, i bleed the air out of my system, for the first four hours. This has worked for me perfectly. And Thanks to Amazon, these thermocouples are accesible. I keep atleast two spares, incase pilot goes out and wont reignite. Get more detail about Honeywell #CQ100A-1039 30" Thermocouple.