Honeywell Thermocouple Q335C1023B

This Honeywell thermocouple, model Q335C1023B, is 900 mm long. It connects to the gas valve using a threaded compression nut. It is designed for atmospheric gas appliances and generates a millivolt signal when heated by the pilot flame. This signal holds the safety valve open, allowing gas to flow to the main burner.

Thermocouples of this type are selected by the length between the hot junction and the connection end, and by the fitting that mates with the gas valve. The connection is typically a threaded compression nut, and the tip must fit the pilot burner assembly. Before ordering, measure the existing length and confirm the valve and pilot burner dimensions. When installing, verify that the millivolt output at the valve end is sufficient for the valve's hold-in current, and inspect the sheath for cracks or corrosion.

A weakened thermocouple may produce a lower millivolt signal, so the safety valve closes even when the pilot flame is normal. This appears as the main burner failing to stay lit. A cracked or corroded sheath exposes the bi-metal junction, leading to premature failure. Check the sheath and measure the millivolt output at the valve end.