Q: My pilot lights and the log set will burn for a period of time (5 to 15 minutes or more), but then shuts down, both burner and pilot. What's going on?
the set works for a period of time, then goes out, all components are
functioning properly. Had either the thermocouple or the valve been
faulty, you would not be able to hold a pilot flame at all.
Therefore, replacement of the components will not be a long term fix
to the problem.
is how a thermocouple works: the thermocouple consists of the "hot
junction" (the tip of the thermocouple where the pilot flame
strikes) and the "cold junction" (near the brass portion of
the thermocouple on the valve side of the bracket). During normal
operation, the pilot flame strikes the hot junction, which creates a
temperature difference across the thermocouple. This results in
millivoltage that flows to the valve, which allows the magnet in the
valve to hold open the plunger so that gas can flow through the valve
to the main burner. This normal operating mode is a "hot tip,
cold base" condition.
the pilot flame goes out, due to being blown out or due to an
interruption in the gas supply, the hot junction cools down,
eliminating the temperature difference across the thermocouple.
Without the temperature difference, no millivoltage flows to the
valve, which causes the magnet to release the plunger and thereby
shut off the flow of gas to the main burner. This is a "cold
tip, cold base" condition.
you are experiencing is the opposite condition. Rather than having a
"cold tip, cold base" condition, you are getting a "hot
tip, hot base" condition, which results in no temperature
difference across the thermocouple and therefore shuts off the flow
of gas to the main burner.
"hot tip, hot base" condition is the result of insufficient
air flow across the base of the thermocouple. Any or all of the
following items may contribute to the "hot tip, hot base"
condition. Correcting the condition may require addressing several
combinations of these items.
glass doors and damper are not wide open during burning (reduces
cooling air flow)
too large of a set is installed for the size of the firebox (reduces
cooling air flow)
One or several of the logs is radiating heat onto the "cold
junction". Remove then reinstall the logs, and/or move the
burner slightly ahead (toward the room).
the pilot/thermocouple assembly is not mounted in the position
specified in the instructions (on a bracket on the side of the
burner, in the path of cooling air being drawn in from the room). The
instruction sheet for the safety pilot control can be viewed online here at www.RasBox.com
the sand in the burner may be piled up too high near the
pilot/thermocouple assembly, thus bringing the main burner flame too
close to the "cold junction".
there is a gas leak at one of the fittings that is igniting and
heating up the "cold junction".
you may need to try several combinations of the above to correct the
"hot tip, hot base" condition.