Pilot and burner shut down unexpectedly
Author: Rett Rasmussen Reference Number: AA-00647 Views: 14268 Created: 2011-10-27 14:54 Last Updated: 2011-10-27 14:56 0 Rating/ Voters

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?

A:  Because 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.

This 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.

When 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.

What 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.

The "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".

Again, you may need to try several combinations of the above to correct the "hot tip, hot base" condition.  

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