Q - My Burner with Valve Vanisher Electronic Ignition turns on fine but shuts down after an hour or two? Why?

A - If any of the components were faulty, it would not hold a pilot for any amount of time.  Here are a few possibilities for your nuisance shutdown.

1. If the burner turns off, but the pilot remains lighted, the temperature of the control module has exceeded its limit of 140 degrees F. The main burner cannot be relighted until the temperature is below 140 degrees F.  Reduced cooling air flow causes of this condition include:
   • glass doors and damper are not wide open during burning 
   • too large of a set is installed for the size of the firebox
   • slots in front face of valve vanisher chassis are blocked

If both the burner and the pilot shut down, potential causes include:

1. Low gas pressure. You need to have 7" water column (1/4 psi) inlet pressure to the system.
2. The Thermocouple is losing its temperature difference. The thermocouple consists of the "hot junction" (the tip of the thermocouple where the pilot flame strikes) and the "cold junction" (where the wires enter the base of the thermocouple.  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 may be 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 or excessive heating 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 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.

It would helpful if you could please email to us at info@rasmussen.biz a photo showing a view of the entire fireplace opening.  We might be able to discern something causal from the photo, or to contact you for more information.