I can light the pilot light with not problem, the burners also will ingnight with no problem. After about 5 minutes the burner, and the pilot light will go out. i can re-light them only to have the same results. i have read about the combustion shutoff system. could this be my problem? I have changed out the thernocouple, and the gas control system only to have the same results.

Check for error codes and service manual on following link:
http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html#intellivent

Replace gas control valve
http://waterheatertimer.org/How-to-replace-Honeywell-gas-control-valve.html

Gene:

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That's an intermittent problem. Could be: dirty filter, dirty blower wheel, dirty secondary heat exchanger, dirty evap. coil, weak or damaged high temp safety, loose wire connection, fault at control board, poor air flow. Just about 100 guesses! Could be's don't solve anything. Check for cleanliness. Clean flame sensor(flame rod that burner flame comes in contact with). These are the first I would try.

i'm thinking after the main valve opens the pressure could be dropping in the pilot tube. this could be due to a dirty, plugged and sometimes spider web inside the pilot tube. rare but does happen, nice warm home for a spider. good luck. if not let me know. could also be weak pilot coil.

Sounds like the igniter needs to be replaced. It is normal for a heater to run five or so minutes that means the thermostat is working. then restart when the temp calls for heat but if the igniter does not work it will not light up again.

The ignition spark needs to jump from the electrode tip to a piece of bare metal on the burner. Make sure the burner area is clean and dry.

Hello KJGreen, It sounds, based on your description, as though the thermocouple is faulty and should be replaced. If the pilot light stays lit and you've left it on for several minutes before turning the knob on to light the log set, then that indicates that the thermocouple is bad.

The thermocouple is a safety device (heat sensor) designed to stop the flow of gas to the burner in the event the gas flame goes out and the pilot goes out. The sensor will only allow the gas valve to open once it has reached the preset temperature.

To confirm that the thermocouple is defective, I would light the pilot light and allow it to remain lit for up to 10 minutes before I attempted to turn on the log burner. If, after 10 minutes, the log burner still won't stay lit, then you know that the thermocouple needs to be replaced. Otherwise, the burner should light, since after 10 minutes, the sensor should be hot enough to signal the gas valve to open.

If you are going to have this replaced, please make sure you use only an approved supplier, since an incorrect replacement part could cause an unwelcome malfunction of the gas log burner. Safety should be primary when dealing with open flame burners.

Hope you find this Very Helpful and best regards!

Hello. Begin by using a small brush to clean everything in that area. Then light the pilot and light the burner. If it still acts up, change the thermocouple. Regards, joe

Set t/stat to 250 degrees. If it does not shut off after about 5 minutes, replace t/stat.If shuts off, must be calibrated Pilot may be dirty, flame weak. If flame strong, replace thermopile(pilot generator). If weak, must remove and clean pilot orifice inside pilot.

If the burners do not fully ignite, the sensor will shut the furnace down. The lack of sufficient burner flame is a gas valve or gas pressure problem within the valve.
Replacing the gas valve with the original one may get you going. I surely suggest a professional service company for safety reasons and a thorough inspection. A 20 year old furnace is past it's normal life span.

Usually delayed lighting on a standing pilot system is caused by a gas pathway that is dirty from spilled food or grease. If you can remove the burner assembly and clean it that will correct the problem. If there is a removable plate above it then you can remove that and get access instead of removing it. Pilot ignitions work by creating a column of gas between the pilot and the burner that is established when the burner is turned on kind of like a fuse. A small tube my be used to establish the column of gas between burner and pilot and it may get blocked with food that overflows. Sometimes the pilot is just located right next to the burner and may get partially blocked or the orifice in the burner next to the pilot may be blocked. Cleaning all this helps. Be very careful with gas. Work in a well ventilated area. Turn on gas very briefly to run an ignition test. Turn off the gas and vent it off if it fails to light in about 5 seconds. Unburned propane drops to the floor and collects. Natural gas rises. Be aware of this when you ventilate.