I have a richmond 5v40-36f water heater & when i light the pilot it stays on for 20-30 seconds,then when i turn it to ON it lights the burner for 3-5 seconds then goes out,i replaced the thermacouple & the same problem still exsists

SoundS like the thermocouple needs replacing .Pilot not holding .

Remove the pilot cover on the outside of the water heater. Turn the gas valve knob to "Light". Using a long fireplace match or long stem lighter light the pilot. Turn the knob to "run" or "on". Replace the cover. If it will not light or stay lit, you may need a new thermocouple. They are not that expensive, but replacing them yourself can be a pain.

Troubleshoot begins following page:
http://waterheatertimer.org/troubleshoot-flammable-vapor-on-gas-water-heater.html#FV

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Make sure the thermocouple is deep into the blue part of the flame. If it is good, then the only part left is the defective main gas valve.

Is the thermocouple secure, routed properly, no kinks, no sharp bends? Is the end of the unit properly aligned so the hottest part of the pilot flame is hitting the unit. Did you hold pilot switch in the light position for at least 30 seconds, before you set the gas valve to on? If all above OK, try this. Light the pilot using barbeque lighter once it lights keep the lighter on the t-couple for 30 seconds, then set the valve to ON and keep the lighter where it is. Once the main burner lights, remove the lighter. If the burner and pilot go off, your gas valve is bad.

I suspect a bad thermocouple

The pilot lights then heats/sends a signal to open gas for burner then(burner)should stay on but if valve shuts the gas off,then have your unit checked. What ever you do,DON'T MESS WITH IT IF GAS/FLAME ON!!!!!. These systems come with a safety feature that allows gas to go on only when safe!!!!

MAKE SURE THE BURNER TUBE AND ORIFICES ARE CLEAN. MAKE SURE YOU HAVE THE PROPER LP PRESSURE.

resetting your lock out for richmond power vent