I am having a problem with a dayton(3E229D) gas heater. It has a standalone pilot that is lit and stays lit. The problem is that the burners don't ignite. I turn the heat on at the thermostat the fan comes on after a delay but the burners are not kicking on. After reading a few DIY guides people suggested checking the thermostat by jumping the red and white wires together. Others the solved the problem by resetting the vent limit switch. Or is the gas valve bad andneeds replacing?

Sounds like the thermocouple is shot

if the pilot stays lit and main burners wont fire you don't need to replace the pilot generator! you have an open safety, poor electrical connection or a problem within the tstat controls

chech the flame sensor or thermocouple which ever it is equipt with. Sensors can be cleaned by lightly sanding them and therocoulpe's need to be replaced.

Most heaters has a pilot proving sensor of one type or another, it will be in direct contact with the pilot flame to prove that the pilot is lit and then will allow for the main gas to the burners to open. If the proving device has failed then it needs replaced.

If this is a standing pilot system, turn the gas valve to pilot position. push in the knob & hold while lighting pilot...keeping holding knob down for a minute... this will pick & hold the gas valve. Let go of the knob & see if the pilot stays lit, if it does, then turn the knob to on position & the furnace should start if there is a call for heat from the thermostat

Most gas water heater problems can be fixed with yearly service that cleans burner and burner parts, checks gas pressure and bleeds air out of gas line, cleans air intake, and checks for venting and air intake problems.
Open following links for resources:
http://waterheatertimer.org/pdf/Pilot-light-will-not-stay-lit.pdf
http://waterheatertimer.org/pdf/Attic-Ventilation-and-Pilot-Outage-in-Gas%20Water-Heaters.pdf
http://waterheatertimer.org/How-to-install-gas-water-heater.html#troubleshoot

Find where the gas line connects to the gas valve on the back of the heater, from the gas valve, follow a small aluminum tube going into the back of the heater. There may be a removable door to access the pilot assembly. If there is no aluminum tube, the unit has electronic ignition and no pilot lite. Turn the gas knob to off, then to pilot. Hold the gas valve in and with a long match or gas grill lighter, light the pilot. Continue to hold the knob in for about 30 seconds. After that, release the knob and check to make sure the pilot stays lit. If so, rotate the knob to ON. The thermostat will kick the burners on.

Normally you turn the gas valve to the pilot position, light the pilot, hold down the gas valve knob for 30-40 seconds and release. If the pilot will not stay lit replace the thermocouple. These are available ay Home Depot.

Ray