Jayss wrote:First you need to check the thermopile at the gas valve and make sure you at least 500 millivolt.
3 points for Jayss! Make sure the TP leads are not connected to the gas valve when testing. If its a good solid 500 or more look for loose/corroded connections. Heck, tighten them all anyways. The diaphragm in the gas valve can stick closed if it remains shut for a long time. A **gentle** tap with a screwdriver handle under the gas valve might free it. If so turn the thermostat up/down several times to help make sure it stays that way. DO NOT try to disassemble the gas vlave.
Firing the boiler with low water will flash the water to steam which is an explosion, especially in the confined area of a heat exchanger.