These boilers start up by means of a "standing pilot". A standing pilot is a small flame which is on all the time, similar to many gas stoves and water heaters. When the thermostat on the boiler calls for more heat, the main gas valve opens, gas is let into the burners and ignited immedietely by the standing pilot. Because the standing pilot flame is on all the time, the boiler uses slightly more fuel than an electronic ignition boiler (about 1-2%). However, the initial investment cost of a boiler with a standing pilot is a bit lower than the cost of an electronic ignition boiler.