[LHaven]

What happens at that point depends on the layout of your rig. The main task of the purge is to fill the long feeder line with gas. In my rig, the range is just ahead of the furnace and the water heater is way aft. If both of those are working, only the short stub actually feeding the furnace itself needs a purge, and that is taken care of by the first ignition attempt. Now, if your furnace is distant from every other gas appliance in your rig, you may need two or three ignition attempts to purge the shunt, but this is something your furnace typically does automatically anyway by trying to start a few times before giving up.

The typical startup routine of a furnace is to turn on the fan, check for good airflow and no overheat, turn on the gas, try igniting it for 20 seconds or so, and if no ignition, turn off the gas, then the fan. Then typically it will wait some period (60 seconds?) and try again. It will do this two or three times, then give up. If you really have a difficult purge condition, you may have to turn the thermostat off furnace at that point, then turn it back on to begin the cycle again. If it still fails the second set of cycles, your problem is probably not purge.