My 20+ year-old Laars EPC-II natural gas heater does not start up correctly. It tries to ignite for about 20 seconds, then shoots a ball of fire out the front (if the cover is removed), and ignites. It runs normally once ignited.
I have uploaded a video here: http://youtu.be/oCW3pGmQb8E
What happens typically (and in the video) is that the heater tries to ignite twice and fails. Then on the third try, the ignition spark ignites the gas that has accumulated (I think). Then the heater runs normally. If you turn the volume way up on the video you will hear the ignitor.
Start-up is not always consistent. Sometimes the heater lights on the first or second try; sometimes it tries several times and seems to give up.
How do I fix this?