I never let my Yellow Flame idle longer than 15 minutes. With the big firebox and the grates just sitting in the middle you will pull virtually zero air through the grates with natural convection and the fire goes out. Treat it like a stoker stove. No combustion fan running, your fire goes out.
Your going to have to cycle every 15 minutes or so or get the Keystoker conversion fan kit. It's a kit that has you cut a small hole in the combustion fan housing and mount a small blower. It runs continously providing some combustion air so the fire doesn't go out at idle. See this link Keystoker k6 combustion blower
I opted to not cut into my combustion blower housing and just stuck pins in my Intermatic c8865 60 minute cycle timer so it cycled the boiler on every 15 minutes. Much easier than cutting holes in perfectly good fan housings.
New Beginning Church