I sealed combustion boiler is probably as safe to run in your shop as anything else with a motor. Do you shut off your air compressor when painting?
I remember reading some scientific data on what it takes to have an explosion in a shop. What i got out of it is the fumes would have to be so thick you wouldn't want to be in the space.
Seriously, i would not concern myself about it. The chances of blowing yourself up are very low. I have painted cars and other equipment in my shop with an 80% furnace with open burners. Get some decent fans to ventilate and it will be fine.
I remember on a wood working group they were taking about shop explosions from dust. Someone posted information that showed the dust in the air would have to be thick, like a fog to be dangerous. Guys were concerned about shop heaters and dust. I used to do HVAC service and we maintained equipment at a commercial wood working shop and the unit heaters would have 2-3 inches of saw dust inside, all over. Shop vac them out, tune them up and go clean them out nest year!