Normally residential tech venturing into the commercial forum here on hvac-talk for the first time due to running a commercial call today.

In addition to the simple moving of a thermostat and placing it in a lockable cover, they asked me to take a look at their beast of a Hastings heater. Well, after I consulted my company owner, who also happens to be my dad, about it since he has eons of commercial experience, he informed me that what I was being told was a large hanging warehouse heater is actually a makeup air unit. It's two large intake ducts are just open now as the connections that used to go through the roof have been eliminated years ago. The large amount of exhaust/vent air that used to be in the building is gone as well. They have been using this 836,000 btu beast for at least the past couple of years as a large warehouse heater heating about 8000 sq. ft. with 20 foot ceilings. And in normal Hastings fashion the unit does not vent out exhaust, it is just moving its air over open flame.

I wanted to double-check what my dad left them with which is that this unit is only designed to be used as a makeup air unit and if it is no longer pulling its intake air from outside and running along with adequate building exhaust then it is dangerous to use.

Would you agree with this or do these units run so clean that there is no danger of carbon monoxide and other combustion byproducts building up in this now not very well ventilated space?