Bidding a job to replace old oil system. Million btu propane up flow furnace? Don't even know where to look. My local supply house say no carry it
Engineered air may have something.
They have condensing models up to 5000mBTU, but they're custom built, and expensive
Is that the input of oil furnace? I would look at replacing w/ 2 pairs of twinned 150 btu furnaces, I bet the output btu would be close enough. Has a heat loss be done to determine actual load? Ducting should probably be seperated into zones anyway.
Residential units probably don't have the blower power for commercial use. Most of them are designed for .5 static where commercial can be several times that.