You have omitted some critical information. How many watts? Volts? Some will tell you any fixed in place appliance must be hard wired. The same people will put a cord cap on a disposal. Heck, a garage door opener is fixed in place and has a cord. Of course, it is factory installed. That will be another issue. It wasn't factory installed so you violated the listing. Who cares really. As long as you have accounted for the proper wattage and voltage it will work.

I don't know why whether something is fixed in place can't be cord and plug connected. I've certainly got tons of builtin kitchen appliances that come from the factory with plugs on them.

Unless the manufacturer forbids it there is nothing wrong with a cord and plug connected furnace as long a the circuit and cord are sized properly for the load. The circuit may be required to be dedicated depending on the load. Where I live virtually all gas furnaces, dishwashers, disposals, microwave hoods, window AC's and swamp coolers are cord and plug connected, and yes it qualifies as the disconnect.

That's exactly how my shop heater is connected, as done by a "real" electrician and passed by both the plumbing and electrical inspector.

The heater is just 120 @ 3 amps - It is natural gas fired and has 2 fans on it, one for combustion air and one for the combustion air. I did put it on a dedicated 20 amp circuit.

When it comes to the electrical connections the manufacturer specifications say that it is recommended to be on a separate, grounded circuit with a disconnect means as close to the unit as possible. There was no maximum breaker size listed, unless it is lighting I prefer to use 12 gauge wiring so I put it on a 20 amp breaker. This will be inspected but I just wanted to make sure I didn't make a big mistake with the cord, but as long as that counts as a disconnect means I think I should be OK based upon teh responses here.