I know it's a violation of the NEC. What I'd really like to know is what (if anything) is actually wrong with doing it. NEC is supposed to be about preventing fires. How is having a plug-in furnace a fire hazard?
Could it have something to do with grounding?
Are you concerned about melting the hose?
Could it have something to do with the furnace being connected to metal ducts and metal ducts in the rooms? If you have A/C, there's also the lineset.
Washers are connected to plumbing (refrigerators too if they have icemakers).
I suppose you could do something like that. It may cost more than having
2 tanks, which could run your generator for longer.
Twist-lock? So maybe the problem has something to do with the plug falling out (or coming loose) when it shouldn't.
Are you thinking of freeze damage, or something else?
I doubt it has anything to do with grounding. I think it has everything to do with plug and cord connected appliances, eqpt being a more common source of fires, compared to permanently wired ones. The code isn't specifically addressing furnaces, it's permanently installed eqpt in general. When you have things connected on cords, people tend to start using extension cords, draping them anywhere. And IDK how far back that code goes, but older furnace stacks, steam boilers, etc could have temps where if a cord comes in contact, it could melt the insulation. As I said, if it's done right, it should not be a problem and some places allow it.
From a practical standpoint, it's irrelevant to me. Rather than rewire the furnace to cord and plug, I'd always opt for an interlock on the panel and an inlet, so you can power anything in the house.
That is one danger.
A plug that's only partially pulled out can generate enough heat to start a fire - particularly in a device that draws a lot of amps.
Perhaps freezers should be hard wired, like furnace?
How about an electric dryer?
Even more critical, and even more need for hard wiring.
The plug for my dryer is harder to get to, requiring moving the dryer (which requires moving the washer, because of limited space). It also takes more force to pull out.
The tank is.. but.. A modern car has a friendly electric fuel pump inside the tank which is controlled by a fat relay that's usually under the hood. If you take the fuel line off of your carb/injection rail, and bypass the relay, it'll happily pump fuel at a good rate until the tank runs dry, the battery fails, or you turn it back off.
Don't try to start your car to trick into turning the pump on for you to drain gas though. Just ask the relay to do it. You don't need the car getting upset thinking it's losing a lot of fuel and shutting things down to prevent a fire.
Depending on how far it's pulled out and the load present, I'd be more concerned with an unexpected electrical fire that the breaker isn't going to notice is a problem (no short circuit or overload, no trip-basic breaker; I'm not including Arcfault/GFCI breakers in this). If nobody is home, this could be a real problem in a very short period of time. More so if this receptacle is located near easily flammable items.
Once the plastic on the receptacle itself catches, and, it will.. if someone isn't around, that fire isn't going to remain confined to the box.
Freeze damage and flooding are certainly two other valid concerns though. Then you have the issue of people who will borrow the outlet in partially finished basements and 'forget' to plug the original cord back in all the way. Give someone a place to plug something in, they usually will. Even if they shouldn't!
Hmm.. It's probably because your electric ignition systems aren't pulling any significant amps on the dedicated circuit (should be) I'm assuming? the NG furnace uses the electricity for an ignitor and to run the blower(s)?
If so, that shouldn't exceed the circuits capacity. I'd make one suggestion though. Remove the 3way plug, remove the single outlet, replace with double outlet and faceplate. It was originally a single outlet to discourage you from trying to use that (should have been) dedicated circuit for anything besides what was originally plugged into it. Stapling the wires when routed properly once you plug them back in can also reduce stress on the connection and ensure they will never accidently become unplugged.
Plug both back in as normal. It's a better connection for both devices. If the single 3way comes out, you lose power on both as it is now. If one comes out of a brand new outlet, it doesn't automatically take the other with it in the suggestion I made.
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