As was already said, it would depend on the manufacturer's instructions.
Here we hard wire them in off of a single pole switch that is used as the disconnect. Usually located at the bottom of the stairs so if the furnace is on fire you don't have go close to it to cut the juice. It is on its own cicuit. We put a red switch plate on it that says "emergency" so you remember what it is for and don't accidentally turn it off.
http://www.tnb-canada.com/en/catalogues/online/comresconstruction/pdf/c1/boxes_18_eng .pdf This is code here in Canada when the panel, which would normally be the disconnect is in the vicinity of the furnace. They don't want you to have to walk by the furnace to disconnect it if there is an emergency. Allot of times the panel is in the vicinity of the furnace so that is why we do it here.
No switch required if the panel is located between the furnace and the point of entry to the area where the furnace is located.
I know you are not from Canada but thought I would write it anyway because I like the reasoning behind our rule.
[This message has been edited by RobbieD (edited 01-19-2007).]
