If my regular propane tank for the house runs out can I use a BBQ tank as a temporary fix? What will I need to do this?
This generally won't work all that well. While both large and small tanks contain propane, the large tank can deliver more propane to appliances than the small one can.
Propane is supplied as a liquid under pressure. The pressure in a propane tank is pretty much an indication of its temperature, not the amount of propane in it. When propane is drawn off by an appliance, the propane in the tank boils to make more gas - and that boiling lowers the temperature of the tank. A small tank, especially a small tank in winter, can become so cold that it cannot effectively supply gas.
This PDF (which unfortunately lacks page numbers to refer to) lists vaporization rates for various sized containers (100 lbs being the smallest, 5 times your typical BBQ cylinder) at different temperatures on the 6th page - at 40F the 100 lb can deliver 61,500 BTU/hr, at 20 F 43,300, and at 0F 25,000 - A pound of propane is 21,600 BTU, a gallon is 91,000 BTU.
It's possible in an emergency but it will not last long. Making the connection is somewhat difficult if you don't have the parts. The regulators are different threads. Tanks use national pipe thread after the regulator and cylinders (bbq tanks) use cylinder threads(M something or whatever) . How much propane you can get out of the cylinder depends on how cold it is outside and how fast it is being drawn off. If you managed to get it connected you could place the cylinder in a warm water bath after it cools. You could even connect inside directly to the furnace but you will still face the cooling issue. Also propane mishandled is EXTREMELY DANGEROUS. Do not attempt any suggestions from anyone unless you are experienced with hydrocarbons. 1lb of propane contains about 20,000 btus so a 30lb cylinder on a 80,000 furnace theoretically should last about 7 and a half hours. You'll have to fight to keep it warm the whole time. Seriously don't attempt anything that you are not comfortable with. It may even be illegal to bring a cylinder inside. If you are comfortable with it and want a satisfactory result,connect 4 or 5 cylinders together and the freezing problems disappear. We do that often on construction sites.