In article , Tom Zillig wrote:

I'm wondering how to make an old propane furnace in my shop run on
20 LB cylinders, and need to understand the stepdown of the regulator
and where to get one inexpensively. I already have all of the fittings
to attach the tank directly to the furnace.

There are TWO regulators on a propane system. The first stage drops
the tank pressure to 10PSI ( this is known as "high" pressure ) and
the second regulator drops the pressure to the required level for the
appliance. There are two types of "low" pressure regulators - a 2PSI
version for large heating units and an "11 inches of water" version
for most everything else.

Fisher makes both of these in many models depending upon how much gas
needs to be delivered. You do know that the tank has a reverse thread
on it, don't you?

http://www.fisherregulators.com/lp/pdf/lp31slct.pdf

Try eBay for regulators but only buy new in box units.