Cracked Heat Exchanger, Soot Again - HELP!!

I have two propane furnaces (both Nordyne, both installed 4 years ago). The one upstairs is a smaller unit (only heating 1500 sq. feet) and has never given me a moment of trouble. The heating unit downstairs is located in my basement and is a much larger unit. My basement is simply a dirt floor and stays slightly damp most of the year.
Just FYI: My propane storage tank is very old. The propane lines coming into the house and supplying all my appliances are new (replaced 2 years ago).
Okay, last winter I noticed a funny smell in the house. Turns out the entire downstairs heating system was coated in soot and the heat exchanger was cracked. The heating people replaced the heat exchanger.
Four weeks ago, I noticed the funny smell again. Guess what! Soot and a cracked heat exchanger.
Also, I was told by the heating people that allowing my propane tank to go below 30% causes reduced pressure and incomplete combustion. He told me this possibly caused the problem. Is this true? I really believe the problem is with the heating unit itself because my upstairs unit is fine and clean.
Is it possible that something else is wrong with the heating unit (i.e. pressure regulator) that could cause this?
Thanks for any help you can give.
Holly
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Could be a bad pressure regulator, but I would focus on the propane lines. Flush them well with denatured alcohol and see if that fixes it.
If the problem still exists, get a new digital thermostat and run a ground loop to the nearest water pipe. You can do this yourself--it isn't nearly as hard as it sounds. The more expensive digital thermostats have the "G" terminal and the instructions are printed in the manual inside the package.
Let us know how it turns out.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.