OPTION A: Small Propane Furnace prototype

This post will be updated as progress is made.

Theory
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Create a mold for a small gas furnace that can be reused.

Cylinder
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PVC pipe as the outside cylinder form.

Inside cylinder form.  On the first go I will use option 2 which is not easy to repeat because it may be a lot of work and foam to create the core.  But it should be the easiest to remove so lets hope I am wrong.

OPTION 1: Make the core from a split PVC pipe that can be removed without damage to it or the refractory.  Split the pipe in 4 parts with the saw cuts orientated so the saw blade is always in parallel planes and the pipe not rotated.  This will make two sides that can be pulled to the center, no draft on the cuts.  Support the inside of the pipe with discs with two opposing flats.   When a disc is rotated about the axis of the flats they fall out.  Problem is finding a properly sized pipe.  Maybe have to make one from wood.

OPTION 2: Make up a Styrofoam core for the inside that can be melted out with solvant.  Hot melt glue it to the base and use spacers for centering that can be removed as the refractory is filled.   Styrofoam insert for the burner hole.  The core will be layers of Styrofoam cut to shape and laminated.  If needed they can be turned on the lathe.

Create a lip around the top of the refractory cylinder.  Ensure it is flat so the lid fits well.  Cylinder is cast upside down so the lip is on the bottom.

Base
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Base is a disc.  A groove for the cylinder to rest in.  Top surface has a center flat for the plint surrounded by a sculpted/slanted ring and a drain hole to deal with spills.  All Styrofoam inserts.  Maybe make the base and top lip the same size as the outside skin which will be riveted sheet metal from a water heater.  A little unsure about this not wanting to crack either the lip or base.

I am thinking of casting a robust sub base from aircrete which is aireated Portland cement.  Maybe it can be cast in the same form as the base with a ring added for additional height.

Top
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Top is another disc but with the exhaust vent hole.  This will be a Styrofoam insert in the mold. 

The space between the cylinder wall and outside skill will be filled with perlite that has been sifted to remove the fines.  This will allow reuse of the perlite should the furnace liner fail.   The question is how much space do we want between the outside of the refractory cylinder and the outside skin.

Prep Work
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Build a foam generator for making aircrete.

Make a bow for cutting Styrofoam to a specific height using blocks/sticks as guides.

Mold Implementation
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About the same as above but with less musing.   Maybe the best way to do this is from the bottom up.   So lets start with the base and sub base.

Sub Base
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A board with rings to form the cavity for the disc.  Make a removable top ring to reduce the height for use in casting the base.  Styrofoam plug for the drain hole.

Base
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Remove the top ring from the sub base mold.  Cut a Styrofoam hoop the same dimension as the base of the cylinder.  Maybe a wee bit larger because the cylinder can be mortared in.  Attach that to the base form which is a depression for a disc

Cylinder
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The cylinder will be cast upside down.  Start by making the form for the top lip with a central support for the outside PVC wall

Cut the large PVC pipe such that two opposing sections can be pulled out.  Its not important now but I may reused this pipe latter as the inner cylinder form for a larger furnace.

I plan on using the blade from a battery powered skill saw.  These blades are narrower.  The PVC will fit back together better.

Lid
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The form for the lid is a disc with a Styrofoam plug for the exhaust vent.