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Converting an M1937 Fire Unit to Propane
BTU Rating of the Burners (Part 3)

By David Jarvela

Click on the thumbnail for a larger view of each photograph.

This decided, I set about trying to find the BTU rating of the fire unit because I would need to know the rating to properly size the orifice. After some online research, I found, run by Steve Karoly. After posting a message, Steve contacted me and sold me a manual for the M1937. The manual did not contain the BTU rating, but I was on the trail.

A converted M1937 fire unit
The converted M1937 fire unit working at peak efficiency. The flame reflected into the camera lens and tinted the picture blue.

On a hunch, I talked to the service technicians at one of the local propane dealers, and they agreed with me that by size alone the unit appeared to be in the neighborhood of 50,000 BTUs. The technicians gave me several gas orifices to try in my conversion. Some came from a Reznor natural gas unit heater they had converted to propane. Note: The hole in a natural gas orifice will be larger than a propane orifice of the same BTU rating because the natural gas is run at a lower pressure than propane.

At this point I had to figure out how to run the propane piping through the air shutter while leaving the shutter in place and functional. The orifice was to be inserted through the tube in the panel the flame valve assembly is normally inserted into. The flame valve assembly holds the air shutter in place in normal use. I had a little advice from some of the technicians, but at this point I was on my own due to the liability. After a few failed attempts, I talked to a plumber I know who suggested using washers and a sleeve inside the flame valve assembly tube to hold the orifice and air shutter in place. This suggestion worked, and I used it in both conversions.

<<= Previous | Next =>>


  1. Converting an M1937 Fire Unit to Propane

  2. Fire Unit Conversion

  3. BTU Rating of the Burners

  4. Supplies and Tools

  5. Conversion Procedure

  6. Flame and Pressure Control

  7. Safety Tests

  8. Cooking with the propane burners

David Jarvela is a native of White Pine, Mich. He currently lives in Hoyt Lakes, Minn. Questions or comments? E-mail him at:

Copyright 2000 by David Jarvela. All rights reserved. Used with permission.

October 2000

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