Remarkably, the furnace in my 1967 Airstream travel trailer checked out fine during the 2004 refurbishment, and has been used for the last 16 years without any significant repairs. But after the pilot light started having trouble staying lit near the end of last season, following a checkout I decided it was better to replace the old Suburban furnace with Suburban’s newest, matching model rather than repair the old one.
Although the old & new furnaces are functionally identical, the new unit is shorter and thinner. Also, the old style forced heated air out of a rectangular hole on the bottom into a plenum instead of out of round holes on the sides. Fortunately, the new furnace is backwards-compatible in that it had a blocked-off rectangular hole on the bottom. The Internet sold me cover plates for the round holes.
Another design change over the last half century is that the old furnace slid into its outer housing which is permanently mounted atop the plenum. The new furnace has no plenum.
The housing needed to be removed in order to properly align the new furnace with the existing air intake & exhaust ports. Unfortunately, the enclosure had been placed in the Airstream before the cabinets were installed. Short of removing the cabinet something would need to be cut to remove it. I chose to cut the housing with a cutting wheel on an angle grinder.
Worked great! Unfortunately, I forgot the gas line was near one side. Oops.
After cutting the housing remains off of the plenum, the next step in aligning the new furnace was to make a cardboard pattern of the exterior ports with respect to the plenum.
The plenum was then placed back in the Airstream. The holes in the cardboard were found to be one inch lower than the holes on my American classic. After noting that, the block-off plate on the bottom of the new furnace was addressed.
A one-inch thick board the width of the plenum and depth of the back-of-the plate-on-the-furnace was then screwed to the top of the plenum. The hole on the top of the plenum was then modified to match the dimensions of the outlet hole on the bottom of the furnace.
Individual pieces of sheet metal were then cut & bent both to block off areas of the old hole which were no longer needed and to direct air from the furnace into the plenum.
After firmly mounting both the plenum & furnace in the Airstream, the new work was sealed with aluminum tape.
Although the ports on the old & new furnaces were the same diameter, the distance between them was not. Ovalizing the bottom port with a Dremel tool resolved the issue.
No one will ever see it, though, because the cover plate does its job quite well.
After the old furnace gave me trouble last season it was pulled out and thoroughly inspected. To my surprise, the space beneath the burner was completely filled with an old yellowjacket nest.
Although the nest was partially blocking the burner’s inlet air, it was not responsible for the pilot not staying lit. That problem is probably due to either a bad thermocouple or gas control valve neither of which are available now new. But to guard against future nests, screens were installed over the ports.
The old furnace’s pilot light had to be lit by hand as needed and was accessed by removing the front panel. The new furnace has auto-ignition. Hopefully it will be a long time before I need to remove its front panel.
As hoped, the new furnace ran like a champ after the gas & electrical connections were made. It’s much quieter than the old one even though I never considered the old one a problem.
We’re now ready for cold weather again!
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