I wasn't thrilled to have the 'recalled' NT-22 furnace in my '69 Safari:
You can see in the photo above that the rubber combustion tube (cause of the recall) has a gaping hole in it. Many folks have successfully replaced this tube with new material but you're still kinda stuck with a 40 year old pilot light furnace... I decided to lurk on ebay and ended up with a "new" Suburban Nt-30sp from an rv salvage company for 300 bucks, which was about half the cost of a new store-bought one.... Inspection confirmed that the furnace had not been used and was, for all purposes, like new.
After pulling the old furnace I slipped the new one in the existing (larger) box and hooked it up to test. The burner came on for 10 seconds but wouldn't stay on. I emailed the ebay seller with my hunch that the board was bad and he was decent enough to send me a new Suburban board.
That false start killed my momentum and I put the furnace in the corner by the washing machine until my wife gently suggested I needed to get that "thing" installed and out of the house. She's only right about 99% of the time, which confirms she is, in fact, human ;-)
Here's a shot of the space after pulling the old furnace:
That supply line snaking up over the furnace (and it does get hot up there) really creeped me out (but the new furnace allows for "zero clearance" on top, btw) I pulled it. You can see the duct opening at the bottom, leading to fore and aft floor vents.
So I bench tested the new furnace after replacing the board (not hard, just a few plugs and connectors) and it worked fine. I was ready to install. I cut away the top and sides of the old furnace enclosure with my $20 mini grinder from Harbor Freight (best twenty bucks you'll ever spend!) and put two pieces of leftover 17" italian tile to raise the new furnace up to correct level for the intake/exhaust tubes to mate with the two holes in the exterior:
You can also see where I added a strip of aluminum at the top rear of that space to some wood that came really close to the top of the furnace box. Only took a few minutes, "zero clearance" notwithstanding...
Slid the new furnace in and made sure the tubes were properly aligned. I fabricated a new mini-duct out of flashing to move the hot air from the inside-front of the furnace down to the duct opening. It's a bit funky but works fine with a piece of aluminum tape on the right. Secured the furnace to the cabinet with 2" wood screws. The tight fit and security screw on the exhaust vent also stabilizes the furnace to trailer.
The exterior vent cover is a beautiful piece of industrial design, IMHO. It's a tricky little devil to install, though, but a fellow forums user turned me on to this little home-made wire tool that I used to PUSH the spring clip rearward onto the flange, which is the way to do it:
The finished installation (temporary wing nuts until the (lost) ones arrive from Suburban. Nice and toasty inside!