Cleaning and Refurbishing My Suburban NT-35K Furnace

[Steve Bryant]

This thread is to document how I cleaned and refurbished the Suburban NT-35K furnace on my 1991 Airstream. This is not a how to thread, it’s merely meant to supplement the information found in the Suburban Dynatrail Furnaces Service Manual, Suburban Manufacturing Company Product Presentation document on RV Furnaces, the Suburban NT-35K Exploded Parts Diagram and other documents that I’ve located on the internet; many of which are available from Bryant RV Services (see http://bryantrv.com/docs.html for links to many valuable manuals on RV appliances). Note: I am not affiliated with Bryant RV Services in any way … the name of their company happens to be my last name/surname.

Suburban Manufacturing Company no longer supports this furnace as it has been out of production for a number of years. My furnace has very few operating hours on it. It had some superficial rust on it and dirt daubers/mud daubers/mud wasps had decided to make it their home. I needed to evict them and remove the evidence of their former habitation!

Disclaimer: You must ask if you are qualified to service any appliance safely! This is true for electrical as well as gas appliances or any other device used on an RV or any other vehicle. Your safety and the safety of those around you are of paramount importance!

I am a former diesel truck mechanic and a retired electrical engineer (also aviation electronics and instrument repairman). I have done a number of projects of this type in the past. I want to show you my approach to this working on this furnace. This is the basic approach I use to repair anything, although I seldom document things as well photographically unless it’s for a forum or something of the sort.

[Steve Bryant]

First, I recommend that you read this post by catson4 http://www.airforums.com/forums/f427/suburban-nt35k-furnace-update-as-requested-by-peter-forwood-48103.html#post665213 and take a look at this thread http://www.airforums.com/forums/f427/ignitor-carbon-poor-heat-73628.html.

My symptoms were that the fan motor was initially badly out of dynamic balance (vibrated like a son-of –a-gun, then smoothed out within a few minutes) and the igniter gap closed up with soot quickly after cleaning on two subsequent days. The soot is evidence of a rich air to fuel ratio/mixture (too much fuel or insufficient air/oxygen) and the vibration that went away was because the mud daubers had a bunch of mud on the combustion squirrel cage that cleared itself after vibration (more to come).

[Steve Bryant]

Let’s begin. After removing the furnace, I took a bunch of pictures before and during disassembly to document how the unit went together, physical condition, etc. I also had previously printed the Suburban Maintenance Manual, Parts Diagram, etc. and placed in a notebook for reference during disassembly.

[Steve Bryant]

Getting to the heart of the matter, here are the former dwellings and deceased descendants of my friends the mud daubers (or dirt daubers as we called them in my native East Tennessee).

[Steve Bryant]

After disassembly of all the major parts, I cleaned the heat exchanger, burner assembly, squirrel cage blowers (main air fan and combustion fan) using brushes, shaking/rolling/etc. of the heat exchanger and compressed air to get all the clay and insect corpses out. Note: After I evaluated the heat exchanger integrity, I took it to a reputable RV repair facility and had them evaluate the heat exchanger. They concurred that it was in good shape.

[Steve Bryant]

I evaluated and reused two gaskets (here’s the combustion fan cover gasket that I reused as an example).



I was able to buy a couple of the correct gaskets for this furnace and modify a couple of currently available Suburban gaskets for the induction tube running between the combustion fan and the beginning of the combustion chamber. These gaskets were round foam gaskets of the proper diameter (ID and OD) and intended for the same purpose. I only had to trim them and make a couple of new screw holes and glue them on with copper-impregnated gasket adhesive.





The third gasket I had to make from scratch out of some “Mr. Gasket” brand exhaust manifold sheet material from an auto parts store.

[Steve Bryant]

Then I prepped and painted the cabinet and a bit of the heat exchanger. First, I wire brushed and cleaned with a paint prep solvent (Imperial Cleanwood shown in black and yellow can in one of the pictures below). Then I painted the rusted cabinet and a small area of superficial rust on the heat exchanger as a preservation measure).

The black paint is Eastwood Exhaust Manifold/Header Paint (used minimally to maintain as much heat exchanger efficiency as possible).




I sprayed the cabinet inside and out with Rustoleum brand paint as shown.


I let the paint cure for two days and then I tried to do some heat cure to the heat exchanger with my heat gun. This was fairly ineffective because the paint still outgassed a good bit when I operated the furnace. I stayed out of the trailer for a day except for occasional checks. Then I shut down the furnace and opened a couple of windows while running my Fantastic Vent fan overnight to clear the air.

[Steve Bryant]

During reassembly, I referred to the Suburban Maintenance Manual, Illustrated Parts Diagram and my previously taken pictures that were now on my laptop. As an aside, I use my table saw as a work bench much of the time.



I used a 1/8” drill bit to gage and set the recommended spark gap.

[Steve Bryant]

As I was reassembling the furnace, I also tied out the harnesses better than they were originally to straighten them up and to avoid the possibility of future chaffing. Here, I used a black wire harness lacing cord that was a staple in aircraft wire harness construction forty years ago (prior to the adoption of zip-ties/tie wraps). I have some of the cord and I like to use it sometimes, although it’s no better or worse than the plastic zip-ties.

[Steve Bryant]

At the end of the day, an ounce of prevention is worth a pound of cure. I installed mud dauber screens on all three portals.

[Steve Bryant]

Here’s the Suburban back in its home. Note: had the cabinet not been rusted, I would have only had to remove one 5/16” head sheet metal screw from the lower left front of the cabinet (below the gas shutoff valve) and I could have pulled the innards out of the furnace after shutting off and disconnecting the gas and electric wires.

[ROBERTSUNRUS]

Hi, nice job Steve. Looks like it was mostly clean and paint, no major problems.

[HiHoAgRV]

Great thread!!!!!

[Steve Bryant]

Bob and Vernon,
Thanks very much. I had fun doing the thread. This whole project didn't take very long and I planned to do the thread while I was doing the work on the furnace. Then, I just compiled the pictures that I wanted to use and wrote the whole thing out in Microsoft Word (including the image links) and just pasted the posts into this thread.

I really like Air Forums very much! I've been active on several other fora over the last several years and I'm a fairly inactive moderator on one forum. I really like the spirit of sharing here and the lack of one oneupmanship stuff that can and does go on in some other fora.

All my best,

Steve

[Steve Bryant]

This is kind of a postscript to my thread. I wasn't pleased with the way the furnace mated to the combustion inlet and outlet pipes as the furnace was about a quarter of an inch below the mating (female) tubes attached to the flanges on the exterior skin of the trailer. Also, the LP Gas supply for the stove was routed beneath the Zip Dee chair storage compartment along with a wire cable which didn't give enough top clearance to the furnace.

I re-routed and the gas feed and wire cable to the interior side of the Zip Dee Chair storage compartment and clamped them securely in place. Then I removed the carpet from the sub floor beneath the furnace along with any remaining staples, etc. I made some sample shims that were 3/8 of an inch thick and temporarily put them beneath the furnace and that seemed about right. Then I riped a couple of slabs off of a 2X4 about and milled them to 7/16" thick using my jointer and thickness planer. I experimented with the thickness and found that 5/16 was just right. I then screwed some side rails onto the 5/16 thick slabs to act as alignment guides for the furnace and allowed about 1/8" clearance on each side of the furnace. Now I can slide the furnace home easily and I'm not putting any strain on the intake and exhaust tubes.

[dryenko]

Where did you obtain the mud dauber screens, and are they stainless steel?
Thanks BC

[Steve Bryant]

BC,
I bought mine at a local RV dealer (same folks who inspected my heat exchanger). They are a Camco brand which I used before on an Alumalite trailer that also had a Suburban furnace. Here's a link to the type of screens Amazon.com: JCJ M-300 Mud Dauber Screen for RV furnace Outside Fitting: Automotive. Camco and some other suppliers make a wide variety of these screens.

[goransons]

Thanks Steve for this thread, I've decided to do the same on our NT-30, ordered my parts today after a tear down. Your pics REALLY helped!

[0557]

I have 2 of those furnaces in my 92 limited.
Both are running but the motors are squeaking in both units. I was thinking about lubing them when I remove my units
For a refurb as well, or should I replace them. Nice pics of your work btw.

[Steve Bryant]

Goransons and 0557,
Thanks for the kind words! I really wanted to take the extra time to document the work in pictures and words so that others could hopefully have an easier time. In the end, it probably helped me to stay organized and the disassembly photos really helped in the reassembly.

On the motor squeaks, that sounds like a bushing squeaking. These motors have permanent "lifetime" lubrication. Also, I only think that a few sources have replacement motors (as I recall). I think that I would try to find someone who had new motors and get the price for a pair plus shipping. Then, I'd find an electric motor shop that could press on new bushings and get a price for that in order to make my decision.

Steve