RE: [VACList] Furnace Repairs

[RJ.Dial@xxxxxxx]

Hi Rick,
Depends on which furnace you have.  If it is the Suburban NT-20 that 
resides on the floor in your cabinetry, with ducts that then provide 
heat put to small plastic grills and has two blower motors, by all 
means have it replaced with a new unit - they fit in almost the same 
space with almost the same hook-ups.  Those early sealed combustion 
units gave people fits.

On the other hand, if it is one of the passive heat Utah Hydro's,
http://vintageairstream.com/archives/%2768GT2.jpg

those units are fully repairable/restorable, and add value to the 
trailer.

If by chance it is a desirable Utah Hydro, a couple of items in your 
post jump out:

> The furnace in my '66 GlobeTrotter is mostly original. I 
> can't tell the
> brand. The controls and the thermostat are not original 
> because they don't line up with the holes in the cover. 
Someone may have adapted a new control module, which is OK, but it has 
to be matched for the burner size and type of thermocouples used.  A 
correct installation will also result in the pilot and thermostat 
control knob to line up - someone may have did a do-it yourself special.

> The chimney seems to have been a double
> walled pipe, the exhaust being the inner pipe and fresh air 
> coming in the
> larger pipe. The outer pipe is completely missing and the 
> inner pipe is in
> bad shape, but still works.
> I was planning to rebuild the chimney/vent, but recently 
> another problem
> developed. The furnace now burns dirty which causes the outer 
> skin, over the
> exhaust, to turn black. 
This is caused by not enough air supply/too much LP flow to the burner. 
 It may be because of that replaced control being the wrong type, or 
may have developed an internal leak.  The inner exhaust/outer fresh air 
supply tubes are actually an engineered component. The hot exhaust air 
creates a pulling draft for the incoming fresh air.  You should restore 
that design before going further. You can see a typical install in the 
install manual for its baby brother Hydroflame:
http://airstream.org/members/documents/apmanuals/HydroFlame.pdf

> I cleaned the burner, which was dirty with rust in
> some of the ports, but it didn't help. 
Rust on a burner is OK, as the act of burning LP will produce water. 
Most were designed to rust effectively without altering performance or 
safety.  But you do need all the ports open - plugged ports will cause 
sooting. What you do not want to rust is the burner can!  This can 
occur if rainwater is allowed to go in/down the exhaust/inlet pipes.  
Any thing more than surface rust (loss in thickness), should have a new 
burner can fabricated.

> I can't find any adjustment on the
> controls other than the pilot adjustment. I hear a slight 
> hissing from the
> controls/burner area which I didn't hear when the unit was 
> burning clean.

There is only the pilot adjustment on these controls. The pressure 
regulation and on/off diaphragm regulation is fixed by design.  You 
could have a diaphragm  failure, and the only remedy is to replace the 
control assy.  Small appliance parts stores are the best bet.  There 
are two mfr's that have units that adapt to RV passive heat LP 
furnaces.  (Being here at work, I can't remember the names).

There are two Airstream manuals on the VAC website with info on the 
Suburban NT-20 and the Hydroflame:
http://airstream.org/members/documents/tlmanuals/%276626%27OverlanderOwnMan.pdf
http://airstream.org/members/documents/tlmanuals/65CaravelManual.pdf

As 
with any combustion device, always verify annually that the combustion 
chamber and exhaust flues are 100% sealed tight, and always leave a 
window slightly open for ventilation.

Best of luck,
RJ