Suburban furnace question

[Charles in SC]

Hello all, I have been installing a Suburban SF-30 propane furnace and cannot find anything in the destructions that says what the output ducts need to be made of other than how much free flow area it needs. I seem to remember from a couple of s&s units that I have had in the past that the ducts were just clothes dryer type duct. Does anyone here know if that is correct?
Thanks in advance!

[Lin]

You might want to go a step up from the cheapest dryer exhaust duct, which seems to be very thin, white plastic.  There is foil covered flex duct easily and cheaply available.

[pvcces]

Charles, I think that your local RV store has what you want in stock, if they sell the furnaces. Dryer duct is light duty, but on the other hand, it is made to be used with 150 degree air temperatures, so it might not be a problem.

Your furnace has a high limit cutoff to reduce the risk of fire, so I doubt that it is a big deal. Do you have the furnace to hose adapters already? If you don't, the RV store should have some that will fit. They also should have several kinds of registers that will make the installation very simple.

Good luck.

Tom Caffrey

[niles500]

Ductboard is quieter, insulated ..... and yes, more expensive, harder to fabricate/install - FWIW

[JohnEd]

Every time you make a 90 degree turn you cut the flow thru the pipe by 30% or some very large number.  Nick would know.  For every so many feet of pipe you diminish the flow by X percent.  There is no way to get around that flex white duct but if you can put so;id galvanized heater pipe in a spot....do it.  The clamps are cheap and you can even use tie wraps.  Don't insulate the pipes/duct and it will warm the areas in which it is run...back of cabinets and under bunks etc.  The runs cannot be of more than a certain length if metal and shorter if flex.  See the manual and talk to RV installers.  More than one hopefully and still you have no iron clad guarantee that the info is correct.

You need to use all the ports in the furnace.  If you can only exhaust into the living room with the front ports then run both into the living room.  Adjust air flow with the register.  The more air you have flowing over the heat exchanger the more efficient the furnace will be and the quieter by a little.

There are two safety thermostats.  One checks the temp of the heated air and the other checks the temp of the heat exchanger.  Air to slow and the temp of air and exchange will both go high and trip.  This causes the furnace to cycle like it had reached temp and the fan runs on after gas shutdown so it cools the furnace.  Problem is that those safety devices are not intended to operate at that rate and fail early.  Keep your air flow up.  For a idea of how efficient these furnaces are just go outside and feel the temp of the furnace exhaust.  Be nimble cause it won't take all that long to get the idea and insufficient air will up that temp....although that won't be a problem.

Pay attention to the part of the book that describes the size and color of the flame.  I think the furnace is the most critical propane device in your inventory.  You should, IMHO, set your gas regulator for proper furnace flame....then re jet the other stuff to optimize their individual performance at that pressure.  If the gas range is to low just drill out the gas orifice with a teensy weensy drill bit and adjust the new flame with the knob.  Been going strong for 18 years and adjusting for the furnace solved problems.