Surbuban Furnace

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Lowell

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Aug 15, 2005
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Tempe, AZ
While camping this past week, we were able to stay warm but it seemed that the furnace was not putting out as much air volume or as warm air as recall from past trips.  The furnace started fine and ran until the thermostat shut it off upon reaching temperature.  I plan to take the inside cover off the furnace tomorrow to see it I can detect anything.  The floor grates have no adjustment and are full open.  I think it is only a one speed blower.  I read the library furnace notes but didn't see anything that jumped out at me.
 

Alfa38User

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Oct 4, 2007
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6,676
You obviously had enough volume on the combustion air side for the furnace to start or else the sail switch would have prevented it for lighting. However, you may have a detached vent or some blockage in the heated air side but not enough to slow the fan substantially. (There is 1 single speed fan motor with two turbines on the shaft, 1 for combustion air and 1 for hot air circulation).
 

bailer6334

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Mar 13, 2010
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Prescott, Arizona
I just had the same problem. Up to recently everthing was fine. This last trip I felt not enough heat coming through the ducts. Had the unit into the dealer to check brakes, bearings, so I asked them to check the furnace output. They told me several of the ducts had come loose and alot of air loss from other holes. Cost me $80 to have it fixed, but feel it was well worth it.
 

Gary RV_Wizard

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Feb 2, 2005
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At our Silver Springs FL home
Since the furnace runs, you can be sure the motor is working and at its proper speed because there is a safety device (the sail switch) that verifies that before combustion takes place.  The furnace fan is single speed and the same motor powers the combustion air intake/exhaust and circulation air.

Check the air return and ducts for blockage or loose connections. Or crushed air tubes if they are the usual flexible hose type. One possibility is that critters, e.g. mice, have made a nest in the ducts, blocking air flow. You can disconnect the duct tubes at the furnace and check air flow there, in and out. If you have good output there, you know the inlet is ok. If poor there, then work on the inlets.

Another possibility is that one or more duct outlets is closed or broken. The small round heater outlets are notorious for that, so check each one if you have that type.
 

Lowell

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Aug 15, 2005
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Tempe, AZ
I pulled off the duct covers and used a flashlight and mirror to check the long duct that runs down the center of the TT.  Everything I could see was clear.  I can't see into the short leg of the duct that goes from the furnace to the long duct.  I pulled off the cover of the furnace and didn't see any issues. Everything was very clean.  With the cover off (and gas off), I had my wife turn on the furnace and the blower was certainly moving a lot of air. 

So maybe there isn't anything wrong except my memory. I just think we could feel more air moving and warmer air in the past.  It did keep us warm an I know it got down into the 20s.  But it ran forever.

Thanks to all who commented.
 

Gary RV_Wizard

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Well, if you have good air flow near the furnace but not so much coming from the outlets, the problem has to be somewhere in between. Could a duct be crushed somewhere, e.g. behind a cabinet or drawer? Or the outlet itself partially closed?
 

Lowell

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Aug 15, 2005
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Tempe, AZ
Gary, the only part I couldn't check was a 1-2 ft. section of duct that goes from under the furnace to the long duct that runs under the floor of the trailer.  I don't have any way to access or view that part of the duct work short of removing the furnace itself.  That "T" intersection is under the floor and about 3-4 feet from the closest grate.  Looking under the TT, all duct work is covered by a fabric panel that runs from frame side to frame side. I hate to cut into that.  When I said I had good air flow, what I meant was with the cover panel off the furnace(not the decorative inlet panel but the actual furnace panel so that I could see the heating sections) the air flow from the blower was quite good but I was just feeling the air blowing around the heating sections.

I think I will pull these panels off again and see if I can detect the amount of cool air inflow into the furnace. 
 

Lowell

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Aug 15, 2005
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Tempe, AZ
Maybe a sketch of the layout would make things more clear.
 

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Lowell

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Aug 15, 2005
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Tempe, AZ
Well, I checked out everything again and it seems to be working, just not putting out as much air as I remember.  ;D  I'm not positive it was putting out more air in the past but my wife & I both think so.  But as long as it keeps us warm, I don't think I will do anything else for now.

Thanks for all the comments.
 

kenz

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Jan 1, 2012
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199
Location
Michigan
It could be possible that the motor isn't running as fast as it should be, but still moving enough air to satisfy the sail switch. The switch is there only to make sure there is enough air flow to purge any fumes from the fire box before ignition, and to make sure the fire box stays cool. Sounds weird, but if there was no air flow the burner would get so hot it would start things on fire. Should be able to feel a noticable amount of air coming from the floor vents when the furnace is running.

Another possibility is the propane tanks. Tanks need to be purged of air and moisture when new, and few if any are actually done (most dealers sell the trailer with full propane tanks, but won't spend the money to have them purged). Then they need to be purged every so many years to eliminate moisture/condensation from the tank. hard to explain but basically it lowers the BTU's and the propane doesn't burn as hot. Check by lighting a burner on the stove. The flame should be a pretty blue color with none or very little yellow on the very tip of the flame. If it's mostly yellow with hardly any blue than a purge should be done. This was explained to me by my local propane guy. My flame was pretty much all yellow. After being done I did notice a difference in how the furnace, water heater, and stove run. The furnace air seemed hotter, and my gas use went down a little. And the cost at the time was only $3 a tank. Not sure if this may be the fix, but cheap enough to try.
 

Jammer

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Dec 20, 2009
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1,491
With squirrel-cage blowers driven by DC motors the airflow is proportional to the square of the voltage applied.  So a fairly slight voltage drop can cause a considerable change in airflow.

Often, there is a noticeable change in airflow when connecting to shore power after running on battery for a time, because the voltage jumps from around 12.6 to around 13.6.  So that may be what you remember.  Or there may be wiring problems.  You can measure the voltage at the motor to find out.  It should be 13.0 or higher with shore power.

Also the bearings usually are the first to go in these things and can result in an RPM drop.

So those are some possibilities.

But if the air seems cooler then probably you have an air leak somewhere in the ducts.  In RV ductwork the air cools remarkably quickly along the length of the duct as heat is lost through the duct wall to the surrounding air.  The temperature drop is greater when there's less airflow.  Could be mice, a connection that pulled loose, tape that came off, whatever.  There's really no substitute for pulling the ducts out and looking at them if you think there's a problem.
 

Lowell

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Aug 15, 2005
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Location
Tempe, AZ
Thanks for all the comments.  I put the TT back in storage so it may be a while before I do any more checking.  In general, we only use the furnace a couple of nights a years, usually late October in the high country.  When we were observing this, we were plugged into 30 amp service.  And when I plugged into 30 amp service at home and took the face off the furnace, the blower was moving a lot of air.  The long section of the duct is certainly open as I can see from one end to the other with a mirror and flashlight.  The short section of the duct from the furnace to the long duct  is not visable to me and the only way to get access would be to tear off the cover under the TT.  I'm not ready to do that.  If there is a leak in that duct, perhaps it would be indicated by some heat or billowing of the cover under the TT.  I may go out to the strage unit in a couple of months when it gets cold in in AZ and see if I can detect anything.
 

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