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Old 11-20-2019, 10:58 AM   #1
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Adjusting Furnace Burner?

I've discovered a problem with my Suburban 35-FQ RV furnace. Actually, the problem seems to be with the ducting which appears to be insufficient and causing the burner to cycle--the furnace cannot aspirate as needed so the burner overheats, then the safety mechanism shuts down the burner (fan keeps running) until it cools off and the burner fires up again, the burner cycling off and on every couple minutes even while the RV temperature is well be low what has been set on the thermostat.

A Suburban tech said this shouldn't be and that it will wear out the safety switch that shuts down the burner when it gets too hot. It appears the furnace is under-ducted and cannot aspirate sufficiently causing the excessive burner cycling.

One fix would be to add additional ducting to allow the furnace to better breathe which so far is my plan, but I'm wondering if there might be a way to make the burner less hot via some adjustment mechanism somewhere so that adding ducting might become unnecessary. I rather expect not but thought I would ask. (It is not easy to get a Suburban tech on the phone which is one reason I am asking here.)

The SF-35FQ, BTW, must be installed and removed from inside the RV and it looks like it might be necessary to remove the refrigerator which sits just above it in order to do that... not sure.
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Old 11-20-2019, 11:13 AM   #2
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Propane flow is FIXED.....based on LP SYstem Pressure of 0.4 psi (11"WC), furnace burner and size of orifice used


Several things can cause 'limiting' (operating on the High Temp Limit Switch)
*FAN .....not moving sufficient air thru heat exchanger (loose air wheels/broken vanes/nests)

*Intake/Exhaust Tubes obstructed (Mud Dabbers etc)

*Return Air blocked (Items stored/placed in return pathway)
*Discharge ducts (not enough ducts/crushed-collasped ducts/too many turns etc)
*Overfiring (HIGH LP System Pressure..Regulator bad or out of adjustment)



Here is the Installation Manual for your Suburban Series
Note: FOUR 4" ducts are minimum requirement
http://techsupport.pdxrvwholesale.co...ions-SF-FQ.pdf
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Old 11-20-2019, 11:27 AM   #3
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Same issue with my Fleetwood, and I've had the ductwork "fixed." I believe I might try smaller orifices to see if that helps. No adjustment you can do, just swapping orifices.
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Old 11-20-2019, 11:48 AM   #4
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Same issue with my Fleetwood, and I've had the ductwork "fixed." I believe I might try smaller orifices to see if that helps. No adjustment you can do, just swapping orifices.

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Old 11-20-2019, 11:54 AM   #5
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Quote:
Originally Posted by Old-Biscuit View Post
Propane flow is FIXED.....based on LP SYstem Pressure of 0.4 psi (11"WC), furnace burner and size of orifice used


Several things can cause 'limiting' (operating on the High Temp Limit Switch)
*FAN .....not moving sufficient air thru heat exchanger (loose air wheels/broken vanes/nests)

*Intake/Exhaust Tubes obstructed (Mud Dabbers etc)

*Return Air blocked (Items stored/placed in return pathway)
*Discharge ducts (not enough ducts/crushed-collasped ducts/too many turns etc)
*Overfiring (HIGH LP System Pressure..Regulator bad or out of adjustment)



Here is the Installation Manual for your Suburban Series
Note: FOUR 4" ducts are minimum requirement
http://techsupport.pdxrvwholesale.co...ions-SF-FQ.pdf
Thanks!

I already have the installation manual, and I'm aware that at least 4 of the 11 ports on the furnace must be used but more is better, and that Arctic Fox used only four, and that the ductwork has 90 bends where it enters the living compartment which reduces the air flow to what may be below the minimum required.

The furnace fresh air intake and exhaust outside looks clean and unobstructed. The return air path around the furnace appears unobstructed. I have not checked the regulator nor do I have the means by which to do so but it is relatively new and I am not experiencing any problems of which I am aware with any of the other gas appliances including the fridge, water heater, stove, oven or catalytic heater. I realize the problem could be something other than the ducting, one of the things you mentioned perhaps, but the most likely candidate would seem to be insufficient ducting.

Being that there is no adjustment for the burner I'll probably install another duct and see if that fixes the problem unless another test or fix becomes available to me.
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Old 11-20-2019, 12:31 PM   #6
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Originally Posted by RussOnTheRoad View Post
Thanks!

I already have the installation manual, and I'm aware that at least 4 of the 11 ports on the furnace must be used but more is better, and that Arctic Fox used only four, and that the ductwork has 90 bends where it enters the living compartment which reduces the air flow to what may be below the minimum required.

The furnace fresh air intake and exhaust outside looks clean and unobstructed. The return air path around the furnace appears unobstructed. I have not checked the regulator nor do I have the means by which to do so but it is relatively new and I am not experiencing any problems of which I am aware with any of the other gas appliances including the fridge, water heater, stove, oven or catalytic heater. I realize the problem could be something other than the ducting, one of the things you mentioned perhaps, but the most likely candidate would seem to be insufficient ducting.

Being that there is no adjustment for the burner I'll probably install another duct and see if that fixes the problem unless another test or fix becomes available to me.

Therein lies your problem
4 ducts are minimum but those 90* turns are a choke point.....especially when using 'flex duct'
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Old 11-20-2019, 12:47 PM   #7
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Therein lies your problem
4 ducts are minimum but those 90* turns are a choke point.....especially when using 'flex duct'
Yes, 4 is the minimum but Suburban recommends more.

It appears the ducting used by Arctic Fox in my RV is rectangular rigid metal (as opposed to round flexible tubing) except, as I have noticed, for a round flexible piece that comes off the metal ducting to the bedroom and goes down to the holding tank compartment. It's where the metal ducting enters the living compartment that the air takes a 90 turn. I believe this 90 bend occurs at all 5 of the floor registers. The only outlet port on the furnace that was used by Arctic Fox in this installation is the one on the bottom of the furnace which counts as 4 of the round ports found on the sides of the furnace, however, as noticed, there are 90 bends at all of the registers which I think probably reduces the effective venting to less than the 4 required.

One of my chores today is to continue to explore installing additional ducts. I have one plan I think I may be able to pull off myself and another as well, but both bear additional scrutiny before I embark on the project or hire it done.
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Old 11-20-2019, 12:51 PM   #8
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Quote:
Originally Posted by RussOnTheRoad View Post
I've discovered a problem with my Suburban 35-FQ RV furnace. Actually, the problem seems to be with the ducting which appears to be insufficient and causing the burner to cycle--the furnace cannot aspirate as needed so the burner overheats, then the safety mechanism shuts down the burner (fan keeps running) until it cools off and the burner fires up again, the burner cycling off and on every couple minutes even while the RV temperature is well be low what has been set on the thermostat.

A Suburban tech said this shouldn't be and that it will wear out the safety switch that shuts down the burner when it gets too hot. It appears the furnace is under-ducted and cannot aspirate sufficiently causing the excessive burner cycling.

One fix would be to add additional ducting to allow the furnace to better breathe which so far is my plan, but I'm wondering if there might be a way to make the burner less hot via some adjustment mechanism somewhere so that adding ducting might become unnecessary. I rather expect not but thought I would ask. (It is not easy to get a Suburban tech on the phone which is one reason I am asking here.)

The SF-35FQ, BTW, must be installed and removed from inside the RV and it looks like it might be necessary to remove the refrigerator which sits just above it in order to do that... not sure.
My Dutch star you can remove the unit all from the out side------ remove inside grill under refrigerator and 2 or 3 screws thru the metal housing into floor the rest from outside.
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Old 11-20-2019, 08:09 PM   #9
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My Dutch star you can remove the unit all from the out side------ remove inside grill under refrigerator and 2 or 3 screws thru the metal housing into floor the rest from outside.
That may be true for your Dutch Star but Suburban makes a line of furnaces that are installed and removed from the inside only. My RV has one of those. There is no way it could be removed from the outside without cutting a hole in the RV.
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Old 11-26-2019, 01:42 PM   #10
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Solution Found

UPDATE TO MY ORIGINAL POST:

Basically, I came to conclude that unless my RV had some obstruction(s) in the furnace ducting I didn't find that it was insufficiently ducted at the factory, and that because it could not sufficiently vent the hot air it was making into the living compartment of the RV its burner was overheating causing the high temp limit switch to shut down the burner until it cooled of at which time it would reignite. This created a perpetual on-again-off-again cycle that would occur as long as the furnace was in use.

So, I installed another duct to in order to see if that would cure the problem. It did.
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Old 11-26-2019, 02:01 PM   #11
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Thanks for the Update and your resolution!!


Hot air has to MOVE.....
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Old 11-26-2019, 02:15 PM   #12
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Quote:
It appears the furnace is under-ducted and cannot aspirate sufficiently causing the excessive burner cycling.
Last winter I was kind of paying attention to the front furnace on my Southwind and I wondered if it was doing this same thing. It seemed to cycle off-on long before the temperature was up to the set point.

ANYWAY, A few weeks ago while I had all the drawers out and cupboards open, I noticed how easily the furnace louver vents could be closed without even touching them.

I decided to remove the adjustable louvers out of the vents so they are wide open all the time and cannot be adjusted. I'll pay close attention to it this winter and if need be, add another duct vent.

NOTE - I was going to remove all of the adjustable louvers, even from the rear furnace. However, the louvers for the rear furnace are not adjustable and are wide open all the time.

With my new ECC unit, I can set the set points precisely and monitor what the temperature is.

Waiters ECC – Replacement for the Intellitec ECC – 1999 Southwind 35S

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Old 11-26-2019, 02:21 PM   #13
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Last winter I was kind of paying attention to the front furnace on my Southwind and I wondered if it was doing this same thing. It seemed to cycle off-on long before the temperature was up to the set point.

ANYWAY, A few weeks ago while I had all the drawers out and cupboards open, I noticed how easily the furnace louver vents could be closed without even touching them.

I decided to remove the adjustable louvers out of the vents so they are wide open all the time and cannot be adjusted. I'll pay close attention to it this winter and if need be, add another duct vent.

NOTE - I was going to remove all of the adjustable louvers, even from the rear furnace. However, the louvers for the rear furnace are not adjustable and are wide open all the time.

With my new ECC unit, I can set the set points precisely and monitor what the temperature is.

Waiters ECC – Replacement for the Intellitec ECC – 1999 Southwind 35S

..
Sure sounds like your RV has the same problem.

My Arctic Fox 29-5T came from the factory with 5 floor registers and if I recall only two of them had louvres that could be used to restrict the air flow from them. When I asked why only two and not all five I was told that it could lead to a situation whereby the furnace could have the outflow restricted too much.

IMHO, if an RV maker creates a situation whereby the owner could accidentally over-restrict the venting of the furnace then there is a problem in the manufacture of that RV.
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Old 11-26-2019, 02:30 PM   #14
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Quote:
Sure sounds like your RV has the same problem.
I think so. I'll see if removing the adjustable louvers solves it. They were very restrictive, even when wide open.
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