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Old 11-27-2019, 10:41 AM   #1
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Question for 29-5T Owners

This is a question for owners or recent model year Arctic Fox 29-5T 5th wheels: does your furnace burner cycle off and on repeatedly before your RV reaches the temperature set on the thermostat? Before you answer, know that this is something that coud be happening without you being aware of it and some careful attention and experimentation may be required to know for sure.

Quoting another post: "We think we found a problem with our 2018 29-5T that other owners of the same and possibly other RVs are almost certainly experiencing and about which they may have no idea. We lived in our RV for over a year before even noticing the symptoms that helped us diagnose and repair the problem.

We noticed that our furnace burner was cycling on and off when we didn’t think it should. When the furnace ignites the burner should remain lit until the RV comes to the temp set on the thermostat. Then the burner should shut off followed a minute or so later by the furnace fan once the unit has cooled off a little. The burner should come on once, stay on, and only shut off when the RV has come to the set temperature. The burner shouldn’t be on-again-off-again, on-again, off-again, etc. while the RV is still in the process of coming to the temp set on the thermostat.

When turned on the fan for our furnace would come on, then a few moments later the burner would ignite, but rather than staying lit until the RV reached the temperature set on the thermostat the burner would cycle on and off every few minutes. The fan remained on but the burner cycled on-again-off-again. Placing our digital meat thermometer’s probe over the register in front of the fridge revealed that the air coming out of the furnace was bouncing back and forth between about 140 degrees when the burner kicked in and 175 degrees when the burner would then turn off. The fan kept running the whole time. One day it occurred to us that it didn’t make sense for the burner cycle on and off constantly like that, but it took us over a year to notice and wonder about this.

Here is why we believe it was happening, why it was a problem, and how we fixed it: We think that Northwood may not have provided ample ducting for the furnace to expel the heat it makes and that this caused the heat to back up inside the furnace which tripped the high temperature limit switch, a safety mechanism that turns the burner off when it is overheating. According to Suburban this switch is not made for frequent cycling and would likely burn out prematurely unless the situation was remedied. So, we added another duct to the furnace to allow it to better aspirate and this solved the problem. Now the furnace burner comes on once and remains on until the RV reaches the temp set on the thermostat at which time the burner shuts off. Once on, once off, as opposed to on-again-off-again, etc.

Suburban requires that 4 of the 11 outlets on the SF-35FQ furnace be used in order to allow it to properly expel the hot air it makes so that the burner doesn’t overheat. Well, Northwood used four (there is one big, rectangular outlet on the bottom of the furnace that counts as four of the circular outlets on the sides and that is what Northwood used), but Northwood also placed 90 degrees bends in the ducting wherever it attaches to the registers in the floor. These 90 degrees bends reduce the airflow and work against the minimum requirement for outlet ducting. Suburban says “Avoid making any sharp turns in the duct system. Sharp turns will increase the static pressure in the plenum area and could cause the furnace to cycle.” That seems to be exactly what was going on in our RV. It would appear then, that our 29-5T and all others like it may be insufficiently ducted. The bends wouldn’t matter if there were enough ducts to allow the furnace to aspirate sufficiently, but there aren’t. For sure, other things like damaged ducting, crud or critter nests in the ducting can cause the furnace burner to overheat and cycle as ours was doing.

Do you have a 29-5T (or any other RV for that matter)? Run your furnace for 20 minutes or so with a thermometer over one of the registers. Listen carefully to see if you can hear the burner turning on and off [the fan remaining on] and make note of the thermometer readings. If your burner turns off and on repeatedly your furnace may be headed to the repair shop prematurely."

So, I wonder if other owners of late model 29-5T fivers are having the same issue or if it is isolated.
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Old 11-27-2019, 10:44 AM   #2
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How close to the furnace is the thermostat?
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Old 11-27-2019, 10:49 AM   #3
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Regardless of Brand/Model of furnace....

One only needs to stand next to furnace intake/exhaust and listen if furnace is 'limiting'------operating on the High Temp Limit Switch

(Lack of airflow thru heat exchanger)
*Room Air Wheel loose
*Exhaust Port obstructed
*Overfiring----orifice or high LP pressure
*Bad High Temp Limit Switch
*Blocked Return Air
*Discharge obstructed, insufficient ductwork
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Old 11-27-2019, 11:00 AM   #4
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Quote:
Originally Posted by Old-Biscuit View Post
Regardless of Brand/Model of furnace....

One only needs to stand next to furnace intake/exhaust and listen if furnace is 'limiting'------operating on the High Temp Limit Switch

(Lack of airflow thru heat exchanger)
*Room Air Wheel loose
*Exhaust Port obstructed
*Overfiring----orifice or high LP pressure
*Bad High Temp Limit Switch
*Blocked Return Air
*Discharge obstructed, insufficient ductwork
I am trying to find out if this is a common problem with 29-5T fivers. If it is then it may point to a design or manufacturing flaw.
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Old 11-27-2019, 11:24 AM   #5
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Quote:
Originally Posted by 450Donn View Post
How close to the furnace is the thermostat?
Thank you for that. My purpose in this thread, however, is not to troubleshoot a problem but rather find out if other owners of the same make and model have or are experiencing the same issue.

That said, it seems as if you are wondering if the thermostat is causing the burner to cycle. That's not what caused this problem. The thermostat could be set at 75º, for example, but showing the temp inside the RV to be 65º, so it wouldn't be shutting down the burner before the RV reaches 75º. In fact, the thermostat shuts down the furnace at the temperature to which it has been set. There is no indication of a malfunction with the thermostat or that it is shutting down the burner because it thinks the RV has come to temp.

The problem was for me that the burner was overheating and tripping the high temp limit switch because the furnace couldn't expel the heated air adequately. Installing another outlet duct fixed that problem.

Again, I am merely asking other 29-5T owners if they are seeing similar behavior with their furnaces because if they are it may point to a manufacturing issue. I am not trying to troubleshoot a problem in my RV, one that I have already dealt with successfully.
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Old 11-27-2019, 12:55 PM   #6
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Can you share pictures of your fix so that other owners, if faced with this issue, can do the same?


Thanks!



//KMac
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Old 11-27-2019, 01:34 PM   #7
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Originally Posted by kmac7800 View Post
Can you share pictures of your fix so that other owners, if faced with this issue, can do the same?


Thanks!



//KMac
I would be happy to but I'd prefer to do that in another thread and keep the focus here on the question in the original post.
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Old 11-28-2019, 01:43 PM   #8
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Originally Posted by kmac7800 View Post
Can you share pictures of your fix so that other owners, if faced with this issue, can do the same?


Thanks!

//KMac
Here you go...
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