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Old 10-07-2014, 05:04 PM   #1
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Suburban Furnace Shuts Off

I’m hoping someone can help with my furnace problem.

The furnace has begun to cycle off on what I assume is high limit and will only run again when the t’stat is manually cycled to “off” then back to “heat.”

I have determined that when the door to the furnace is removed and allowed to blow directly into the cabin Vs the ducts, it runs as designed.

I have:
Cleaned the burner and inside of the heat exchanger
Replaced the high limit switch
Cleaned the fan
Confirmed no obstruction in the intake or exhaust vent(s)
All ducts are clear and unobstructed
Air flow seems to be the same as it’s always been
Replaced the propane regulator

With the furnace door on and air forced through the ducts, I get a discharge temp at the floor register of ~ 175°. Not sure if this is normal or too high. I do see where the vinyl flooring at the furnace door has been heated to the point of discoloration and has curled up.
Would a low voltage cause a problem such as reduced CFM or overcurrent? Seems to be normal air flow and the voltage is ~12VDC while the fan is running. (Shore power)

Can’t seem to find anyone with enough knowledge of these symptoms. They are parts changers. It would seem to me that if the unit works, it can be made to work properly without changing out the entire unit.

I would really appreciate any suggestions.
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Old 10-07-2014, 05:42 PM   #2
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Firdt it is not the hi Limit sw as that would cycle on it's own with no resetting of the t stat. How long does it run? Also which model is it because they wire some of them different? 1st eliminate the thermostat on the side of the furnace should be a 3 prong plug with 4 wires one red one yellow and two blue connect the 2 blue wires together the furnace should now run until you unhook them. If the furnace works then your issue is in the t stat not the furnace. If it runs a while and quits then we start testing. This is where model info comes in handy. As in does it have a fan relay on the board or a separate one? If it is on the board then the only things you can check are hi limit and sail sw. to start but also you need to know if the blower motor is getting to full RPM. If the motor slows down the sail sw can open and that would shut it off. However if it has external fan relay as long as the t stat called for heat the fan would continue to run. The board will not attempt to start or run if the sail and limit sw are not correct. One thing I have seen in older units is the motor slows down as it warms up which opens the sail and shuts off the heat.
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Old 10-07-2014, 06:22 PM   #3
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I have an SF30. (One red one yellow, two blue) I have eliminated the stat so it's not the issue. I have replaced the high limit sw which apparently wasn't the issue. When the cover is off the fan speed actually slows down which would indicate an obstruction in the duct. However, I have visually inspected every inch (mirrors, flashlight and disassembly where needed). When the cover is off I get about a 145 deg discharge air temp. Unit runs for several minutes until the stat is satisfied then cycles off....then back on when the stat call for heat. With the cover on, the fan runs noticeably faster. I have 12,2v at the wires feeding the unit while the fan is running (cover on or off) 13v when the fan stops. With the cover on I will get around 175deg at the floor register. Pretty darn hot. Too high? When it shuts off before the stat is satisfied it shuts off the burner first, runs a while, then shuts the fan off (normal sequence of operation). The only change is cover on-too hot, cover off runs correctly but heats the main cabin only and not the forward bedroom (5h wheel).
Any ideas?
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Old 10-07-2014, 06:33 PM   #4
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Then your building pressure which means a restriction even if you can't find it. Most RV's have close to the bare minimum in duct work as to air flow specs. I have seen people add booster fans to their ducts kinda like a bilge blower. Also what size is the ductwork.
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Old 10-07-2014, 06:45 PM   #5
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Have you tried removing the floor registers? First thing I would do is remove the register in the bedroom and see if that helps air flow. You need 56sqin of free blow air for that furnace. One odd issue I saw a few years ago was the customers wife covered a register with a book this caused the furnace to cycle on the limit sw for a few days which caused the heat exchanger in the furnace to swell up and block airflow in the unit itself. So looking in with the rear door off do you have good clearance around the heat exchanger?
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Old 10-07-2014, 07:53 PM   #6
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This only started last winter while my dad was visiting and it got down to 20°. (Murrphy's Law). The ducts are completely clear with no modifications from the original 2005 construction of the trailer and it's worked great for 8-1/2 years.
The heat exchanger is normal with no cracks, leaks or any other malfunctions. I have cleaned the burner, replaced gaskets, cleaned the fan squirrel cage, verified the orifice is clear and blown out the heat exchanger. The duct runs under the floor and looks like 10" x 2". As it runs forward there are two floor registers before feeding a 4" flex that feeds the 10" x 2" floor duct that feeds the toilet room and bedroom. Again, it worked great for 8 years. Not sure how to measure the CFM to confirm adequate flow with the cover on. (Or off) it works great with the cover off.
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Old 10-07-2014, 09:03 PM   #7
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Un hook the flex tube and run it The flex tubes can plug very easy if it runs OK with it off then you know what is wrong if it feeds 2 rooms I would suggest you find a way to make it 2 separate tubes one to each room.

A 10 x 2 duct is 20 sq in so that in itself is a obstruction. Usually with a small duct we like to run a couple of 4in tubes straight to the furnace to give it more airflow. It could very well be that time has caught up to a marginal install.
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Old 10-08-2014, 08:18 AM   #8
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Excellent observation. I will dive into it today and let you know what I find.
Thanks for your suggestions.
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Old 10-08-2014, 08:23 AM   #9
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I have another question. Do you know what control or safety would cause the furnace to shut down and not restart without a manual cycle of the stat Heat/Off switch? You mentioned earlier that the high limit would cycle back on automatically so it apparently is not that.
Just curious.
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Old 10-08-2014, 08:57 AM   #10
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I know you have checked the ducts ect but I sounds to me like some sort of air flow problem 175 at the outlet sounds high.i suspect there is a secondary temp limit that shuts the unit down lee
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Old 10-08-2014, 09:36 AM   #11
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Quote:
Originally Posted by Oilburner53 View Post
Excellent observation. I will dive into it today and let you know what I find.
Thanks for your suggestions.
Quote:
Originally Posted by hometool View Post
I know you have checked the ducts etc but I sounds to me like some sort of air flow problem 175 at the outlet sounds high.i suspect there is a secondary temp limit that shuts the unit down lee
Oilburner53
Insufficient air flow, (resulting in high temp shutdown), can be caused by restricted, (aka: NOT ENOUGH), "intake air" as well..... (NOT ONLY by restricted/blocked "output ducts and/or heat registers").
Have you verified/determined that the cold air return is not restricted/blocked?
Mel
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Old 10-08-2014, 09:57 AM   #12
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Hometool, I suspect there may be a sensor on the board that is cutting out requiring a manual reset. Just thought I'd ask to see if anyone knew for certain.

Mel s, I plan on removing the furnace this morning to determine all is clear. I suspect it is, since it runs fine with the discharge cover off. However, putting the cover back on combined with a slight blockage on either the combustion intake or the return air intake could put it over the edge.

Also, I know these are 12v systems, but they typically operate closer to 13-14 which I'm not getting. I may also have a voltage problem causing a slow fan speed.

Thanks for all the input. You know you can think things through, but sometimes someone else can can say something that will make sense or cause you to think of something else.
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Old 10-08-2014, 10:55 AM   #13
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Good luck with a solution.
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Old 10-09-2014, 11:19 AM   #14
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Ok, latest update.....
All ductwork clear, fan runs normally, air flow at furthest floor vent seems as good as it ever was. I removed the 4" flex duct connecting the lower floor duct from the upper floor duct feeding the toilet room and bedroom. No change. furnace still went into lockout. Floor vent discharge temp around 175°.
Disassembled furnace, blew out the heat exchanger. Really didn't get any debris or soot. Once I re-installed the furnace I removed one of the duct connection plugs on the cabinet to allow increased air flow. The discharge temp at the open duct connection was somewhere around 190°. I then observed the burner flame through the sight glass. nice blue flame. However, the flame seemed large and extended well up to the top of the heat exchanger (beyond where I could see through the window). Pretty sure that's not right.
I have ordered a new burner and orifice which I will swap out when it arrives. we'll see if that makes a more acceptable flame. Really hope this solves the problem. Next would be the gas valve. I'll probable spend more time and money on parts replacement than just replacing the whole darn furnace.

Thanks again for everyone's input. It's really helped me whittle away at this problem. I'll let you know what happens next.
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