Hello all. i have a Suburban NT-30SP furnace that I am having problems with. I noticed when I was using it that I would hear a click and the air from the vents cooled down then another followed by the sound of the flame relighting. I cleaned all the ducts out and eliminated what I could to see if it was a air circulation problem and it still happened. I also cleaned the exhaust and air intake. I removed the front cover from the furnace and It stayed running. Once it is put back on it starts cycling again. As of yesterday I removed the furnace from the camper all together and bench tested it with no ducts attached and sure enough It still cycled the burner. I suspected the limit switch so I borrowed a meter from work that measures temperature and got some interesting and somewhat concerning readings. Coming from the air outlets themselves I measured around 174 degrees F. when the over limit switch clicked off. Next to the switch about 145 degrees F. So that tells me that the switch is probably fine. I don't think the blower motor is bad as it sounds fine and has no problem starting, But I suppose I shouldn't rule it out yet. I read elsewhere that the burner orifice can go bad and allow more gas to get in. is this true? It is not a power issue as it happens in or outside the camper and with different power supplies. I'm not sure what else it could be. Any thoughts.
Suburban Furnace Overheating
- Thread starter cyborg555
- Start date
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Lou Schneider
Site Team
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Welcome to The RV Forum!
Here's a link to the service manual if you don't have it yet:
http://www.bdub.net/manuals/Suburban_Service_Manual.pdf
The only way the gas orifice can expand is if someone ran a wire through it to clean out a clog. Have you confirmed the propane regulator delivers 11 - 14 inches water column while the flame is lit? If the pressure is high the flame will burn too hot.
It pretty much has to be either too much flame inside the heat exchanger or not enough airflow outside it. Check the gas pressure and make sure you have the minimum number of ducts and total square inches of duct area as shown in the manual.
Here's a link to the service manual if you don't have it yet:
http://www.bdub.net/manuals/Suburban_Service_Manual.pdf
The only way the gas orifice can expand is if someone ran a wire through it to clean out a clog. Have you confirmed the propane regulator delivers 11 - 14 inches water column while the flame is lit? If the pressure is high the flame will burn too hot.
It pretty much has to be either too much flame inside the heat exchanger or not enough airflow outside it. Check the gas pressure and make sure you have the minimum number of ducts and total square inches of duct area as shown in the manual.
Frank B
Well-known member
I was going to suggest that this was normal for that furnace, as mine does the same. However, if you were able to fix it, then mine may have the same problem!
You say that the burner was cycling, on and off. How long did it go between Cycles? Was this fairly rapid, as in several times per minute, or are we talking maybe 5 minutes or more at a time before it would cycle?
I have always considered this to be a limited airflow situation in our unit, as the furnace fan can't seem to blow enough air through the ducts to keep the burner cool, and therefore the high limit switch turns off the gas. Therefore, I am not getting full value from the btu output that this furnace is capable of.
Anyway, thank you for posting back already. And please do let us know how quickly the burner was cycling. That will make a difference to me if I have to troubleshoot my furnace or not.
You say that the burner was cycling, on and off. How long did it go between Cycles? Was this fairly rapid, as in several times per minute, or are we talking maybe 5 minutes or more at a time before it would cycle?
I have always considered this to be a limited airflow situation in our unit, as the furnace fan can't seem to blow enough air through the ducts to keep the burner cool, and therefore the high limit switch turns off the gas. Therefore, I am not getting full value from the btu output that this furnace is capable of.
Anyway, thank you for posting back already. And please do let us know how quickly the burner was cycling. That will make a difference to me if I have to troubleshoot my furnace or not.
Old_Crow
Well-known member
Frank, ours does the same thing. If I don't have the furnace on at night and the coach temp drops into the 50's when I turn the furnace on in the morning sometimes I'll hear the flame cycle a couple of times before the coach gets up to temp.
Once the coach reaches the set temp, the furnace cycles normally and I don't hear the flame dropping out.
I also have felt this was normal up to now.
Once the coach reaches the set temp, the furnace cycles normally and I don't hear the flame dropping out.
I also have felt this was normal up to now.
Frank B
Well-known member
Wally:
Our burner kicks in and out regularly during the whole heating process. I have always assumed that this is an airflow problem with the furnace being able to give more heat that the forced air circulation can carry away into the coach. I saw that as a design issue with our coach, and not a furnace problem. I added some extra ducts to improve airflow, but never found that it made much of a difference.
As I say, I have considered this normal operation of this furnace in this installation. If the OP got his to burn steadily without the need for the high temp cutoff to control the max heat of the burner, then I'd be interested in knowing more.
Our burner kicks in and out regularly during the whole heating process. I have always assumed that this is an airflow problem with the furnace being able to give more heat that the forced air circulation can carry away into the coach. I saw that as a design issue with our coach, and not a furnace problem. I added some extra ducts to improve airflow, but never found that it made much of a difference.
As I say, I have considered this normal operation of this furnace in this installation. If the OP got his to burn steadily without the need for the high temp cutoff to control the max heat of the burner, then I'd be interested in knowing more.
Old_Crow
Well-known member
Well, like I said, ours only does it on initial startup when it has to run longer than it normally would to obey the t/stat commands.
Once the coach is up to temp, the burner cycling stops. At this point I'm not going to worry about it. I have other fish to fry at the moment.
Once the coach is up to temp, the burner cycling stops. At this point I'm not going to worry about it. I have other fish to fry at the moment.
Gary RV_Wizard
Site Team
I'm not familiar with a furnace part of that name. Do you mean the High Limit Switch, which is mounted on the heat exchange and senses if the furnace is overheating? It will indeed cause the burner to cut off until it cools a bit, but it's a very simple thermostat device and rarely fails.
"High Limit Switch, which is mounted on the heat exchange and senses if the furnace is overheating? It will indeed cause the burner to cut off until it cools a bit, but it's a very simple thermostat device and rarely fails." But what if it does fail? Mine does the same thing as the ones You folks are talking about.I installed a hard wired electric heater and only use the furnace when awake.
MoonStream Vintage
New member
I had very similar problems with a relatively new Suburban NT-30SP furnace. The problem arose suddenly where the furnace would go into hard lockout mode after about 6-10 minutes of running. With the front cover removed, the unit would run indefinitely (until the coach reached the thermostat's set temp). For this reason, I assumed the high limit switch was putting the unit into lockout. However, this was not the case - it was one of the two solenoids on the gas valve that was failing intermittently - and only when it got warm. The loss of gas put out the flame, which couldn't be relit and so the unit would go into lockout mode.
That was the short answer, but I will provide details below for anyone experiencing similar problems:
The furnace is a Suburban NT-30SP, bought new in 2017 and installed in a shell-off '75 Airstream Argosy renovation. Spent a full winter living in the trailer (lows around 0 degrees F) with no issues. Then sat winterized for two winters. Got it back up and running for about 2 months in the winter (again, lows around 0 degrees F) with no issues. Winterized it, it sat for 2 weeks, then tried to boot it back up in December and the furnace would not run for more than 10 minutes before going into a hard lockout. Again: with the front cover removed, the furnace would continue running fine and not enter lockout mode.
Tried heating the coach with electric heaters before starting it up again - didn't make a difference.
Cleaned and then tested the limit switch in my oven with a meat thermometer and multi-meter - it did not lose continuity up to 215 degrees F, so I assumed the high limit switch itself wasn't malfunctioning.
In case a mouse got in somewhere or something and airflow was being obstructed, I took apart the entire furnace and vacuumed it, blew compressed air through the manifold per manual's instructions. Did the same to the 4 ducts. Re-measured cross sectional area to verify I was within spec on that. The unit was fairly new, and so was really quite clean all around. There were no obstructions in the ducts either. Reassembled, and the problem persisted. (Also, the intake screen for this unit is huge - about 24" wide by 16" tall, and anyway, all these tests were done with that screen removed.)
The water heater had been making some loud noises, and I wanted an auto-changeover regulator with a color indicator (which the existing one lacked), so I bought a new regulator for the trailer without testing any pressures. The problem persisted.
Got a digital manometer from a friend and checked the coach's propane system's pressure - 14.15 inches of H2O. That's a tiny bit higher than the furnace's stated range of 11" to 14" H2O. Without adjusting that regulator, I tested the pressure coming out of the orifice downstream of the gas valve - 12.05" H2O. That's quite a bit higher than the 10" to 10.5" quoted by someone in one of these forums as being from the manufacturer. The gas valve does indeed have its own regulator built into it, and printed on the side of the gas valve are the words Reg 10.2", which I assumed meant it was supposed to further regulate the gas pressure down to 10.2" H2O - though now I'm not so sure. Anyway, I bought a new gas valve, which comes with the 2 solenoids already attached. Unfortunately, the package got crushed in the mail and both solenoids looked broken, so I swapped them out for my original ones on the gas valve I was replacing. Reassembled, and measured gas pressure coming out of the orifice - even with the new gas valve, it was still at 11.46" H2O. Reassembled the burner tube, and some new data: this time the furnace had trouble lighting, but on the 9th try (3rd attempt of the 3rd cycle), it finally lit, but unfortunately went into lockout about 2 minutes after I replaced the front cover.
I adjusted the trailer's regulator, counter-clockwise 2.5 full rotations (a little at a time while measuring) until I got the system's pressure down to 13.2" H2O on the main tank and 12.7" H2O on the auxiliary tank (those auto-changeover regulators lose a little pressure when they changeover). With the water heater running there was a 0.2" H2O drop. Hooked the gas back up and tried again - this time the furnace refused to light and I gave up after 9 tries (3 cycles of 3 attempts).
I removed the gas valve and solenoids and bench tested them off my car battery. One of the two would not click with 12.5V applied, and I could not blow through the valve with 12.5V connected to both. Of the two crushed ones that arrived in the mail attached to the new gas valve, it turns out that one still worked and the other did not. So I used the 2 good ones I had - one from the old gas valve and one from the new gas valve. Applying 12.5V to both opened the valve and I could blow through it. The polarity on the new one was backwards from what was stated on the bracket that holds them in place, so I amended with a sharpie.
I went back and reassembled and tested pressures again - for science. Coming out of the orifice, pressure decreased with every iteration until plateauing at 10.75" H2O. (First cycle was 11.2, 11.05, 10.85, second cycle was 10.82, 10.75, 10.75" H2O). Reassembled with the burner tube and everything worked and the unit us no longer going into lockout.
I haven't looked into it (though I'd like to), but maybe solenoid's start to fail with higher heat? - not heat so high that the high limit switch thinks it's dangerous, but some level of heat between cold and normal operating temperature?
To recap - it was a solenoid on the gas valve failing intermittently and not an overheating situation caused by some malfunctioning of air flow or gas flow.
Hope this helps some people out there. Cheers.
That was the short answer, but I will provide details below for anyone experiencing similar problems:
The furnace is a Suburban NT-30SP, bought new in 2017 and installed in a shell-off '75 Airstream Argosy renovation. Spent a full winter living in the trailer (lows around 0 degrees F) with no issues. Then sat winterized for two winters. Got it back up and running for about 2 months in the winter (again, lows around 0 degrees F) with no issues. Winterized it, it sat for 2 weeks, then tried to boot it back up in December and the furnace would not run for more than 10 minutes before going into a hard lockout. Again: with the front cover removed, the furnace would continue running fine and not enter lockout mode.
Tried heating the coach with electric heaters before starting it up again - didn't make a difference.
Cleaned and then tested the limit switch in my oven with a meat thermometer and multi-meter - it did not lose continuity up to 215 degrees F, so I assumed the high limit switch itself wasn't malfunctioning.
In case a mouse got in somewhere or something and airflow was being obstructed, I took apart the entire furnace and vacuumed it, blew compressed air through the manifold per manual's instructions. Did the same to the 4 ducts. Re-measured cross sectional area to verify I was within spec on that. The unit was fairly new, and so was really quite clean all around. There were no obstructions in the ducts either. Reassembled, and the problem persisted. (Also, the intake screen for this unit is huge - about 24" wide by 16" tall, and anyway, all these tests were done with that screen removed.)
The water heater had been making some loud noises, and I wanted an auto-changeover regulator with a color indicator (which the existing one lacked), so I bought a new regulator for the trailer without testing any pressures. The problem persisted.
Got a digital manometer from a friend and checked the coach's propane system's pressure - 14.15 inches of H2O. That's a tiny bit higher than the furnace's stated range of 11" to 14" H2O. Without adjusting that regulator, I tested the pressure coming out of the orifice downstream of the gas valve - 12.05" H2O. That's quite a bit higher than the 10" to 10.5" quoted by someone in one of these forums as being from the manufacturer. The gas valve does indeed have its own regulator built into it, and printed on the side of the gas valve are the words Reg 10.2", which I assumed meant it was supposed to further regulate the gas pressure down to 10.2" H2O - though now I'm not so sure. Anyway, I bought a new gas valve, which comes with the 2 solenoids already attached. Unfortunately, the package got crushed in the mail and both solenoids looked broken, so I swapped them out for my original ones on the gas valve I was replacing. Reassembled, and measured gas pressure coming out of the orifice - even with the new gas valve, it was still at 11.46" H2O. Reassembled the burner tube, and some new data: this time the furnace had trouble lighting, but on the 9th try (3rd attempt of the 3rd cycle), it finally lit, but unfortunately went into lockout about 2 minutes after I replaced the front cover.
I adjusted the trailer's regulator, counter-clockwise 2.5 full rotations (a little at a time while measuring) until I got the system's pressure down to 13.2" H2O on the main tank and 12.7" H2O on the auxiliary tank (those auto-changeover regulators lose a little pressure when they changeover). With the water heater running there was a 0.2" H2O drop. Hooked the gas back up and tried again - this time the furnace refused to light and I gave up after 9 tries (3 cycles of 3 attempts).
I removed the gas valve and solenoids and bench tested them off my car battery. One of the two would not click with 12.5V applied, and I could not blow through the valve with 12.5V connected to both. Of the two crushed ones that arrived in the mail attached to the new gas valve, it turns out that one still worked and the other did not. So I used the 2 good ones I had - one from the old gas valve and one from the new gas valve. Applying 12.5V to both opened the valve and I could blow through it. The polarity on the new one was backwards from what was stated on the bracket that holds them in place, so I amended with a sharpie.
I went back and reassembled and tested pressures again - for science. Coming out of the orifice, pressure decreased with every iteration until plateauing at 10.75" H2O. (First cycle was 11.2, 11.05, 10.85, second cycle was 10.82, 10.75, 10.75" H2O). Reassembled with the burner tube and everything worked and the unit us no longer going into lockout.
I haven't looked into it (though I'd like to), but maybe solenoid's start to fail with higher heat? - not heat so high that the high limit switch thinks it's dangerous, but some level of heat between cold and normal operating temperature?
To recap - it was a solenoid on the gas valve failing intermittently and not an overheating situation caused by some malfunctioning of air flow or gas flow.
Hope this helps some people out there. Cheers.
