RV furnace

[wrenchrob]

I know this is not bike related BUT.... I am wondering if there are any RV techs out there. I have a Suburban NT-30sp furnace in my RV and am having troubles with it. I have ruled out propane supply as I can run my water heater, 3 burner stove and oven at the same time. When i request heat the blower motor will start running, (sounds and feels quite strong), I can hear the ignitor click than a thud from the solenoid, than the burner lights and will burn for maybe about 5 seconds and than the burner will go out. It will repeat this 2 more times than the furnace fan will shut off after a couple minutes. Can anyone help me out on this one? Rob

[Black Owl]

By no means an RV tech. I get to look at them after the heaters work too well....

But, if it has been sitting for a while, you might want to check for spiders nesting in the ignitor or the burner.

They can be a real PITA......

[Trader]

I"m not a furnace tech but I THINK the problem may be with your thermocouple.

 

IT senses the heat, and ensures that the burner is lit. If it doesn't register, then it shuts off the gas flow.

 

I doubt it's the spider problem because the burner IS lighting.

[Guest scarylarry]

 

The orifice tube are known for spiders nest..

[wrenchrob]

I"m not a furnace tech but I THINK the problem may be with your thermocouple.

 

IT senses the heat, and ensures that the burner is lit. If it doesn't register, then it shuts off the gas flow.

 

I doubt it's the spider problem because the burner IS lighting.

Thermocouple you say. That sounds like I the problem i could be dealing with. When it lights the flame looks good (nice blue colour) and all holes in burner seem lit. I am making this tomorrows project to tr and get it going. My 12 year old daughter having a few friends over this coming friday for her 13th birthday and they are sleeping in the trailer so I want to get it going by than.

[Ozlander]

On my suburban, the spark electrode is also the flame detector. It didn't like having a mud dabber nest on it.

 

Same results as yours.

[wrenchrob]

On my suburban, the spark electrode is also the flame detector. It didn't like having a mud dabber nest on it.

 

Same results as yours.

Thanks for your input. Will pull the electrode out next and have a look.

[Flyinfool]

I just had to fix my heater for Bongobobny.

It was dust and crud on the sail switch not letting it close. But mine just had fan and no light at all.

[wrenchrob]

I just had to fix my heater for Bongobobny.

It was dust and crud on the sail switch not letting it close. But mine just had fan and no light at all.

Hey Jeff. I have heard about this sail switch and was looking for one but could not see it. I dont think thats my problem because i dont think it will light with a failed S S but would like to find it and see how it works.

[uncledj]

I'm not an RV tech, but I am a HVAC Tech.

What seems to be happening is that it's going through an igniton trial, but not sensing the flame. It burns for 5 seconds waiting for the igniton module to sense flame, and if it doesn't it'll shut down to keep from allowing raw gas to build up. (safety issue)

The most likely cause will be an oxidized flame rod. It'll look like a thermocouple, but with just a single wire coming off of it.

Clean it with steel wool or a scotchbrite pad....avoid using sandcloth.

If that doesn't do it for ya, make sure any ground wiring is intact, and making good connection.

The flame sensing circuit will actually prove flame by passing a small bit of electricity through the flame to ground, and back to the igniton module. (Probably between 0.5 and 2 micro amps)

If none of that does it for you, you may be due for a new igniton module.

PM me if you need further advice.

[wrenchrob]

I'm not an RV tech, but I am a HVAC Tech.

What seems to be happening is that it's going through an igniton trial, but not sensing the flame. It burns for 5 seconds waiting for the igniton module to sense flame, and if it doesn't it'll shut down to keep from allowing raw gas to build up. (safety issue)

The most likely cause will be an oxidized flame rod. It'll look like a thermocouple, but with just a single wire coming off of it.

Clean it with steel wool or a scotchbrite pad....avoid using sandcloth.

If that doesn't do it for ya, make sure any ground wiring is intact, and making good connection.

The flame sensing circuit will actually prove flame by passing a small bit of electricity through the flame to ground, and back to the igniton module. (Probably between 0.5 and 2 micro amps)

If none of that does it for you, you may be due for a new igniton module.

PM me if you need further advice.

Thanks. going to tackle this tomorrow. Yuou just might here from me. Will post my findings and/or frustrations as project advances.

[Flyinfool]

Hey Jeff. I have heard about this sail switch and was looking for one but could not see it. I dont think thats my problem because i dont think it will light with a failed S S but would like to find it and see how it works.

 

On my furnace it is a small micro switch with a square piece mounted to the lever, when the fan is blowing the wind pushes it hard enough to close the switch. Mine is really buried and was hard to get to, so I can not get a pic of it without significant disassembly.

[Nemo]

I also am not a tech, but the first item that drew my attention was that you mentioned that the blower ran and sounded good. But, the blower shouldn't come on until after the flame has gotten the plenum hot enough to blow hot air. So, the item in between the thermostat and the blower (and gas valve), may be defective.

[Marcarl]

I also am not a tech, but the first item that drew my attention was that you mentioned that the blower ran and sounded good. But, the blower shouldn't come on until after the flame has gotten the plenum hot enough to blow hot air. So, the item in between the thermostat and the blower (and gas valve), may be defective.

Not necessarily,,,, my blower comes on first and then then the ignition and flame.

[Guest scarylarry]

The blower comes on first and goes off after the heat is satisfied and the burner is off, it will take a minute or 2 for the blower to off after the burner has shut off, and the same on the start up, blower comes on for a min or 2 then the burner..

 

This is for safety reasons

[uncledj]

Are y'all talking about the furnace blower which distributes the heated air throughout the camper, or are you talking about the combustion blower, which creates a draft for combustion...???

 

Furnace blower will be energized either through a time delay, (delay on upon a call for heat, then delayed off after loss of call) or through a temperature switch, which would allow the plenum to heat up a bit, then shut down the blower after it cools. For a camper, I'd bet it'll be a time delay style, but I'm not sure.

 

A combustion blower will typically run for 30 seconds or so as a pre purge,..as a safety measure, to clear the heat exchanger of any raw gas before the ignition sequence begins, then will continue to run for as long as the unit is firing, and in some cases (rarely) they'll have it set up to run a post purge as well.

[wrenchrob]

In this case there is only one blower for the combustion gas and the forced air throught the ducts. I know it is suppose to run for a bit before and after flame and that is where this so called sail switch comes into effect to verify there is adequate air flow for the combustion gas before lighting the burner. I just got to figure out why burner only lights for about 5seconds. As mentioned before there has to be something to do with a flame sense or heat sense or something like that. sensor? wiring? circuit board? Having breakfast and going to go tackle this monster. If you dont hear from me again you know what likely happened. Propane and electricity. :225::225:

[MiCarl]

I had similar symptoms with mine. Turns out it was dirt behind the gas orifice.

 

The furnace in my RV has an optical flame sensor. The dirt was cutting the gas back to where the flame didn't give enough light for the sensor to detect.

 

By the way - here is how I got a hint about the problem. This test is probably a bad idea and someone will probably flame me. My furnace would run normally if I used my hand to block most of the combustion intake air. My conclusion was that the furnace functioned ok with a bright orange flame. I assumed I would find some sort of nest blocking the flame sensor, but after some poking around found the dirt behind the orifice.

 

One more thing, and yours may be the same. My RV has ducted heat as opposed to the older style where the furnace face sticks into the living quarters. The case around the furnace is connected/aligned with the duct openings. It's tough to get out and back in. It turns out that the case can be left in place. There is only a screw or two that will let you pull the internals out as one assembly.

Edited September 9, 2012 by MiCarl

[dacheedah]

They make a brush on a flex rod to clean those out, not expensive and I believe it's required pm anyways. Usually a screw holding it, brush and vac then try again, doesn't take much debris to cause an issue.

[saddlebum]

What is the make and model number of your heater. This is important because different makes use different systems to detect flame on. This is to avoid a build up of raw gas. Normally they will have either a thermocouple or light sensor to detect a proper flame. The combustion blower (not Air circulation fan) comes on prior to ignition to purge the combustion chamber of any raw gas fumes (to avoid an explosion) after a given time the ignitor will start sparking then gas is intrduced and you have flame on. Once the gas is ignited the thermocouple or Light sensor will detect that there is a flame. If no flame is detected then the system is shut down. If you can get hold of the schematics for your make and model you will be able to temporarly by pass these sysems (for diagnostic purposes only) to see if this is the problem. Another thing to look for is restricted fuel supply. Enough fuel may enter to start a flame but not to maintain it for long.

[wrenchrob]

What is the make and model number of your heater. This is important because different makes use different systems to detect flame on. This is to avoid a build up of raw gas. Normally they will have either a thermocouple or light sensor to detect a proper flame. The combustion blower (not Air circulation fan) comes on prior to ignition to purge the combustion chamber of any raw gas fumes (to avoid an explosion) after a given time the ignitor will start sparking then gas is intrduced and you have flame on. Once the gas is ignited the thermocouple or Light sensor will detect that there is a flame. If no flame is detected then the system is shut down. If you can get hold of the schematics for your make and model you will be able to temporarly by pass these sysems (for diagnostic purposes only) to see if this is the problem. Another thing to look for is restricted fuel supply. Enough fuel may enter to start a flame but not to maintain it for long.[/QUIt It is a Suburban NT 30 sp. I have removed the burner this morning and it is as clean as a whip, romeved the cover on the ouside of the trailer to access the sail switch and tested it while operating the fan and it is ok, bypassed the limit switch and no differnt, on the scematics i found it shows a flame sensor with the electtrode but mine does not have one at this location, I hve been monitoring the operation this morning and have discovered that the burner is burning nicely for 5 seconds as long as the ignitor is sparking and once the ignitor stops sparking the flame also stops so I am thinking there must be a flame or light sensor somewhere. unfortunately the access is terrible. cant even seee the gas valve or circuit board. going to contiplate this for a bit yet this morning before pulling the unit out. i think i will go remove the burner again and try to see inside the furnace housing with a mirror for any other sensor.

[saddlebum]

maybe this will help.

 

http://bryantrv.com/docs2/docs/ntseries.pdf

 

Or try the attached PDF file.

[wrenchrob]

maybe this will help.

 

http://bryantrv.com/docs2/docs/ntseries.pdf

 

Or try the attached PDF file.

Thanks saddle. I really apprecite you working with me on this one. I got that file already and it is slightly generic or outdated or something. On pages 15-17 it talks about a flme sensor near the electrode and i dont have one of those flame sensors near the electrode or near the burner. I removed the burner and looked inside housng and no wires or sensors. That file you sent was a good one though it helped verify thoperation of the limit switch and the sail switch and... Ayways im removeng the entire unit now to work on it, service it on the bench. Going to hook up the bbq propane tank and a 12v power supply. Cant access any of the components to test or replace so might as well just remove it. After this i will be a half A$$ pro at RV furnaces. Will post some pictures later when IM set up on bench. Got it all disconnected now except ssomething is caught on the back of the unit. Just came into get a little mirror and a cold beer.

[uncledj]

Sometimes they use the electrode as the flame sensor.

It can do double duty if it's set up that way. Make sure it's clean. If the unit is sparking and lighting, you need not worry about the sail switch. If it wasn't making, it wouldn't try to light at all.

[wrenchrob]

Sometimes they use the electrode as the flame sensor.

It can do double duty if it's set up that way. Make sure it's clean. If the unit is sparking and lighting, you need not worry about the sail switch. If it wasn't making, it wouldn't try to light at all.

I am starting to wonder about the electrode. i have the furnace on the bar and cover off,bbq tank hooked up, battery hooked up and it still does the same thing. There is a limit seitch, sail switch and the electrode only as sensors/switches goes. I can keep it going by applying 12v to thesolenoid on the valve, works great. It is shutting the 12vjsupply to the solenoids after 5 seconds and ignitor stops sparking. I verified 12v coming out of the limit switch at all times the blower is running . So i am down to either the board or this single terminal electrode. I cant see how this can be a flame sense compnent with only 1 wire but hey who knows? Obviously Im going to buy an electrode and try it.( Wish I would have done that before removing the furnace) To the best of your knowledge can this electrode send a signal amp. back to the board?