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Furnace trouble


aunut

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I tried to turn my Suburban furnace on this week (first time since last year) and it would not light. I followed the instructions and turned it on several times. I could hear the lighting mechanism clicking, but it would not light. I did notice wasps going in and out in July while on a trip.

The furnace is almost new and worked fine last time out last year. My hot water and fridge work fine on propane.

Do the wasps build a dirt nest and block the gas? If so, how do I clean it off? The access panel on the exterior is very tight and I didn't want to force it off after removing the screws.

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Wasps do tend to build nests in furnaces as they are for some reason attracted to the propane odor. It would take several times of trying the furnace to get any air out of the lines which could accumulate over the summer. I always light the stove top first and let it burn for several minutes, just to be sure that as much air as possible has been vented, then try the furnace. I would try at least 5 times before I gave up, and 10 might be in reason.

 

Does the blower work as it should? The blower must come on for about 30 seconds before the gas is turned on by the control system and the ignition circuit fires. I would also suggest that you use a good light and look into the burner area and the burner tube to make sure that there are no nests in them, as either location could cause it to fail to light. If there is propane turning on, you should be able to smell it near the orifice location when it doesn't light.

 

The access panel on the exterior is very tight and I didn't want to force it off after removing the screws.

I'm not quite sure just what you are asking. Most Suburban furnaces are accessed from inside of the RV. There is a video on youtube that may be of some help.

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The blower works fine. I have gotten a whiff of gas outside at the vents. I'll try several more times, but if the gas is getting to the fridge and hot water tank, which are near the heater, it should be the same effect as lighting the stove, shouldn't it. I tried a flashlight, but really couldn't see anything.

I can't really tell, but it seems like I hear the furnace light, then almost immediately go off.

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I can't really tell, but it seems like I hear the furnace light, then almost immediately go off.

If you wait until it is dark and then try it, you should be able to see the flame if it lights at all. Before that I would try to make sure that things are clean inside.

 

whburner.jpg51iANYenMhL._AA160_.jpg

Could you post some pictures of the furnace that we are discussing? The second picture is of some stainless steel screens for a Suburban furnace exhaust to prevent the entry of nesting bugs.

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The owners manual specifically says "do not use the screens".

 

You can use the screens. Just keep them clean or change them often. The main concern is blockage and possibly posing a fire hazard if they are not kept clean, but it's perfectly safe to use them with a little bit of routine maintenance.

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I have used the screens for many years to keep mud daubers out and had no problems with the wasps or using the furnace. Make sure the 2 screws are very loose and wobble then pry the access cover off. If you have seen wasps entering the furnace they have probably make a mud nest inside. Our furnace access is through the outside panel. Greg

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On the picture issue, I'm trying to be sure of just what furnace you actually have. Most Suburban furnaces do not have an outside access panel but only have the exhaust ports on the outside of the RV and then must be accessed from the inside or removed for most work. If yours is by Suburban it probably looks like one of these two.

51oDeNc4GuL._AC_UL160_SR160,160_.jpg51fa-J6ROEL._AC_UL160_SR160,160_.jpg41rniE9SypL._AA160_.jpg Suburban exhaust/intake port.

If so you probably only have the small ports for air intake and exhaust as shown on the upper left of the first unit and on the right side of the second, which are on the outside of the RV. The port is removable and must be removed in order to remove the furnace from the RV. If you can gain access to the other side of the furnace inside of your RV, only then can you do much work on it. There are models of the second with outside access, but can't find any pictures.

 

On the other hand, if you happen to have an Atwood furnace, those do have an outside access panel that can be removed and they look like the pictures below.

71770-1n.jpg If yours has a panel like this, the two screws at the top release it and you can then remove it for access.

The owners manual specifically says "do not use the screens".

There is good reason for that statement and it is in all of the owner's manuals from both furnace manufacturers and with good reason. Their concern is that you might use ordinary metal window screen and those will plug up very quickly. Nearly all dealers I know of will sell and recommend the screens made for that purpose because they are made of stainless steel (thus are not cheap) and they are easily removable should the furnace need to be serviced, and they also prevent bug intrusion. Bug nests are by far the most common cause of RV furnace failures. If you have doubts, ask the service department of your dealer about the use of them and if they sell/install them. Also, as if they ever denied any warranty work because of them. The new RV which we now own came from the dealership with screens installed. Because it is such a problem here, that dealer simply puts them on all of the new RVs that they sell.

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The first simplest thing to do if the blower is operating at speed, you hear a click witch is the power board telling the gas valve to open, you smell gas for a short time and you think it is igniting.

If all of the above is true then remove the power board and use a pencil eraser to polish the contact area on the power board connection. If that does not work (15 percent chance ) then if you have a spare power board try it and that may fix the problem(50 percent chance).

But most likely problem is that the power board is not sensing flame.

The furnace needs pulled out and all screws involved in holding the igniter I place needs taken apart and put back together. Pain in your back side but that is probable what is wrong.

Much easier if the is an outside cover panel.

If the furnace needs pulled out from inside make sure it is test for operation several times before installing it and it still not operational.

Also if there are bug nest they need removed also before it will work properly.

 

Safe Travels, Vern

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I have a Suburban SF-30 furnace, which is one of the most popular and common furnaces out there. It is accessed, serviced, and removed from the outside of the coach.

The picture I posted to the right is an SF30 but I could not find any pictures of it with the cover in place.

 

56910-11n.jpg

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On the picture issue, I'm trying to be sure of just what furnace you actually have. Most Suburban furnaces do not have an outside access panel but only have the exhaust ports on the outside of the RV and then must be accessed from the inside or removed for most work. If yours is by Suburban it probably looks like one of these two.

51oDeNc4GuL._AC_UL160_SR160,160_.jpg51fa-J6ROEL._AC_UL160_SR160,160_.jpg41rniE9SypL._AA160_.jpg Suburban exhaust/intake port.

If so you probably only have the small ports for air intake and exhaust as shown on the upper left of the first unit and on the right side of the second, which are on the outside of the RV. The port is removable and must be removed in order to remove the furnace from the RV. If you can gain access to the other side of the furnace inside of your RV, only then can you do much work on it. There are models of the second with outside access, but can't find any pictures.

 

On the other hand, if you happen to have an Atwood furnace, those do have an outside access panel that can be removed and they look like the pictures below.

71770-1n.jpg If yours has a panel like this, the two screws at the top release it and you can then remove it for access.

 

There is good reason for that statement and it is in all of the owner's manuals from both furnace manufacturers and with good reason. Their concern is that you might use ordinary metal window screen and those will plug up very quickly. Nearly all dealers I know of will sell and recommend the screens made for that purpose because they are made of stainless steel (thus are not cheap) and they are easily removable should the furnace need to be serviced, and they also prevent bug intrusion. Bug nests are by far the most common cause of RV furnace failures. If you have doubts, ask the service department of your dealer about the use of them and if they sell/install them. Also, as if they ever denied any warranty work because of them. The new RV which we now own came from the dealership with screens installed. Because it is such a problem here, that dealer simply puts them on all of the new RVs that they sell.

Mine is the middle one and there is an access panel on the exterior. I've tried to remove it, but, after removing all the screws, it is still very tight and I don't want to force it.

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Mine is the middle one and there is an access panel on the exterior. I've tried to remove it, but, after removing all the screws, it is still very tight and I don't want to force it.

By middle, I mean the flat looking one with the motor on the left and ports on the right. I can't get to it on the inside. I can see it under my fridge when I remove the vent leading to my water heater.

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That should be the SF-30 series, also shown in #11. As I look at the Suburban Furnace Service Manual, it seems that there are four screws that hold the cover in place. It looks like the vent tube & exhaust cap is attached to the removable panel on yours and must come off with the cover. I find no mention of the process of taking the cover off, but would bet that you need to find something plastic to use and gently pry it loose from the side of the RV, as long as all 4 screws are removed. (See figure 3 on page 7 of the linked manual)

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That should be the SF-30 series, also shown in #11. As I look at the Suburban Furnace Service Manual, it seems that there are four screws that hold the cover in place. It looks like the vent tube & exhaust cap is attached to the removable panel on yours and must come off with the cover. I find no mention of the process of taking the cover off, but would bet that you need to find something plastic to use and gently pry it loose from the side of the RV, as long as all 4 screws are removed. (See figure 3 on page 7 of the linked manual)

Thank you. Very helpful. I'll let you know how it goes.

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  • 2 weeks later...

Well, I couldn't find anything, so I took it to Camping World where I bought the coach. They have assured me that it is the sail switch and I ordered a new one through them for $12. However, they want 1.5 hours @ $130/hr to install it because they say the furnace has to be removed. I don't assume this is an operation I should try myself (I'm good at taking things apart, just not putting them back together). Is this something that is much simpler than what they say it is?

 

By the way, the 1.5 hour charge is on top of the approx. $100 they charged for the diagnosis.

 

All is pretty bad since I have used this furnace no more than 4 days since I purchased the coach new.

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Yes, the sail switch could be the problem. It could also be a loose connection. Our front furnace would try and try to light, but never got lit. I finally started tracing wires and checking voltages when I came to a terminal that was loose. When I touched that terminal (I didn't know it was loose) I got a small spark from making a connection and the furnace started working. I pulled the power, tightened the connection, and all was well. When it happened again last year I knew what to do.

 

This reminds me that I should go out tomorrow and check that connection before it gets cold.

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Well, I couldn't find anything, so I took it to Camping World where I bought the coach. They have assured me that it is the sail switch and I ordered a new one through them for $12. However, they want 1.5 hours @ $130/hr to install it because they say the furnace has to be removed. I don't assume this is an operation I should try myself (I'm good at taking things apart, just not putting them back together). Is this something that is much simpler than what they say it is?

I think that it is time for you to get someone else to give you an opinion. Looking at the schematic shown on page 31 of the service manual in the link from my previous post, you will see that the sail switch if not closed will prevent the opening of the gas valve and there would be no spark. That is also stated in the trouble shooting guide on page 28 of the same service manual. Before you let them do the work, ask them if there will be any charge should the switch not solve the problem.

 

Just as a test, you could try using one of the long butane lighters like are used to light a propane barbecue.

41qMRae3SnL._AA160_.jpg

With the lighter burning, hold it up where the spark probe is located and have a helper turn up the thermostat to call for heat. If there is any propane at all flowing, you should see it flare even if it doesn't light. I would also observe the sparking with it dark as it makes it possible to see where it is sparking. You should also remove the spark probe and clean it carefully with a product like Scotch Brite or similar product. While it is out, check the gap which should be 1/4", +/- 1/16". Before you do this, take time to read through the sequence of events found on page 25 to understand what should be happening. You can also use your volt meter across the terminals of the gas valve to see if it is being supplied 12V power but failing to open, if there is not gas to light with the butane lighter. If it does light, that indicates that the probe is the problem.

 

Have you by any chance watched the burner area with low light just to see if it may be that the flame is being ignited, but the probe is failing to detect the heat from that flame? You should be able to see the burning gas and if it lights, but the spark continues and then the valve shuts and flame goes out, that is an indication of failing of the heat sens part of the ignition probe.

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I think that it is time for you to get someone else to give you an opinion. Looking at the schematic shown on page 31 of the service manual in the link from my previous post, you will see that the sail switch if not closed will prevent the opening of the gas valve and there would be no spark. That is also stated in the trouble shooting guide on page 28 of the same service manual. Before you let them do the work, ask them if there will be any charge should the switch not solve the problem.

 

Just as a test, you could try using one of the long butane lighters like are used to light a propane barbecue.

41qMRae3SnL._AA160_.jpg

With the lighter burning, hold it up where the spark probe is located and have a helper turn up the thermostat to call for heat. If there is any propane at all flowing, you should see it flare even if it doesn't light. I would also observe the sparking with it dark as it makes it possible to see where it is sparking. You should also remove the spark probe and clean it carefully with a product like Scotch Brite or similar product. While it is out, check the gap which should be 1/4", +/- 1/16". Before you do this, take time to read through the sequence of events found on page 25 to understand what should be happening. You can also use your volt meter across the terminals of the gas valve to see if it is being supplied 12V power but failing to open, if there is not gas to light with the butane lighter. If it does light, that indicates that the probe is the problem.

 

Have you by any chance watched the burner area with low light just to see if it may be that the flame is being ignited, but the probe is failing to detect the heat from that flame? You should be able to see the burning gas and if it lights, but the spark continues and then the valve shuts and flame goes out, that is an indication of failing of the heat sens part of the ignition probe.

The tech did a test. He covered the exhaust and it lit. When he released it, it went out and just kept clicking. He guaranteed me that it was the sail switch. I just assume that this is something an amateur should not try do, so unless there is a simple way to replace it myself, I will let them do it.

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The tech did a test. He covered the exhaust and it lit. When he released it, it went out and just kept clicking. He guaranteed me that it was the sail switch. I just assume that this is something an amateur should not try do, so unless there is a simple way to replace it myself, I will let them do it.

Interesting test. I'll be very interested to hear the results that you get as he does the work. It is a test that I've not heard of but need to think about. The sail switch is one that has a moving vane in the air path for combustion air, into the furnace and moved by the internal blower. If air movement is too little for safe operation, the switch is there to remain open and so prevent the lighting of the furnace. Low battery voltage is a very common cause of this type of failure as the furnace blower speed is determined by the voltage that is applied to it, so as the battery voltage drops, the blower turns more slowly and the air movement decreases. Once the blower slows sufficiently the sail switch (normally open, held closed in operation) fails to close and the furnace does not light, or will turn off if the switch opens while it is in operation.

 

I'm not sure why blocking the output of the blower would make the sail switch close, but since I have never tried that test, I'll not say that it couldn't. I wonder where Vern is, since he is a certified RV tech?

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Interesting test. I'll be very interested to hear the results that you get as he does the work. It is a test that I've not heard of but need to think about. The sail switch is one that has a moving vane in the air path for combustion air, into the furnace and moved by the internal blower. If air movement is too little for safe operation, the switch is there to remain open and so prevent the lighting of the furnace. Low battery voltage is a very common cause of this type of failure as the furnace blower speed is determined by the voltage that is applied to it, so as the battery voltage drops, the blower turns more slowly and the air movement decreases. Once the blower slows sufficiently the sail switch (normally open, held closed in operation) fails to close and the furnace does not light, or will turn off if the switch opens while it is in operation.

 

I'm not sure why blocking the output of the blower would make the sail switch close, but since I have never tried that test, I'll not say that it couldn't. I wonder where Vern is, since he is a certified RV tech?

He showed me the test and I heard the furnace light and felt the heat coming out. That's all I know about it. I quizzed him several times and he said there was no doubt what the problem is.

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I'm wondering if you have been back to CW yet? I sent a copy of your post as well as a link to this thread to my friend Chris Bryant, of Bryant RV to see if he had heard of the "test." I will post the response that he sent back. ;)

 

Hi Kirk,

I do not think that is a valid test at all for a sail switch- the sail switch is not in the combetion air flow at all.

If it lit with the vent blocked, it needs cleaning. What I have found for this is that small bugs (smaller than the screens) have partially plugged the orifice, so there isn't enough fuel to ignite- if you cut off the air flow, the air fuel ratio becomes good, and it fires, but it blows out right away.

That access door will come off- the vent tubes are both a tight slip fit, and have to be "worked" a bit back and forth, up and down to slide off. The vent can be left screwed in to the panel, I believe.

 

Hope this helps!

Feel free to post it however- thanks!

 

--

Chris Bryant

http://bryantrv.com

http://rx4rv.com

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Don't know if the OP has had the problem resolved yet, but my SF42 Suburban FINALLY ignited when the 4th tech service (over 3 months) to try to get it to ignite said the problem was too much air being blown over igniter when propane released, so there was never enough propane available to ignite with the spark: just blows propane outside (I could always smell it). He put an air flow restrictor in, and it does work, ignites and heats every time. Not sure this is a good long-term solution, so Tiffin has agreed to send me a new furnace under their warranty. At least I have real heat for the moment. Tech thinks a manufacturing defect, but Suburban claims it bench tests fine, not their problem, just my problem because it never works when installed in an RV (Suburban also had the gall to tell me I could not expect the furnace to ignite at altitudes over 4,500 feet!!!)

 

Rick

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I'm wondering if you have been back to CW yet? I sent a copy of your post as well as a link to this thread to my friend Chris Bryant, of Bryant RV to see if he had heard of the "test." I will post the response that he sent back. ;)

Interesting. And, no, I haven't been back yet. They had to order the switch. I wonder if this is a scam by the tech. When I told them I saw wasps, they said it would just be a $136 service fee which would include cleaning any insects or nests. It would be simple to fool someone like me when there is no way to actually see the switch.i think I'll call Suburban and ask them about the 'test'.

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