Suburban furnace works sometimes, not now

[Naturescalin]

I need some sidewalk superintendants.
My Excella, 31' furnace won't go on now. Sometimes when I have guests in it it has just blown cold air. I figured it was out of gas so changed tanks and it was o.k. But, now I just filled up the tank: nothing but cold air. Switched tanks: cold air. What's a guy got to do to get her to sing? See attachment pictures if necessary.
Thanks for any ideas about how to get the thing to work. I don't think I need to replace the furnace, just don't know where to start the process of elimination/problem solving.
I'll work on the soup while you all work on pointing me in the right direction. I'll keep my eyes open.

Thank you, thank you.
Brian

[Naturescalin]

Somehow, my diligent sidewalk superintendants I did not get attachments all there. I'll try to add the main one.


Brian

[alan]

I recently had a similar problem. The technician cleaned the ignitor and temporarily it worked ok. After one week, the problem recurred. I took it back for additional service. It appears there may be a problem with voltage drop. With everything up to snuff (battery, converter), the furnace works. When the weather warms up, I will take a look at the 12V wiring, looking for corrosion in the terminals and fuse connections.

[thenewkid64]

Here is the start up sequence, there are multiple safety interlocks that will prevent ignition, including low voltage.

http://www.airforums.com/forums/138293-post5.html

[Naturescalin]

One detail, the fan is blowing cold air. I hear the click of the gas opening up but nothing happens, no heat just cold air. Got another one of those old threads - I know I've seen one months and months ago, but not sure where or how to look.

Thanks so much for your help.
Brian

[lewster]

Hey Naturescalin,

If you have power to the blower and you can hear the gas valve opening, you apparently have 12VDC to the furnace. I could not tell from your back picture whether you have a re-igniter system or a module board. Plus you could have a blockage in your gas orifice located inside the heater box where the burner is or another type of blockage that is preventing the propane from getting thru the orifice to the igniter electrode. Can you hear the electrode sparking?

You really have to get deeper into the unit to further diagnose the problem. If you have never done this before, I would suggest not making yours the first attempt! (LOL)

IMHO, it does look like it is ready for an overhaul though.

[eubank]

Most of these units have what's called a sail switch, which detects blowing air and turns on the gas. (You don't want to have gas burning without a working blower.) The sail switch itself is pretty much as the name implies: It's a simple switch operated by a lever with a sail on the end: Air blows on the sail, and the lever moves to operate the switch.

I don't know how many of these things I've "repaired." It's quite a job, but I take them out, blow out any dust with compressed air, and reinstall. Also happened with our own rig: Took it out and found it really dirty. Ordered a new one (they're not very expensive) and cleaned ours up for the meantime. That was several years ago, the cleaned one is still working fine, and the new one is still packed in its original shipping box.

Of course, you can get this done at an RV repair shop. The big cost will be the labor.


Lynn

[Naturescalin]

So, is there any blow by blow description of what one must do to take apart and clean the thing?

Brian

[davidz71]

I had mine rebuilt about 3 yrs. ago and it had multiple problems. The furnace would ignite the first time but would not ignite thereafter. I pulled the unit out and took it into the local RV shoop. First of all, mud daubbers had built a number of nests inside the combustion chamber. They were cleaned out by the tech and I bought a screen to fit over the outside exhaust port. Second, the 12 v. motor was not always making it up to speed to trip the sail switch which opens the gas. I was hearing the igniter clicking off and on but it would ignite the propane because there was none. I was asked if it was not igniting on just the two 12 volt batteries or if hooked up to shore power. Sometimes it was both. He replaced the motor. He then found a bad board and replaced it. It bench tested fine so I put it back in. A month later I was having a problem again. Seems the temp limiter switch was allowing the unit to get too hot and it ruined the new board. He put in a lower temp limiter and replaced the board. Price came to $384 for the rebuild and I can't remember what the limiter switch was.

Moral of the story is: If I had known it would be over $350, I would have bought a new heater.

[dwightdi]

Lots of possibilities for problems. First thing to do is listen for the spark. If no spark but the gas valve came on your sail switch is ok but your ignition board is mostlikely bad. The original boards are also tempermantal to low voltage and easily damaged by over voltage. You really need to check that there is at least 12 volts at the furnace connector when the unit is fully on and the fan running. If that is OK you most likely need a new board. Dinosaur boards are cheaper and better. You can buy them direct over the net for a little over $100. A new furnace will set you back $500 or so but be all new and you do not have to worry about what will go wrong next. The fan motors frequently lose a bearing because they are not lubricated and that will be over $100 plus labor. It is no big deal to pull and reinstall the furnace yourself.

[wingfoot321]

Davidz71's advice is right on. Your furnace is 22 years old.

You might pull it yourself and have it bench tested. Maybe one repair of $100 would be ok but, if it is like mine was, it can get to 250-350 in a hurry.

Camping world will probably have a sale sometime this winter at about $500 for a new 30-35000btu furnace.

[eubank]

Originally Posted by Naturescalin

So, is there any blow by blow description of what one must do to take apart and clean the thing?
Brian

You mean the sail switch? If it's an NT24 (like ours), you turn off the gas, disconnect the wiring, take out all the screws (including the one that leads from outside), and remove the whole unit. Then remove screws and take the "guts" of the heater out of the metal box. The sail switch is located on the back side of the heater near the fan. Remove the tiny screws and take out the switch. Use compressed air to blow it out really well. Reverse the process.

As others have noted, you also need to check it for overall cleanliness, including from any dreaded mud dauber nests, which are deadly (literally).

It sounds very much like a sail switch problem, but it could also be the board. (Ours is so old that there isn't any circuit board, knock on wood.)

Orrrrrr, bring the unit to Angel Fire, and I'll fix it for you. All you have to do is to return the favor by fixing our lingering tail droop problem.


Lynn

[Naturescalin]

Sorry for absense. Fighting other fires.
I will pull the unit and clean it out. It seems from all the suggestions that the signal to fire up or the igniter is at fault. Is there a way to check specific parts of the board? I guess when I get it out I'll better be able to ask more questions. Thanks again for all the help. I'll post the solution.
Brian

[davidh]

Hi Naturescalin. Before you pull your furnace remove your electronic igniter first its a hole lot simpler to check before removing furnace. wright below the inspection window is your igniter remove both wires then romve the to screws pull out igniter and check for corrosion on igniter tip moisture is usuly the bigest problem with the or else you may see that part of the tip is coroaded off if this is not the case clean up the tip with some fine sandpaper and reinstal and check to see if it will ignite burner.the reason I say this is you may see it igniting through the inspection window but if the tip is bad it might not have enough spark to ignite burner. this happend to me but I ended up removing whole furnace when I did not need too. Its a quick check before you half to dig deeper..

[Darol Ingalls]

I suspect you have something stuck in the main orifice to the firebox. You have to remove the furnace to get to it. Consult your owners manual for the furnace, not the trailer. There is a steel gas pipe that bends around the rear of the fan chamber and goes into the firebox through a square plate with four screws. In order to remove this plate, you have to disconnect the pipe further back. There is a brass orifice at the end of this pipe inside the firebox with a tiny round hole. Spiders and other friends enjoy filling up these small holes and there just isn't enough gas pressure to clean them out. Best advise: get it working and then run the furnace at least once every two weeks to keep the buggers out.

[Silverhobby]

All of the above is good advice. Be sure upon re-installation to check for propane leaks by performing a drop pressure test, or test propane lines with propane liquid leak detector. Kev

[mandolindave]

I was reminded that the furnace blows cold air at the beginning and end of it's cycle. So I waited .......still cold. Then I tried burning the stove top
to make sure gas was in the lines. That did the trick. I suspect it is dirty
and I will probably experience further problems shortly.