Furnace Question

[Rusty_Recycled_Toy]

Ok, so this might be a no brainer for some of you, but it's bugging me.

I've been breathing some new life into this old '78 Itasca. It's time to tackle the furnace.
I took the unit ( Coleman ) out of the MH. Used compressed air to blow off the circuit board,clean out the fans,blow dirt dauber nests out of heat chamber.

Last week I installed the unit back into the MH. Hooked the wiring and gas line back up. Followed manufacturers start-up procedures to the letter.

Low and behold, as I watched through the sight glass the silly thing fired up. ;D

It started getting warmer and warmer. Then it went out. Then it lit back up.Then it went out, and didn't start back up.

I have repeated this scenario numerous times. It will light and burn for approx 1min. go out for about 10 seconds, light for 30 seconds,then go out.
What am I missing?
I've been reading some furnace posts and was thinking possibly the regulator? I mean sometimes 28 year old things get clogged/worn out.
When I light the burners on the stove, the more burners I have lit the 'better' the flames. With just one burner going it is almost like it is trying to blowitself out. Is this some indication that the regulator is malfunctioning?
So what would the general consensus be here? Bad regulator? Or do you think it is specifically furnace related?

Thanks,
Ed

 

[Shayne]

NO knowledge of your problem per se but I'd check the Thermometer and clean it, then check each wiring connector. Other thatn that when it comes to furnace and water heater, I call someone.  I don't mess with propane.

 

[Jeff]

Ed:

There is an overheat limit switch that could be bad or a problem with the board in your furnace.  A slow fan or obstructed duct could cause the overheat.

If you Google your brand of furnace you should be able to find service manuals for your unit and the troubleshooting section may help.

 

[Karl]

A slow fan or obstructed duct could cause the overheat.

Possible, but not likely. If it were a slow fan, the sail switch would keep it from lighting at all. Obstructed duct - same. Could be a disconnected duct that allowed the heat to recirculate in the furnace area, but his mention of the flame "trying to blow itself out" leads me to think it's either the regulator OR the propane tank had a lot of air in it and wasn't purged before refilling. What say you, Ed - did you purge it? Regulators are cheap - about $10 or less. 

 

[Rusty_Recycled_Toy]

Possible, but not likely. If it were a slow fan, the sail switch would keep it from lighting at all. Obstructed duct - same. Could be a disconnected duct that allowed the heat to recirculate in the furnace area, but his mention of the flame "trying to blow itself out" leads me to think it's either the regulator OR the propane tank had a lot of air in it and wasn't purged before refilling. What say you, Ed - did you purge it? Regulators are cheap - about $10 or less. 

I say, no. I did not purge the tank.
When I bought the unit it was already 3/8 full. No telling how long that propane has been in there though. Which leads me to wonder, does propane go bad over time? It is quite possible that propane is more than 2.5 years old.
The previous owner told me when I bought the unit that all he ever did to it was try to fill the fuel tank. I say try because he said he stopped at $65. :

As for the fan running slow, no. I promise you that the fan is moving plenty of air. That was one of the issues I addressed while I had it out of the MH.( I actually wondered after reinstalling it if it was moving too much air.)


Ed

 

[Bob Buchanan]

It started getting warmer and warmer. Then it went out. Then it lit back up.Then it went out, and didn't start back up.

I have repeated this scenario numerous times. It will light and burn for approx 1min. go out for about 10 seconds, light for 30 seconds,then go out. What am I missing?

Couple of thoughts. On a '94 and fairly new MH at the time, I had such an intermittent problem. They changed the board, but it continued to occur. A fellow in Reno found the culprit in the sail switch -- in that the paddle part could not function properly. The weather had something to do with it in that when cold (such as in Reno vs. down in the valley in Sacto) stray grease in the area would get hard and cause the sail to stick and not move freely. So my first question would be, when you see it stop after 1 minute, are you able to observe the sail switch actually opening? On my current furnace, one can view the sail switch. The sail itself on the '94 had worked it's way loose, so even tho the fan would cause the switch to close initially, the sail would then turn and not hold the air to keep the switch closed.

Secondly, on my current MH, there is a 1 by 1.5' hole in the MH floor under the wardrobe cabinet just over and to the left of the furnace. It's purpose is air return. For some reason, I found it obstructed. So was also wondering if all of your return air channels are open and unobstructed. Would imagine there is some kinda cut off if that air cannot return thru the ducts or other openings provided.

 

[Karl]

Bob,
Good point about the air returns. If the rig was left sitting for an extended length of time, mice and other critters may have built nests there.
Ed,
If you haven't already seen one, a sail switch is a small (about 3/4" square or similar) piece of metal attached to a microswitch which itself is installed in the combustion air side of the blower assembly.... just in case you were looking for a large object.
Propane, unless contaminated by something else, won't 'go bad'. I'm favoring the obstructed air flow and/or the regulator. And no, too much air flow wouldn't be your problem... unless the gas/air mixture is too lean. The mixture can be adjusted by moving the sleeve on the burner tube. Start by moving it to the closed position (slots are covered by the sleeve) until you get a yellow flame, then back it off (open up the slots) until you get a nice, even, blue flame. 

 

[Rusty_Recycled_Toy]

I noticed the sail switch when I had the unit out. Wasn't sure that's what it was, but thought it probably was. I'll watch it the next time I turn the furnace on and see if I can see it operate.

I think There is plenty of fresh air getting to the unit for the air returns. One side of it is open to the under seat storage in the rear bunk. Currently there is no lid on said storage. Allowing me access to the furnace.

 

[Gary RV_Wizard]

I'd bet on the high limit switch, which shuts down the burner if the heat exchanger gets too hot.  The high limit switch cuts out the ignition and the furnace will re-ignite as soon as the limit drops below its threshold. This is called "cycling on the High Limit" and is a fairly common problem. The fix is usually improved airflow in/out of the exchanger, but sometimes it's a bad limit switch. I realize you have looked at all that stuff, but it's still the most likely cause for the symptoms you describe. Sometimes having the furnace partially exposed, or having a duct connected or not connected can be enough to alter the heating in the exchanger.

We had a case here recently where a new furnace had somewhat similar symptoms when tested. Turned out the external exhaust & air intake tube/face plate had not been installed and this affected combustion  air enough to cause intermittent failure in the burner itself.  Did you have the exhaust & air intake stuff hooked up when you did your test run?

Propane doesn't go bad, nor is purging required after the initial fill of the tank when new. However, an old tank can accumulate moisture and oils from the propane and sometimes a glob of it gets sucked into the regulator or even all the way through the lines to the furnace burner. That will block propane flow and can cause no start or intermittent flame-outs, especially in a high-flow device like a furnace. If neither of the above things solve the problem, check the regulator and lines for a black oily goop. Or just replace the regulator - they are cheap enough and hard to see inside to check anyway.

 

[RLSharp]

I'd bet on the high limit switch, which shuts down the burner if the heat exchanger gets too hot. 

Gary,

I don't think the high limit switch will get too hot in one minute--even if it is defective. Ed indicates that the furnace shuts down the first time in about one minute and then in less than a minute on the next shutdowns.

I would guess that the flame sensing electrode is either corroded or not in the correct position to properly sense the flame. If this is the case the circuit board will put the furnace in lockout. A bad regulator would be my next guess. He needs to have the gas pressure checked. Erratic gas pressure will produce an erratic flame which the flame sensor will not be able to sense.

Richard

 

[Gary RV_Wizard]

I don't think the high limit switch will get too hot in one minute--even if it is defective. Ed indicates that the furnace shuts down the first time in about one minute and then in less than a minute on the next shutdowns.

If you are right about the one minute for the first failure, I would have to agree. However, as I read his description it took awhile for the first failure ("...started getting warmer and warmer").

 

[Rusty_Recycled_Toy]

OK, Here is something I noticed.

Obviously I have to have the front cover off the furnace in order to be able to see the sight glass. Last night on a whim, As soon as I saw the burner ignite, I closed the lid. The timing on the shutdowns has been consistent from the git go.
So I closed the front cover, counted to ten, and open the cover again to get a peak at the sight glass. The burner was already out. Had not even cycled long enough to heat up the heat chamber.

So I followed The shutdown procedure listed on the lid. Waited the prerequisite 5 minutes. Then started the start up procedure.

Without the front cover on it burns approx one minute before going out. With the front cover on it goes out within 10 seconds.


Looking through the sightglass while it is burning I can see that the entire burner has good blue flames along the entire length.
At the front of the burner on the left there are 3 thin metal 'rods'. All 3 of these rods extend into the flame and glow cherry red while the burner is burning. When the burner goes out, it does not relight until all 3 rods have quit glowing. THEN it relights. Rods glow, it goes out.

I'm not sure if I mentioned this, but we are dealing with a Coleman Presidential Electronic Ignition furnace.

To answer a question, yes. The exhaust and intake piping is installed and outside cover is screwed into place.

But here is my new question.

If I were to again remove the furnace, how likely would it be that I damage something by removing the sight glass (4 screws) and blowing air through to sight glass opening in hopes of blowing 'crud' out the exhaust?

As I stated earlier, There were dirt dauber nests in the chamber closest to the exhaust. I broke them up with a stainless welding rod and then blew air in (after shaking pieces out)to get smaller pieces out.
When I did this, A LOT of crud came out. But what if, some of it was lodged further into the heat chamber(s)? I would think that this might obstruct airflow.

Can I blow air in through the sightglass hole in hopes of forcing any sediment out the exhaust or do you think this may cause damage to the heat chamber?

It's just a thought.
Ed

 

[Gary RV_Wizard]

I'm not sure if I mentioned this, but we are dealing with a Coleman Presidential Electronic Ignition furnace.

Well that's a new one on me. Every RV furnace I've ever seen was either a Suburban or an Atwood, which have similar designs.  Didn't know Coleman ever made one and am not sure our previous comments are applicable to a Coleman design. Anybody ever work on one of them???

Those glowing rods may be part of the heat exchanger mechanism and might have some kind of limit switch function as well. Might also just be coincidence they glow/cool along with the cycling, i.e. it is an effect rather than cause of the problem.

 

[Karl]

Ed,
Does it look like the attached picture? If so, the rod connected to the thick red wire is the ignition spark wire. The other two are the flame sensor leads. When gas is burning, it ionizes the flame and those rods will pass a small current through it which is detected by the control board. If they are dirty or knocked out of place, they won't function and the flame will go out. Try removing the burner assembly and clean all dirt/carbon from them. When fired up, make sure both rods are In the flame; not just close to it and make sure they're not too far apart. You can experiment with them placement to get the right placement, but before you do that, make sure the coaxial cable that plugs into the board is clean and sound.

 

[Karl]

Ed,
Almost forgot. Blowing air through the unit shouldn't do any damage unless there is severe rusting. Maybe running a soft, flexible bottle brush in the intake and exhaust ports might help.

 

[Bob Flight]

Ed,
We had a '77 that had the same sort of problem, but once out stayed out.  The time interval to going out wasjust as it started to produce noticeable heat.  When we got the RV, the furnace had not been used and water found its way into the combustion chamber.  I think it was a Coleman furnace, but not sure.  It had a sight glass that screwed on like a bottle top, but it was angled down at a 45 deg angle & hard to see in.  The water caused excessive rust in the chamber.  I cleaned it out by using a small brass brush on a stick (toothbrush size), then vacuuming out the rust with a small tube (~1/2") attached to the vacuum hose.  Then I found that the fan forced flue exhaust caused the pilot to go out by blowing the pilot away from the thermocouple.  The thermocouple thought the flame was out and shut the furmace down.  I also found that the gasket on the sight glass access plate leaked.

Cleaning, adjusting the location of the pilot deflector, and resealing the gaskets allowed the furnace to work for many years.

Hope this helps.

 

[Rusty_Recycled_Toy]

Those glowing rods may be part of the heat exchanger mechanism and might have some kind of limit switch function as well. Might also just be coincidence they glow/cool along with the cycling, i.e. it is an effect rather than cause of the problem.

I was guessing that at least one of the rods was a thermocouple, and I was thinking that the other two might be the ignitor. I am probably at least partially wrong though.

Posted by: Karl 
Ed,
Does it look like the attached picture? 

Hard to tell. Mine is still inside the furnace, but kinda not really. (was that a vague answer or what?)


Through the use of modern technology I attempted to make a short video through the sightglass of what happens when it lights. But apparently when you have a 25mph wind blowing directly into the intake/exhaust it is more than somewhat reluctant to light. Figures.

I am going to pull the unit back out again tomorrow night when I get home and do a very thorough cleaning. I can't help but think I may have knocked some crud deeper into the unit the last time I had it out. That thought has been nagging at me for 2 days now.

I'll keep you posted,
Ed

 

[Karl]

Ed,
The igniter is only a single rod which makes a spark between it and the burner tube. The other two are the flame sensors. Trust me on this. Coleman doesn't use a thermocouple or heat expansion tube on anything but their standing pilot models.

 

[Gottasmilealot]

A few thoughts...

...I believe the rods are your heat thermocouple sensors that make the fan come on after the heat is produced, and also keep the fan running for a little while after the burner shuts down until the heat dissipates.  The other is probably be the safety that shuts the gas value if it doesn't sense heat from ignition.  These are probably working OK because the gas valve is not opening again until the heat is gone so you don't blow yourself up trying to introduce gas into the hot chamber when relighting. (hate it when that happens). 

...Does your nice blue flame change if the fan blowers comes on?  Maybe it doesn't stay lit long enough to tell.  You need to check your heat exchanger to be sure your heat exchanger doesn't have a hole in it. It was probably rusted over the years, and the the process of cleaning it up knocked the rust out, exposing a hole (or 2 or 3), especially if you were poking around with a rod trying to loosen bees mud. A hole in the heat exchanger will change the air current in the combustion chamber, drawing the burner flame towards the hole(s), which probably causes it to go out.  When you open the sight glass, the air is drawn in through there, allowing the flame to burn longer until the fan kicks on, then it will be snuffed out.  The flame goes out, and the gas valve correctly shuts as the operating safety. In you position, I would put a drop of peppermint oil in the combustion chamber through the sight glass hole and immediately shut it.  Turn on the unit's fan, and if you smell peppermint coming from you ducts, the exchanger is bad and the heater must be replaced.  I hope that's not the case, but you have to know, because a defective heat exchanger will allow carbon monoxide into the RV.  Not good for waking up in the morning.

...If your exhaust is restricted with crud, the high limit switch would shut down the unit, but the fan would come on before that to move air into the RV.  Checking outside to see if the exhaust is blowing strong should help you to determine if it's restricted.

...turn on your range burners and water heater, then light the furnace.  They should all be able to operate.  If the don't, have the regulator checked, and check for a valve that's not open all the way at your tank.

This is probably a situation that needs professional evaluation unless you are comfortable and qualified to do it yourself.  You sound like you're handy, but be careful. Good luck...

 

[Karl]

Robert,

Is it a stackable w/d?

Good thought, but he doesn't have a standing pilot light to go out. It's electronic ignition.

Gottasmilealot,

...I believe the rods are your heat thermocouple sensors that make the fan come on after the heat is produced, and also keep the fan running for a little while after the burner shuts down until the heat dissipates.

Unless the sail switch is sticking shut, the furnace will not ignite until after the blower is running for several seconds. Typically, the furnace motor powers both the combustion air blower and the heat circulating blower from a common shaft, one on each end. That's why most furnaces blow cold air for a minute or two when they turn on - the furnace hasn't ignited yet and isn't producing heat.

Just to clarify some terminology, if I may: A thermocouple is made of two dissimilar metals welded together at one end and may be contained in a tube-type shield to prevent contamination. When heated, it will generate a small voltage in proportion to the heat applied (it's not in direct proportion to the heat, but along a curve) which the control board determines is high enough to indicate there is a flame present, and allows the gas valve to remain open. Sensor rods on the other hand, will pass a small current produced by the control board from one to the other through the ionized flame (acting like a conductor of sorts), and the board senses this current flow. No flame; no current flow, and the unit shuts down. Standing pilot models use a capillary tube which contains a gas or liquid sealed inside it. When heated by the pilot light, the gas/liquid expands and pushes on a diaphragm on the other end of the tube to operate a switch which turns on an electric gas valve, or operates the gas valve directly - no electricity required.