HELP!! Furnace frustration!

[Forrest]

I have an original Duo-Therm Model 15T direct vent furnace. It will not stay lit. I can get the pilot light going, and it will stay lit until I turn the thermostat on. The pilot light will light the burner nicely, but then 30 to 60 seconds later, with the burner still going, the pilot light goes out. There is an immediate "click" as the automatic safety control shuts off the supply of gas and the burner goes out too.

The frustration is that this was the only utility in the trailer that functioned when I bought it. Since then I've dismantled, cleaned and repaired everything (including the furnace) and now everthing but the furnace works. I even replaced the thermostate and burner assembly with "new" old stock. The flame for the pilot and the burner burn a nice crisp blue. I did replace the old LPG regulator with a new Marshall. The Marshall seems to working fine - all the other utilities (including the original refrigerator) work, but I wonder if it is letting gas through at too high a pressure. Laying on my side to watch the pilot light in the furnace it did sometimes appear that the gas fueling the burner blew the pilot light out. At first I figured this was just air in the line that hadn't purged, but I think by now all the air in the line is gone. Any ideas about this????

[thenewkid64]

You may need to check the actual gas pressure in the line. This is best done with a pressure gauge designed for propane. You should have 11 column inches of pressure.

The furnace is a large draw source so when the main burner kicks in it may be drawing the presure down to the point the pilot will not stay lit. There should be an adjustment for this on the regulator you installed. They say they are pre-set from the factory but they usally require some type of adjustment.

Any RV service place will have the tools to test this, or you can buy the tool. But for a once a year thing (you are suppose to check this yearly) I don't think it is worth the investment.

[smily]

I literally have the exact same problem!

But right around the entrance where the pilot tube and the thermocouple enter the heater body there is a "gap" around the old asbestos gasket. This gap allows the air from the fan motor to be sucked into the gap. You can see an erratic flame at the pilot when this occurs. You can actually see the flame "follow" the draft of the air leaking around this gap.

I always wondered why there was a greyish matter around this area that certainly did not look OEM. It was Muffler Mender Compound. It was placed by a PO. The substance had gotten brittle and flaked away with vibration.

Today, I put some new Muffler Mender compound around this area and sealed the gap.

Like brand new money!!!!!!!

I am going to go out and turn off the furnace right now. It has ben running for4 several hours now.

If your Pilot light has an erratic flame and it appears to be flaming backwards, (towards the pilot tube entrance fitting). this may be your problem.

One other symptom I have seen is that the thermocouple is not close enough to the pilot flame to maintain the "heat" applied to the thermocouple. I slightly bent the pilot shroud towards the thermocouple and this fixed my problem.

Hope you can fix yours with this advice.

Smily

[Forrest]

Thanks for the suggestions. I'm going to build a simple manometer tomorrow to see if I'm getting 11 water-column inches of gas pressure. That seems like a good place to start. If it turns out I'm getting the correct amount of pressure then I'll try to figure out if a draft is blowing the pilot out. However, I don't have a fan on my furnace. It works on convection alone. I thought that maybe my leaving the little access door open (to light the pilot) was creating a draft that blew out the flame, but it did the same thing when it was closed. Anyway, I'll post to let the both of you know how it goes.

Thanks.

[Chas]

Talk about frustration, I can definitely recommend to make sure you have a screen on your furnace vents. I tried to fire up the furnace on my recently acquired Overlander. The pilot lit just fine but when I turned the stat to heat all I got was a muffled pop and some noticable groaning from the converter. I pulled the furnace and found a huge colony of mud dauber nests. Spent hours disassembling the unit, shaking and tapping the heat exchanger, cleaning the blower wheels and putting it all back together. Another tip, I noticed the heat output at the rear was low, almost non-existent. I pulled the duct apart under the bed storage bins and found little mouse nests of pink insulation. Seems the little critters had been in the bellypan and pulled the pink stuff up into the duct through the holding tank heat scoops. The nests were almost completely blocking the ducts.

Chas

[Forrest]

I made a simple water column pressure gauge from scrap tubing and peg board. I hooked it up to the gas supply for one of the stove top burners. It gave a reading of about 12 1/2 inches. So, I was getting too much pressure and the gas from the burner was blowing out the pilot light. I made the adjustment to the regulator and now the furnace works great! The nice thing about the water column gauge I made is that it is large enough to see the reading through the front window. I was able to adjust the regulator while directly reading the gauge. The peg board holes are exactly 1" apart, and I simply ran red wire trash bag ties through the holes to attach the tubing. Then I added green food colored water to the tubing. It looks almost like a Christmas decoration doesn't it?

[MsLed]

I'm really impressed by your homemade fix! I also replaced my regulator w/a Marshall. Unfortunately I put pipe dope too close to the threads and it started leaking from the tank pointer lever (nothing like a technical term) or atleast that's the only thing I could come up w/to cause that failure. I've since replaced it with an identical unit, gotten my alum tanks filled, recerted and OPDs installed- ran the stove about ten minutes and the furnace maybe five hours and I'm outta LP. Somethin' just ain't right. I guess I should go around w/soapy water and check all the couplings and connections?
Maybe after that I can make a homemade pressure checker and use a hint of red food coloring so it'll be pink. TeeHee.
Any helpful hints'll be greatly appreciated. Thanks, Debi AKA MsLed

[Forrest]

Hello MsLed,

Do you have a regulator that has a red/green indicator on top of it(red indicates your tanks are empty, green indicates the tank still has LPG)? As long as your tanks are not empty and if you have an indicator you can check for leaks by closing the valves on the tanks and all the shut off valves for your appliances (e.g. your stove, and refrigerator - on the water heater and furnace just turn the control to off so that there is no pilot light lit). At this point the indicator should still be green. Leave the system like this for at least 30 minutes or more. When you return if the indicator is still green then there are no leaks (between the regulator and appliances). If it's red there is a leak, get out the soapy water and check for bubbles. If there is no leak, but your tanks still become mysteriously empty then they may be leaking at the tank valve itself - take the tank(s) in for service.

[MsLed]

Hey - thanks for the Propane class. Musta' died from that in a previous lifetime 'cause it sure holds my attention. Gotta get 'em filled before I can check 'em to watch 'em leak again. I JUST had the tanks recertified so I'm thinkin' it's "somewhere else". A couple of my valves (HWH for one) are really tight - Ya think I can WD40 it? Or should I replace it? Is pipe dope not to be used at all? I vaguely remember the container saying it was OK for it.
I've been practicing senior moments here lately.
Thanks again !

[Forrest]

Most gas line fittings have flare fittings and need no pipe dope. They should be snug, but over tightening them can rip the flare on the tubing and cause a leak. Pipe fittings can be assembled with dope, but I prefer the yellow teflon tape made for gas. WD-40 on the tight valves is okay, but I'm not an expert on any of these things. I don't mind following my own advice, but gas can be dangerous. If you have any doubts perhaps you should have things serviced by professionals.

Forrest

[hex]

Hey Forrest very interesting water column device. You did not explain how much water to add, isn't that critical?
Did you happen to run a 2nd test at another gas outlet? Would a partial crimp in a line increase exit pressure (like a thumb over a water hose) ?



Chas great tips, is there not a screen or something to protect from criters entering the outlet scoops from the basement? Is the complete duct run (except the scoops) above the floor? I always thought the ducts were in the basement with only short returns up to the room registers??
Also what is a blower wheel?And how much crud came out of the exchanger?

[Chas]

Yes grasshopper, hex,

I will try to hit on a few of your queries, the heat duct in mine runs above the floor, just beneath the cabinets and under the bed storage bins terminating under the shower. I am not sure if there are screens in the scoops that feed the basement tank areas, I assume that is where the pink insulation came from. The blower wheels, two on a common shaft, are part of the furnace, they pull room air over the heat exchanger in order to heat it and also, separately, pull combustion air from the outside and exhaust it back out again. There were a lot of dauber nests in it, I was almost to the point of just replacing the whole furnace but feel that I got almost all of them out without damaging the heat exchanger. I think someone had taken the furnace out before and failed to put the screens back when they reinstalled it.

Chas

[ALANSD]

I asked this before and got no replies= when you turn the air adjustment screw out to make the flame in the furnace burner more blue (to eliminate sooting) and the screw is now all the way out...what else needs adjustment? Would I check the gas pressure as above, or is it related to the air intake..my ignitor seems to be fairly new from the look of the part. Thanks

[Forrest]

Hex,

First, Bob Livingstone's book, RV Repair & Maintenance Manual, has a nice drawing showing how to hook up a water-column gauge and what it looks like. He simply states that it should be "filled midway," and that with 11 column inches of gas pressure "the water in the gauge is pushed down 5 1/2 inches ... and up the other tube 5 1/2 inches." He writes as though there are two tubes, but of course there's really just the one tube, bent in a U-shape, either out of plastic or glass tubing. The main thing is that the device has to be tall enough to measure a difference in water level on one side to the other of 11 inches. If you put in too much water some of it will be pushed out the open end of the tube. If you put in less than 11 inches of water (I suspect) it will be pushed completely out of the tube - resulting in gas leaking out (just turn off the burner control if this happens).

I don't believe that a crimp increases pressure at the gauge. When you put your thumb over the end of a water hose the pressure stays the same, but the velocity of the water coming out increases. I think I remember that from one of my high school science classes, but perhaps someone with a better understanding of such things can post a note for us.

[Forrest]

There's a more detailed explanation with diagram at this web page: http://www.rverscorner.com/articles/manometer.html

If you have Bob Livingston's book the diagram is on page 2.5.

Compare the diagram at the web page with the one in Livingston's book! They are identical, but both are copyrighted.

[Sasquatch]

I too have problems with the heater pilot staying lit. I got the trailer from my Dad last summer and he said the furnace gave him fits. I haven't worked on it yet since I've been restoring the rest of it. I had the outside shrouds off when I polished my trailer. I installed stainless steel screen with a similar mesh size as you have on window screen on all of the vents to the outside to keep the critters out. When I pulled the outside shrouds off I found dirt dobbers, red wasps and spiders in mine. I know this will affect the functionality of the unit due to air flow restrictions. Maybe that had something to do with it not working for Dad. I'll give it a try in the near future and see. I tried to cover every other hole in my trailer with screen also. Roof sewer vents, hot water heater vents, refrigerator vents, A/C vents on the old Duo Therm. Every thing I pulled off the trailer was full of critter nests! I also had a couple of mouse nests inside the trailer so I tried to caulk and screen everything.

[roessler]

Actually, there is an increase of water pressure at the end of the hose, thats why the stream of water can shoot further out. The physics of the situation is exchanging flow rate for pressure, i.e., converting kinetic energy (flow) to static energy (pressure). Of course, the pressure is limited to, say in a home, the pressure coming from the city water main.

[Forrest]

Are we comparing apples to oranges though? The original question was if a crimp in the gas line increases or decreases the gas pressure at the exit (at the appliance?) Does the water pressure in a hose compare to the gas pressure in a line. Do they share the same physics? I'm not saying that I know one way or the other, but it seems to me the two might be quite different.

[wallybyam]

Actually water shoots further out when you pinch off a garden hose because of (assuming that flow rate stays the same) the increased velocity necessary to maintain a given gpm through the smaller orrifice at the pinched end. Guage pressure would increase because of the added friction loss at the smaller end, which is a result of the increased velocity of the flowing liquid.

Water and gas do not have the same dynamics because of the different compressability factors of the two.

Bruce