Duo-Therm gravity furnace not working properly - Fiberglass RV


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Old 10-18-2010, 02:42 PM   #1
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Question Duo-Therm gravity furnace not working properly

Hi all,
I have a problem with a Duo-Therm 65512-2 gravity furnace. I bought the trailer (a 1980 Trillium 4500) this past weekend and was assured that the furnace worked. However, it does not appear to work all the time, which is a problem. I have been unable to find a manual or advice on this particular furnace, although Raya may have a manual and is looking into it. There is a listing in Google for BolerRV to have one, but that website (csc.laurentian.ca) appears to be not functioning.

This is a gravity type furnace with no fan. The pilot light lights easily and stays on. There is lots of propane. The actual heater seemed to work once (after a delay of several minutes) and then went off and won’t fire up again, very odd. It has a simple coil-spring-type RV thermostat on the wall of the trailer to govern the temperature.

I also own a Boler trailer with a different type of gravity furnace (Wagon Master) that works perfectly and does not delay at all, so I am stumped. I have a technical background and a degree in electronics, but I have not checked any of the connections in the furnace yet because I have not had time.

I am hoping that this furnace does not require electricity to run it, since there is no fan. At present there is no battery on the trailer, but I do have the original Basler model CP20 power converter that allegedly converts 120V to 12V (this is another thing I will be checking out in the weeks to come, as I noticed the ground plug on the main electrical input to the trailer was snipped off). I had the 120V plugged in when I was trying the furnace.

In my research on the internet, I have run across the possibility that the thermostat is powered by the output from the thermocouple, and also that the pilot light hole must be closed for proper operation because there is a sensor on the controller that thinks that the pilot is out if any air crosses it, however I don’t know if either of those suppositions is correct. Neither would have mattered for the furnace in the Boler.

Any advice you can give about what might be the problem would be appreciated.

Thanks,
Rick G in Edmonton
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Old 10-18-2010, 03:40 PM   #2
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Hi Rick,

I do have the paper manual still (it is the digital copy of it I made that I don't have access to until I get my other computer). I have the paper manual right in front of me now.

The furnace manual shows that it does not need electricity. There is an air adjustment screw on the 65212 model, but that is apparently to be adjusted if you have flame lifting from burner (too much air), or yellow/soot flame (too little air).

I do not see anything about troubleshooting your problem; only about the pilot light and what to do if it goes out.

The instructions are only two pages (plus a couple of more pages of installation, but that is more about clearances and such), so there is not much there!

Is there anything specific you would like me to look up for you? I have the pages right in front of me here.

Raya

PS: After reading your question again, I should add that it does say "6) Close lighter door after lighting heater."

It also notes that some of the heaters have thermostatic control and some don't. The "different" instructions for the thermotstatic control are to use it to set the temperature, and that you should allow the pilot to burn for two minutes before rotating dial to "on" position. Could the latter be your problem?
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Old 10-18-2010, 03:46 PM   #3
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Quote:
Originally Posted by Raya L. View Post
Hi Rick,

I do have the paper manual still (it is the digital copy of it I made that I don't have access to until I get my other computer). I have the paper manual right in front of me now.

The furnace manual shows that it does not need electricity. There is an air adjustment screw on the 65212 model, but that is apparently to be adjusted if you have flame lifting from burner (too much air), or yellow/soot flame (too little air).

I do not see anything about troubleshooting your problem; only about the pilot light and what to do if it goes out.

The instructions are only two pages (plus a couple of more pages of installation, but that is more about clearances and such), so there is not much there!

Is there anything specific you would like me to look up for you? I have the pages right in front of me here.

Raya
Is there anything about the 65512 model specifically, or about the thermocouple in relation to the thermostat? Since a thermostat is just an electric on/off switch, I don't know what electrical signal is being controlled by it. It might be the same as the thermal bulb type temperature sensor in non-thermostat furnaces, but the electrical signal must come from somewhere.

I will look at mine again tonight and try to figure out the details of the signal path, and of course also try to make it work.

Thanks,
Rick
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Old 10-18-2010, 04:01 PM   #4
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Okay, well why don't I just type these in. Here is the Owners Operating Instructions for Direct Vent furnaces, minus the standard safety disclaimers and that sort of thing:

1) Heaters with thermostatic controls, adjust thermostat or temperature control dial to "off" position.

2) Be sure all shut-off valves in supply line are open.

3) Turn gas control dial to "off," open lighter door and wait 10 minutes.

4) Rotate dial to "pilot," depress dial while lighting pilot burner. This is easily accomplished by using a wooden kitchen match and attempting to get the flame to the pilot burner before the phosphorous is completely burned from the tip of the match.

5) Keep dial depressed for 1 minute after pilot starts burning, and rotate dial to "on" position. (On heaters equipped with Millivolt Control, allow pilot to burn for 2 minutes before rotating dial on "on" position.)

6) Close lighter door after lighting heater.

7) Heaters with thermostatic controls, adjust thermostat or temperature control dial to desired temperature.

8) If pilot goes out, open lighter door and wait 10 minutes before repeating lighting procedure. If pilot needs adjusting, see Pilot Adjustment (for modulating and millivolt controls).

9) For complete shut down, push in on dial and rotate to "off" position.

10) If more heat is desired of heaters equipped with a thermostatic controls, turn thermostat or temperature control dial to a higher setting [duh!]

Pilot Adjustment
Remove pilot adjusting cap and adjust screw so flame completely envelopes the end (approximately 3/8") of thermocouple.

[Then they show a diagram of the two controls. The Millivolt Control is like a square box proportioned like a duplex outlet box in a house, with a small dial on the top on one end (side that would have outlets on receptacle) and the rest flat area.

The Modulating Control is more square, with a round outline on one side face and two knobs on top. It has 1-5 numbers on the dial for settings.]

Air Adjustment
An air adjustment screw is provided on some heaters. The 65211, 65311, 65411, 65510, 65512, 65515, and 65517 series heaters do not have an adjusting screw.

Too much air = Firing at orifice - Flame lifting from burner
Too little air = Yellow flame - Soot

There are diagrams showing where the screw is, but it's kind of hard to describe them. I will try:

On 65210 and 21R series, it looks like it is near a bottom pan corner and there is a little lock nut.

On 65212 series, there is a metal box/corner, and on the longer vertical face there is a little screw just to the lower right of the middle. There is something above it that is kind of square/round - maybe the pilot hole flap? It has a lock nut. Around the corner on the shorter vertical face there is a wire coming out.

That is basically the whole operating instructions.

Form number 19-424-7
Duo-Therm, Division of Motor Wheel Corporation
Printed U. S A. 1/75
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Old 10-18-2010, 04:55 PM   #5
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Smile

Hi Raya,
That is great! There are two clues in your typing (which I appreciate) that point the way.

First is the description of a “Millivolt control”. What the heck is that? Well Wikipedia has this to say:

Millivolt thermostats</SPAN>

As illustrated in the use of the thermostat above, the power is provided by a thermocouple, heated by the pilot light. This produces little power and so the system must use a low power valve to control the gas. This type of device is generally considered obsolete as pilot lights waste a surprising amount of gas (in the same way a dripping faucet can waste a huge amount of water over an extended period), and are also no longer used on stoves, but are still to be found in many gas water heaters. (Their poor efficiency is acceptable in water heaters, since most of the energy "wasted" on the pilot light is still being coupled to the water and therefore helping to keep the tank warm. It also makes it unnecessary for an electrical circuit to be run to the water heater. For tankless (on demand) water heaters, pilot ignition is preferable because it is faster than hot-surface ignition and more reliable than spark ignition.)

This means that the electrical signal to the thermostat is provided by the heat generated within the furnace. They also have a photo of a typical millivolt thermostat that looks almost identical to mine. The link is here: http://en.wikipedia.org/wiki/Thermostat#Simple_two_wire_thermostats

The second point is in the lighting instructions, where it says that if you have a millivolt control you should wait a couple of minutes on Pilot before turning the control to On. I did not do that because I didn’t know about the millivolt control.

So, I have things to try tonight to see if it works. Thanks for your help.

Rick G
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Old 10-18-2010, 11:38 PM   #6
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Okay, excellent! We are getting somewhere

I used to have a propane wall furnace at home that had what must have been a millivolt thermostat. I had forgotten about it, but I remember that one reason I had that furnace was it required no electricity (way out in the country where power outages are common in winter), and I asked "But, but.... the thermostat has wires going to it?" They said that the furnace itself "generated" that tiny bit of electricity. It did have a pilot light. I may even remember that they said "millivolts."

You probably aren't worrying about this anyway, but I wouldn't give much thought to the gas "inefficiency" of it. Because you usually light the furnace around the time you are going to use it and don't just leave the pilot running for weeks on end.

So.... I hope the two minutes does the trick! Wouldn't that be sweet?

Raya
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Old 10-19-2010, 10:11 AM   #7
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OK, so I got the furnace working last night and it works very well. It did not work at first, but after doing the procedure below it now works properly. Ultimately I think it came down to bad connections on the thermostat, but here is what I did and learned.

Since I didn't know what the problem was, I decided that this would be a good opportunity to take the whole furnace apart and learn what goes in it, clean it all out and set it up so I won’t have to do it again for a few years. So, I turned off the propane and disconnected the gas line, removed the pilot and thermopile and removed the burner assembly from the furnace. I separated the burner from the control unit, which was attached by one bolt. The burner assembly was filled with rust, which flowed out like fine sand. I doubt that this was the problem, but there was enough in there to possibly obstruct some of the gas flow.

The pilot light orifice looked good, and the main burner orifice also looked good although I was surprised at how big that hole was, larger than some others I have seen. In any case, neither orifice was the problem. The gas tubes also looked clear. I blew out the burner assembly a bit so that the top ridge of holes was not obstructed.

Then I reassembled the burner and assembly and turned to the thermostat and thermopile. Yesterday I did not know what a thermopile was, and it is basically a monster thermocouple, being made of several thermocouples in series to create a higher (but still low) voltage. They are also called “pilot generators” in the sales websites. One of the reasons I wanted to take the furnace apart was because I was puzzled that there only seemed to be two assemblies for the pilot, the pilot light and the thermopile, when I thought there might have been 3. Also, the pilot light seemed to burn down onto the thermopile below it, which I thought was odd. However, on investigation it turned out that this was the original setup and was the way it was designed.

When I measured the resistance in the thermostat, it was in the range of 150 ohms, which seemed high to me. Since it is just a switch, it should be close to 0. So, I filed away at the points where the elements make contact, carefully, and also sprayed it with electronics contact cleaner. When I tested it again, it was less than 1 ohm, which is about right. I also cleaned the electrical connections for the thermopile as much as I could.

Then I lit the pilot light, and tested the thermopile output. When I did this before disassembly, I was getting about 210 millivolts with the thermostat disconnected, and about 110 across the thermostat when connected. From the literature on the internet, the thermopile should be giving about 250 to 740 millivolts (that is 0.25 to 0.74 Volts) when disconnected. After reassembly, the thermopile was giving about 450 millivolts when the thermostat was disconnected, and about 30 across the thermostat when connected. This seemed about right as there should not be much voltage drop across the thermostat since it has very low resistance.

So, then I connected everything up and started the furnace according to the instructions as noted in the previous posts. I also found the metal nameplate for the furnace underneath it, where it had broken off from its position and slid at some point in the past. Lo and behold, it worked perfectly. I ran it for about an hour to be sure that the thermostat would go on and off at room temperature, then shut it down since we are not actually camping at present.

Also, when I reassembled the control unit, I bent the gas line for the pilot a bit so it would be out of the way when turning the main control knob (making sure not to crimp the gas line). Before it was right over top of the knob and it got hot in operation, meaning that I would burn my fingers trying to turn the furnace off.

So, thanks for your help Raya, it appears that the problem was solved. Pictures below.

Rick G
Attached Thumbnails
furnace control assembled.JPG   furnace burner assembly.jpg  

duotherm burner top.JPG   furnace control4.jpg  

furnace control3.JPG   furnace nameplate.jpg  

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Old 10-19-2010, 01:00 PM   #8
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Excellent!

And it's so nice that you took the time to write such a good follow-up. Future searchers will be very happy to read that!

Raya
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Old 12-07-2015, 02:42 PM   #9
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I've got the exact same heater, but mine will light upon signal from thermostat, but the flame is so small (maybe 1/8 inch) that it does not get the camper any warmer..Any clues on this one? Thanks
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Old 12-08-2015, 05:46 PM   #10
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never mind

the little orifice inside the burner which is maybe 1/32 of an inch was clogged with rust particles. I removed it and shook out the rust and reinstalled and everything works fine..thanks fiberglassrv for all the pics and info..Tobin
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