DuoTherm furnace - Post 1: Jerry-rigged sail switch

[Stereo]

I have an old Skamper (early 80s?) with a DuoTherm 66012-045 9k btu furnace. It has an electrode rather than a thermocouple. Sometimes the furnace starts right up when we call for heat. Other times, the fan runs but the pilot doesn't light. Because our problem is intermittent and we never let our battery run down below 60%, I don't think inadequate voltage is our issue.

I thought the issue might be inadequate gas flow. I've been told that spiders can get into the gas line and cause partial or complete blockage and wasps can build nests in the intake or exhaust ducts. All electrical connections also must be clean and tight, especially the sensor/electrode wire connection because it sends a microamp reading to the circuit board when the flame is burning and any impedance to this tiny amperage flow will cause the board to shut things down. So I decided to take out the furnace and clean it up. In doing so, I think I might have found what's causing the intermittent problem.

A previous owner or mechanic either needed to replace the sail switch and couldn't find the right part (DuoTherm is no longer in business), or he thought he could make the existing one work "better" by gluing an extension made from a piece of beer can onto the "flag." However, the extension was touching at the base, so I'm guessing it might at times have caught/dragged.





I won't know if the sail switch is the source of my problem until I put it back together and I won't do that until I can get the right sail switch. This switch is a critical safety device that prevents the furnace from running if the current isn't strong enough to power the blower fan adequately. I'm not comfortable using a jerry-rigged switch. What's on there now is a 3 amp switch made by Cherry Electric. I believe the part No. is E22. The mount holes were not altered but the arm's been bent and the flag has glue on it now that I removed the extension (which I thought was just pinched on). So I need a new flag switch.

Does anyone have an owner's manual for this furnace so I can get a part number? Or do you know a good source for "ancient" parts?

I knew nothing about how a furnace works before I pulled mine out but I got some advice from xdbx and read up on it on various websites. I want to be of help to other people who want to do furnace maintenance so I'm going to follow this post with some additional ones. The first will show pictures of my furnace. I couldn't post any more pics so I had to make a third post with questions I have. The fourth post explains what I've learned about how a furnace works. That one will includes a wiring schematic of my DuoTherm.

 

[Stereo]

DuoTherm furnace - Post 2: Photos of disassembly

Here's how I went about disassembling my furnace, a DuoTherm 66012-045 with electrode.

1. I took off the cover plate (just yank) and unscrewed four screws that held the lip of the furnace to the face of the cabinet. Here's what you'll see. Please let me know if I've mislabeled anything.



2. I opened the electrical junction box and disconnected the wires supplying power to the furnace. I then unplugged the wire connector to more easily access the gas connection and disconnected it. I pushed the wires and gas supply line out of the furnace box so I could slide it out.

3. The furnace just slides into the outside air vent, but it was pretty stuck, perhaps from a little corrosion. I tried unscrewing the exterior cover plate to see if there was another screw underneath, but in the end, some additional wiggling allowed it to come loose.



4.Once out, I unscrewed the front panels that were covering the electrodes and the burner and removed them. The burner just slips out after taking off a tension washer.





5. I also uscrewed the cover to the gas valve 'cause I didn't know what it was at the time.



6. I then removed the rear panel (more screws) which exposed the first squirrel fan that pushes air into the combustion chamber. That just pulled off to allow the next cover panel to be removed.



7. Removing the second panel exposed another access panel and some mud dauber nests which weren't causing any problems.



8. Attached to that panel was the second squirrel fan which blows air across the heat exchanger into the camper. I could now see the sail switch which a former owner or mechanic had added an extension to. It's a critical safety device so I'll need to replace it - if I can find the part.





I've just gotten a message that I can't add more photos so I'll have to ask my questions in the next post.

 

[Stereo]

DuoTherm furnace - Post 3: Re-assembly and maintenance questions

I've now got the furnace apart but I need your ideas on a few things.

1. How do I clean out the gas line? There are two right-angle turns, one at each end, so I can't thread anything through to clean it out. I did blow compressed air through the line as best I could. Also, the orifice is pretty unaccessible. It's all the way at the back and there are no access panels. I ended up taping a sewing needle on the end of a stick and reaching through from the front to poke through the orifice to ensure it wasn't blocked. I just don't know if this is sufficient.

2. The gaskets behind the front access panels are shot. What material can I use to make new ones? It has to sustain very high temperatures.



3. I'm wondering if the blower fan motor is sealed or whether there's a way to lubricate it. Are the black dots access covers? If it can be lubricated, how do I do that? (oil type and amount)



4. The ID panel says the furnace is "For use from 0-4500 feet." Are there any safety issues associated with using it as up to 12,000 feet?

Thanks for your help.

 

[IdaSHO]

Good luck running anything worth a darn on propane at 12k

Most start having trouble above 7k and give up around 10k



At the very least you will have to alter/modify the unit.
You will need to adjust the manifold pressure and swap out the orifice, as well as run an adjustable regulator on the tank.





On the problem at hand, I STRONGLY suggest you find a way to get that orifice out to clean it.
The vast majority of furnace problems I encounter are related to blockage or partial blockage of the orifice.

 

[IdaSHO]

Do you happen to have the model number of the unit?

There are loads of manuals floating around on the internet for old furnaces.

 

[Stereo]

DuoTherm furnace - Post 4: How a furnace works

I wanted to understand how a furnace functions so I would have an easier time troubleshooting it. The sequence was made pretty clear here http://www.rverscorner.com/, but I reworded it so it made more sense to me. I do think the sequence described on their website is a little different than what my wiring schematic shows, but then again, I am challenged to follow electrical diagrams since they don't have arrows to show direction of flow.

Here's my furnace's wiring diagram:



What follows is my understanding of how the furnace and its safety mechanisms work:

• The thermostat calls for heat (In our case, we thought the thermostat might be the problem so we replaced the thermostat with a simple switch since we only use the furnace to take the chill off in the early morning or night.)
  
  
• Electrical current passes through the wire that runs to the furnace, through a fuse, and on to a fan relay which handles the relatively high current needed to run the blower motor. (It also allows the blower to run for 45 to 90 seconds after the thermostat is satisfied so that excess heat in the chamber can dissipate before the blower stops.)
  
  
• Current then flows to the blower motor which drives two squirrel cage fans. One fan sends air into the combustion chamber, the other blows air through the heat exchanger to provide heated air to the camper.
  
  
• (I'm not totally sure about the next sequence of events. Based on my furnace's schematic, current then appears to flow to a control module/circuit board. A description on the website above suggests that the current first goes to the sail switch and then to the control module. I'm guessing current actually flows through the control module to the sail switch while the module also sends current to the gas valve and igniter as described in the next bullet points.)
  
  
• The air flow generated by the blower flows across a sail switch which is open when the furnace is off. (It gets it's name from the "sail" or paddle that is attached to the switch mechanism.) If enough "wind" is generated, it pushes the switch closed, thus allowing electrical current to flow to the next component in line. If the fan does not come up to 75 per cent of it's design speed, the sail switch will not close. Inadequate air flow can be caused by a low battery, restricted return air inflow, restricted or inadequate outlet vents, restricted combustion air inlet or exhaust, or a faulty sail switch.
  
  
• If the sail switch closes, current is sent to a limit switch (and a control module/circuit board if it didn't start there; again, this is the order that I'm confused by.) The limit switch is a simple temperature-controlled switch that monitors the combustion chamber heat level. If the temperature of the combustion chamber exceeds the preset limit, the switch will open and disrupt the flow of current to the circuit board (Is this after passing through the fan relay as suggested by my schematic?), shutting down the main burner. Once the chamber temperature cools sufficiently, the limit switch resets. This initializes the ignition sequence and starts another cycle.
  
  
• After about a 15-second delay to allow the blower to purge the combustion chamber of any unburned gases, the control module sends power to the gas valve which opens to let fuel pass to the burner.
  
  
• At the same time that current is sent to the gas valve, the control module also sends high voltage pulses to the electrode assembly. This device allows current to jump the 1/8th inch gap between the electrode probes causing an ignition spark that ignites the gas/air mixture.
  
  
• The control monitor evaluates the burn cycle through the electrode assembly which acts as a sensor that detects the presence of the flame. If the sensor does not detect a satisfactory flame within about 10 seconds, the control monitor then shuts off the gas valves and discontinues the ignition spark.
  
  
• Depending on the control monitor/circuit board design, it will try for ignition up to three times. Beyond that point, it goes into lockout mode and will not retry for ignition until reset BUT the blower will continue to run.

Please let me know if any of this information is wrong so I can correct it.

 

[IdaSHO]

That diagram makes me love my old gravity furnaces so much more.


So darn simple.....

I just finished restoring the one Im using in my custom camper.

It is an old Hydroflame 8,400BTU

I also retrofited a new milivolt valve for it, to allow a traditional wall thermostat to be used.


No noise, no 12v needed, just heat

 

[Stereo]

Good luck running anything worth a darn on propane at 12k. On the problem at hand, I STRONGLY suggest you find a way to get that orifice out to clean it.

The furnace has actually done a great job keeping us warm and we're rarely below 9500 feet. It's just that it doesn't always fire up but that hasn't been related to altitude.

I did manage to poke a needle into the orifice by taping it to a stick and reaching through the front of the furnace.

 

[Stereo]

Do you happen to have the model number of the unit?

It's a DuoTherm 66012-045. I searched the internet using that number but didn't get any hits. Is there a specific site where I should look?

 

[Stereo]

I just finished restoring the one Im using in my custom camper.[/IMG]

Nice work!! Where do you find those kinds of furnaces? Junk yards?

 

[IdaSHO]

You asked, I answered. If it hasnt been a problem for you, then why even ask?


By the way, not always firing IS a sign of high altitude.

High altitude reduced the pressure output of the tank. due to lower atmospheric pressure.

Dropping pressure with the same size orifice = lower flow.


And if it is having trouble lighting because of low flow you are also wasting propane as the flame goes out and the valve waits for the thermocouple to cool down.
The valve will remain open allowing propane to spill into the furnace until the thermocouple is has cooled.

 

[IdaSHO]

Nice work!! Where do you find those kinds of furnaces? Junk yards?

Scouring craigslist normally.

I have 3 of them now. One in the old camper, one in the new, and one for parts if I need them.


Good old school furnaces.

 

[Haf-E]

I picked up a nice non-electric propane furnace from a sale at Lowe's in the spring a couple of years ago - works really well for heating a loft apartment space in a shop building I have.

Its a direct vent type unit - so the burner's intake and exhaust are sealed off from the inside air and go through a single "coaxial" pipe through the wall. Doesn't add moisture to the interior air and should be safer - less chance of CO poisoning.

I think it would be a good option to consider for a camper since it uses no electricity. One nice feature on the one I have is that the burner turns down to a lower level before going to pilot only so it cycles on and off less. Mine has only been used at low elevations so I can't say much about high altitude operations unfortunately. I did have the pilot clog once due to foreign material getting into the propane line - so a filter by the furnace is a good idea to consider - especially if the propane line is disconnected when not in use - it will save a lot of time messing with it trying to get it to light or stay light on the pilot...

 

[Stereo]

You asked, I answered. If it hasnt been a problem for you, then why even ask? By the way, not always firing IS a sign of high altitude. High altitude reduced the pressure output of the tank. due to lower atmospheric pressure. Dropping pressure with the same size orifice = lower flow. And if it is having trouble lighting because of low flow, you are also wasting propane as the flame goes out and the valve waits for the thermocouple to cool down. The valve will remain open allowing propane to spill into the furnace until the thermocouple has cooled.

Please re-read my post. I asked if it was a safety issue to operate the furnace at high altitude. I had assumed that altitude was not the issue since I thought that if it was, the furnace would not work at all. You have educated me otherwise.

That said, I don't have a thermocouple. I have an electrode assembly. From what I understand, propane will not continue to be delivered to the furnace if combustion fails because of the safety mechanisms, though I could be wrong since all this is new to me.

Now that I know that the issue of intermittent lighting could be altitude, I'd like to ask 1) how do I adjust the manifold pressure (actually, I need to first know where the manifold is) and 2) How do I know what size orifice to get if I decide to change it out?

 

[Stereo]

Scouring craigslist normally.

What search words have you found to be successful?

How wide are these type furnaces?

Would they also have an issue with altitude? I'm guessing so since you still have combustion.