DuoTherm furnace - Post 1: Jerry-rigged sail switch

[Stereo]

Source for old service manuals

I found a copy of the owner's manual for my heater at www.bryantrv.com, specifically http://bryantrv.com/docs2/docs/operating/dt6600.pdf. It's the best source yet for guessing the age of my Skamper. The manual is dated 1979. (I couldn't locate a service manual which I assume would have a parts diagram.)

It provides orifice sizes for high altitude. 65 for 7000-9000 feet, 66 for 10,000 feet, 67 for 11,000 feet (as opposed to 61 for 0-4500 feet for which it was mfg). The manual also says "the gas input to the burner should be reduced 4% for each 1000 feet above sea level when the unit is used at elevations above 4500 feet" lest there be a build up of carbon monoxide that might make its way back into the camper. Fuel efficiency will also improve.

"Also, check gas pressure when derating the unit. First, check piping system pressure and adjust to 11" w.c. at tank regulator. Then check manifold pressure at the pressure tap location on the outlet end of the gas control on the furnace. Adjust to 10.5 w.c." Does anyone know how to do this (tools, procedure)?

Here are some other sources for camper manuals:

http://www.premiermobileservice.com/rv-owners-manuals.html
http://campingconnectionoutlet.com/w/5/rv-appliance-manuals-service

 

[Stereo]

Need ideas for cleaning rust off a heat exchanger

What are your recommendations for cleaning rust off a furnace heat exchanger?

I finally figured out how to take more of my DuoTherm furnace apart so I can now change out the orifice and clean more of the electrical contacts. I'll post photos of the dismanteling later. For now, I would appreciate your recommendations for cleaning the rust off my heat exchanger. I don't see any cracks so I'm hoping it's worth the effort of trying to get down to bare metal wherever I can reach. However, I came across one post that cautioned against scratching the metal, saying that would reduce the effectiveness of the exchanger. I don't really see why 'cause as far as I know, the heat is just conducted across the metal so I don't understand why a bit of abrasion would be harmful to the process.

Here's the outside of the exchanger with a small spot near the top that I scrubbed clean quickly with a green kitchen sponge scrubby.



Here's my attempt at cleaning the main body with 00 steel wool. After several minutes in one small spot, there is still a lot of pitting.



The inside "tube" of the heat exchanger where the fan blows through has about the same amount of rust. The inside of the combustion chamber also has a layer of rust.

Thanks for any tips you can provide about what to use on the rust. Note that I won't be able to get inside with any tools

 

[IdaSHO]

That is just surface rust. Nothing to worry about.

If you really want to treat it, try some OSPHO.


Regardless, even after you get rid of it it will only return. It is a product of the heat cycles and condensation.

 

[Stereo]

Nothing to worry about.

Thank you for the assurance.

 

[Stereo]

try some OSPHO.

I've been reading about OSPHO as a rust converter. Sounds like great stuff. If I were to use it on the heat exchanger, I assume I should rinse it off first, but it still sounds like a residue would be left behind. Would there be any concerns about odor or noxious fumes due to the high heat in the exchanger?

Reviews suggest OSPHO is a very effective product. I'll start using it on my truck when it gets scratches the I don't attend to right away.

 

[IdaSHO]

It is good stuff, when used correctly. It wont handle rust scale, just surface rust. But it takes care of surface rust very well. I use it often, mainly for prepping raw metal for paint. It converts any and all corrosion prior to priming.

As far as chemical off gassing goes, I would fire up the furnace and let it get nice and hot, BEFORE reinstalling into the camper.
If that isnt possible, make sure to run it with all the windows and doors open before using the camper.

 

[Stereo]

Furnace orifice size - Change it or not?

High altitude reduced the pressure output of the tank due to lower atmospheric pressure. Dropping pressure with the same size orifice = lower flow. /QUOTE]

As I've been reading more about natural gas appliances and altitude, it appears the possible source of ignition problems at high altitude is a deficiency in oxygen rather than insufficient gas delivery pressure. The pressure regulator on the tank maintains the LP gas output pressure at all altitudes. (That's the purpose of a regulator.) However, insufficient oxygen may hamper the burn.

My furnace manufacturer recommends smaller orifices (larger numbers) at the burner to reduce the fuel delivery. It also suggests reducing the gas delivery pressure at the furnace using the gas control on the furnace rather than the gas regulator on the tank. These things are said to 1) reduce the amount of CO emitted during the burn and 2) increase the heat output received from a given unit of fuel (i.e. improve efficiency.) These issues to not appear to relate to my problem with intermittent firing at start-up.

Most people on other forums who discussed supposed altitude problems other people complained about said they had NO problem at altitude. Some, however, suggest trying to increase exposure to outside air to provide the furnace with more oxygen (which I can't do since the vent inlet is fixed). I wonder if some people's rigs (mine?) have a more constricted vent system that limits air intake more than others.

I'm on the fence about what orifice to use. Mine is NOT easy to get to so I don't want to have to change it with any kind of regularity. During the summer, we mostly camp at 10,000 or higher (recommended orifice size 65). However, there will be times when we'll do road trips below 5000 (mfg installed orifice at 61). An orifice size of 64 (4500 TO 6500 feet) would split the difference.

When I bought my first vehicle back in the 80's, my dealer changed out the orifices in my carburetor for ones better suited to our altitude (5800). I drove my truck back to Ohio one summer for an internship and could hardly drive the thing. Is it better to have too small or too large an orifice?

As to my own furnace issue, I'm guessing it's either a faulty sail switch (soon to be replaced) or dirty contacts or wrong gap on the electrodes. Or maybe it IS just not enough power off my battery causing the blower speed to be too low. I do not know what the minimum battery charge is for the furnace to start normally. I'm assuming 60% is enough but maybe not.

 

[85_Ranger4x4]

Anybody else been down this road in the last 8 years?

I found this thread but I can't find the listed switch anywhere.