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Jammer · 05-13-2014, 02:10 PM

The Duo-therm Glen Aire vertical gravity furnace was used in certain Avion trailers, and C-11 pickup campers made under both the Avion and Cayo nameplate. It is the ideal boondocker's furnace since it requires no electricity. It is a sealed-combustion design that pulls in outside air through a concentric roof vent, making it superior in every way to catalytic heaters and even to the Panel-ray heater. Because it is controlled by a wall thermostat, it has advantages over the Newport Propane Fireplace made by Dickenson Marine.

These heaters are built to last and are worth saving.

Over the years the gas valves will stick, so the valve knob won't pop back up under spring pressure after lighting the pilot. The valves aren't repairable but are easy to replace. Also, these were manufactured as 18,000 BTU/h input (12,000 BTU/h rated ouptut) heaters. That's fine for larger trailers, but too much for the smaller trailers and the C-11, and will lead to large temperature swings. It is possible to reduce the firing rate.

This week I disassembled my furnace, cleaned it, and replaced the gas valve.

Suitable gas valves are readily available. It is important to use a low-profile valve that will fit the space. I used a Robertshaw 710-502 valve. This valve ships with a regulator intended for the lower pressures used with natural gas but can be converted to unregulated with cover place kit 1751-007. Or, there is a propane regulator available as part 1751-013. The original gas valve in these furnaces was unregulated, but using a regulated valve provides an easy way to reduce the firing rate if desired, by reducing the manifold pressure a little.

I needed a 1/2" input and 3/8" output. Usually the valves come with 1/2" taps in 3 directions and a set of reducing bushings and plugs.

I use and recommend supplyhouse.com as a parts source, but there are many places to order these.

The burner, pilot, valve, and baseplate assembly are removed from the heater as an assembly. Here are the steps:

1) remove the bottom access grill as you would when lighting the pilot.
2) Remove the two screws attaching the sheet metal diffuser housing and trim. Carefully remove the diffuser and trim by pulling up and out. There is a lip at the top that secures the assembly to the wall.
3) Remove the blower assembly and thermostatic switch. Though it is a tight fit it should just slide out. It may be necessary to remove a screw holding the thermostatic switch in place.
4) Shut off the propane supply and bleed the lines, either by lighting a stovetop burner, or by venting them outdoors in a safe location.
5) Disconnect the flare nut holding the copper supply line to the valve, using a backup wrench on the fitting.
6) Remove the thermostat wires from the gas valve.
7) While supporting the valve, remove four nuts holding the base plate to the heat exchanger.
8) Carefully lower the valve, burner, and baseplate assembly.

Some units may have an asbestos seal between the baseplate and the heat exchanger. Don't disturb it or used compressed air on the gasket. I cleaned the burner outside, trying to disturb the gasket as little as possible.

The pilot and thermopile generator can be replaced at this point if necessary. Mine was still good.

There was quite a bit of rust and debris that came out of the heat exchanger. I vacuumed it up with a shop vac. I inspected the interior of the heat exchanger with a mirror and also put a light inside to check for cracks or pinholes.

I removed the gas valve and output elbow, and installed the new gas valve using a paste type pipe dope suitable for gas. Rectorseal is my favorite but they don't carry it at Menards. I used the top outlet, which makes for an easier pilot light run but required me to extend the supply line by a couple inches so it would reach.

I was able to re-use the aluminum pilot line by cleaning it up, gently re-bending it using a tubing jig, and connecting it to the new location.

Photos to follow

Jammer · 05-13-2014, 02:23 PM

Showing the original valve just after removing the baseplate and burner assembly. The blower assembly has been slid out to the left.

Jammer · 05-13-2014, 02:25 PM

After cleaning the burner and replacing the valve, I testing the assembly by setting it on a mop bucket and connecting a low-pressure propane line I use for my gas grill. The burner itself is heavy cast iron and will last for centuries.

Note how the pilot line has been rerouted.

At this point I also leak-tested all the fittings using a commercial bubble-producing gas leak test solution.

Jammer · 05-13-2014, 02:28 PM

Reinstalled in the furnace. Everything just fits, but I had to add a swivel and union to the gas line for it to reach. Had I used the original setup with the outlet to the right and then an elbow, the gas line would have reached but the pilot line would have been difficult to route.

Jammer · 05-13-2014, 02:37 PM

So, as I noted upthread, 18,000 BTU is way too much for my C-11 and it makes for wild temperature swings at night, very noticeable when sleeping on the cabover bed. I experimented with the firing rate while the burner was on the mop bucket, and the burner will still ignite reliably and burn clean at much lower rates. There are two ways to reduce the firing rate. One is to change to a smaller orifice. That would be easy enough to do while everything's apart since the orifice is a standard one and readily accessible. The other way is to reduce the manifold pressure.

Again, the original stock setup used an unregulated valve so the manifold pressure was 11", give or take whatever the tank regulator was kicking out. For the time being I have left the natural gas regulator in the valve in place. At its factory setting, 3.5", there was clearly not enough heat. The regulator is easy to adjust with just a screwdriver and I've turned it up about as far as it will go. I haven't measured the gas pressure but it's probably 6-8" or so. I have a propane regulator on order, which comes with a stiffer spring so it can be set for higher pressures, but it's unclear whether I'll use it. It's easy to install without removing the gas valve from the furnace, just a matter of 4 screws.

I ran the furnace for a while this morning and on a 45 degree day was able to get it up to 80 degrees inside, so we'll have plenty of heat for any weather we'll encounter between now and the end of October. When winter weather returns I'll try the furnace in cold Minnesota conditions and decide whether to make any changes.

OrquiAH · 01-17-2015, 09:39 AM

Thank you for posting these instructions. We just bought a Silver Streak 1963 Sabre that has a Glen-Aires wall furnace model 17-SWT (18,000 BTU?). We have no literature other than how to install it (no user guide, no parts, no nothing). We have been struggling with getting this baby to fire up (new thermostat, blowing out the lines, etc.) and have been reluctant to tinker with it because we are newbies to this and have no directions or owner's manual to assist us. Your retrofit using the Robertshaw valve may solve our problem. We can get the pilot to stay on but the furnace will not fire up (doesn't matter what temperature the thermostat is set to) and occasionally there will be a flash, as if the unit wants to fire up, and then nothing. We suspect valve problems. Your directions on how to dismantle the base plate and burner assembly (ours looks just like your picture and the number on it is A0D-PG9A26) is a lifesaver for us. Did you use the Cover Place kit #1751-007 at first and then decided to switch to the Propane regulate #1751-013? These parts are readily available for about $100 plus shipping from supplyhouse.com.

M2HB · 01-18-2015, 08:43 PM

I haven't had to rebuilt mine yet, but these are great instructions and part #s.

Great job.

PaulnGina · 01-18-2015, 08:51 PM

Wow!!! What a great write-up and information dump!!

I really like the old Cayo/Avion truck campers. Some day, I'd like to add one to my back yard to be able to use when the 34W doesn't "fit".