Fuel burning heater warning

[cranetruck]

Over the last few years, I have spent a lot of time on the fuel burning heater and have come to consider it fairly safe to operate, in particular with diesel fuel.
My 8x8 was never equipped with any other type of heater, so there wasn't much of a decision here.

Any malfunction that I have run into, was easily attributed to old and corroded units and I have been playing with more than 1/2 dozen units made by Stewart Warner and Benmar types.

The military technical manuals rarely explain anything, beyond statements like "replace" or inform next higher level of maintenance and as we all know that can become frustrating at times. Typical instruction for starting the fuel burning heater is shown in the image below.

The point is that fuel can get past the combustion chamber and into the discharge air on a brand new heater too. I posted a note on this some time ago, but decided at the time that the leak was due to corrosion. Not so, an NOS Benmar heater will leak if it doesn't start after the the first few minutes. Only a thin mesh type gasket is used and with age, it will deteriorate.

If the fuel control valve is functioning, but the heater fails to start, it will dump 1/4 cup (60 cc) of fuel into the combustion chamber within 3 minutes and some of this fuel may leak into the fresh air ducting downstream from the heater.

The seal between the combustion chamber and the fresh air, blowing past the heat exchanger is not under a lot of compression and fuel will leak past this joint under the right circumstances in both SW and Benmar units. The Stewart Warner heaters are safer when operating in the vertical position in this respect, leaving all the unburnt fuel in the combustion chamber only, if there is a leak. The gasket here is a silicone o-ring plus a fiber "washer", more substantial than the Benmar gasket.

If gasoline is used for fuel, I'd be seriously concerned about using these heaters if they didn't start right away (within 60 to 90 seconds).

So, if the heater doesn't start according to instructions, you'll have to take it apart and remove any fuel that may be left in it or "notify maintenance" as it were. A possible solution for the Benmar heater is to use a high temperature "stove" cement or similar. The SW gasket is much less likely to be effected by age.










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[steelsoldiers]

Good points, Bjorn! I had not considered the unburned fuel after a failed starting sequence.

I also plan on keeping a CO monitor in my truck when I will be using the heater for an extended period. That will make me feel better about using it, especially when parked with little air circulating through the cab. I'm going to install a heater kit in my bed eventually for camping. The CO detector will be handy there to make sure that I wake up in the morning.

 

[cranetruck]

Chris, read the spec on the CO meter you get, most designed for home use, will need a lot of time before setting off the alarm, the equivalent of a person getting a long enough dose to cause harm, perhaps 45 minutes or so.
I'd opt for an actual CO meter, which will give you an instant response.

When I restarted my heater with fuel left in the combustion chamber from a previous failed start, the air was mixed with blue-gray smoke from the heated excess fuel, certainly a good indicator.

 

[stampy]

You can pick up great CO meters in anyaircraft supply shop catalogs like sportys or wag aero, they are made to warn you well before you pass out or get sick.

 

[Stalwart]

Thanks for the heads up Bjorn, I was afraid of the flambe effect and never tried to start mine. I later heard the the gassoline ones were the real danger but I'd do numerous test firings outside the truck before using one inside it.

 

[dk8019]

Good info Bjorn! I understand that gasoline burning heaters were pretty common not to long ago, I hate to think what one of these might turn into if it doesn't start up correctly, or if something else goes amiss.

 

[cranetruck]

I'm aware that this may have limited interest, but it's still a bit of a problem for me.
The concern is that I ended up with exhaust fumes and fuel in the "fresh air" duct of my Benmar installation and the sealing of the joint between combustion chamber and the fresh air stream is the problem.
The Stewart Warner has the superior solution as I see it, a silicon O-ring with a protective fiber washer vs the Benmar woven fiberglass (apparently) cord.
The SW vaporizer is located below the joint (assuming an upright installation), so if there is no ignition, excess fuel ends up in the combustion chamber. On the other hand, the Benmar vaporizer is above the joint and any excess fuel can easily be trapped in this V-shaped joint and soak through the fiberglass "washer" and end up in the fresh air stream.
In both cases, the joint is under limited or little compression, due to the way the heater shells are joined (the screws don't pull the sections together).

I would suggest that before these heaters are used after sitting in storage for 20 or 30 years, this seal is checked and the gaskets replaced, at least the fiber washer in the SW units and for sure the wimpy Benmar fiberglass washer, which was nothing more than a broken string in the 3 units I have looked at...

Edit: ...and, because of its location in relation to the vaporizer, the SW joint may not be running as hot as the Benmar joint, offering more choices as to the material used for the gasket.

 

[Audiocontr]

Sorry to bring this thread up again. Has anyone located a viable replacement gasket for the Benmar? Mine has an odor, but no leaking fuel.

 

[cranetruck]

Sorry to bring this thread up again. Has anyone located a viable replacement gasket for the Benmar? Mine has an odor, but no leaking fuel.

Yes, got this from McMaster Carr and installed it. Has worked good for about a year now. Don't know the stock #...
When you install it, use long carpenter's style clamps to compress the heater "shells" (housing sections), the clamps holding it together can then be installed when the gasket is under compression. By themselves, the clamps do not squeeze the gasket.