A friend of mine recently ran into a problem with her Atwood water heater not starting—the ignitor wouldn't spark. She checked the fuses (both the one inside the coach and the 2A fuse on the water heater's chassis), but they were good. She went out and ran a pipe cleaner through the burner tube, flushing out an indignant beetle, but still the ignitor wouldn't spark. That's when I got called in.
Taking a look at the water heater, I noticed a small diode-like electronic part encased in a clear plastic sleeve, hanging out in midair a few inches above the burner tube's air slots. The plastic sleeve was charred and bubbled on the underside. When I saw that, I was pretty sure I knew what the problem was. As a test, I bypassed the part with clip leads, and sure enough, the water heater fired up as soon as she turned it on.
What was this tiny mystery part? According to the Atwood service manual, the device in question is a "thermal cut-off," Atwood part #93866. The manual has this to say:
In other words, if the tube is partially blocked, the flame can "flash back" instead of firing down the burner tube. If that happens, the heat rises and triggers the thermal cut-off mounted above it, in order to turn off the heater before the misdirected flames can do serious damage. Here's what it looks like when that happens:
What Atwood doesn't say is that there's another scenario where flames can emerge from the air slots: when propane pressure is low. That could happen if you run out of propane; if you take on a load of propane laced with butane in a warm part of the country and then drive to a cold area where the butane won't vaporize, causing a pressure drop; or if your propane regulator is faulty, as once happened to another friend of mine. Any of those scenarios can cause a flashback that will fry the thermal cut-off and stop the water heater from igniting.
Finding the part
Like a fuse, the thermal cut-off is a one-shot device... so if it blows, it must be replaced. This is where it gets interesting. A Google product search on 'atwood thermal cut-off' turned up prices ranging from $16.00 to nearly $23.00. I knew that a simple part like this couldn't possibly cost that much—somebody, probably Atwood, was tacking on huge markups.
By cross-referencing the part numbers on my own undamaged thermal cut-off, I was able to locate the part at Digi-Key for just $1.60—a far cry from the RV dealers' pricing! Here's a link to the item. (You can also find it at Amazon, but it's ten bucks and change there.)
For the record, it's an NTE Electronics #NTE8096 thermal fuse, designed to carry 15A and cut off at 98 degrees Celsius. It's not something you can find in a local Radio Shack store, so I carry a few spares with me just in case. You might want to do likewise.
To be fair, Atwood's $20.00 replacement part includes two thermal fuses (worth five bucks), some plastic sleeving, and a couple of 3/16" crimp-on lugs (worth a buck, maybe)—but I'm sure most of us can scrounge those items for a lot less than Atwood is charging!
So if your water heater won't ignite, open the cover and check this part to see whether its clear plastic sleeve is burned or melted. If so, suspect either low gas pressure or (more likely) an obstruction such as a wasp or spider nest. Make sure any obstruction is cleared before replacing the part or trying to use the water heater.
Note: as mentioned, it's possible to jumper around the thermal cut-off and get the heater working again, but you risk a fire if the "flashback" condition recurs. If you do resort to this, be VERY sure that you've removed the cause of the flashback problem before you try jumpering. It's far better to carry spare thermal cut-offs and stay safe!
An ounce of prevention
How can you keep insects from getting into the water heater in the first place? With a protective screened cover, available from just about any RV supply vendor. But there's controversy over these. Some manufacturers say not to use them, claiming that they block airflow through the heater.
Here's my take: first, the coarse screen is far less of an impediment to airflow than the narrow slots of Atwood's metal grille. Because of its dished shape and the fact that it's several inches larger than the factory-supplied grille, the screen has a much larger open area. In short, I don't think there's enough air restriction here to worry about. If you're really worried about airflow, take out the Atwood grille and use the screen instead.
I've been using these protective screens for more than ten years on my water heater and my furnace (yes, you need both!), but I've never seen or heard of a case of overheating or other problems caused by one of these screens. On the other hand, I've seen several cases of damage caused by insects, and heard of others—including more than one fire.
Bottom line: the risk of damage from insects is far greater than the risk of any problem caused by a protective screen of this type. I strongly recommend installing them on both the water heater and the furnace.