Water heater thermal cut-off November 16, 2008, 02:46:51 pm Yahoo Message Number: 97304My friend Erik recently ran into a problem with Gertie's water heater, and in troubleshooting it, he and I learned a few things--including a way to save twenty bucks. :-)The symptom was no spark/no ignition, following an episode of low propane pressure due to a faulty regulator. Examining the water heater, Erik 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. Bypassing this device with clip leads got the water heater working again.Checking the Atwood service manual, I found that the device in question is a "Thermal Cut-off," Atwood part #93866. The manual has this to say: THERMAL CUT-OFF DEVICE Current Atwood direct ignition water heaters are equipped with a thermal cut-off device. This device is located on the incoming power wire and is connected to the thermostat. The thermal cut-off is designed to permanently break circuit and shut down the water heater before excessive heat can cause damage due to obstructions in the main burner tube or flue tube caused by spiders or mud wasps. These obstructions can cause the main burner flame to burn outside the main burner tube. When the flame or the heat from the flame contacts the thermal cut-off, the circuit will open. In other words, if the tube is partially blocked, flame can shoot out of the air slots 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. What Atwood doesn't say is that there's another scenario where flames can emerge from the air slots: when propane pressure is low, as in Erik's case. 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 regulator is faulty, as in Erik's case.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. Erik and I knew that a "jelly-bean" 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 undamaged thermal cut-off, Erik was able to locate the part in the Newark Electronics catalog for just $1.64 in quantities of 1-24:http://tinyurl.com/6bzhg>orhttp://www.newark.com/31C5993/electrical-circuit-protection/product.us0?sku=nte-electronics-nte8096>For the record, it's an NTE Electronics #NTE8096 thermal fuse, designed to carry 15A and cut off at 98 degrees Celsius. Seems like a good thing to have spares of, so I ordered a few each for me and Erik. You might want to do likewise. :-)To be fair, Atwood's $20.00 replacement part includes two thermal fuses (worth three 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! Summary: 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. 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--see this website for a vivid description of the risks involved!http://www.pjrider.com/WHFlashback.htm>Far better to carry spares and stay safe. :-)Andy Baird http://www.andybaird.com/travels/
Re: Water heater thermal cut-off Reply #1 – November 16, 2008, 03:53:21 pm Yahoo Message Number: 97307Sorry, folks, I left off the last character of a TinyURL again. :-(The thermal fuse is at http://tinyurl.com/6bzhg7>.Andy Baird http://www.andybaird.com/travels/
Re: Water heater thermal cut-off Reply #2 – November 17, 2008, 09:40:56 am Yahoo Message Number: 97319Hey Andy, Many thanks for writing up the excellent analysis of the water heater problem and money saving source for the part to correct it. The Newark catalog brings back a lot of memories as it was one of my prime sources for project parts when I was engineering for Bell Labs
Re: Water heater thermal cut-off Reply #3 – November 17, 2008, 10:07:39 am Yahoo Message Number: 97320QuoteLike 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. Erik and I knew that a "jelly-bean" part like this couldn't possibly cost that much--somebody, probably Atwood, was tacking on huge markups. The problem I had with this cut-off was that it would accumulate moisture inside that plastic tube usually on damp days. I would have to go out a physically wiggle this for ignition but the fuse never did blow. I tried sealing this tube off but that was not much help. Probably other damage was done by that time. I ordered a 2 pack card I believe from Amazon for Joe Hamm
Re: Water heater thermal cut-off Reply #4 – November 17, 2008, 10:54:18 am Yahoo Message Number: 97321Joe, Try putting some silicone grease on the connections. This will prevent problems with water and corrosion. It's often used in aviation for harsh environments.bumper
Re: Water heater thermal cut-off Reply #5 – November 17, 2008, 11:46:07 am Yahoo Message Number: 97323Quote Joe,Try putting some silicone grease on the connections. This will prevent problems with water and corrosion. It's often used in aviation for harsh environments.Quote bumper Thanks for the tip. Sounds better than me out there in my PJ's on a cold morning doing my thing in order to take hot shower. Joe
Re: Water heater thermal cut-off Reply #6 – November 25, 2008, 03:16:36 am Yahoo Message Number: 97522Another reason for the water heater thermal couple is as a safety device when you forget and drive down the road with the hot water heater on. Yep, it crisps right up when the airstream and wind divert that little flame from its intended path. It's always a good idea to carry a spare.Lori and the girls21 ft SOB[Non-text portions of this message have been removed]
Topic: Water heater thermal cut-off (Read 16 times)
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Topic: Water heater thermal cut-off (Read 16 times)
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