First, thanks to all who helped put this together - John, Erik, Judy and Andy. They picked up a number of errors and omissions that helped me make it better.
This is targeted at water heaters from Atwood, used in Lazy Daze motor homes. It is NOT applicable to other makes of heaters. It is not intended to be a detailed or comprehensive guide to troubleshooting your heater. Rather, the intent is to allow you to become familiar enough with the electronics in the system to understand what is going on, and possibly to identify a failed component if professional help is not available. Other articles address flushing the heater, and I do not plan to address issues with propane delivery here.
Now the disclaimers - Combining propane and electricity is dangerous. Do not mess with the system if you do not feel able to do so safely. Read your Atwood manual for more detail. If you do not have one, you can download one from http://atwoodmobile.com/manuals/waterheaters/MPD%2093756%20SP%2011.19.07.pdf. Their manual has step by step instructions for troubleshooting, and it is recommended that that be your primary resource.
Stay within your skill set. If you are not comfortable with any of these steps, let a pro do the work.
Do not work on the water heater when the water is hot.
You may want to take pics of everything before you start, but if you only deal with one component at a time, getting things back together is pretty simple.
These water heaters run on 12 volts for the controls and propane for the heat. There are some 120v heaters out there, but this does not address them. They have a confusing array of wires running here and there. When the heater isn’t working right and you are well away from help, it can be a challenge to figure it out. This article is intended to give you a basic understanding of the parts, what they do, and perhaps how to check them if you are having problems.
Looking at the heater, in the top right corner is the circuit board. It is the brains of the operation. It senses what is happening elsewhere in the system and does things like allow propane to flow or not, send a spark to light the propane, monitor the water temperature, and monitor the flame.
To the left of the Board is the brass pressure relief valve. It has two functions. If there is too much pressure in the heater, such as from water that is overheated and boiling, it releases pressure rather than allowing the pressure to build and cause a steam explosion. It is also used to restore or maintain a pocket of air at the top of the heater chamber which cushions the chamber and your plumbing from overpressure as water heats and expands.
Left of the pressure relief valve is the exhaust vent.
Below the pressure relief valve are two thermostats and a device (thermal cutoff) in a clear plastic tube. The left thermostat, covered by black foam tape, is labeled ECO (Emergency Cutoff). It shuts down the system if heater temperatures are too high. It is normally in the closed position, conducting electricity through it, but if it senses temperatures that are too high it will open, cutting off flow of propane.
The right thermostat is the one the system uses to control the water temperature. When the water is cold, this thermostat is closed, conducting 12 volts through it, and current can flow to the system. When the water reaches the designated temperature the thermostat opens, cutting off the electricity, and the system cuts off the flow of propane.
Floating about in front of the thermostats is a little electrical part encased in a clear plastic tube. This is a thermal cutoff. If there is too much heat in the area behind the thermostats, perhaps from a blocked flue or exhaust vent, a propane leak or some such, this part melts, cutting off power to the system.
Andy Baird’s “Eureka” talks about the thermal cutoff. He said, ”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. But I was able to locate the part in the Newark Electronics catalog for just $2.16—a far cry from the RV dealers’ pricing! Here’s a link to the item.
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 four 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!”
Below the thermostat and ECO is the propane solenoid valve. To the right is the mix tube or burner, which mixes air with the propane and turns and directs the air and propane mixture into the burn chamber. In front of the mix tube outlet, at the entrance to the burn chamber, is a pair of contacts from a white ceramic ignitor, which acts like a spark plug.
Finally, above and left of the solenoid valve is a plug which allows draining of the heater for maintenance such as flushing or storage.
OK, here we go. When you switch the heater on and the water in the heater is cold, 12 volts should go first to the thermal cutoff, then to the thermostat, then to the circuit board. That condition is signaled by a red and a green light by the switch over the stove. If the thermal cutoff is melted, or if the thermostat is in the open position, you will see only a green light on the switch above the stove, not the usual red and green lights on startup.
When the board receives a 12 volt signal, it accepts that as meaning that the thermal cutoff and the thermostat are closed. Then the board thinks it is safe to begin lighting the burner to heat water. It signals with a red and green light by the switch, and begins the startup procedure, sending voltage through the ECO to the solenoid. If the ECO is closed, that voltage reaches the solenoid, and the solenoid allows the flow of propane.
The board now sends a high voltage pulse to the ignitor. Between pulses it measures the resistance across the ignitor’s 1/8” gap. That resistance tells the board whether there is a flame or not. It should try three times to light the burner. If no flame is sensed, it will shut down the propane and cease further attempts. If a flame is sensed, the lights by the switch go to green and it continues the burn until the thermostat opens, at which time it cuts off the flame.
So - what can go wrong? Let’s assume here that you do not have dead batteries, and that you have ample propane. What next?
First, there are a lot of spade connectors in the system. They are subject to corrosion. If they become corroded, the board will not get good information and the heater will not work right. So, start by making sure all the connections are clean and free of corrosion. Connectors can be cleaned with contact cleaner, with a fine abrasive such as a coarse eraser or fine sandpaper, or by other similar means. When cleaning the contacts on the board, be gentle. That material is thin. You do NOT want to abrade it away.
Next, if you have a multimeter, you can check the thermal cutoff, the thermostat and the ECO for an open circuit. All should read closed or very low resistance. If any read open, you found the problem. Replace the part. If you do not have a multimeter, and you have a helper, you can test them by bypassing each of these items, one at a time, by unplugging the spade connectors from the device and connecting the two wire ends together. WARNING - DO NOT USE BYPASSING TO OPERATE THE HEATER, just to troubleshoot it. The recommended way is to have a helper inside, with the switch off. Make the bypass connection and have the helper turn the switch on. If the heater lights, you found the problem. Turn the switch off immediately (you disconnected a safety device) and reconnect the failed device until you can get a replacement. Do the thermal cutoff first, then the thermostat, then the ECO.
The Atwood manual gives places where voltage can be checked with a multimeter. You could have a working ECO, but corroded contacts, leading to low voltage at the solenoid. If you have a multimeter, do those voltage tests.
OK, you checked and the thermal cutoff, thermostat and ECO and all are showing closed or good, but you have no flame. The voltage at the solenoid is good. Is the solenoid operating? You may be able to hear gas flow if it is. My hearing is not good enough for that, but yours might be. Make sure the mix tube is clear of debris, spider webs, etc., then try again. If the tube is removed to clean, it should be put back in same location and make sure the flame is adjusted to provide the most efficient air gas mixture. The color of the flame is the indication of this by seeing a good blue flame with a tinge of yellow on the flame tip. Too much air will be noticeable by a louder burn and yellow flame. This adjustment is done by loosening the screw at the beginning of the tube and making the air gap there larger or smaller, then tightening the screw. This setting should not be done at higher elevations.
At this point, some guesswork is involved. If you see no spark at the ignitor electrode tip, then either the ignitor or the board is bad. If you see a spark but no flame, you may not be getting adequate propane flow, indicating a problem with the solenoid, you may not be getting air mixed with propane, or the spark may be weak or in the wrong place. If you see a flame that burns briefly before going out, either the ignitor or the board is probably bad, but a blocked mix tube or propane orifice could be to blame.
Let’s look at the ignitor. You should hear a tic - tic - tic sound as the ignitor shoots a spark across that 1/8” gap, and unless you are in bright sunlight you should be able to see the blue spark. You might even see a jet of flame come on, burn briefly, then go out. Remember, the ignitor does more than just throw a spark. It also tells the board when a flame is present. A dirty ignitor, corroded electrodes or a crack in the ceramic can result in improper resistance, shutting down the gas flow. The ignitor can be removed fairly easily. The Atwood manual says, “The gap between the sparking probe and the ground probe should be 1/8”. The probes should be clean and free of cracks, flaking and corrosion. Position the probes so that they are in the path of the gas flow. Cracks in the ceramic insulator can also be the source of an intermittent problem. To check for cracks insert a fiber washer or any other type of insulation material in the 1/8” gap between the rods. Remove the gas valve from the circuit and turn the unit on. If you see a spark jumping from the ceramic to the ground rod or bracket, replace the spark probe.”
If you service the ignitor electrodes, after cleaning the igniter rods the gap adjustment should be made with two pairs of pliers, one to hold the straight rod right above the ceramic to not put undue stress on it while bending the curved rod with the other pair. If you have gotten the ceramic wet, it is somewhat porous and may take a while to dry completely. Rain won’t do it, but direct water spray, such as your hand slipping when flushing the heater can do it. Guess how I know that… Until it is dry, it may send a bad resistance reading to the board, preventing the heater from working. A day of dry conditions should deal with a moisture problem.
OK, the thermal cutoff is good, the thermostat and ECO are good, and you see nothing wrong with the ignitor, but you still have no hot water. The remaining problems are gas flow and a bad board. A board is not cheap, but can be replaced fairly easily. It is NOT RECOMMENDED that you mess with the solenoid or the propane orifice. There is simply too much risk if you mess something up. If you are confident that propane is flowing, but you have no flame, your problem is pretty much narrowed down to the ignitor or the board. The ignitor is cheap. Replace it. Still no good? Now, finally, it is time to try replacing the board.
Hopefully, this will help you understand the heater’s steps and may give you a clue what to look for if you are having problems. Again, stay within your skill set on this. With some understanding and a sense of how the parts work together, you may be able to figure it out. If not, get thee to a qualified service person.
Ken Fears - September 2016
As Editor I can not resist adding a few very minor points to this fine article.
Note the nylon drain plug in the picture. A suggested improvement. One way to easily remove the drain plug is with a socket wrench with an extender and a swivel. That allows you to get around the gas plumbing.
Also note the position of their air/fuel “mixer tube”. It's all the way to the left. That position may yield the bluest flame. You need to know this if you disassemble the tube for cleaning. That needs to be done every few months of use. A few minutes with a wire brush and WD40 will clean it up.
I have my spare thermal fuses in a ziplock bag taped to the inside of the heater cover, so I can find them if I ever need one.
The adjustable thermostat allows you to lower the temperature to the level you find most useful. Some set it so that no cold water needs to be mixed. We keep ours at 120. I think the factory setting is about 140. How do you know what the temp is? You add an aquarium thermometer to the tank. I guess we need to add a How To for that Andy B improvement.