LP Appliances- pilot type water heaters

In my last blog entry, I touched on the theory behind using a pilot and thermocouple as a flame safety device in an LP appliance. This week we’ll look at how it works in the most common RV application- an LP water heater.

The 2 main RV water heater manufacturers are Atwood Mobile Systems and Suburban Manufacturing. I’ll look at an Atwood model (because that’s what I have on the lot to photograph!), but the problems, fixes and maintenance are largely the same for both manufacturers.
The main parts in a pilot type water heater are the gas valve- which performs a dual duty- as a thermostat, maintaining the desired water temperature, and as a safety valve, shutting off the flow of LP gas when the pilot is out, the manifold and orifice (the manifold is the brass part right under the gas valve), the burner tube, and the pilot assembly (of course, the tank and all of the case are also important!)

In the picture to the right, you can see the probe which is inserted in to the water heater tank to sense the temperature of the water. On this particular valve, the temperature is set by a lever on the top- other models use a knob on the front for temperature adjustment. LP gas enters the valve, goes through the safety valve assembly, then goes through the thermostatically controlled main valve, then out to the orifice and main burner.

You can see from this picture the electrical connection I talked about last week- the current from the thermocouple has to go through the wires, which lead to a thermal fuse, also known as an “ECO” or Energy Cut Off” fuse. This fuse will blow when the water temperature reaches an unsafe level, interrupting the flow of gas to the main burner and pilot, shutting the water heater off. If this fuse is bad, the valve must be replaced. Another type of failure is when the thermostat valve doesn’t shut completely off, leaving a small flame burning at the orifice- again, it’s time for a new valve.

Next we come to the main burner assembly. Even though the pressure of the LP gas is very low, when it comes out of the orifice and enters the burner tube, the flow is enough that it pulls air in to burn. LP gas has a fairly small range of air to fuel that it burns well in- this adjustment allows you to fine tune the air mixture. There is a similar arrangement (though not adjustable) for the pilot assembly.

If the main burner adjustment is off, the burner is dirty (even by a spider web), or the pilot air inlet is dirty, the result will be incomplete combustion- a yellow flame and soot buildup.

To clean and adjust the main burner, a burner tube cleaning brush can be used- slide the “air shutter” open, run the brush through, and readjust the shutter. To adjust the shutter (air mixture), I like to slide the shutter closed until the flame is yellow and “lazy”, the slide it open [b]just[/b] until the flame starts to roar. On Suburban models, the flame is supposed to roar (but is non-adjustable), on Atwood models, it isn’t.

One last look at a pilot assembly- here you can see the tube, the orifice and the burner. It’s worth noting that the orifice is replaceable, though often it is better to simply replace the whole assembly (getting a new thermocouple is the proccess).

If the pilot flame is burning yellow, it is usually enough to simply blow out the assembly, by applying compressed air to the air inlet hole- 90% of the time, this will clean the assembly well enough to get rid of the problem

So- the problems with a pilot type water heater can be:

Pilot won’t stay lit:

• Too small a pilot flame (clean)
• Bad thermocouple (replace)
• Bad gas valve (replace)

Burns, but creates black soot:

• Dirty/Blocked Burner Tube (Clean)
• Dirty Orifice (have cleaned by qualified person)
• Bad LP pressure (have set by qualified person)

I hope this touches on the main causes of the dreaded “cold shower” when using a pilot type water heater.

Questions? Comments? Discuss in this RV.net forum thread.